Question

On heating an aliphatic primary amine with chloroform and ethanolic potassium hydroxide, the organic compound formed is:
  (A) an alkyl cyanide
  (B) an alkyl isocyanide
  (C) an alkanol
  (D) an alkanediol

Answer 1

Hint: Primary amines (both aromatic and aliphatic) when heated with chloroform and alcoholic KOH, isocyanides or carbylamines are formed and the reaction is called a carbylamine reaction. The newly formed product has an offensive smell. This reaction is answered only by primary amines.

Complete step by step answer:
  - In the given reaction heating of an aliphatic primary amine with chloroform and ethanolic potassium hydroxide takes place. This reaction is known as carbylamine reaction or as Hofmann’s isocyanide test.
-The carbylamine reactions are effective only for primary amines. Thus it’s used to detect the presence of primary amines. If the primary amine is present, there will occur a formation of carbylamine or isocyanide which can be easily recognized by its extremely foul odour.
-The Hofmann isocyanide test or carbylamine reaction will not give a foul smell with secondary or tertiary amines since they do not undergo the carbylamine reaction.
 - In the carbylamine reaction a mixture of primary amine (aromatic or aliphatic) and chloroform is boiled with alcoholic potassium hydroxide to form alkyl isocyanide or carbyl amine. The reaction for this test is written below
$$R-N{H}_2+CHC{l}_3+3KOH\stackrel{Heat}{\to }R-NC+3KCl+3H_{2}O$$

 This reaction is recognized as an isocyanide test or Hoffmann's carbylamine reaction and the compound formed as product $R-NC$ is the alkyl isocyanide. Let’s take the example of methyl amine which is a simplest aliphatic amine. When it is treated with chloroform and ethanolic potassium hydroxide the following reaction is taking place.

$$C{H}_3-N{H}_2+CHC{l}_3+3KOH\stackrel{heat}{\to }C{H}_3-NC+3KCl+3H_{2}O$$
  Where the product formed is methyl isocyanide ($C{H}_3-NC$).
So, the correct answer is “Option B”.

Note: Keep in mind that this test is not given by secondary and tertiary amines. The classification of amines into primary, secondary and tertiary are done according to the number of carbons bonded directly to the nitrogen atom. Primary amines will have one carbon bonded to the nitrogen and secondary amines will have two carbons bonded to the nitrogen whereas tertiary amines have three carbons bonded to the nitrogen.