IUPAC Nomenclature of Branched Chain Hydrocarbons
In branched chain hydrocarbons, one or more alkyl groups are present as side chain attached to the main straight chain of carbon atoms. The carbon atoms of the side chain constitute alkyl groups. These alkyl groups are written as prefixes in the IUPAC name.
Alkyl Group
An alkyl group is obtained from an alkane by removing one hydrogen atom. Since the general formula of alkane is CnH2n+2, the general formula of alkyl group is CnH2n+1.
The alkyl groups are generally represented by R– and named by replacing the suffix ane of the corresponding alkane by yl. Alkyl group are named as word root + yl.
| Parent Chain | Alkyl Group | Name |
| Methane | CH3- | Methyl |
| Ethane | C2H5- | Ethyl |
| Propane | C3H7- | Propyl |
| Butane | C4H9- | Butyl |
Naming Rules
Branched chain hydrocarbons are named using the following rules in IUPAC system.
Rule 1: Longest Chain Rule
According to this rule, the longest possible chain of carbon atoms is considered and the compound is named as the derivative of the corresponding alkane. If some multiple bond is present, the selected chain must contain the carbon atoms of the multiple bond.
The number of carbon atoms in the selected chain determines the word root and the saturation or unsaturation will determine the suffix.
Rule 2: Lowest Number or Lowest Sum Rule
The longest carbon chain is numbered from one end to another and the positions of the side chain are indicated by the number of carbon atoms to which these are attached.The numbering is done in such a way that:
- The substituted carbon atoms have the lowest possible numbers.
- The sum of numbers used to indicate the positions of various alkyl groups must be the lowest.
Rule 3: If some multiple bond is present in the chain, the carbon atoms involved in the multiple bond should get the lowest possible numbers.
Rule 4: Naming of compounds with one alkyl group as the substituent (side chain)
The name of a substituted hydrocarbon consists of the following parts:
Position of substituent - Name of substituent, Word root, Suffix
Rule 5: Naming the same alkyl groups at different positions
If the compound contains more than one identical alkyl groups, their positions are indicated separately and the prefixes di (for two), tri (for three) are attached to the name of the substituents. The positions of the substituents are separated by commas (,).
Rule 6: Naming different alkyl substituents
If there are different alkyl substituents present in the compound, their names are written in the alphabetical order. However, the prefixes di, tri, etc. are not considered in deciding the alphabetical order.
