Question

Sol. Key concept: Count constitutional isomers of a primary amine without counting stereoisomers.

A saturated primary amine has the form R–NH₂. For C₅H₁₃N, R is a C₅H₁₁ group. Count inequivalent positions at which one hydrogen can be replaced by NH₂ on each five-carbon alkane skeleton.

The n-pentane skeleton gives three: pentan-1-amine, pentan-2-amine and pentan-3-amine.

The 2-methylbutane skeleton gives four: 2-methylbutan-1-amine, 3-methylbutan-1-amine, 3-methylbutan-2-amine and 2-methylbutan-2-amine.

The 2,2-dimethylpropane skeleton gives one: 2,2-dimethylpropan-1-amine. Thus the total is 3 + 4 + 1 = 8, option (c). “Primary” means nitrogen is bonded to one carbon group; that carbon itself need not be primary.