Nucleophilic Addition-Elimination
in Aldehydes and Ketones
Aldehydes and ketones react with a number of ammonia derivatives in the presence of acids to form addition products.
Some ammonia derivatives are given below :
- NH2OH (hydroxylamine)
- NH2-NH2 (hydrazine)
- C6H5NHNH2 (phenylhydrazine)
- NH2CONH2 (Semicarbazide)
pH of acid catalysts
In reactions of aldehydes and ketones with ammonia derivatives, weak acids are used as catalysts. Due to lone pairs on oxygen, the carbonyl group reacts with weak acids.
The above reaction yields the following product which exhibits resonance.
Due to resonance, the positive charge on carbonyl carbon increases and, consequently, the carbon becomes more susceptible to a nucleophilic attack. The reaction, however, is pH dependent and takes place at pH 4 to 5 only. This is due to the following reasons :
- When pH is very low : The ammonia derivatives being basic in nature form their respective ammonia salts and, consequently, lose their nucleophilic nature.
- When pH is high : At higher pH, the carbonyl group will not be sufficiently protonated.
Reactions of Aldehydes and Ketones
with Hydroxylamine
Aldehydes and ketones react with hydroxylamine (NH2OH) to form oximes.
Reaction of Aldehydes and Ketones with Hydrazine
Aldehydes and ketones react with hydrazine (NH2−NH2) to form hydrazones.
Similarly, aldehydes and ketones react with phenylhydrazines (C6H5NHNH2) to form phenylhydrazones.
2,4-DNP Test
Aldehydes and ketones react with 2,4-dinitrophenylhydrazine to form yellow, orange or red ppt. This reaction is used for distinction of aldehydes and ketones from other compounds and is known as 2,4-DNP test or Brady's test .
Aliphatic aldehydes and ketones give yellow ppt with 2,4-dinitrophenylhydrazine.
We get red ppt with aromatic aldehydes and ketones.
Reactions of Aldehydes and Ketones with semicarbazides
Aldehydes and ketones react with semicarbazide (NH2CONHNH2) to form semicarbazones.
Questions
Question : There are two −NH2 groups in semicarbazide. However, only one is involved in the formation of semicarbazones. Explain why.
Answer : The lone pair on NH2 group directly attached to the carbonyl carbon is involved in resonance.
As a result, electron density of this NH2 group decreases; therefore this NH2 group does not act as a nucleophile. The lone pair of other NH2 group, which is attached to NH, is not involved in resonance; hence this NH2 group acts as a nucleophile.