AS Organic Chemistry - Nomenclature

• Introduction
• Naming Alkanes
• Naming Alkenes
• Naming Alcohols
• Naming Aldehydes and Ketones
• Aldehydes
• Ketones
• Naming Carboxylic Acids
• Esters
• Naming Halogenoalkanes
• Naming Cyclic Compounds
• Cycloalkanes
• Cycloalkenes
• Cyclic Esters (Lactones)
• Structural Isomers
• Chain (Branched) Isomers
• Positional Isomers
• Functional Group Isomers
• Stereoisomers
• Geometric Isomers
• Optical Isomers
• Nomenclature Summary Table

Nomenclature - Introduction

The names of simple organic compounds are composed of two parts -

(i) a prefix which comes from the number of carbon atoms in the longest straight chain in the molecule.

(ii) a suffix which shows which type of different organic compound it is.

Nomenclature - Naming Alkanes

With alkanes this suffix is ___ane.

Examples -

ethane	octane

Nomenclature - Naming Alkenes

With alkenes this suffix is ___ene.

Examples -

ethene	but-1-ene

Nomenclature - Naming Alcohols

With alcohols this suffix is ___anol.

Examples -

ethanol	propan-1-ol

Nomenclature - Naming Aldehydes and Ketones

With aldehydes this suffix is ___anal.

Examples -

ethanal	butanal

With ketones this suffix is ___anone.

Examples -

(i) prefix

- this depends on the number of carbon atoms in the longest chain

(ii) position number

- count from the nearest chain end to the carbonyl carbon atom

(iii) suffix

- add __anone around the position number at the end of the prefix

Exemplar compounds -

propanone :
pentan-2-one :

With carboxylic acids this suffix is ___anoic acid.

Examples -

With esters this suffix is ___anoate.

Examples -

With haloalkanes the basis for the name is the parent organic group (e.g. alkane, alkene, etc.)

(i) Position number -

Count the number of carbon atoms from the nearest chain end to where the halogen is attached.

(ii) Prefix -

Second comes the prefix based on which halogen element is present,

(iii) Suffix -

Simply add the normal unchanged alkane (or alkene) name onto the suffix (see naming alkanes in the alkanes page).

Exemplar compounds -

bromoethane	1-chlorobutane

Some examples to try out -

They are called alkanes as they do not have any carbon-carbon double bonds. The name is based on the alkane with the same number of carbon atoms. Cyclo__ is added as a prefix to the whole name.

Examples -

cyclopropane	cyclopentane

They are called alkenes as they have a carbon-carbon double bond. The name is based on the alkene with the same number of carbon atoms. Cyclo__ is added as a prefix to the whole name.

Examples -

cyclopropene	cyclohexene

Examples -

oxiran-2-one	dihydrofuran-2-one

This is also known as branching in a molecule.

When the alkane, or any other organic molecule, is not just a simple straight chain of carbon atoms joined together the names become a little more complex.

The longest connected chain of carbon atoms must be found as before and the alkane (alkene, alcohol, etc.) name generated as usual. For example, with 5 carbon atoms and 12 hydrogen atoms :

5 carbon atoms long is a pentane	4 carbon atoms long is a butane	3 carbon atoms long is a propane

Then the name for the pendent group is found, again by counting the number of carbon atoms present, and used as a prefix.

The pendent group is itself prefixed by a number denoting which carbon atom it is attached to, followed by a hyphen.

If there is more than one of the same pendent group then each group is numbered, with a comma used to separate the numbers. The group is also prefixed by di__ to denote two of them, tri__ to denote three of them, tetra__ to denote four of them, penta__ to denote five of them, etc.

Continuing the previous examples :

No pendent groups = pentane	1 pendent methyl group on the 2nd carbon = 2-methylbutane	2 pendent methyl groups on the 2nd carbon = 2,2-dimethylpropane

N.B.: The numbers used to indicate the positions of the pendent groups must be the lowest numbers possible, so always check them from both ends of the molecule.

Further examples -

2,3-dimethylbutane	3-methylbut-1-ene	2-methylbutan-1-ol

Some examples to try out -

There are a number of different ways of representing an organic molecule, and all of them will probably examined in AS and A2 question papers.

The simplest is a molecular formula, which is simply the number of each type of atom totaled up.

A structural formula separates the atoms into groups, such as CH₃ and CH₂ parts of a molecule.

A displayed structure shows all the bonds between the atoms, including all the bonds going to hydrogen atoms.

A skeletal structure is similar to the displayed structure, though all the hydrogens are removed, including any bonds to hydrogen atoms, and the labels for carbon atoms are removed as well. Labels for atoms other than carbon are kept (see the nomenclature sections of other homologous series for examples of their skeletal formulae).

e.g. for propane,

molecular formula :	C3H8
structural formula :	CH3CH2CH3
displayed formula :
skeletal formula :

and for 3-ethylpentane the skeletal formula would look like this -

Nomenclature - Summary Table

Homologous Series Name	General Formula	Functional group	Exemplar Compound
			Name	Formula	Structure
Alkanes	CnH2n+2		methane	CH4
Alkenes	CnH2n		ethene	CH2CH2
Alcohols	CnH2n+1OH		propan-1-ol	CH3CH2CH2OH
Aldehydes	CnH2n-1HO		butanal	CH3CH2CH2CHO
Ketones	CnH2nO		pentan-2-one	CH3COCH2CH2CH3
Carboxylic Acids	CnH2n-1OOH		hexanoic acid	CH3CH2CH2CH2CH2COOH
Esters	CnH2nOO		propyl butanoate	CH3CH2CH2COOCH2CH2CH3
Halogenoalkanes	CnH2n+2X		4-chlorooctane	CH3CH2CH2CHClCH2CH2CH2CH3
Cycloalkanes	CnH2n		cyclononane	C9H18
Cycloalkenes	CnH2n-2		cyclodecene	C10H18
Cyclic Esters (Lactones)	CnH2nCOO		oxacycloundecan-2-one	C9H18COO

written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Tuesday, 24^th January, 2012

Created with the aid of ACD/ChemSketch Freeware 12 and