Question
Explain the Hofmann bromamide degradation reaction. How can you use it to prepare aniline from benzamide? What is special about the product amine compared to the starting amide?
(JEE Main 2021, similar pattern)
Solution — Step by Step
The Hofmann bromamide degradation converts a primary amide (RCONH) to a primary amine (RNH) with one less carbon atom:
The carbon of the C=O group is lost as NaCO.
- Base removes N-H proton: RCONH + NaOH → RCONH + HO
- Bromination of nitrogen: RCONH + Br → RCONHBr + Br
- Loss of HBr (Hofmann rearrangement): RCONHBr + NaOH → RCON: (nitrene intermediate) → R-N=C=O (isocyanate) — the R group migrates from C to N
- Hydrolysis of isocyanate: R-N=C=O + 2NaOH → RNH + NaCO
The key step is the 1,2-shift of the R group from carbon to nitrogen, forming an isocyanate intermediate.
Benzamide (7 C) → Aniline (6 C). One carbon is removed as carbonate.
Why This Works
The driving force is the rearrangement step where the R group migrates to the electron-deficient nitrogen. This migration preserves the configuration of R (it migrates with its bonding electrons), making it a concerted 1,2-shift.
The Hofmann degradation is one of the few reactions that shortens the carbon chain. This makes it useful in synthesis when you need to go from a carboxylic acid derivative (amide) to an amine with one fewer carbon — a transformation that is otherwise difficult to achieve.
Alternative Method — Curtius and Lossen Rearrangements
Similar reactions that also go through isocyanate intermediates:
- Curtius: Acyl azide (RCON) → isocyanate → amine
- Lossen: Hydroxamic acid derivative → isocyanate → amine
All three share the same key step: migration of R from C to N.
The Hofmann degradation is a favourite for CBSE 3-mark questions: “Convert propanoic acid to ethylamine.” Answer: First convert propanoic acid to propanamide (using NH), then Hofmann degradation. This two-step conversion (acid → amide → amine) is a pattern worth memorising.
Common Mistake
Students forget that the product amine has one less carbon than the starting amide. If the starting amide is CHCONH (2C), the product is CHNH (1C), not CHNH. The carbonyl carbon is lost. This carbon-count error leads to wrong products in synthesis problems.