IUPAC Nomenclature

IUPAC nomenclature uses the longest continuous chain of carbon atoms to determine the basic root name of the compound.
The root name is then modified due to the presence of different functional groups which replace hydrogen or carbon atoms in the parent sturcture.

There are a number of different ways to modify the root name to indicate the functional groups present.

• Substitutive : (most common) : the highest priority functional group modifies the suffix of the root name, while all other groups, or substituents, are added as prefixes to the root name.

• Functional group : names the compound based on the highest priority functional group, i.e. as an alcohol, ketone, alkyl halide, etc.

• Replacement : used to indicate when an atom, usually carbon, is replaced by another atom.

• Conjunctive : used to combine named subunits (i.e. cyclohexanecarboxylic acid).

• Common or trivial : due to widespread use, some compunds with simple names have been adopted into basic IUPAC nomenclature.

These pages focus primarily on the substitutive and functional group nomenclature but also include examples of all systems in cases where the name is generated by a combination of methods.

Remember:

• that organic molecules can in general be either chains (also known as acyclic) or cyclic or a combination of both. In most cases this doesn't make a difference.  The general rules for cyclic systems will be developed for cycloalkanes and can be applied to other scenarios.

• molecules are not restricted to a single functional group, they can have several functional groups. A common example are amino acids which have both an amine and a carboxylic acid present. 
  

However, the most important feature of nomenclature is that given a complete name, a single unique structure can be drawn

Basic rules

The IUPAC systematic name of an organic compound can be constructed based on a series of steps and rules:

• Identification of the principal functional group and substituents

• Identification of the longest continuous chain containing the principal functional group.

• Assign locants (i.e. numbering) to the principal functional group and substituents.
  

Misconception Alert! When assigning the numbers (i.e. the locants) while naming an organic compound there is NO rule based on summing the numbers.

The steps and rules are summarised below, more details are provided as the cases are encountered.

Principal Functional group	The principal functional group is used to define the class the compound belongs to e.g. an alcohol, ROH The principal functional group is the highest priority functional group. Functional group priority is discussed later. The principal functional group is usually given the lowest locant possible. When a group is named as a substituent, the priority order does not apply.
Longest chain	The longest continuous chain containing the principal functional group defines the root name.  Other groups attached to this chain are called substituents. If there are two chains of equal length, then the choice that gives the simplest substituents is chosen.
Numbering  (i.e. assigning locants)	The numbers that define the positions of the principal functional group and substituents are called locants. Compounds are numbered from one end of the longest continuous chain. The locants are assigned such that the principal functional group gets the lowest possible locant. If this results in a "tie" then the first point of difference rule is applied so that the first time a difference in numbering occurs, then the method that gives the lower number at this first difference is used. In the event that there is no first point of difference then alphabetisation is used. Note that a locant can only be omitted if they are a. "1" and b. would be unambiguous (see later).

What's in a name?

The IUPAC name of an organic molecule is assembled from components that describe various features and parts of the molecule.

Functional group suffix
This is added to the end of the name based on the principal functional group.

Root
This defines the number of atoms (usually carbon atoms) in the longest continuous chain that contains the principal functional group.

Substituent prefix
Any groups other than the principal functional group appended to the root chain are called substituents, i.e. they have replaced an H atom on that root chain. 
Substituents are added to the beginning of the name and are listed in alphabetical order.

Multiplier
If a functional group or substituent occurs more than once, a simple multiplier (e.g. di, tri, tetra, etc.)  is used to indicate how many times it occurs.

Locants
Locants are numbers (or occasionally letters e.g. N-) that define the position of the principal functional group and substituents.  Typically there needs to be a locant for each functional group and each substituent. The 1993 modifications requires that the locant for the principal functional group is placed before the functional group suffix, e.g. pentan-2-ol, see below. Note that the functional group priority order (see functional group page) only applies if the group is being named as the principal functional group (i.e. it contributes as the functional group suffix), and the priority order does not apply if the group is named as a substituent.

The basic structure of the IUPAC name is shown schematically below : 

Functional Groups

Here is a list of the more important functional groups arranged in decreasing priority order for a nomenclature perspective.

Note that aromatic systems (arenes) such as a benzene ring should also be thought of as a functional group, but they don't fit into the priority order list shown below. 

This priority order is important in nomenclature as the higher priority group is the principal functional group and it is typically numbered such that it has the lowest number (the lowest locant). 

You need to learn to recognise these functional groups not just for nomenclature but in order to recognise their reactions later. 
In each case the fundamental functional group unit is shown, it is this that you need to be able to recognise - pay attention to the atoms involved and the bonding patterns. 
Note the "R" is used to represent generic groups based on C (such as a methyl group) or just H.

All the 3D-JSMOL structures can be manipulated using your computer mouse (e.g. zoom, rotate)

Functional Group	Formula	Structure
Carboxylic Acids
Acid Anhydrides
Esters
Acyl Halides
Amides
Nitriles
Aldehydes
Ketones
Alcohols
Thiols
Amines
Ethers
Sulfides
Alkenes
Alkynes
Alkyl Halides
Nitro
Alkanes

Root names

Here is a list of the root names for naming the parent hydrocarbon chains of C1 to C12 and a few other biologically important chains.
This list is very important as it provides the foundation for all IUPAC nomenclature : you will have to learn the names.
You can see models of C1-C10 by using the buttons above the box.

C1 = meth- C2 = eth- C3 = prop- C4 = but- C5 = pent- C6 = hex- C7 = hept- C8 = oct- C9 = non- C10 = dec- C11 = undec- C12 = dodec- C16 = hexadec- C18 = octadec- C20 = icos-

Alkanes

Nomenclature	Formula	3D structure
Functional group suffix = -ane  Substituent name = alkyl  Structural unit : alkanes contain only C-C and C-H bonds. Note: alkanes are the simplest organic compounds and are the minimum present in an organic molecule.

Simple alkane chains without branches are named using the appropriate root name plus the suffix -ane:	Propane  CH3CH2CH3
Simple cyclic alkanes  without branches are named using the prefix cyclo plus the appropriate root name plus the suffix -ane:	Cyclopropane

Simple Branched Alkanes

Simple branched alkanes are really alkanes with alkyl substituents.  The principles covered here will apply to other substituted systems in general.

The substituent is named in a similar way to the parent alkane. It is named based on the number of carbon atoms in the branch plus the suffix -yl.   

CH3-	methyl
CH3CH2-	ethyl
CH3CH2CH2-	propyl
CH3CH2CH2CH2-	butyl
CH3CH2CH2CH2CH2-	pentyl

In general an alkane type substituent becomes an alkyl group (often represented as "R-")

The name is then constructed in the standard way with locant + substituent prefix + root name  based on the basic rules. 
When constructing the name, hyphens separate locants and letters, commas separate locants. No spaces.

Consider the following examples: 

Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C5 therefore root = pent The branch is a C1 alkyl group i.e. a methyl group The first point of difference rule requires numbering from the right as drawn, the substituent locant is 2- 2-methylpentane	CH3CH2CH2CH(CH3)2
Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C5 therefore root = pent The branch is a C2 alkyl group i.e. a ethyl group The substituent locant is 3-  3-ethylpentane	CH3CH2CH(CH2CH3)CH2CH3
Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C6 therefore root = hex There are two substitutents, both C1 alkyl group i.e.  methyl groups The first point of difference rule requires numbering from the left as drawn, the substituent locants are 3-  and 3- 3,3-dimethylhexane	CH3CH2C(CH3)2CH2CH2CH3
Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C6 therefore root = hex There are two substitutents, both C1 alkyl group i.e.  methyl groups The first point of difference rule requires numbering from the left as drawn, the substituent locants are 2-  and 3- 2,3-dimethylhexane	(CH3)2CHCH(CH3)CH2CH2CH3
Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C6 therefore root = hex One C1 alkyl group substituent i.e. a methyl group One C2 alkyl group substituent i.e. an ethyl group The first point of difference rule requires numbering from the right as drawn, the substituent locants are 2-  and 4-  4-ethyl-2-methylhexane	(CH3CH2)2CHCH2CH(CH3)2

Common names for alkyl substituents

Certain alkyl substituents are very common, and you should be able to recognise and name them quickly, these are listed below.
In each case, the structural formula of the substituent is given where the C that is attached to the rest of the molecule is in bold. A line drawing of a generic example and the molecular models of the series of alkyl bromides are also available.

Notes:

• the the prefix iso is not hyphenated.
  

• the symbol "R" is commonly used to generically represent an alkyl group, e.g. R-X
  

Structural formula	Name	Symbol	Generic line drawing
CH3-	methyl	Me
CH3CH2-	ethyl	Et
CH3CH2CH2-	propyl	Pr
(CH3)2CH-	isopropyl	iPr
CH3CH2CH2CH2-	butyl
CH3CHCH2CH3	sec-butyl or s-butyl
(CH3)2CHCH2-	isobutyl
(CH3)3C-	tert-butyl or t-butyl	tBu

Complex names for alkyl substituents

The list of common alkyl substituents is quite short and it is therefore quite limited in its scope. In order to deal with other substituents (but it can also be applied to the common branched substituents too), there is a methodology for dealing with more complex substituents.  This essentially involves treating the substituent as if it were a molecule on its own, see below.

For the substituent:

• determine the point of attachment to the chain that defines the parent root (i.e. the longest continuous chain)

• the first carbon in the substituent is regarded as the C1 of the substituent

• from C1, find the longest continuous chain originating from C1 - this is the root for the substituent.

• identify any substituents off this chain

• list these alphabetically with appropriate multipliers

• insert locants as required remembering that the point of attachment is defined as C1.

Now include this "complex" substituent in the overall name...

• the complex substituent appears in brackets proceeded by its locant
  

• the complex substituent is alphabetised based on the first letter of the name in the bracket : this includes the multiplier e.g. di

• this is because the term in brackets is the name of the whole complex substituent and not several substituents.

• syntax: locant numbers separated from letters by hyphens, e.g. 1-methyl, locant numbers from locant numbers by commas e.g. 1,1-dimethyl, no spaces. 
  

Consider the following examples:

First work out the name of the complex substituent - this is shown in bold in the figure: The longest chain in the substituent is C3 therefore it's a propyl substitent There is one substituent off this chain, a C1 alkyl group i.e. a methyl group The methyl group is on C2 on the complex substituent Complex substituent name : 2-methylpropyl
Now work on the whole structure: Functional group is an alkane, therefore suffix = -ane The ring structure is C6 therefore root = hex There is one substituent: a complex substituent (see above) = (2-methylpropyl) Note the syntax: no space or hypen to separate the letters of the substituent and parent root.   (2-methylpropyl)cyclohexane

 

First work out the name of the complex substituent - this is shown in bold in the figure: The longest chain in the substituent is C2 therefore it's an ethyl substitent There are two substituents off this chain, both C1 alkyl group i.e. methyl groups Both methyl groups are on C1 on the complex substituent Complex substituent name : 1,1-dimethylethyl
Now work on the whole structure: Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C9 therefore root = non There are two substituents:  simple substituent is a C2 alkyl group i.e. an ethyl group a complex substituent (see above) = (1,1-dimethylethyl) The first point of difference rule requires numbering from the left as drawn, the locants are  4- and 5-  Alphabetisation requires that the complex substituent be listed first since "d" is before "e". Note the syntax: locant numbers are separated from letters with hyphens, locant numbers are separated from each other with commas.  4-(1,1-dimethylethyl)-5-ethylnonane

Of course, the common alkyl substituents can also be named in this more systematic way.  Both methods are used and you should be familiar with both methods.  The  following table list the common alkyl substituents with both naming methods.

Alkyl group, R-	Common name	Complex name (if different)
CH3-	methyl
CH3CH2-	ethyl
CH3CH2CH2-	propyl
(CH3)2CH-	isopropyl	(1-methylethyl)
CH3CH2CH2CH2-	butyl
CH3CH2CHCH3	sec-butyl or s-butyl	(1-methylpropyl)
(CH3)2CHCH2-	isobutyl	(2-methylpropyl)
(CH3)3C-	tert-butyl or t-butyl	(1,1-dimethylethyl)

Remember that if a structure has two chains of equal length, then the choice that gives the simplest branches is chosen, see below for an example.

Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C6 therefore root = hex The lower choice is correct - it has more but simpler substituents There are three simple substituents a C2 alkyl group i.e. an ethyl group two C1 alkyl groups i.e. methyl groups The first point of difference rule requires numbering from the top right as drawn, the locants are  2-, 3-  and 4-  Alphabetisation requires that the simple substituents be listed as ethyl before methyl  3-ethyl-2,4-dimethylhexane

or

Substituted Cycloalkanes

Substituted cycloalkanes are named in an analogous fashion to regular alkanes (also referred to as acyclic alkanes, i.e. non-cyclic alkanes).

In principle, a substituted cycloalkane could be name in two ways, either as an alkyl substituted cycloalkane, or as a cycloalkyl substituted alkane

• When the ring size (e.g. number of C atoms) is larger than the longest continuous chain, then the ring becomes the parent and hence the system is treated as a alkyl cycloalkane.

Other rules:

• If a ring is monosubstituted then no locant is required since the substituent must be at C1.

• If a ring is polysubstituted then each substituent requires a locant to unambiguously assign their relative positions. 
  

Ring is smaller than the chain, so the chain is the parent. Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C4 therefore root = but The branch is a C3 cycloalkyl group i.e. a cyclopropyl group The first point of difference rule requires numbering from the right as drawn, the substituent locant is 2-  2-cyclopropylbutane
Ring is larger than the chain, so the ring is the parent. Functional group is an alkane, therefore suffix = -ane The ring is C6 therefore root = cyclohex The branch is a C2 alkyl group i.e. a ethyl group Since it's monosubstituted, no locant is required since it is unambiguous (i.e. the ethyl group will define the C it is attached to as C1) ethylcyclohexane
Ring is larger than the chain, so the ring is the parent. Functional group is an alkane, therefore suffix = -ane The ring is C6 therefore root = cyclohex One branch is a C2 alkyl group i.e. an ethyl group The other branch is a C1 alkyl group, i.e. a methyl group Since the first point of difference doesn't resolve the numbering, the alphabetisation order is used. 1-ethyl-2-methylcyclohexane

Polycyclic alkanes

There are four basic types of polycyclic systems based on how the rings are connected together.  There are slight differences in the way in which they are named.
The systems can be further classified according to the number of rings present (e.g.  2 = bicyclic, 3 = tricyclic, 4 = tetracyclic etc.). The examples shown below are all bicyclic systems.
If you mouseover the images, then the common atoms and bonds will be highlighted. 

(1) Substituted systems The ring systems have no common atoms. In naming, it should be treated as a subsituted cycloalkane where the smaller ring is regarded as a substituent of the larger ring. The simplest example is the C6 system shown to the right.
(2) Spiro ring systems Spiro ring systems share a single common atom.  Hence the rings join at a single "point". The simplest example is the C5 system shown to the right.
(3) Fused ring systems Fused ring systems share two common atoms in one common bond, hence the rings share one side. The simplest polycyclic system is the C4 system shown to the right where the rings share two atoms (one common side).
(4) Bridged ring systems Bridged ring systems share more than two common atoms. The simplest polycyclic system is the C5 system shown to the right where the rings contain three common atoms (two common sides).  If we view the two rings as been the one on the left and the other on the right, then the highlighted C atoms are common to both (mouseover image).

Spirocyclic alkanes

• The root name is based on the number of C atoms in the ring structures.

• The prefix spiro[x.y] is added where x and y are the number of atoms in the links as defined below.

• The size of the rings is indicated in square brackets by counting the number of atoms in the links that make each ring, x and y, excluding the spirocenter.

• The number of atoms in the links is listed smallest first, i.e. spiro[x.y] where y > x

• For substituted spiroalkanes, the rings are numbered starting on the smallest ring adjacent to the spirocenter.

• If that does not resolve the numbering, then the other rules for determining locants are applied in order (i.e. principal functional group, first point of difference, alphabetisation).

Show spirocenter Show C in shorter link, x Show C in longer link, y

STUDY TIP In spiro systems, number the smallest ring first spiro = smallest

Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix spiro The ring structure is C5 therefore root = pent Excluding the spirocenter, there are two C atoms in each ring = [2.2] spiro[2.2]pentane
Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix spiro The ring structure is C8 therefore root = oct Excluding the spirocenter, one ring has two C atoms and the other has five C atoms = [2.5] spiro[2.5]octane
Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix spiro The ring structure is C9 therefore root = non Excluding the spirocenter, one ring has three C atoms and the other has five C atoms = [3.5] There is a C1 substituent = methyl group The lowest locant (first point of difference) = 5 5-methylspiro[3.5]nonane
Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix spiro The ring structure is C9 therefore root = non Excluding the spirocenter, one ring has three C atoms and the other has five C atoms = [3.5] There is a C1 substituent = methyl group The lowest locant (first point of difference) = 1 1-methylspiro[3.5]nonane

Fused and bridged bicycloalkanes

• The root name is based on the total number of C atoms in the ring structures.

• The prefix bicyclo[x.y.z] is added where x, y and z correspond to the number of atoms in the "links" as defined below.

• The size of the rings is indicated in the square brackets by counting the number of atoms in the links that make each ring, x, y and z, excluding the bridgehead atoms

• The number of atoms in the links are listed largest first, i.e. bicyclo[x.y.z] where x > y > z 
  

• For substituted bicycloalkanes, the rings are numbered starting at a bridgehead atom and proceeding round the rings, the largest ring first.

• If that does not resolve the numbering, then the other rules for determining locants are applied in order (i.e. principal functional group, first point of difference, alphabetisation).
  

  Show bridgehead C  Show C longest link, x  Show C in medium link, y  Show C in shortest link, z

• For example, in the structure shown above, the bridgehead atoms are C1 and C6. The three links are x= C2,C3,C4,C5, y = C7,C8 and the final link z has no atoms in it, so this is a [4.2.0] system.

STUDY TIP In bridged systems, number the biggest ring first bridged = biggest

Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix bicyclo The ring structure is C4 therefore root = but The links contain 1, 1 and 0 carbon atoms = [1.1.0] bicyclo[1.1.0]butane
Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix bicyclo The ring structure is C7 therefore root = hept The links each contain 2, 2 and 1 carbon atom = [2.2.1]  bicyclo[2.2.1]heptane
Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix bicyclo The ring structure is C10 therefore root = dec The links each contain 4, 3 and 1 carbon atom = [4.3.1]  bicyclo[4.3.1]decane
Functional group is an alkane, therefore suffix = -ane The ring structure requires the prefix bicyclo The ring structure is C9 therefore root = non The links each contain 3, 3 and 1 carbon atom = [3.3.1]  There are two C1 substituents  i.e. dimethyl groups First points of difference implies locants are 1,3- 1,3-dimethylbicyclo[3.3.1]nonane

Alkenes

Nomenclature	Formula	3D structure
Functional group suffix = -ene  Substituent name = alkenyl  Structural unit :  alkenes contain C=C bonds.

• The root name is based on the longest chain containing both ends of the alkene unit, the C=C.

• The chain is numbered so as to give the alkene unit the lowest possible numbers.

• The locant for the first carbon of the alkene is used in the name.

• If the position of the alkene is unambiguous, the locant is not required, see examples below.

• As we will see later, in certain cases we will also need to specify the stereochemistry. We are going to ignore stereochemistry on this page to deal with the basics first.
  

Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C3 therefore root = prop In order to give the alkene the lowest number, number from the right as drawn The C=C is unambiguously between C1 and C2 therefore the locant isn't required  propene	CH3CH=CH2
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C4 therefore root = but The C=C is between C2 and C3 so the locant is 2-  but-2-ene or 2-butene	CH3CH=CHCH3
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C4 therefore root = but In order to give the alkene the lowest number, number from the right as drawn The C=C is between C1 and C2 so the locant is 1-  but-1-ene or 1-butene	CH3CH2CH=CH2
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C6 therefore root = hex The system is cyclic there prefix = cyclo The C=C is unambiguously between C1 and C2 therefore the locant isn't required. cyclohexene
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C5 therefore root = pent In order to give the alkene the lowest number, number from the right as drawn The C=C is between C2 and C3 so the locant is  2- The substituent is a C1 alkyl group i.e. a methyl group The methyl group locant is 4- 4-methylpent-2-ene or 4-methyl-2-pentene
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C6 therefore root = hex The C=C is between C3 and C4 so the locant is  3- The substituent is a C1 alkyl group i.e. a methyl group The first point of difference rule requires numbering from the right as drawn to make the methyl group locant 3-  3-methylhex-3-ene or 3-methyl-3-hexene

Cis- and trans- terminology

If alkenes have two different substituents at each end of the C=C then they can exist as stereoisomers (as geometric isomers).
This is because there is restricted rotation of the double bond due to the pi bond which means they don't readily interconvert.

Examples:

• all terminal alkenes i.e. those with a C=CH₂ unit can not exist as cis- and trans- isomers.

• similarly, all 1,1-symmetrically disubstituted alkenes i.e. those with a C=CR₂ unit can not exist as cis- and trans- isomers.

• the terms cis- and trans- are assigned based on the relative arrangement of the alkyl groups that form the root name on the alkene unit. In general terms, that means including the principal functional group.

• alkenes with the R₁-CH=CH-R₂ unit can exist as cis- and trans- isomers.

If we consider the general alkene unit shown below, then the alkene can exist as cis and trans isomers only if R₁ is not equal to R₂ AND R₃ is not equal to R₄. 

There are two ways to name these types of isomers, one is the cis / trans method which is described here, the other is E / Z method that is described on the next page.

Misconception Alert! cis ¹ Z and trans ¹ E In general terms there is NO specific relationship between cis and trans / E and Z as they are based on fundamentally different naming rules.

1,2-disubstituted alkenes are described as:

• cis- if the two alkyl groups, R-, that form the longest chain (i.e. the root name) are on the same side of the C=C

• trans- if the two alkyl groups, R-, that form the longest chain (i.e. the root name) are on opposite sides of the C=C.

• these terms are inserted into the name as prefixes.
  

cis-	trans-

For example,  but-2-ene, where both R = methyl :

trans-but-2-ene
cis-but-2-ene

Tri- or tetrasubstituted alkenes are described as cis- and trans- based on the relative arrangement of the groups that form the parent hydrocarbon carbon chain that gives the root name. In the example shown the below, the longest carbon chain that gives the root name is highlighted in blue:

trans-3-methylhex-3-ene		cis-3-methylhex-3-ene

E- and Z-nomenclature of alkenes

On the previous pages, we looked at naming alkenes as cis- and trans-.

Misconception Alert! cis ¹ Z and trans ¹ E In general terms there is NO specific relationship between cis and trans / E and Z as they are based on fundamentally different naming rules.

It is important to note that the two methods are different (i.e. they are based on different rules) and they are NOT interchangeable, see below for an example. 
The cis- / trans- style is based on the longest chain whereas the E/Z style is based on a set of priority rules.  
You need to know both styles.

cis-but-2-ene or (Z)-but-2-ene	cis-2-chlorobut-2-ene or (E)-2-chlorobut-2-ene

The E- and Z- style is more reliable and particularly suited to tri- or tetra-substituted alkenes, and especially when the substituents are not alkyl groups. 

The Cahn-Ingold-Prelog priority rules are used for naming geometric isomers (e.g. E- or Z-alkenes) and other stereoisomers (see later).

In order to apply the Cahn-Ingold-Prelog priority rules to alkenes:

• Imagine each alkene as two pieces, each piece containing one of the sp² C atoms

• Assign the priority (high = 1, low = 2) to each atom attached to each sp² C based on atomic number 
  

  Subrules:

• Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H

• Same atom attached ? By moving out away from the C=C one atom at a time, locate the first point of difference and apply the priority rule there.
  
  

• Determine the relative position of the two higher priority groups

• If they are on the same side then it is a (Z)-alkene (German; zusammen = together) 

• If they are on opposite sides then it is an (E)-alkene (German; entgegen = opposite)
  
  

• If there is more than one C=C that can be E/Z, then the locant and the stereochemistry of each alkene needs to be included e.g. (2E,4Z)-
  

Example: but-2-ene

			<img src="http://www.chem.ucalgary.ca/courses/351/WebContent/orgnom/alkenes/alkenes-31d.gif" alt="relative positions" width="210" height="96" "="">
Step 1: Split the alkene	Step 2:  List the attached atoms looking for the first point of difference. Here we have C and H atoms attached.	Step 3: Assign the relative priorities.  Since the atomic numbers C > H then the -CH3 group is higher priority.	Step 4: Look at the relative positions of the higher priority groups: same side = Z, hence (Z)-but-2-ene
Step 1: Split the alkene	Step 2:  List the attached atoms looking for the first point of difference. Here we have C and H atoms attached.	Step 3: Assign the relative priorities.  Since the atomic numbers C > H then the -CH3 group is higher priority.	Step 4: Look at the relative positions of the higher priority groups: opposite side = E, hence (E)-but-2-ene

Example: 3-methylpent-2-ene

Step 1: Split the alkene	Step 2:  Assign the relative priorities.  The two atoms attached to the left end C are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority.	Step 3: The two atoms attached to the right end C are both C. So we need to look at the atoms these C are attached to. List them out...we have C (H,H,H) and C (C,H,H)systems. This means that at the first point of difference (see the arrow), a C > H meaning that the ethyl group is higher priority.	Step 4: Look at the relative positions of the higher priority groups : opposite side = E, hence(E)-3-methylpent-2-ene.

---

Alkenes as substituents

• In some cases, a group containing an alkene may need to be treated as a substituent.  
  

• In these cases the substituent is named in a similar fashion to simple alkyl substituents.

• The method is required when the alkene is not the priority group.

• The substituent is named in a similar way to the parent alkene. 
  

• It is named based on the number of carbon atoms in the branch plus the suffix -yl. i.e. alkenyl

• There are two common names that are widely used:
  

Alkenyl group	Common name	Systematic name
CH2=CH-	vinyl-	ethenyl
CH2=CHCH2-	allyl-	2-propenyl
CH3CH=CH-		1-propenyl

Functional group is a (tri) alkene, therefore suffix = -ene The longest continuous chain with 2 C=C is C6 therefore root plus "a" = hexa There are two alkenes in the parent chain, so insert the multiplier di Number from the right as drawn to give the C=C the lowest locants : therefore 1- and 4- The line drawing shows a trans- or E-alkene at C4. The substituent is a C2 alkenyl group i.e. an ethenyl group The substituent is on C3  trans-3-ethenylhexa-1,4-diene or (E)- 3-ethenylhexa-1,4-diene

CH3CH=CHCH(CH=CH2)2

Polyenes

The term polyene simply implies the presence of several alkenes. To be more specific, a diene has two C=C, a triene has 3 C=C etc. 
Polyenes are named in a similar manner to alkenes themselves.

• The root name is based on the longest chain containing both ends of all the alkene units.

• The chain is numbered so as to give the one of the alkene units the lowest possible number (i.e. first point of difference).

• The locant for the lowest numbered carbon of each alkene is used in the name.

• The appropriate multiplier (i.e. di- for two, tri for three) is inserted before the -ene suffix.

• In order make the name pronounceable,  -a- is inserted after the root.

• If there is more than one C=C that can be E/Z, then the location needs to be included with the locants and listed in numerical order, e.g. (2E,4Z)-

• Or, if using cis/trans- then the terms are listed in locant order e.g. trans,cis-

Functional group is an alkene, therefore suffix = -ene There are two alkenes, so insert the multiplier di The longest continuous chain is C4 therefore root plus "a" = buta The C=C units are both at the ends so we can number from either end Locants for C=C units are therefore 1- and 3- buta-1,3-diene or 1,3-butadiene	CH2=CHCH=CH2
Functional group is an alkene, therefore suffix = -ene There are two alkenes, so insert the multiplier di The longest continuous chain is C5 therefore root plus "a" = penta The first point of difference requires that we number from the right as drawn Locants for C=C units are therefore 1- and 3- The C3=C4 alkene has E stereochemistry (E)-penta-1,3-diene or (E)-1,3-pentadiene	CH3CH=CHCH=CH2
Functional group is an alkene, therefore suffix = -ene There are two alkenes, so insert the multiplier di The longest continuous chain is C5 therefore root plus "a" = penta The substituent is a C1 alkyl group i.e. a methyl group The first point of difference doesn't distinguish the C=C So, need to apply the first point of difference to the alkyl substituent. The first point of difference requires that we number from the left as drawn The methyl group locant is 2- Therefore the locants for C=C units are 1- and 4- 2-methylpenta-1,4-diene or 2-methyl-1,4-pentadiene	CH3C(CH3)=CHCH=CH2
Functional group is an alkene, therefore suffix = -ene There are two alkenes, so insert the multiplier di The longest continuous chain is C7 therefore root plus "a" = hepta The first point of difference requires that we number from the left as drawn Therefore the locants for C=C units are 2- and 4- The C2=C3 alkene has E stereochemistry The C4=C5 alkene has Z stereochemistry (2E,4Z)-hepta-2,4-diene or trans,cis-hepta-2,4-diene

Alkynes

Nomenclature	Formula
Functional group suffix = -yne  Substituent prefix = alkynyl  Structural unit :  alkynes contain C≡C bonds.

• The root name is based on the longest chain containing both ends of the alkyne unit, the C≡C.

• The chain is numbered so as to give the alkyne unit the lowest possible number.
  

Functional group is an alkyne, therefore suffix = -yne The longest continuous chain is C4 therefore root = but The first point of difference rule requires numbering from the right as drawn, the substituent locant is 1- but-1-yne or 1-butyne	CH3CH2C≡CH
Functional group is an alkyne, therefore suffix = -yne The longest continuous chain is C4 therefore root = but The substituent is a C2 alkyl group i.e. a ethyl group The first point of difference rule requires numbering from the right as drawn, the substituent locant is 2-  but-2-yne or 2-butyne	CH3C≡CCH3

Polyynes

The term polyyne simply implies the presence of several alkynes. To be more specific, a diyne has two C≡C, a triyne has three C≡C etc. 
Polyynes are named in a similar manner to alkynes themselves and to polyenes.

• The root name is based on the longest chain containing both ends of all the alkyne units.

• The chain is numbered so as to give the one of the alkyne units the lowest possible number (i.e. first point of difference).

• The locant for the lowest numbered carbon of each alkyne is used in the name.

• The appropriate multiplier (i.e. di- for two, tri for three) is inserted before the -yne suffix.

• In order make the name pronounceable,  -a- is inserted after the root.

Functional group is an alkyne, therefore suffix = -yne There are two alkynes, so insert the multiplier di The longest continuous chain is C6 therefore root plus "a" = hexa The first point of difference requires that we number from the right as drawn Locants for C≡C units are therefore 1- and 4- hexa-1,4-diyne or 1,4-hexadiyne

CH3C≡CCH2C≡CH

Enynes

The term enyne simply implies the presence of both an alkene and an alkyne.
Enynes are named in a similar manner to alkenes, alkynes and polyenes.

• The root name is based on the longest chain containing both ends of the alkene and alkyne units.

• The alkene suffix "ene" minus the last "e" is inserted after the root with its locant before the root.
  

• The alkyne suffix, "yne" is added at the end with its locant preceding it.
  

• Hence alkene +  alkyne = enyne.

• The chain is numbered in accord with the first point of difference rule to either the alkene or alkyne units the lowest possible locant.
  

• The locant for the lowest numbered carbon of the alkene or alkyne is used in the name.

• If there is a choice, then the C=C takes priority and is given the lowest locant.
  

Functional groups : alkene and alkyne, therefore suffix = -en and  -yne The longest continuous chain is C6 therefore root = hex The first point of difference requires that we number from the left as drawn Locant for C=C is therefore 1- Locant for C≡C is therefore 4- hex-1-en-4-yne	CH2=CHCH2C≡CCH3
Functional groups : alkene and alkyne, therefore suffix = -en and  -yne The longest continuous chain is C6 therefore root = hex The line drawing shows a cis or Z-alkene The first point of difference requires that we number from the right as drawn Locant for C≡C is therefore 1- Locant for C=C is therefore 4- cis-hex-4-en-1-yne or (Z)-hex-4-en-1-yne	CH3CH=CHCH2C≡CH
Functional groups : alkene and alkyne, therefore suffix = -en and  -yne The longest continuous chain is C5 therefore root = pent The first point of difference fails since C=C and C≡C are equally distant from the ends. Therefore the C=C takes priority and requires that we number from the left as drawn Locant for C=C is therefore 1- Locant for C≡C is therefore 4- pent-1-en-4-yne	CH2=CHCH2C≡CH

Haloalkanes / Alkyl halides

Nomenclature	Formula	3D structure
Functional group suffix =  halide (i.e. fluoride, chloride, bromide, iodide) Substituent name = halo- (i.e. fluoro, chloro, bromo, iodo) Structural unit :  haloalkanes contain R-X where X = F, Cl, Br, I etc. Notes :  Haloalkanes can also be named as alkyl halides despite the fact that the halogens are higher priority than alkanes. The alkyl halide nomenclature is most common when the alkyl group is simple.

Haloalkane style: 

• The root name is based on the longest chain containing the halogen. 
  

• This root give the alkane part of the name.
  

• The type of halogen defines the halo prefix, e.g. chloro-
  

• The chain is numbered so as to give the halogen the lowest possible number

Alkyl halide style:

• The root name is based on the longest chain containing the halogen.

• This root give the alkyl part of the name.

• The type of halogen defines the halide suffix, e.g. chloride
  

• The chain is numbered so as to give the halogen the lowest possible number.

Haloalkane style:  Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C2 therefore root = eth The substituent is a bromine, therefore prefix = bromo No locant is required since the -Br location is unambiguous (i.e. substitution at either carbon gives the same molecule) bromoethane Alkyl halide style: The alkyl group is C2, it's an ethyl  The halogen is a bromine, therefore suffix = bromide ethyl bromide	CH3CH2Br
Haloalkane style:  Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C3 therefore root = prop The substituent is a chlorine, therefore prefix = chloro The first point of difference rule requires numbering from the right as drawn, the substituent locant is 1- The locant is required to distinguish between 1- and 2-chloropropane 1-chloropropane	CH3CH2CH2Cl
Alkyl halide style: The alkyl group is C4, it's a tert-butyl  The halogen is a bromine, therefore suffix = bromide tert-butyl bromide Haloalkane style:  Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C3 therefore root = prop The substituent is a bromine, therefore prefix = bromo There is a C1 substituent = methyl The substituent locants are both 2- 2-bromo-2-methylpropane	(CH3)3CBr
Haloalkane style:  Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C4 therefore root = but The substituent is a bromine, therefore prefix = bromo Since bromine is named as a substituent, the alkene gets priority The first point of difference rule requires numbering from the left as drawn to make the alkene group locant 1-  Therefore the bromine locant 4- 4-bromobut-1-ene	CH2=CHCH2CH2Br

Alcohols

Nomenclature	Formula	3D structure
Functional class name = alkyl alcohol e.g. ethyl alcohol Substituent suffix = -ol e.g. ethanol Substituent prefix = hydroxy-   e.g. hydroxyethane Structural unit :  alcohols contain R-OH

• The root name is based on the longest chain with the -OH attached.

• The chain is numbered so as to give the alcohol unit the lowest possible number. 
  

• The alcohol suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -ol = -anol or -ene + ol = -enol.
  

Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C2 therefore root = eth No locant is required since the -OH location is unambiguous (i.e. substitution at either carbon gives the same molecule) ethanol	CH3CH2OH
Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C3 therefore root = prop It doesn't matter which end we number from, the alcohol group locant is 2- propan-2-ol or 2-propanol  (or isopropanol)	CH3CH(OH)CH3
Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkene therefore -ene The longest continuous chain is C4 therefore root = but The first point of difference rule requires numbering from the right as drawn to make the alcohol group locant 1- Hence the alkene locant is 3- but-3-en-1-ol or 3-buten-1-ol	CH2=CHCH2CH2OH

Diols (or polyols)

The term diol simply implies the presence of two alcohols. Polyols contain two or more -OH groups.

• The root name is based on the longest chain containing both the alcohol groups.

• The chain is numbered so as to give the one of the alcohol groups the lowest possible number (i.e. first point of difference).

• The appropriate multiplier (i.e. di- for two, tri for three etc.) is inserted before the -ol suffix or before the root.
  

Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkane therefore -ane There are two alcohols, so insert the multiplier di The longest continuous chain is C2 therefore root = eth Locants for -OH units are 1- and 2- ethane-1,2-diol or 1,2-ethanediol	HOCH2CH2OH
Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkane therefore -ane There are two alcohols, so insert the multiplier di The longest continuous chain is C3 therefore root = prop Locants for -OH units are 1- and 2- propane-1,2-diol or 1,2-propanediol	CH3CH(OH)CH2OH
Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkane therefore -ane There are two alcohols, so insert the multiplier di The longest continuous chain is C4 therefore root = but Locants for -OH units are 1- and 4- butane-1,4-diol or 1,4-butanediol	HOCH2CH2CH2CH2OH
Functional group is an alcohol, therefore suffix = -ol Hydrocarbon structure is an alkane therefore -ane There are two alcohols, so insert the multiplier di The ring is C6 therefore root = cyclohex Locants for -OH units are 1- and 2- cyclohexane-1,2-diol or 1,2-cyclohexanediol

Thiols

Nomenclature	Formula
Functional class name = alkyl mercaptan  Substituent suffix = -thiol e.g. ethanethiol Substituent prefix = sulfanyl-   e.g.  Structural unit :  alcohols contain R-SH Note: Thiols are the sulfur analogues of alcohols

• The root name is based on the longest chain with the -SH attached.

• The chain is numbered so as to give the thiol unit the lowest possible number. 
  

• The thiol suffix is appeneded after the hydrocarbon suffix : e.g.  -ane + -thiol = -anethiol or -ene + thiol = -enethiol.
  

Functional group is a thiol, therefore suffix = -thiol Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C3 therefore root = prop It doesn't matter which end we number from, the alcohol group locant is 2- propane-2-thiol  or  2-propanethiol

CH3CH(SH)CH3

Ethers

<td font-weight:="" bold;"="" style="vertical-align: middle; width: 400px;">Functional class name = alkyl alkyl ether e.g. ethyl methyl ether

Substituent prefix = alkoxy-   e.g. methoxyethane

Nomenclature	Formula	3D structure

Examples of how each of the types of nomenclature are applied are given below. No one method is more correct than the other, but some guidance on the common practices is given.  

Note : the examples chosen are to illustrate the naming subsystem.  

In practice, the examples labelled (*) would probably all be named as alkyl alkyl ethers.

"Simple" ethers

• If both groups are simple alkyl groups, then the ether is usually named as alkyl alkyl ether

• The alkyl groups are listed in alphabetical order
  

• If the two alkyl groups are the same, then it's a dialkyl ether

Functional group is a simple ether, therefore use alkyl alkyl ether First substituent is C1 alkane therefore alkyl = methyl Second substituent is C2 alkane therefore alkyl = ethyl ethyl methyl ether	CH3CH2OCH3
Functional group is a simple ether, therefore use alkyl alkyl ether Both substituents are C2 alkane therefore alkyl = ethyl Since there are two alkyl groups, multipler = di diethyl ether	CH3CH2OCH2CH3

 "Intermediate" ethers

• If one of the groups is more complex then the ether group is usually treated as an alkoxy substituent (i.e. R-O-).

• The ether is treated as a substituent (i.e. it is not the principal functional group).

• The more complex group (i.e. longer chain, principal functional group, more branched, other substituents) defines the root.

Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C3 therefore root = prop Ether substituent is a C1 alkoxy = methoxy The first point of difference rule requires numbering from the right as drawn to make the group locant 1- 1-methoxypropane *	CH3CH2CH2OCH3
Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C3 therefore root = prop Ether substituent is a C1 alkoxy = methoxy Numbering makes the group locant 2- 2-methoxypropane *	CH3CH(OCH3)CH3
Functional group is an alkene therefore suffix = -ene The longest continuous chain is C4 therefore root = but The C=C is the principal functional group and needs the lowest locant The C=C is between C1 and C2 so the locant is 1- Ether substituent is a C1 alkoxy = methoxy Numbering makes the alkoxy group locant 4- 4-methoxybut-1-ene	CH3OCH2CH2CH=CH2
* In reality, these examples are simple enough that they would typically be named as alkyl alkyl ethers

"Complex" ethers

• If both groups are complex then the ether can be named using -oxa which treats the oxygen as a substituent in the chain.

• The ether is treated as a substituent (i.e. it is not the principal functional group).

Hydrocarbon structure is an alkane therefore -ane Ether is treated as a chain substituent, therefore prefix = -oxa- The longest continuous chain (including the O)  is 5 therefore root = pent Numbering from the left as drawn to make the oxa group locant 2- 2-oxapentane *	CH3OCH2CH2CH3
Hydrocarbon structure is an alkane therefore -ane Ether is treated as a chain substituent, therefore prefix = -oxa- The longest continuous chain (including the O) is 4 therefore root = but The substituents are C1 alkyl groups i.e. methyl groups There are two methyl groups i.e. dimethyl The first point of difference rule requires numbering from the right as drawn to make the methyl group locants 2,2-and the oxa locant 3- (i.e. 2,2,3- is preferred to 2,3,3-) 2,2-dimethyl-3-oxabutane *	CH3OC(CH3)3
* In reality, these examples are simple enough that they would typically be named as alkyl alkyl ethers

Epoxides

Nomenclature	Formula	3D structure
Functional class name = alkene oxides   e.g. ethene oxide Substituent suffix = -ene oxide e.g. ethene oxide Substituent prefix = epoxy-   e.g. epoxyethane Note: The term oxirane is also used to describe epoxides.

• Epoxides are cyclic ethers, a 3 membered ring (see above diagram). Their reactivity is such that they are essentially a separate functional group.
  

• There are two methods for naming epoxides:

• as the oxide of the corresponding alkene (this relates to a method of synthesising them).

• using the prefix epoxy- to indicate the epoxide as a substituent.
  

Alkene oxide

• The root name is for the corresponding alkene (think of removing the oxygen and adding a C=C at that location).

• Add the suffix oxide. 

• This is common for very simple epoxides.
  

Epoxy-

• The root name is based on the longest chain with the two C-O bonds attached.

• The chain is numbered so as to give the epoxide unit the lowest possible locant (again like alkenes)
  

• The epoxide prefix is inserted prior to the root name along with both locants e.g. 1,2-epoxypropane.

• Both locants are included since this method is also used for naming other cyclic ethers. 
  

Alkene oxide style: Functional group is an epoxide, therefore suffix = -ene oxide The longest continuous chain is C3 therefore root = prop Location of "alkene" is unambiguous, so no locant needed. propene oxide Epoxy style: The longest continuous chain is C3 therefore root = prop The epoxide is a substituent therefore prefix = epoxy Number to give epoxide (only group present) the lowest locants = 1,2- 1,2-epoxypropane
Alkene oxide style: Functional group is an epoxide, therefore suffix = -ene oxide The longest continuous chain is C6 therefore root = hex The system is cyclic therefore prefix = cyclo Location of "alkene" is unambiguous, so no locant needed. cyclohexene oxide Epoxy style: The longest continuous chain is C6 therefore root = hex The root system is cyclic therefore prefix = cyclo The epoxide is a substituent therefore prefix = epoxy Number to give epoxide (only group present) the lowest locants = 1,2- 1,2-epoxycyclohexane
Alkene oxide style: Functional group is an epoxide, therefore suffix = -ene oxide The longest continuous chain is C6 therefore root = hex There is a C1 alkyl substituent = methyl The first point of difference rule requires numbering from the right as drawn to make the "alkene" locant = 2- Hence the methyl group locant = 5- 5-methyl-2-hexene oxide Epoxy style: The longest continuous chain is C6 therefore root = hex There is a C1 alkyl substituent = methyl The first point of difference rule requires numbering from the right as drawn The epoxide is a substituent therefore prefix = epoxy Number to give epoxide (only group present) the lowest locants = 2,3- 2,3-epoxy-5-methylhexane

Thioethers or sulfides

Nomenclature	Formula	3D structure
Functional class name = alkyl alkyl sulfide e.g. ethyl methyl sulfide Substituent suffix = sulfide Substituent prefix = alkylthio-   e.g. methylthioethane Substitutive = -thio- Structural unit :  thioethers contain R-S-R

• Thioethers are named in much the same way as their oxygen cousins, ethers.

"Simple" thioethers

• If both groups are simple alkyl groups, then the thioether is usually named as alkyl alkyl thioether

• The alkyl groups are listed in alphabetical order
  

• If the two alkyl groups are the same, then it's a dialkyl thioether

Functional group is a simple thioether, therefore use alkyl alkyl sulfide First substituent is C1 alkane therefore alkyl = methyl Second substituent is C2 alkane therefore alkyl = ethyl ethyl methyl sulfide	CH3CH2SCH3
Functional group is a simple thioether, therefore use alkyl alkyl sulfide Both substituents are C1 alkane therefore alkyl = methyl Since there are two alkyl groups, multipler = di dimethyl sulfide	CH3SCH3

"Intermediate" thioethers

• If one of the groups is more complex then the thioether group is usually treated as an alkylthio-   (i.e. R-S-) substituent.

• The more complex group (i.e. longer chain, more branched, other substituents) defines the root.

Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C3 therefore root = prop Substituent is C1 alkane therefore alkyl = methyl The first point of difference rule requires numbering from the right as drawn to make the group locant 1- 1-methylthiopropane

CH3CH2CH2SCH3

"Complex" thioethers

• If both groups are complex then the ether can be named using -thio

Hydrocarbon structure is an alkane therefore -ane Functional group is an thioether, therefore prefix = -thio- The longest continuous chain (including the S)  is 5 therefore root = pent Numbering from the left as drawn to make the thio group locant 2- 2-thiopentane

CH3SCH2CH2CH3

Amines

Nomenclature	Formula	3D structure
Functional class name = alkylamines or alkanamines Substituent suffix = -amine e.g. ethylamine Substituent prefix = amino-   e.g. aminoethane Structural unit (depending on R, R' and R") primary amines contain R-NH2 secondary amines contain R2NH tertiary amines contain R3N.

There are slight differences between amines depending on whether they are primary, secondary or tertiary (based on number of R groups attached to the N)

Primary amines

• Primary amines have one alkyl group attached to the N.

• The root name is based on the longest chain with the -NH₂ attached.

• The chain is numbered so as to give the amine unit the lowest possible number. 
  

• The amine suffix is appended to the appropriate alkyl root or alkana- root.

Functional group is an amine, therefore suffix = -amine Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C4 therefore root = but The first point of difference rule requires numbering from the right as drawn to make the amine group locant 2- butan-2-amine or 2-butylamine (or sec-butylamine)

CH3CH2CH(NH2)CH3

Secondary amines

• Secondary amines have two alkyl groups attached to the N.

• The root name is based on the longest chain with the -NH attached.

• The chain is numbered so as to give the amine unit the lowest possible number. 
  

• The other alkyl group is treated as a substituent, with N as the locant.

• The N locant is listed before numerical locants, e.g. N,2-dimethyl....
  

• The amine suffix is appended to the appropriate alkyl root or alkana- root.

Functional group is an amine, therefore suffix = -amine Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C2 therefore root = eth N-methylethylamine  or N-methylethanamine

CH3NHCH2CH3

Tertiary amines

• Tertiary amines have three alkyl group attached to the N.

• The root name is based on the longest chain with the -N attached.

• The chain is numbered so as to give the amine unit the lowest possible number. 
  

• The other alkyl groups are treated as substituents, with N as the locant.

• The N locant is listed before numerical locants, e.g. N,2-dimethyl....

• The amine suffix is appended to the appropriate alkyl root or alkana- root.

Functional group is an amine, therefore suffix = -amine Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C1 therefore root = meth Name : N,N-dimethylmethylamine or N,N-dimethylmethanamine or trimethylamine

(CH3)3N

Diamines  (or polyamines)

The term diamine simply implies the presence of two amines. Polyamines contain two or more amine groups.

• The root name is based on the longest chain containing both the amine groups.

• The chain is numbered so as to give the one of the amine groups the lowest possible number (i.e. first point of difference).

• The appropriate multiplier (i.e. di- for two, tri for three etc.) is inserted before the -amine suffix or before the root.

• For secondary or tertiary amines, the N locant is used in the same manner as for amines.

• If more than one N is substituted, then use N', N" etc.

• The N locant is listed before numerical locants, e.g. N,N',2-trimethyl....

Functional group is an amine, therefore suffix = -amine There are two amines, so insert the multiplier di The longest continuous chain is C2 therefore root = eth Locants for -NH units are 1- and 2-  1,2-ethyldiamine or ethyl-1,2-diamine	H2NCH2CH2NH2
Functional group is an amine, therefore suffix = -amine There are two amines, so insert the multiplier di The longest continuous chain is C3 therefore root = prop Locants for -NH units are 1- and 3- There is a C1 substituent = methyl The methyl group is located on the amine, so locant = N N-methyl-1,3-propyldiamine or N-methylpropyl-1,3-diamine	H2NCH2CH2CH2NHCH3
Functional group is an amine, therefore suffix = -amine There are two amines, so insert the multiplier di The longest continuous chain is C2 therefore root = eth Locants for -NH units are 1- and 2- There is are three C1 substituents = trimethyl The methyl groups are located different amines, so locants = N-, N- and N'- N,N,N'-trimethyl-1,2-ethyldiamine or N,N,N'-trimethylethyl-1,2-diamine	(CH3)2NCH2CH2NHCH3

Aldehydes

Nomenclature	Formula	3D structure
Functional class name = ? Substituent suffix = -al  e.g. ethanal Substituent prefix = oxo-

• The root name is based on the longest chain including the carbonyl group.

• Since the aldehyde is at the end of the chain, it must be C1. 
  

• The aldehyde suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -al = -anal or -ene + al = -enal etc.

Functional group is an aldehyde, therefore suffix = -al Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C3 therefore root = prop propanal

CH3CH2CHO

Ketones

Nomenclature	Formula	3D structure
Functional class name = alkyl alkyl ketone Substituent suffix = -one  e.g. propan-2-one Substituent prefix = oxo-

• The root name is based on the longest chain including the carbonyl group.

• The chain is numbered so as to give the ketone carbonyl the lowest possible number. 
  

• The ketone suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -one = -anone or -ene + one = -enone etc.
  

Functional group is a ketone, therefore suffix = -one Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C5 therefore root = pent The first point of difference rule requires numbering from the left as drawn to make the ketone group locant 2- pentan-2-one or 2-pentanone

CH3CH2CH2C(=O)CH3

Carboxylic Acids

Nomenclature	Formula	3D structure
Substituent suffix = -oic acid  e.g. ethanoic acid Substituent prefix = carboxy

• The root name is based on the longest chain including the carboxylic acid group.

• Since the carboxylic acid group is at the end of the chain, it must be C1.

• The carboxylic acid suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -oic acid = -anoic acid etc.
  

Functional group is a carboxylic acid, therefore suffix = -oic acid Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C4 therefore root = but butanoic acid

CH3CH2CH2C(=O)OH

Esters

Nomenclature	Formula	3D structure
Functional class name = alkyl alkanoate Substituent suffix = -oate

• Esters are alkyl derivatives of carboxylic acids.

• The easiest way to deal with naming esters is to recognise the carboxylic acid and the alcohol that they can be prepared from.
  

• The general ester, RCO₂R' can be derived from the carboxylic acid RCO₂H and the alcohol R'OH.
  

• The first component of an ester name, the alkyl is derived from the alcohol, R'OH portion of the structure.

• The second component of an ester name, the -oate is derived from the carboxylic acid, RCO₂H portion of the structure.

• Alcohol component

• The root name is based on the longest chain containing the -OH group.

• The chain is numbered so as to give the -OH the lowest possible number.

• Carboxylic acid component

• The root name is based on the longest chain including the carbonyl group.

• Since the carboxylic acid group is at the end of the chain, it must be C1.

• The ester suffix for the acid component is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -oate = -anoate etc.

• The complete ester name is the alkyl alkanoate

Functional group is an ester, therefore suffix = -oate The alcohol component here is methanol, so the alkyl = methyl The acid component here is propanoic acid, so propanoate methyl propanoate
Functional group is an ester, therefore suffix = -oate The alcohol component here is 2-propanol (or isopropanol), so the alkyl = 2-propyl or isopropyl The acid component here is propanoic acid, so propanoate 2-propyl propanoate or isopropyl propanoate
Functional group is an ester, therefore suffix = -oate The alcohol component here is methanol, so the alkyl = methyl The acid component has a longest continuous chain that is C3 so the root =prop With the acid group at C1, there is a methyl group at C2 = 2-methyl methyl 2-methylpropanoate
Functional group is an ester, therefore suffix = -oate The alcohol component here is 2-propanol (or isopropanol), so the alkyl = 2-propyl or isopropyl The acid component has a longest continuous chain that is C4 so the root =but With the acid group at C1, there is a methyl group at C2 = 2-methyl 2-propyl 2-methylbutanoate or isopropyl 2-methylbutanoate

Amides

Nomenclature	Formula	3D structure
Functional class name = alkyl alkanamide Substituent suffix = -amide

• Amides are amine derivatives of carboxylic acids.

• The root name is based on the longest chain including the carbonyl group of the amide group.
  

• Since the amide group is at the end of the chain, the C=O carbon must be C1.

• The amide suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -amide = -anamide etc.

• If the amide nitrogen is substituted, the these substituents are given N- as the locant.

• The N- locant is listed first when the same substituent occurs on N and other locations, e.g. N,2-dimethyl
  

Functional group is an amide therefore suffix = -amide Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C4 therefore root = but butanamide	CH3CH2CH2C(=O)NH2
Functional group is an amide therefore suffix = -amide Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C4 therefore root = but The nitrogen substituent is C1 i.e. an N-methylgroup N-methylbutanamide	CH3CH2CH2C(=O)N(CH3)H
Functional group is an amide therefore suffix = -amide Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C2 therefore root = eth The two nitrogen substituents are C1 i.e. an N-methyl group There are two methyl groups, therefore multiplier = di- N,N-dimethylethanamide	CH3C(=O)N(CH3)2

Acyl Halides or Acid Halides

Nomenclature	Formula	3D structure
Functional class name = acyl or acid halide Substituent suffix = -oyl halide

• Acyl or acid halides are derivatives of carboxylic acids.

• The root name is based on the longest chain including the carbonyl group of the acyl group.
  

• Since the acyl group is at the end of the chain, the C=O carbon must be C1.

• The acyl halide suffix is appended after the hydrocarbon suffix minus the "e" : e.g.  -ane + -oyl halide = -anoyl halide etc.

• The most common halide encountered is the chloride, hence acyl or acid chlorides, e.g. ethanoyl chloride

Functional group is an acyl halide therefore suffix = -oyl chloride Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C2 therefore root = eth ethanoyl chloride	CH3C(=O)Cl
Functional group is an acyl halide therefore suffix = -oyl chloride Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C4 therefore root = but butanoyl chloride	CH3CH2CH2C(=O)Cl
Functional group is an acyl halide therefore suffix = -oyl chloride Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C3 therefore root = prop The branch is a C1 alkyl group i.e. a methyl group The functional groups requires numbering from the right as drawn, the substituent locant is 2- 2-methylpropanoyl chloride	(CH3)2CHC(=O)Cl

Acid Anhydrides 

Nomenclature	Formula	3D structure
Functional class name = alkanoic anhydride Substituent suffix = -oic anhydride

• As the name implies, acid anyhydrides are derivatives of carboxylic acids.

• In principle, they can be symmetric (where the two R groups are identical) or asymmetric (where the two R groups are different).

• Symmetric anhydrides are the most common, they are named as alkanoic anhydrides

• Asymmetric anhydrides are name in a similar fashion listing the alkyl groups in alphabetical order.

• Cyclic anhydrides derived from dicarboxylic acids are name as -dioic anhydrides.
  

Functional group is an acid anhydride therefore suffix = -oic anhydride Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C2 therefore root = eth ethanoic anhydride	CH3C(=O)OC(=O)CH3
Functional group is an acid anhydride therefore suffix = -oic anhydride Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C4 therefore root = but The other group is C3 = prop butanoic propanoic anhydride
Functional group is a cyclic acid anhydride therefore suffix = -dioic anhydride Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C5 therefore root = pent pentandioic anhydride

Nitriles

Nomenclature	Formula	3D structure
Functional class = alkyl cyanide Functional group suffix = nitrile or -onitrile Substituent prefix = cyano- Notes :  The cyano prefix is used in a very similar manner to haloalkanes The cyano nomenclature is most common when the alkyl group is simple. The nitrile suffix is used in a very similar manner to carboxylic acids.

Cyano substituent style: 

• The root name is based on the longest chain with the -C≡N as a substituent. 
  

• This root give the alkane part of the name.
  

• The chain is numbered so as to give the -C≡N group the lowest possible locant number

Nitrile style:

• The root name is based on the longest chain including the carbon of the nitrile group.

• This root give the alkyl part of the name.

• Since the nitrile must be at the end of the chain, it must be C1 and no locant needs to be specified. 
  

• Nitriles can also be named by replacing the -oic acid suffix of the corresponding carboxylic acid with -onitrile.
  

Cyano substituent style: Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C3 therefore root = prop The substituent is a -CN therefore prefix = cyano The first point of difference rule requires numbering from the right as drawn, the substituent locant is 1- 1-cyanopropane Nitrile style:  Functional group is a -C≡N, therefore suffix = -nitrile Hydrocarbon structure is an alkane therefore -ane The longest continuous chain is C4 therefore root = but butanenitrile

CH3CH2CH2C≡N

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Aromatic Systems (or arenes)

	Formula	3D structure
Benzene Benzene is an important structure. It's the most common aromatic hydrocarbon. Since benzene (and its relatives) have their own characteristic reactions, they are a functional group, often referrred to as arenes	C6H6

• In principle, all the substituents we have encountered can occur as substituents on benzene rings.

• One way to name these is to use the benzene as the root and add the approriate substituent prefix.

Hydrocarbon structure is benzene The substituent chain is C2 therefore = ethyl ethylbenzene	C6H5CH2CH3
Hydrocarbon structure is benzene The substituent is a bromine therefore = bromo bromobenzene	C6H5Br
Hydrocarbon structure is benzene The substituent is a C2 ether therefore = ethoxy ethoxybenzene	C6H5OCH2CH3
Hydrocarbon structure is benzene The substituent is a nitro group therefore = nitro nitrobenzene	C6H5NO2

Common Substituted Benzenes

• There are many common simple substituted benzenes with common names that are also used as part of the IUPAC system, here are the most important ones that you should know.  
  

Common name	Substituted benzene	Formula	3D structure
Toluene	Methylbenzene	C6H5CH3
Styrene	Ethenylbenzene	C6H5CH=CH2
Phenol		C6H5OH
Anisole	Methoxybenzene	C6H5OCH3
Aniline	Aminobenzene	C6H5NH2
Benzoic acid		C6H5CO2H
Benzaldehyde		C6H5CHO
Benzonitrile	Cyanobenzene	C6H5CN

Benzene as a substituent

• In some cases, the benzene ring needs to be treated as a substituent.  
  

• In these cases, the term phenyl, is used to designate the presence of C₆H₅- as a substituent.

• Take care not to confuse terminology (it will be discussed on the next page)
  

•  benzyl substituent = C₆H₅CH₂

•  phenol a compound = C₆H₅OH

•  phenyl substituent = C₆H₅

• The method should be used when the benzene ring is a substituent of the root (the root contains the principle functional group).

Principle functional group is an alkene therefore suffix = -ene The longest continuous chain is C2 therefore root = eth The benzene ring is a substituent therefore = phenyl Location of the phenyl group is unambiguous phenylethene	C6H5CH=CH2
Principle functional group is an alkene therefore suffix = -ene The longest continuous chain is C3 therefore root = prop The benzene ring is a substituent therefore = phenyl Numbering from the right as drawn to give the alkene the lowest locant = 1 Phenyl locant = 3 3-phenylpropene	C6H5CH2CH=CH2
Principle functional group is an alcohol therefore suffix = -ol Hydrocarbon structure is an alkane therefore -an- The longest continuous chain is C2 therefore root = eth The benzene ring is a substituent therefore = phenyl Numbering from the right as drawn to give the -OH the lowest locant = 1 Phenyl locant = 2 2-phenylethanol	C6H5CH2CH2OH

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Phenyl or benzyl?

• Phenyl and benzyl are not the same and can not be used interchangeably. 
  

• Take care not to confuse terminology : 
  

•  benzyl substituent = C₆H₅CH₂

•  phenyl substituent = C₆H₅

• Here are some examples:

phenyl bromide	C6H5Br
benzyl bromide	C6H5CH2Br
ethyl phenyl ether	C6H5OCH2CH3
benzyl methyl ether	C6H5CH2OCH3

Disubstituted benzenes

• When there are two (or more) substituents, the relative position of the subsituents must be defined.  
  

• There are two methods used based on either numerical locants or specific words for the three possible forms:
  

  
  

• The words can also be shortened to their first letter, i.e. o-, m- and p-

• The terms ortho-, meta- or para- (or their singel letter equivalents) are used as prefixes.

• These terms are ONLY used for benzene systems.
  

• When using numerical locants, the principal functional group is defined to be at C1.

• The numerical locant method is also applicable to other aromatic systems.

Numerical locants method:

Principle functional group is the benzne therefore root = benzene There are two chlorine substituents therefore dichloro The substituent lowest locants = 1 and 2 1,2-dichlorobenzene
Principle functional group is the benzne therefore root = benzene There is a chlorine substituent therefore chloro There is a bromine substituent therefore bromo Numbering from the -Br (due to alphabet priority) clockwise to give the chlorine substituent the lowest locant = 3 1-bromo-3-chlorobenzene
Principle functional group is the aromatic alcohol therefore = phenol The substituent is a chlorine therefore = chloro Numbering from the -OH, counterclockwise to give the substituent the lowest locant = 2 2-chlorophenol

 

Ortho / meta / para method: 

Principle functional group is the benzne therefore root = benzene There are two chlorine substituents therefore dichloro The substituent locants are 1 and 2 therefore ortho ortho-dichlorobenzene or o-dichlorobenzene
Principle functional group is the benzne therefore root = benzene There is a chlorine substituent therefore chloro There is a bromine substituent therefore bromo The substituent locants are 1 and 3 therefore meta meta-bromochlorobenzene or m-bromochlorobenzene
Principle functional group is the aromatic alcohol therefore = phenol The substituent is a chlorine therefore = chloro The substituent locants are 1 and 2 therefore ortho ortho-chlorophenol or o-chlorophenol

Polysubstituted benzenes

• When there are two (or more) substituents, the relative position of the subsituents must be defined.  
  

• Numerical locants are used to specify the locations if there are three (or more substituents).
  

• The principal functional group is defined to be at C1.

• In cases where the root can be defined using common aromatic names this is typically used.

• The remaining substituents are listed alphabetically.

Principle functional group is the methylbenzene therefore root = toluene There is a bromine substituent therefore bromo There is a chlorine substituent therefore chloro Numbering from the -CH3 (priority group at C1) gives the substituents the locants = 2 and 4 2-bromo-4-chlorotoluene
Principle functional group is the aromatic alcohol therefore = phenol There are two C1 substituents therefore dimethyl Numbering from the -OH (priority group at C1) gives the substituent the locant = 3,5 3,5-dimethylphenol
Principle functional group is the aromatic amine therefore = aniline There is a C1 substituent therefore methyl There is a C2 substituent therefore ethyl Numbering from the -NH2 (priority group at C1) gives the substituents the locants = 2 and 3 2-ethyl-3-methylaniline

Stereochemistry

Stereoisomers are molecules that have the same connectivity (i.e. the same pieces in the same order) but differ only in the arrangement of those pieces in space.
For example, but-2-ene, CH₃CH=CHCH₃, has two possible stereoisomers due to the relative spatial positions of the -H and -CH₃ groups on each end of the C=C unit.

methyl groups on the same side
methyl groups on opposite side

Since these types of isomers are different molecules with different properties, they need to have individual and unique names.

The E- / Z- (alkenes) and R- / S- (chirality centers) nomenclature methods for naming stereoisomers are based on the use of the Cahn-Ingold-Prelog priority rules used for naming stereocenters.

These rules are used to establish the priority of the groups attached to the stereocenter and are based on atomic number, and the first point of difference. The rules are outlined below, specific examples are provided within the following pages for the different scenarios, see the links below.

Cahn-Ingold-Prelog priority rules

In order to rank the groups at a stereocenter (e.g. an alkene or a chirality center):

(1) identify the stereocenter
(2) identify each of the atoms attached to that stereocenter
(3) Assign the priority of the atom / group based on the atomic number of that atom. Higher atomic number = higher priority.

If the same type of atom is connected, then we need to look at the next atoms in the groups / chain moving away from the stereocenter until we find the first point of difference. A good method for doing this is to write down the attached atoms and then if needed, a list of the atoms they are attached to in priority order (i.e. atomic number)

Once the difference has been identified, the relative priorites can be determined.

If a multiple bond is encountered, treat it as if the atoms are attached by the same number of single bonds e.g. C=C is treated as 2 x C-C and C=O is 2 x C-O

Study tip for applying the Cahn-Ingold-Prelog priority rules (1) Label the stereocenters with * (2) List out the attached atoms in priority order

Two areas of stereochemistry are described in the following pages:

(1) stereoisomerism at alkenes (e.g. cis- / trans- or E- / Z-)

(2) stereoisomerism at chirality centers (e.g. R- / S-)

Stereoisomers of Alkenes

As we have already described, alkenes with two different substituents at each end of the C=C can exist as a pair of stereoisomers.

The alkene can only exist as stereoisomers if R₁ is not equal to R₂ AND R₃ is not equal to R₄. 

As we have already seen, there are two ways to name these types of isomers: (for a review details follow the links)

• cis / trans method

• E / Z method

The E- and Z- style is more reliable (i.e. potentially less ambiguous) and particularly suited to highly substituted alkenes, especially when the substituents are not alkyl groups. 

The E- and Z-alkene nomenclature system is based on the Cahn-Ingold-Prelog priority rules. They can also be used for naming chirality center stereoisomers (see later). 
In order to apply the Cahn-Ingold-Prelog priority rules to alkenes:

• Imagine each alkene as two pieces, each piece containing one of the sp² C atoms

• Assign the priority (high = 1, low = 2) to each atom attached to each sp² C based on atomic number 
  

  Subrules:

• Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H

• Same atom attached ? By moving out away from the C=C one atom at a time, locate the first point of difference and apply the priority rule there.
  
  

• Determine the relative position of the two higher priority groups

• If they are on the same side then it is a (Z)-alkene (German; zusammen = together) 

• If they are on opposite sides then it is an (E)-alkene (German; entgegen = opposite)
  
  

• If there is more than one C=C that can be E/Z, then the locant and the stereochemistry of each alkene needs to be included e.g. (2E,4Z)-
  

Example: but-2-ene

			<img src="http://www.chem.ucalgary.ca/courses/351/WebContent/orgnom/stereo/alkenes-31d.gif" alt="relative positions" width="210" height="96" "="">
Step 1: Split the alkene	Step 2:  List the attached atoms looking for the first point of difference. Here we have C and H atoms attached.	Step 3: Assign the relative priorities.  Since the atomic numbers C > H then the -CH3 group is higher priority.	Step 4: Look at the relative positions of the higher priority groups: same side = Z, hence (Z)-but-2-ene
Step 1: Split the alkene	Step 2:  List the attached atoms looking for the first point of difference. Here we have C and H atoms attached.	Step 3: Assign the relative priorities.  Since the atomic numbers C > H then the -CH3 group is higher priority.	Step 4: Look at the relative positions of the higher priority groups: opposite side = E, hence (E)-but-2-ene

Example: 3-methylpent-2-ene

Step 1: Split the alkene	Step 2:  Assign the relative priorities.  The two atoms attached to the left end C are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority.	Step 3: The two atoms attached to the right end C are both C. So we need to look at the atoms these C are attached to. List them out...we have C (H,H,H) and C (C,H,H)systems. This means that at the first point of difference (see the arrow), a C > H meaning that the ethyl group is higher priority.

R- and S- nomenclature of chirality centers

The Cahn-Ingold-Prelog priority rules are used for naming chirality centers and geometric isomers (e.g. E- or Z-alkenes)

These rules are used to establish the priority of the groups attached to the chirality center and are based on atomic number, and the first point of difference. 

In the simplest and most common case, a chirality center is characterised by an atom that has four different groups bonded to it in such a manner (e.g. tetrahedral) that it has a non-superimposable mirror image.  Terms such as an asymmetric, stereogenic or chiral center have been used in the past.

The most common chirality center in organic chemistry is a carbon atom with four different groups attached

In order to assign the configuration as R or S:

• Identify each of the chirality centers (most commonly an sp³ C with 4 different groups attached) 
  

Then at each chirality center....

• Assign the priority (high = 1 to low = 4) to each group attached to the chirality center based on atomic number.

• Reposition the molecule so that the lowest priority group is away from you as if you were looking along the C-(4) σ bond. If you are using a model, grasp the lowest priority group in your fist.

• Determine the relative direction of the priority order of the three higher priority groups (1 to 2 to 3)
  

• If this is clockwise then it is the R-stereoisomer (Latin; rectus = right handed) 

• If this is counter-clockwise then it is the S-stereoisomer (Latin; sinister = left handed) 
  

• If there is more than one stereocenter, then the location needs to be included with the locant, e.g. (2R)-
  

Subrules:

• Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H

• If the same atom is attached, then look for the first point of difference by moving out one atom at a time, locate the first point of difference and apply rules there.

• If a multiple bond is encountered, treat it as if the atoms are attached by the same number of single bonds e.g. C=C is treated a 2 C-C and C=O is 2 C-O 
  

HINT:

• at each center, list out the 3 new atoms attached as you move away from the chirality center (see the 2nd example shown below)
  

• list these groups in their priority order (i.e. high to low atomic number)

• use this "list" to locate the first point of difference

Example:  chlorofluoroiodomethane 

The chirality center should be easy to spot, and the four attached groups are in priority order, highest to to lowest: I (purple), Cl (green), F(brown) and H (white) Rotate the image on the left so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority.   Since it decreases clockwise, this is the R configuration.

starting point	identify the chirality center(s	assign the relative priorities then rotate the low priority group away  (to the back)		determine the sense of groups 1 - 3  clockwise = R

Example: cyclohex-2-enol

starting point	identify the chirality center(s) and show the implied H atom	assign the relative priorities.... O > C > C > H Because we have two C groups we need to list the groups the C are attached to in atomic number order (note how the multiple bond is handled)	the first point of difference is the C > H so the C group on the left is the higher priority	with the low priority group already at the back, determine the sense of groups 1 - 3  counterclockwise = S