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Rules for the IUPAC System of Nomenclature

The rules are as given below:
  1. Longest chain rule
    The longest continuous chain of carbon atoms is selected as the parent hydrocarbon. The compound is named as a derivative of the parent hydrocarbon. For example, out of the following two ways of numbering, second one is correct as it involves a longer carbon chain.
  2. Lowest number rule
    The parent carbon chain is numbered in such a way (beginning from either end) so that the other side-chains or substituents linked to the parent carbon chain are indicated by the lowest possible number. For example, II is correct as the methyl substituent gets numbered 2, but I is incorrect as it is 4.
IUPAC names are assigned to alkanes by adding the suffix "-ane" to the root name of the parent hydrocarbon (Table). The side chains and substituents are also indicated by their names.

Table : Root name and name of the corresponding hydrocarbon


Number of carbon atoms in parent chain

Root name

Corresponding alkane

1 meth methane
 2 eth ethane
3 prop propane
4 but butane
5 pent pentane
6 hex hexane
7 hept heptane
8 oct octane
9 non nonane
10 dec decane
  1. Suitable prefix
    In case there are two or more identical substituents, a suitable prefix, di-(for two) and tri-(for three), is placed before the name of the compound, e.g.
  2. Alphabetical order
    In case there is more than one different subsitutent, then the names of the substituents are written in alphabetical order, e.g. 3-ethyl-2-methylpentane. The following two examples are wrongly named.

Name of the alkane

There are five carbon atoms in the longest chain. So it will be named after pentane. Carbon 2 and 4 have one methyl group each. So it is a dimethylpentane. The methyl groups are positioned at carbon atom number 2 and 4. Thus its IUPAC name will be 2, 4-dimethyl pentane


Cyclic alkanes are named by fixing a prefix-"cyclo" before the name of corresponding alkane. The general formula of cycloalkanes is CnH2n.

Names of side chains, if present, in cyclic compounds are prefixed to the name of the cycloalkane.

Functional Groups

Functional groups provide centres of chemical reactivity in molecules. The properties of an organic compound depend largely on its functional group. Compounds having the same functional groups have similar properties because they contain the same group, which is the cause of their reactivity. In a homologous series all the members have the same functional group. Therefore, it is convenient to study organic chemistry by studying the properties of a functional group. These properties will also be the properties of all other compounds containing the same functional group. Some of the common functional groups are shown in Table. The nomenclature of organic compounds becomes similar since compounds having the same functional groups will have the same beginning or ending in their IUPAC names.

For example, names of all alcohols will end in '-ol' and those of ketones will end in '-one'.

Table: Some common functional groups

Nomenclature of substituted Benzene compounds

  1. In mono substituted Benzene compounds, the substituent is placed as prefix to the word, Benzene.


  1. In the case of disubstituted compounds, the position of the substituent is defined by numbering around the ring such that the substituents ore located at the lowest numbers, possible.

  1. The base compound substituent is numbered 1 and then the direction of numbering is chosen such that the next substituent gets the lowest number. The substituents appear in alphabetical order.

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