Out of all isomers of C_4H_11N , how many are primary amines?

The correct answer is Option (2) → 4

A primary amine has one alkyl group on nitrogen, \( \ce{R-NH2} \). For \( \ce{C4H11N} \) the –\( \ce{NH2} \) group must sit on a \( \ce{C4H9} \) group, and there are four \( \ce{C4H9} \) groups (two carbon chains, each with two positions):

• \( \ce{CH3CH2CH2CH2NH2} \) — butan-1-amine

• \( \ce{CH3CH2CH(NH2)CH3} \) — butan-2-amine

• \( \ce{(CH3)2CHCH2NH2} \) — 2-methylpropan-1-amine

• \( \ce{(CH3)3C-NH2} \) — 2-methylpropan-2-amine

So there are 4 primary amines. The other isomers of \( \ce{C4H11N} \) are three secondary amines (N-methylpropan-1-amine, N-methylpropan-2-amine, N-ethylethanamine) and one tertiary amine (N,N-dimethylethanamine), 8 in all.

NCERT (2026-27 edition):
Classification – “Amines are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. If one hydrogen atom of ammonia is replaced by R or Ar, we get RNH₂ or ArNH₂, a primary amine (1°).”