Key ideas
Amines are organic derivatives of ammonia. They are bases, and whether the nitrogen carries one, two or three carbon groups decides how they react in the classic tests.
- A primary amine has one carbon group on N, a secondary has two and a tertiary has three. Count the carbons on nitrogen, not the carbon bearing it.
- Aliphatic amines are stronger bases than ammonia, and aromatic amines such as aniline are weaker because the lone pair is delocalized into the ring.
- Hinsberg test: with benzenesulfonyl chloride and NaOH, a primary amine gives a soluble sulfonamide, a secondary gives an insoluble one, and a tertiary does not react.
- Hofmann bromamide reaction turns an amide into an amine with one carbon fewer. LiAlH₄ reduces amides and nitriles to amines.
Key equations
RNH₂ 1°, R₂NH 2°, R₃N 3°RCONH₂ + Br₂ + 4 NaOH → RNH₂ + Na₂CO₃ + 2 NaBr + 2 H₂ORCN + 4[H] → RCH₂NH₂ (LiAlH₄)basicity: R₂NH > RNH₂ > NH₃ > ArNH₂
Worked example
Hofmann degradation of propanamide, CH₃CH₂CONH₂, gives ethanamine, CH₃CH₂NH₂, one carbon shorter. Reducing propanenitrile with LiAlH₄ gives propan-1-amine, which keeps all three carbons. Of the three nitrogen types, (CH₃)₂NH is secondary and (CH₃)₃N is tertiary.
Common mistakes
- Classifying an amine by the class of the carbon it is attached to. Classify by the number of carbon groups on nitrogen.
- Forgetting that the Hofmann reaction loses the carbonyl carbon, so the amine has one carbon fewer.
- Ranking aniline as a stronger base than methylamine. The ring withdraws electron density from the nitrogen.
Practice
Test yourself with practice questions on this topic: Amines