Question

Give reason for the following:

Nucleophilic addition of ammonia and its derivatives does not occur with carbonyl group in strongly acidic medium.

Hint:

The reaction needs the nitrogen of ammonia (or its derivative) to act as a nucleophile and attack the carbonyl carbon, using its lone pair of electrons.

Answer 1

Concept:
The reaction needs the nitrogen of ammonia (or its derivative) to act as a nucleophile and attack the carbonyl carbon, using its lone pair of electrons.

Step 1:
In a strongly acidic medium, ammonia and its derivatives get protonated at the nitrogen (forming \(R-NH_3^+\)).

Step 2:
Once protonated, the nitrogen no longer has a free lone pair available and carries a positive charge, so it cannot act as a nucleophile to attack the carbonyl carbon.

Answer: In a strongly acidic medium ammonia and its derivatives are protonated, so the lone pair on nitrogen is no longer available for nucleophilic attack on the carbonyl carbon; hence the nucleophilic addition does not occur. (A mildly acidic medium, around pH 3.5, is needed to balance protonation of the carbonyl and availability of the nucleophile.)