NOMENCLATURE
The IUPAC method for naming organic compounds has been explained in the AS notes
for simple organic molecules like alkanes, alkenes, alcohols, aldehydes, ketones and
carboxylic acids.
There are a number of new organic molecules in A2 Chemistry:
Type of compound
Structure of group
Example
R-CN
Nitrile
O
O
H
Ester
H
C
C
O
R
O
C
H
H
H
O
Acyl chloride
O
C
H
Cl
C
C
Cl
H
O
O
C
Acid anhydride
CH3
C
CH3
C
O
O
C
O
O
O
Amide
H
H
C
C
H
H
O
C
H
NH2
C
NH2
Arene
O
N-substituted amide
O
C
CH3
N
C
R
N
H
H
Primary amine
H
Secondary amine
R
H
CH3
N
R
TOPIC 13.4: NOMENCLATURE & ISOMERISM 1
N
C2H5
H
H
C
C
H
H
H
R
Tertiary amine
R
C2H5
C2H5
N
N
CH3
R
C2H5
Alkyl ammonium salt
N
+
Cl
CH3
N
+ C H Cl 2 5
CH3
Nomenclature of groups new to A2
a) Nitriles
Nitriles contain the functional group CN and are named by using the suffix
nitrile after the longest carbon chain.
-
Eg
Ethanenitrile
2-hydroxypropanenitrile
b) Esters
Esters are named using an alkyl prefix to indicate the group attached to the single O
and the suffix –anoate to indicate the group attached to both oxygen atoms.
Eg
O
H
H
C
O
C
H
H
methyl methanoate
The methyl indicates the number of carbons in the chain attached to the single O (ie 1),
and the methanoate indicates the number of carbons in the chain attached to both O
atoms (ie 1).
O
CH3
C
O
CH2
CH2
CH3
propyl ethanoate
Be careful as ethyl propanoate is a different molecule
TOPIC 13.4: NOMENCLATURE & ISOMERISM 2
c) Acyl chlorides
Acid chlorides are named using the suffix: –anoyl chloride
Eg
H
O
H
C
C
Cl
H
ethanoyl chloride
H
H
O
H
C
C
H
H
C
Cl
propanoyl chloride
The group is always at the end of the molecule, so numbering is not necessary.
d) Acid anhydrides
If both alkyl groups are the same, acid anhydrides are named using the suffix –anoic
anhydride
Eg
O
CH3
C
O
CH3
C
O
ethanoic anhydride
If the alkyl groups are different, each must be specified:
Eg
O
H
C
O
CH3
C
O
methanoic ethanoic anhydride
TOPIC 13.4: NOMENCLATURE & ISOMERISM 3
e) Amides
Amides are named using the suffix –anamide
Eg
H
H
C
C
H
H
O
H
C
NH2
propanamide
O
H
C
NH2
methanamide
The group is always at the end of the molecule, so numbering is not necessary.
f) Aromatic alkenes (arenes)
The naming of arenes is not straightforward but here are a few commonly encountered
molecules.
Benzene
nitrobenzene
1,3-dimethylbenzene
phenylamine
TOPIC 13.4: NOMENCLATURE & ISOMERISM 4
Phenol
phenylethene
chlorobenzene
2-hydroxybenzoic acid
g) N-substituted amides
N-substituted amides are named using an N-alkyl prefix to indicate the group attached to
the N only and the suffix –anamide to indicate the group attached to both O and N
atoms.
Eg
O
CH3
H
H
N
C
C
H
H
H
C
H
N-ethyl ethanamide
The ethyl indicates the number of carbons in the chain attached to the N only (ie 2) and
the ethanamide indicates the number of carbons in the chain attached to both N and O
(ie 2)
O
CH3
CH2
C
H
N
C
H
H
H
N-methyl propanamide
h) Amines
Primary amines are named using the suffix –amine.
Eg
H
CH3
N
H
methanamine
Secondary amines are named by using a N-alkyl prefix to indicate the nature of the
shorter chain attached to the N atom. The longest chain is named afterwards, followed
by the suffix –amine.
Eg
H
CH3
N
C2H5
N-methylethanamine
TOPIC 13.4: NOMENCLATURE & ISOMERISM 5
Tertiary amines are named in the same way as secondary amines. The two shortest
alkyl groups are indicated with a N-alkyl prefix.
Eg
C2H5
C2H5
N
CH3
N-methyl- N-ethylethanamine
i) Alkylammonium salts
Alkylammonium salts can be primary, secondary, tertiary or quartenary depending on
the number of alkyl groups attached to the nitrogen atom. They are named simply by
indicating the nature and number of each alkyl group before the ending –ammonium
chloride.
Eg
H
CH3
N
+ C H Cl 2 5
H
N-methylethanaminium chloride
C2H5
C2H5
Cl
-
N,N,N-triethylethanaminium chloride
TOPIC 13.4: NOMENCLATURE & ISOMERISM 6
Nomenclature of compounds containing more than one functional
group.
Many organic compounds contain more than one functional group. Naming more
complex compounds such as these can be quite complicated, and only a few
combinations need to be named at A-level.

If one of the functional groups is an amine or a haloalkane, the molecule is
named simply by adding the necessary prefix:
Eg
H
NH2
H
C
C
H
H
O
C
H
H
Cl
C
C
H
H
OH
O
CH
CH3
C
CH
CH3
OH
OH
Cl
3-aminopropanoic acid
2-chloropropanoic acid
3-chlorobutan-2-ol
 If one of the functional groups is an alcohol and the other is a nitrile or anything
containing a C=O bond, the alcohol group is named using a hydroxy- prefix.
Eg
OH
CH3
H
CH
CH2
C
OH
C
C
H
H
O
C
OH
N
2-hydroxybutanenitrile

H
2-hydroxypropanoic acid
If one of the functional groups is an alkene and the other is anything other than
an amine or a haloalkane, the molecule is named by replacing the –an- part of
the prefix with –en-, including a number if the position of the alkene has to be
specified.
Eg
HO
H
H
C
C
H
H
H
C
but-3-en-1-ol
C
H
H
H
H
C
C
H
O
C
C
OH
H
but-2-enoic acid
TOPIC 13.4: NOMENCLATURE & ISOMERISM 7

Some molecules contain the same functional group twice.
butanedioic acid
butan-1,4-diol
Many other organic molecules can be named in this way. The most important at A2 level
are hydroxynitriles and amino acids. It is also important to remember that the
chemistry of di-functional molecules usually only involves 1 type of functional
group reacting at a time.
TOPIC 13.4: NOMENCLATURE & ISOMERISM 8
ISOMERISM
STRUCTURAL ISOMERISM
Structural isomers are compounds with the same molecular
formula but different structural formulae.
Structural isomers must have at least one bond in their molecules which is different.
Structural isomers are sometimes sub-divided into chain isomers, position isomers and
functional group isomers.
Chain Isomers
Examples of chain isomers are butane and methyl propane.
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
butane
H
H
H
C
C
C
H
CH3 H
H
methylpropane
These isomers occur when the carbon skeleton can be arranged in two or more ways.
Position Isomers
Examples of position isomers are propan-1-ol and propan-2-ol.
H
H
H
H
C
C
C
H
H
OH
H
H
H
H
H
C
C
C
H
OH H
propan-1-ol
H
propan-2-ol
These isomers differ only in the position of the functional group.
Functional Group Isomers
Examples of functional group isomers are ethanol and methoxymethane.
H
H
H
C
C
H
OH
H
H
H
ethanol
C
H
H
O
C
H
H
methoxymethane
These isomers have the same molecular formula but different functional groups.
TOPIC 13.4: NOMENCLATURE & ISOMERISM 9
Stereoisomerism
STEREOISOMERS have the same structural formula. They have the same
number and types of bonds and differ only in their orientation in space.
There are two types of stereoisomerism: geometrical isomerism and optical isomerism.
Only geometrical isomerism will be considered in this module.
Geometrical Isomerism
Geometrical isomerism arises when there is restricted rotation about a bond, for
example around the C=C double bond in alkenes and around C-C single bonds in
cycloalkanes.
Alkenes
There is always restricted rotation around a C=C double bond. The double bond is a orbital formed by the overlap of two 2p-orbitals on the two carbon atoms. If rotation
about a double bond were to take place, it would require the -bond to be broken This
requires an amount of energy not possessed by molecules at room temperature.
-orbital
C
2p
rotation
C
C
2p
C
-bond broken
Restricted rotation gives rise to geometrical isomers only if there are two different
atoms or groups attached to both of the double bond carbon atoms.
For example, but-2-ene exhibits geometrical isomerism, but but-1-ene does not.
H
two
different
groups
CH3
C
C
CH3
H
two
different
groups
E-but-2-ene
CH3
C
CH3
C
H
H
Z-but-2-ene
These two molecules are identical apart from their orientation in space. Both of the
double bond carbon atoms are attached to two different groups. The prefixes E and Z
indicate that the two methyl groups are on opposite sides or on the same side of the
double bond respectively.
H
two
identical
C
atoms
H
CH2CH3
C
but-1-ene
H
TOPIC 13.4: NOMENCLATURE & ISOMERISM 10
Geometrical isomerism is not possible
because one of the double bond carbon
atoms is attached to two identical atoms.
Cycloalkanes
There is always restricted rotation around a C-C single bond in a ring, but this gives rise
to geometrical isomers only if there are two different atoms or groups attached to at
least two different carbon atoms.
For example, 1,2-dimethylcyclopentane exhibits geometrical isomerism but 1,1dimethylcyclopentane does not.
two
different
groups
CH3
CH3
H
CH3
two
different
groups
H
H
H
CH3
Z-1,2-dimethylcyclopentane
E-1,2-dimethylcyclopentane
H
CH3
H
two
identical
groups
CH3
1,1-dimethylcyclopentane
Optical Isomerism
Optical isomerism occurs whenever a molecule lacks a plane of symmetry, a centre of
symmetry or an axis of symmetry. This means that the molecule is non-superimposable
with its own mirror image: it is said to be chiral. There are a number of causes of
chirality, but by far the most common is the presence in a molecule of an asymmetric
carbon atom. An asymmetric carbon atom is one that is bonded to four different atoms
or groups. It is indicated by C*.
TOPIC 13.4: NOMENCLATURE & ISOMERISM 11
If the right-hand molecule is rotated about the vertical axis, it can be seen that it is not
superimposable with the left-hand molecule.
The mirror image forms are called enantiomers.
Enantiomers have identical physical properties except that they rotate the plane of
plane-polarised light in opposite directions. At equimolar concentrations, the rotations
are equal in size. Rotations are measured in a polarimeter. Molecules which are able to
rotate the plane of plane-polarised light are said to be optically active.
One enantiomer rotates the plane of polarisation in a clockwise direction and is said to
be dextrorotatory; this is indicated by the prefix (+) before its name. The other
enantiomer rotates the plane of polarisation in an anti-clockwise direction and is said to
be laevorotatory; this is indicated by the prefix (-) before its name.
Enantiomers have identical chemical properties except in their reactions with other
optically active substances, such as enzymes.
Example 1: 2-hydroxypropanoic acid, CH3CH(OH)COOH
mirror
CH3
C*
OH
CH3
HO
C*
COOH
HOOC
H
H
(+)-2-hydroxypropanoic acid
(-)-2-hydroxypropanoic acid
Example 2: 2-aminopropanoic acid or alanine, CH3CH(NH2)COOH
mirror
CH3
C*
NH2
CH3
H2N
C*
COOH
HOOC
H
(+)-2-aminopropanoic acid
TOPIC 13.4: NOMENCLATURE & ISOMERISM 12
H
(-)-2-aminopropanoic acid
If a mixture contains equal numbers of molecules of both enantiomers, the two rotation
effects cancel each other out. There is no net rotation, and the mixture is, therefore,
optically inactive. Such a mixture is called a racemate.
Single enantiomers occur frequently in biological systems, whereas when an
asymmetric carbon atom is generated during a chemical reaction in vitro, a racemate is
the usual result.
For example, when ethanal reacts with hydrogen cyanide (HCN), it undergoes a
nucleophilic addition reaction to form 2-hydroxypropanenitrile, which contains an
asymmetric carbon atom.
CH3CHO + HCN
CH3CH(OH)CN
The nucleophile is the cyanide ion, CN-, which attacks the carbonyl carbon atom in
ethanal. The carbonyl group is planar, and therefore the attack occurs with equal
probability from above or below this plane. Therefore, both enantiomers are formed in
equal amounts, giving a racemate.
-
:CN
CN
H
H
CH3
C*
OH
C
OH
O
CN
-
:CN
CH3
C*
H
CH3
By looking at the diagram below, it can be seen that the two products are enantiomers.
rotation in the plane
of the paper
H
CN
OH
C*
C*
OH
OH
H
H
CN
NC
CH3
TOPIC 13.4: NOMENCLATURE & ISOMERISM 13
CH3
C*
CH3