Question: 75: Which of the following reactions is not an example for nucleophilic addition – elimination reaction?
Answer: Option (1)
Explanation:
Nucleophilic addition–elimination reactions involve two steps.
First, a nucleophile adds to the carbonyl carbon.
Second, a molecule such as water is eliminated,
leading to the formation of a double bond like C=N.
In reaction (2), hydroxylamine NH_2OH acts as a nucleophile
and adds to the carbonyl group of acetaldehyde.
This is followed by elimination of water to form an oxime.
Hence, it is a nucleophilic addition–elimination reaction.
In reaction (3), phenylhydrazine C_6H_5NHNH_2 adds to the carbonyl carbon of acetaldehyde, followed by elimination of water to form a hydrazone.
Therefore, it is also a nucleophilic addition–elimination reaction.
In reaction (4), ammonia adds to the carbonyl group of acetaldehyde and
then water is eliminated to form an imine.
Thus, this reaction also follows nucleophilic addition–elimination.
In reaction (1), sodium bisulfite adds directly to the carbonyl carbon of acetaldehyde to form a stable bisulfite addition compound.
No elimination of any molecule occurs in this reaction.
Hence, it is only a nucleophilic addition reaction and not a nucleophilic addition–elimination reaction.