Q.Write the structures of all the chain isomers of the saturated hydrocarbon containing six carbon atoms. Write IUPAC names of all the above structures.

Step 1. Recall the isomer count. Table 15.1 (section 15.1.1) states that C6H14 has 5 chain isomers.

Step 2. Draw and name each one. (1) n-Hexane: CH3-CH2-CH2-CH2-CH2-CH3, the unbranched 6-carbon chain. (2) 2-Methylpentane: CH3-CH(CH3)-CH2-CH2-CH3, a 5-carbon chain with a methyl branch at C2. (3) 3-Methylpentane: CH3-CH2-CH(CH3)-CH2-CH3, a 5-carbon chain with a methyl branch at C3. (4) 2,3-Dimethylbutane: CH3-CH(CH3)-CH(CH3)-CH3, a 4-carbon chain with methyl branches at both C2 and C3. (5) 2,2-Dimethylbutane: CH3-C(CH3)2-CH2-CH3, a 4-carbon chain with two methyl branches both at C2.

Step 3. Confirm each is a genuine constitutional isomer. All five share the molecular formula C6H14 but differ in how the carbon skeleton branches -- from the fully unbranched chain (1) down to the two most compact, doubly-branched 4-carbon-chain structures (4) and (5).

Systematically shortening the longest chain by one carbon at a time and placing the resulting methyl branch(es) at every distinct, non-duplicate position, starting from the unbranched 6-carbon chain

• Missing 2,3-dimethylbutane or double-counting it as different from 3,4-dimethylbutane (the same molecule, just numbered from the other end) -- IUPAC numbering always uses the LOWEST possible locants.
• Drawing a 'branch on a branch' (e.g. a methyl on an ethyl side chain), which for hexane's 5 real isomers never actually occurs at this small a carbon count.