3.1 Introduction

Alkanes are hydrocarbons i.e. compounds made of carbon and hydrogen only. There are two kinds of alkanes-linear and cyclic alkanes or cycloalkanes. The linear alkanes have the general formula C_nH_2n+2, while cycloalkanes have the general formula C_nH_2n . Besides the simple monocyclic cycloalkanes, there are other forms such as bicyclic and tricyclic cycloalkanes with bridges containing any or no carbon atoms. Another special class of cycloalkanes is the so called spiroalkanes which have one carbon atom common between two rings.

Alkanes show both constitutional and conformational isomerism. The conformational isomerism manifests itself as early as the second member of the alkane family-ethane while constitutional isomerism first appears for butane. The issue of conformational isomerism is discussed separately under stereoisomerism.

Constitutional isomerism arises due to compounds having same molecular formula but different bond connectivity. Butane has two constitutional isomers: n -butane and iso butane. Similarly, pentane has 3 constitutional isomers: n -pentane, iso pentane and neo pentane (Scheme 1).

Scheme 1

Carbon atoms in alkanes are sometimes labeled as primary (1°), secondary (2°) and tertiary (3°) carbon. This classification is based on the number of carbon atoms attached to the carbon atoms. If a carbon atom is attached to one carbon atom, then it is called a primary carbon. Similarly, a carbon atom which is attached to two and three other carbon atoms is called secondary and tertiary carbon, respectively.

3.2 Nomenclature

Alkanes, the more common ones, are usually referred by their trivial names. However, the IUPAC lays down certain rules for the nomenclature of alkanes. These rules are:

Rule 1. The name of an alkane consists of two parts- a prefix and a root. The prefix will denote the number of carbons present in the chain. Some prefixes for common alkanes are given below along with the number of carbons they represent (Table 1).The root is always ane . The moiety generated by removing a hydrogen from an alkane is called an alkyl group. Its name is given based on the name of parent hydrocarbon from which it is derived.

Table 1