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Nucleophilic Addition-Elimination
in Aldehydes and Ketones

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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 :

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.

Reaction of carbonyl group with weak acid

The above reaction yields the following product which exhibits resonance.

resonance structure of carbonyl carbon

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 :

  1. When pH is very low : The ammonia derivatives being basic in nature form their respective ammonia salts and, consequently, lose their nucleophilic nature.
  2. 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.

formation of ethanal oxime
formation of propanone oxime

Reaction of Aldehydes and Ketones with Hydrazine

Aldehydes and ketones react with hydrazine (NH2−NH2) to form hydrazones.

formation of ethanal hydrazone

Similarly, aldehydes and ketones react with phenylhydrazines (C6H5NHNH2) to form phenylhydrazones.

Reaction of Ketones with Hydrazine

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.

2,4-dinitrophenylhydrazine or 2,4-dnp test

We get red ppt with aromatic aldehydes and ketones.

Brady's test or 2,4-dnp test

Reactions of Aldehydes and Ketones with semicarbazides

Aldehydes and ketones react with semicarbazide (NH2CONHNH2) to form semicarbazones.

Reactions of Aldehydes with semicarbazides
Reactions of Ketones with semicarbazides

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.

Resonance structure of semicarbazide

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.

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