Which of the following amines of molecular formula C4H11N is a second…

The correct answer is Option (1) → N-Methylpropan-2-amine

Amines are classified by the number of hydrogen atoms of ammonia replaced by alkyl or aryl groups, that is, by the number of carbon groups on the nitrogen atom. The position of the nitrogen on the carbon chain does not matter.

• N-Methylpropan-2-amine, \( \ce{CH3CH(CH3)NHCH3} \): Nitrogen carries an isopropyl group, a methyl group and one H — two carbon groups, so it is a secondary amine.

Why the other options are incorrect

Option (2) Butan-2-amine — Incorrect. \( \ce{CH3CH2CH(NH2)CH3} \) has –NH₂ on a secondary carbon, but nitrogen bears only one carbon group, so it is a primary amine.

Option (3) 2-Methylpropan-2-amine — Incorrect. \( \ce{(CH3)3CNH2} \) also has an –NH₂ group, so it is a primary amine, though the carbon carrying it is tertiary.

Option (4) N,N-Dimethylethanamine — Incorrect. \( \ce{C2H5N(CH3)2} \) has three carbon groups on nitrogen and no N–H, so it is a tertiary amine.

NCERT (2026-27 edition):
9.2 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°). If two hydrogen atoms of ammonia or one hydrogen atom of R-NH₂ are replaced by another alkyl/aryl(R’) group, what would you get? You get R-NHR’, secondary amine. … Replacement of another hydrogen atom by alkyl/aryl group leads to the formation of tertiary amine.”