Question

Describe the following giving the relevant chemical equation in each ease.
(i) Carbylamine reaction
(iI) Hofmann’s bromamide reaction

Answer 1

Carbylamine is a reaction of primary amine, chloroform and base for the synthesis of isocyanide. Hofmann’s bromide reaction is used for the conversion of primary amide to a primary amine with one less carbon atom.

Step by step answer: (i) Carbylamine reaction which is also known as Hofmann isocyanide synthesis is the synthesis of an isocyanide by the reaction of a primary amine, chloroform and base. The conversion involves the intermediacy of dichlorocarbene.
This reaction mechanism cannot be used to synthesize isocyanides from secondary or tertiary amines.
When aliphatic and aromatic primary amines are heated with chloroform and ethanolic $KOH$ solution, they form isocyanides or carbylamines which are foul smelling substances.
The reaction can be written as:

$R - N{H_2} + CHC{l_3} + 3KOH \to R - NC + 3KCl + 3{H_2}O$

Given above is an example of this reaction:
Since this reaction is effective only for primary amines, it can be used as a chemical test for the presence of primary amines. When used as a test, then it is called Hofmann’s isocyanide test. In this test, the substance is heated with chloroform and alcoholic potassium hydroxide. If primary amine is present then isocyanide will form which can be easily identified by its extremely foul smell.

(ii) Hofmann’s bromamide reaction includes the use of alkali as a strong base to attack the amide leading to the deprotonation and the subsequent generation of an anion. This reaction is used for the conversion of a primary amide to a primary amine with one less carbon atom. This is accomplished by heating the primary amide with a mixture of a halogen, a strong base and water. The reaction can be illustrated as follows:

$R - CO - N{H_2} + B{r_2} + 4NaOH \to R - N{H_2} + Na2C{O_3} + 2NaBr + 2{H_2}O$

Primary amines made by this technique are not contaminated by secondary or tertiary amines. This reaction is also called Hofmann degradation of amide. Reaction of bromine with sodium hydroxide leads to the formation of sodium hypobromite ( $NaOBr$ ) which turns the primary amide into an isocyanate intermediate.

Note: Carbylamine is a reaction of primary amine, chloroform and base for the synthesis of isocyanide which is a foul smelling substance. Hofmann’s bromide reaction is used for the conversion of primary amide to a primary amine with one less carbon atom. This uses alkali as a strong base to attack the amide leading to generation of anion.