Counting every stereoisomer separately, the number of isomeric amines of molecular formula C4H11N that liberate nitrogen gas quantitatively on…

Only primary aliphatic amines react with nitrous acid to form an unstable diazonium salt that loses $\mathrm{N_2}$ at once, giving alcohols. Secondary amines form yellow $N$-nitrosoamines and tertiary amines only give nitrite salts, so neither evolves nitrogen. List the primary amines $\mathrm{C_4H_9NH_2}$ from the four butyl skeletons. Butan-1-amine is achiral, so it counts 1. Butan-2-amine has a stereocentre at $\mathrm{C_2}$, so it counts 2 enantiomers. 2-Methylpropan-1-amine is achiral, so it counts 1. 2-Methylpropan-2-amine (tert-butylamine) is achiral, so it counts 1. The total is $1+2+1+1=5$. Skeleton isomers of the secondary class (three) and the tertiary class (one) are excluded.