Question

Number of chain isomers shown by Nonane are
(A) $5$
(B) $9$
(C) $18$
(D) $35$

Answer 1

Hint: Chain isomers of an alkane have the same molecular formula but the way their carbon atoms are joined together is different for different isomers. The alkanes which have four or more than four carbon atoms have chain isomers. In these types of isomers, the carbon atoms are bonded together in different ways to produce branches.

Complete answer:
Chain isomerism arises because of the possibility of branching in carbon chains. The alkane butane has two isomers. In one isomer of butane, all the carbon atoms lie in a straight chain whereas in the other isomer the chain is branched. The next alkane pentane has three chain isomers. The next alkane hexane has five chain isomers. The next alkane heptane has nine chain isomers. The next alkane octane has eighteen chain isomers. The next alkane nonane has thirty five chain isomers. Chain isomers mainly differ in their structure with respect to the carbon chain length.
Therefore, option D is the correct answer.

Note:
To draw the chain isomers of a given alkane, first draw the main chain i.e. the straight chain containing carbon atoms. Next, draw a carbon atom chain which has one carbon less than the main parent chain and add a methyl group to the maximum number of positions that are possible. Do not add methyl groups to the end of the chain and ensure that no structures are repeated. Continue subtracting carbons and adding methyl groups until there are no carbon atoms left. Write the IUPAC names of all the compounds. This helps you to check if any structure is repeated.