Question

Write short notes on the following:
(i) Carbylamine reaction
(ii) Hofmann–bromamide reaction
(iii) Diazotisation

Hint:

Carbylamine test \(\Rightarrow\) identifies \(\mathbf{1^\circ}\) amines by isocyanide odour; Hofmann rearrangement \(\Rightarrow\) amide \(\to\) amine with \(-1\) carbon; Diazotisation must be done at \(0{-}5^\circ\mathrm{C}\) and opens many substitution routes via diazonium salts.

Answer 1

(i) Carbylamine (Isocyanide) reaction:
Primary amines (aliphatic or aromatic) on heating with chloroform and alcoholic KOH give foul-smelling isocyanides (carbylamines). Secondary and tertiary amines do not respond, so this is a specific test for 1° amines.
\[ \mathrm{R{-}NH_2 + CHCl_3 + 3\,KOH \xrightarrow[\Delta]{alc.} R{-}NC + 3\,KCl + 3\,H_2O} \] Example: \(\mathrm{C_6H_5NH_2 \xrightarrow[CHCl_3]{alc.\ KOH,\ \Delta} C_6H_5NC}\) (phenyl isocyanide).
(ii) Hofmann–bromamide reaction (Hofmann rearrangement):
An amide reacts with bromine in aqueous/alkaline medium to give a primary amine with \(\mathbf{one\ carbon\ less}\); proceeds via \(N\)-bromamide \(\rightarrow\) rearrangement to isocyanate \(\rightarrow\) hydrolysis. Stereochemistry at the migrating carbon is retained.
\[ \mathrm{RCONH_2 + Br_2 + 4\,NaOH \rightarrow RNH_2 + 2\,NaBr + Na_2CO_3 + 2\,H_2O} \] Key use: chain shortening in synthesis.
(iii) Diazotisation:
Primary aromatic amines react with nitrous acid (from \(\mathrm{NaNO_2/HCl}\)) at \(0{-}5^\circ\mathrm{C}\) to form stable diazonium salts (at low temperature). These are versatile intermediates (Sandmeyer, Gattermann, Schiemann, azo-coupling).
\[ \mathrm{ArNH_2 + NaNO_2 + 2\,HCl \xrightarrow{0{-}5^\circ C} ArN_2^+Cl^- + NaCl + 2\,H_2O} \] Aliphatic diazonium salts are unstable and decompose.