How carbylamine reaction give foul smelling compound?

Carbylamine reaction is in the chapter amines in class 12. It is helpful for the detection of primary amines which give foul smell. Let’s read how and why it give foul smell!

What is Carbylamine reaction?

Carbylamine reaction is the reaction in which aliphatic or aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide give foul smelling compound called alkyl/aryl isocyanides.

This reaction is a test for detection of primary amines only. Secondary and tertiary amines do not give this test. The name of the test performed is called as Carbylamine test.

Carbylamine reaction

R-NH₂ +CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O

Carbylamine reaction mechanism

Carbylamine reaction mechanism is the synthesis of the addition of amines to the intermediate. Here, the resultant intermediate is formed by dehydrohalogenation of the chloroform molecule. This intermediate is called dichlorocarbene. Also, this reaction is referred to as the synthesis of Hoffman isocyanide, where it is the reaction of chloroform, primary amine,  and a base to synthesize these isocyanides. Dichlorocarbene intermediate is quite important for this type of conversion. We cannot use this reaction to synthesize isocyanides from the secondary or tertiary amines.

The first step is given as the dehydrohalogenation (hydrogen halide removal from a given substrate) of chloroform to produce dichlorocarbene intermediate, which is very reactive. The electrophilic dichlorocarbene attacks the primary amine’s nucleophilic nitrogen. The elimination of hydrochloric acid leads to an isonitrile formation. A representation of the mechanism of carbylamine reaction is given below. The carbylamine reaction is used for isocyanides synthesizing from primary amines by using chloroform and a base. Also, the carbylamine reaction is employed to test the primary amine’s presence in a given substrate.

Carbylamine test

Because the carbylamine reaction is only effective for primary amines, we can use it as a chemical test for the presence of primary amines. When used as a test, the carbylamine reaction is also known as Hoffman’s isocyanide test. Here, the test substance is heated with alcoholic potassium hydroxide and chloroform. In the presence of a primary amine case, there will take place an isocyanide (carbylamine) formation, which can be identified easily by its extremely foul smell. This Hofmann isocyanide test does not emit a foul odor either with the secondary or tertiary amines because they do not undergo a carbylamine reaction.

Why does carbylamine test not work with secondary and tertiary amines?

As we all know, the carbylamine reaction is used to detect chloroform and a primary amine. This carbylamine test is used to distinguish primary amine from both secondary and tertiary amines. As per the mechanism, this is one of the best examples of Alpha elimination reaction, where the carbanion is formed in the very first step, which later loses the chloride Ion to form highly reactive dichlorocarbene.

Then, the remaining two chlorine atoms will be removed in the form of HCl. For that, hydrogen is given from the nitrogen in amine. Concerning secondary amine and tertiary amine, nitrogen does not hold more hydrogen atoms, and also, there is expected a sterically highly unstable product if the secondary amines and tertiary amines would be reacted. And notably, alkyl isocyanides are compounds that exhibit unpleasant smelling. Hence, we use this test for the detection of only the primary amines

Questions from IIT JEE

1. On heating an aliphatic primary amine with chloroform and ethanolic potassium hydroxide, the organic compound formed is:

(a) An alkanediol (b) An alkyl cyanide (c) An alkyl isocyanide (d) An alkanol

Solution: Here the reaction occurs is carbylamine reaction, in which the final product is a foul smelling compound called alkyl isocyanide.

Reaction is: R-NH₂ +CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O

Hence the correct option is (c).

2. Which of the following amines can be prepared by Gabriel Phthalimide reaction?

(a) n-butylamine

(b) triethylamine

(c) tert-butylamine

(d) neo-pentylamine

Solution:

Read more: Circle represents most basic atoms in these molecule. Which of the following is correct representation?

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