Question

Write short notes on the following:
i) Carbylamine reaction
ii) Hofmann's bromamide reaction
iii) Acetylation

Hint:

Carbylamine test \(\Rightarrow\) only 1° amines (foul isocyanide smell).
Hofmann rearrangement \(\Rightarrow\) amide \(\to\) 1° amine with \(\mathbf{-1}\) carbon.
Acetylation \(\Rightarrow\) protection/activation control using \(\mathrm{Ac_2O}\) or \(\mathrm{CH_3COCl}\).

Answer 1

i) Carbylamine Reaction (Isocyanide Test):
- When a primary amine (aliphatic or aromatic) is heated with chloroform and alcoholic KOH, it forms an isocyanide (carbylamine) with a very foul smell.
- Secondary and tertiary amines do not give this test, hence it is a specific test for primary amines.
\[ \mathrm{RNH_2 + CHCl_3 + 3\,KOH \ \xrightarrow{\Delta}\ RNC + 3\,KCl + 3\,H_2O} \]
ii) Hofmann's Bromamide Reaction (Hofmann Rearrangement):
- An amide on treatment with bromine and aqueous/alkaline base gives a primary amine with one carbon less (loss of CO, migration of R group).
- Useful for stepping down the homologous series and preparing 1° amines from amides.
\[ \mathrm{RCONH_2 + Br_2 + 4\,NaOH \ \rightarrow\ RNH_2 + Na_2CO_3 + 2\,NaBr + 2\,H_2O} \]
iii) Acetylation:
- Introduction of an acetyl group \((\mathrm{-COCH_3})\) into a compound using acetylating agents such as \(\mathrm{CH_3COCl}\) or \((\mathrm{CH_3CO})_2\mathrm{O}\).
- Commonly used to protect \(-\mathrm{OH}\) and \(-\mathrm{NH_2}\) groups, to reduce basicity/nucleophilicity, and to control orientation in electrophilic substitution.
\[ \mathrm{C_6H_5NH_2 + (CH_3CO)_2O \ \rightarrow\ C_6H_5NHCOCH_3\ (acetanilide) + CH_3COOH} \]
\[ \mathrm{C_6H_5OH + CH_3COCl \ \xrightarrow{pyridine}\ C_6H_5OCOCH_3\ (phenyl\ acetate) + HCl} \]