Question

Carbylamine test is performed in alcoholic KOH by heating a mixture of:
A. chloroform and silver powder.
B. tri halogenated methane and a primary amine
C. an alkyl halide and a primary amine
D. an alkyl cyanide and a primary amine

Answer 1

Carbylamine reaction is a process which is used to synthesis alkyl isocyanides. In this reaction, analyte is heated with alcoholic potassium hydroxide and chloroform.

Complete step by step answer:
Carbylamine test is a process which involves dehydrohalogenation of chloroform in an alkaline medium.
The carbylamine reaction is also known as Hofmann isocyanide synthesis. This reaction is only given by primary amines.
It includes a primary amine, tri halogenated methane and a base to synthesize an isocyanide.
In the carbylamine reaction, the amine is added to the intermediate that is created by the dehydrohalogenation of the tri halogenated methane. This intermediate is known as dihalocarbene. This intermediate is very important for this reaction.
The general carbylamine test can be written as follows:

$\underset { Alkyl\;amine }{ R-N{ H }_{ 2 } } \quad +\quad \underset { Chloroform }{ CH{ Cl }_{ 3 } } \quad +\quad \underset { Alcoholic }{ 3KOH } \quad \longrightarrow \quad \underset { Carbylamine }{ R-CN } \quad +\quad \underset { Potassium\; chloride }{ 3KCl } \quad +\quad \underset { Water }{ 3{ H }_{ 2 }O }$

From the above explanation, we can see that a mixture of tri halogenated methane and a primary amine is heated in alcoholic KOH. Therefore, the correct option is option (B).

Let us now look, how this process takes place by taking the example of aniline.
Firstly, a mixture of chloroform and aniline is heated in the presence of the base, KOH. Here, the dehydrohalogenation, which is the removal of hydrogen form a given substrate, of chloroform takes place which, as a result, gives a very reactive intermediate known as dichlorocarbene. This electrophilic dichlorocarbene is the nucleophilic nitrogen in the primary amine. After this, the base helps to form isocyanide. The reaction involved is given below.

$\underset { Aniline }{ { C }_{ 6 }{ H }_{ 5 }N{ H }_{ 2 } } \quad +\quad \underset { Chloroform }{ CH{ Cl }_{ 3 } } \quad +\quad \underset { Alcoholic }{ 3KOH } \quad \longrightarrow \quad \underset { Phenyl\; isocyanide }{ { C }_{ 6 }{ H }_{ 5 }-\overset { + }{ N } \equiv \overset { - }{ C } } \quad +\quad \underset { Potassium\; chloride }{ 3KCl } \quad +\quad \underset { Water }{ 3{ H }_{ 2 }O }$

Note: Carbylamine tests cannot be used in the synthesis of isocyanides from secondary or tertiary amines. This test can also be used to detect the presence of primary amines.