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Nomenclature and Isomerism

The first three members of this family have same common as well as IUPAC name. The higher members can exist as a number of isomers. The various isomers of butanes and pentanes are distinguished by the use of prefixes n- (for normal), iso- and neo-. Prefix n- is used for straight chain alkanes, iso- is used for the molecules containing

and neo- is used for molecules containing (CH3)C- group at the one end of the chain. Some examples are given below:

Each of the first three members of alkanes; methane, ethane and propane has only one structural formula as shown below:

Thus, no chain isomerism is possible up to propane. The fourth member, that is, C4H10, can have two structural formulae as shown below:

Replacement of hydrogen from first carbon atom of propane by -CH3 gives n-butane while replacement of hydrogen from second carbon atom by -CH3 gives iso-butane.

Thus, butane (C4H10) has two chain isomers n-butane and iso-butane.

Similarly, the next hydrocarbon, C5H12, has three different structural formulae which have been represented below along with their names.

Thus, pentane (C5H12 ) has three chain isomers; n-pentane, iso-pentane and neo-pentane. These chain isomers differ in their physical properties such as boiling point.

From the above discussion, it follows that the number of chain isomers goes on increasing with increase in the number of carbon atoms. For example, C6H14 has five, C7H16 bas nine, C8H18 has eighteen and C10Hn has 75 chain isomers. An alkane with sixty carbon atoms would have millions of chain isomers.

Some alkanes and their common as well as IUPAC names are given in tabular form as follows: