Question

Carbylamine is liberated when..................is heated with chloroform and alcoholic potash:
A. An aldehyde
B. A primary amine
C. A secondary amine
D. A phenol

Answer 1

Hint: The reaction mentioned here is called the carbylamine reaction. This reaction is used as a confirmatory test for a certain type of amine because of its selective nature.

Complete Step by Step Solution:
Carbylamines, also known as isocyanides, are organic molecules containing the isocyanide functional group (\[ - NC\]). They are the products of the carbylamine reaction (also known as Hoffmann isocyanide synthesis). It is the reaction between chloroform (\[CHC{l_3}\]), alcoholic potash (alc.\[KOH\]) and a primary amine.

Image: The carbylamine reaction

The first step of the carbylamine reaction is the dehydrohalogenation of chloroform by the alcoholic potash to produce dichlorocarbene (\[:CC{l_2}\]) intermediate. Since carbenes are electron deficient, they get attacked by the primary amine using the lone pair on its nitrogen atom. Subsequent eliminations of hydrochloric acid molecules lead to the formation of carbylamine. The mechanism is shown below using aniline as an example.

Image: Mechanism of the carbylamine reaction

As per the mechanism shown above, the\[{}^ - CC{l_3}\]carbanion formed in the first step loses a chloride to give dichlorocarbene intermediate which then gets attacked by the nucleophilic nitrogen of the amine to form product II. The remaining two chlorine atoms are removed as\[HCl\]for which a hydrogen atom is supplied by the nitrogen to the carbon atom attached to it as can be seen in the formation of product III from product II and in the formation of product IV from product III.
For the elimination of two\[HCl\]s, two hydrogen atoms need to be supplied by the nitrogen of the amine. This is possible only in primary amines as secondary amines have only a single hydrogen atom and tertiary amines have none. This is why the carbylamine reaction occurs only in primary amines.
Thus, option B is correct.

Note: Since this reaction takes place only with primary amines, it is used to test their presence. The reason for this reaction not working for secondary and tertiary amines is a commonly asked question. To answer it, the mechanism shown above must be known.