Question

Explain Hoffmann bromamide degradation reaction and write the general equation for reaction involved.

Answer 1

Hint: Try to recall that Hoffmann bromamide degradation reaction is used for the preparation of primary amines from primary amides and isocyanate is the intermediate formed during the reaction. Now by using this you can easily answer the given question.

Complete step by step solution:
It is known to you that amides with no substituents on the nitrogen atom react with a solution of bromine or chlorine in sodium hydroxide to yield primary amine through a reaction known as the Hoffmann bromamide degradation.
The general equation for Hoffmann bromamide degradation reaction is as follows:

The general mechanism of Hoffmann bromamide degradation reaction includes following steps:
Step 1: In this step, a strong base (usually an alkali) attacks the amide, this leads to deprotonation of amide and formation of water and an anion of amide.
Step 2: This anion reacts with bromine to form N-Bromamide along with \[B{r^ - }\] anion. This reaction can be seen as an alpha substitution reaction.
Step 3: In this step, another molecule of base deprotonates N-bromamide and leads to the formation of bromamide anion along with water.
Step 4: In this step, the generated bromamide anion undergoes rearrangement in such a way that the R group attached to the carbonyl carbon migrates to nitrogen and simultaneously the bromide ion leaves the compound. This leads to the formation of an isocyanate.
Step 5: In this step, addition of water to isocyanate takes place and it leads to the formation of carbamic acid. This reaction is an example of nucleophilic substitution reaction.
Step 6: Finally, in this step the carbamic acid loses carbon dioxide and the negatively charged nitrogen atom is protonated by water leading to the formation of required primary amine.

Note: It should be remembered that Hoffmann bromamide degradation reaction is used for the preparation of primary aliphatic and primary amines.
Also, you should remember that secondary and tertiary amides cannot be used in the Hoffmann bromamide degradation reaction to produce primary amine.