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Introduction to Nomenclature of Organic Compounds
Organic compounds are named using the IUPAC (International Union of Pure and Applied
Chemistry) system of nomenclature.
The name of an organic compound is constructed from the following parts:
- the functional group(s) present within the compound;
- the root, which is derived from the number of carbon atoms in the main (parent) chain of the compound;
- the position and number of functional groups within the compound.
1. The Functional Groups
A functional group is an atom or group of atoms within a molecule which is responsible for the characteristic properties and reactions of that molecule.
Compounds with the same functional group form a homologous series.
In the names of organic compounds, functional groups are denoted by prefixes and suffixes which precede or follow the root of the compound’s name.
The table below shows information about some common functional groups (R represents a saturated hydrocarbon chain of any length).
|Functional Group |
|alkane||a saturated hydrocarbon||-ane||methane|
X is a halogen (fluorine, chlorine,
bromine or iodine)
Branched alkanes contain hydrocarbon side chains bonded at various positions along the main chain of carbon atoms. The main carbon chain is known as the parent chain and can be identified by finding the longest continuous chain of carbon atoms. Any chains which branch off from the parent chain are known as alkyl chains. The length of the alkyl chain is denoted in the name of a compound by the prefixes shown in the table below.
|Number of Carbon Atoms in Alkyl Side Chain||1||2||3||4|
|Nomenclature of Alkyl Chain||methyl-||ethyl-||propyl-||butyl-|
For example: methylpropane is a saturated hydrocarbon with a parent chain containing three carbon atoms. It has a side chain bonded to the second carbon in the parent chain, containing one carbon atom.
Figure 1: Methlypropane
Cyclic compounds are identified by the prefix cyclo-. For example, cyclohexane is a cyclic alkane.
Figure 2: Cyclohexane
If an organic compound contains more than one functional group, the main functional group is the one with the highest priority:
The functional group with the highest priority determines the suffix which will follow the root
of the molecule in its name.
The prefix for all other functional groups will precede the root in alphabetical order.
2. The Root
Once the highest priority functional group has been identified, the next step is to find the longest continuous chain of carbon atoms which includes the main functional group. This is known as the parent chain. The number of carbon atoms in the parent chain provides the root (sometimes referred to as the base or the stem) of the compound’s name, shown in the table below.
Carbon Atoms in
the Parent Chain
These roots are also used to denote the number of carbon atoms in an alkyl chain in a branched molecule. For example, a molecule whose parent chain has a side chain containing three carbon atoms would have the prefix propyl-.
For example: butanone is a ketone with a parent chain containing four carbon atoms.
Figure 3: Butanone
3. The Position and Number of Functional Groups
Number the carbon atoms of the parent chain, starting at the end of the chain that is closest to a functional group.
The positions of functional groups are denoted with hyphens and numbers within the name of
the compound. There should always be a hyphen between a number and a letter in the name of organic compounds and consecutive numbers should be separated by a comma. For example, 1,4-dichlorobutane and propan-1,3-diol.
The position of each functional group should always be included in the name of a compound unless there is only one possible position for it, then the number is not necessary.
The molecule in Figure 4 contains the OH (alcohol) functional group, so we use the suffix -ol. There are four carbon atoms in the parent chain, so we use the root ‘but’. The OH functional group appears at the end of the parent chain. We must number the carbons in the chain from the end which is closest to a functional group, so in this case we number the carbon atoms from right to left. It would be possible for the OH group to be bonded to the second carbon atom in the chain, so we must include the position number of this group. The molecule in Figure 4 is butan-1-ol.
Figure 4: Butan-1-ol
If the same functional group is present more than once in the same compound, the number of functional groups is represented by the prefixes shown in the table below:
|Number of Occurrences of Functional Group||Prefix|
The molecule in Figure 5 is a halogenoalkane containing chlorine atoms, so the prefix chloro- and the suffix -ane will be used. It has five carbon atoms in the parent chain, so the root is ‘pent’. There are chlorine atoms on the second, third and fourth carbon atoms from either end of the chain. It would be possible for these three chlorine atoms to be in different positions along the parent chain, so we must include their position in the name of the compound. The compound in Figure 5 is 2,3,4-trichloropentane.
Figure 5: 2,3,4-trichloropentane
To determine the systematic IUPAC name of an organic compound, follow these steps:
- Identify the main (highest priority) functional group. This gives you the suffix.
- Count the number of carbon atoms in the longest continuous chain containing the main functional group (the parent chain). This gives you the root.
- Number the carbon atoms in the parent chain, starting at the end that is closest to a functional group.
- Use prefixes, in alphabetical order, to indicate any functional groups which are not the main functional group.
- Indicate the position and number of each functional group. Be sure to use a hyphen before and after numbers within the compound’s name and separate numbers with commas.
- Combine the parts of the name in the order prefix A + prefix B + root + suffix.
Example A contains the C=C (alkene) functional group. This is represented by the suffix -ene.
There are four carbon atoms in the parent chain, so the root is ‘but’.
The C=C functional group appears between the second and third carbon atoms in the chain. The lowest number should be used. It would be possible for the C=C to be between the first and second carbon atoms, so the functional group position must be included.
Example A is but-2-ene.
Example B is a cyclic saturated hydrocarbon. Saturated hydrocarbons (alkanes) are named with the suffix -ane. The cyclic nature of this alkane is denoted by the prefix cyclo-.
There are five carbon atoms in its ring, so the root is ‘pent’.
Example B is cyclopentane.
Example C has two functional groups:
- a chlorine atom, which is represented with the prefix chloro- and the suffix -ane;
- an alkyl side chain, containing one carbon atom, which is represented by the prefix methyl-.
The parent chain (the longest carbon chain) contains five carbon atoms, so the root will be ‘pent’.
The carbon atoms should be numbered from right to left, as shown, because the carbon atom on the right is closest to a functional group.
The methyl group is bonded to the second carbon, so will be denoted by the prefix 2-methyl-. The chlorine atom is bonded to the third carbon atom, so will be denoted by the prefix 3-chloro-. Prefixes must be in alphabetical order.
Example C is 3-chloro-2-methylpentane.
Example D contains two functional groups:
- the R-CO-R (ketone) functional group;
- the OH (alcohol) functional group.
The suffix is determined by the highest priority functional group, in this case the ketone, so the suffix will be -one.
Since the OH group is of lower priority in this compound it will be represented by its prefix,
The parent chain contains three carbon atoms, so the root is ‘prop’.
In a ketone with just three carbon atoms, there is only one possible position for the C=O bond (if it were at the end of the carbon chain the compound would be an aldehyde instead). Therefore, no number is needed to indicate its position.
The carbon chain would be numbered starting from whichever end the OH group is bonded to, the OH group can only be positioned at the first carbon in the chain. So, no number is needed to indicate its position.
Example D is hydroxypropanone.
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