Nucleophiles such as ammonia and its derivatives, \(H_2N-Z\), add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours product formation due to rapid dehydration of the intermediate to form \(-C=N-Z\), where Z = alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.
Consider the following statements about nucleophilic addition to aldehydes and ketones.
(A) During nucleophilic addition, the hybridisation of the carbonyl carbon changes from sp2 to sp3 and a tetrahedral alkoxide intermediate is produced.
(B) In nucleophilic addition, the nucleophile approaches the carbonyl carbon from a direction approximately parallel to the plane of its sp2 hybridised orbitals.
(C) Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons.
(D) The polarity of the carbonyl group is increased in benzaldehyde due to resonance, and hence benzaldehyde is more reactive than propanal in nucleophilic addition reactions.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
In a nucleophilic addition, the nucleophile attacks the electrophilic carbonyl carbon and a proton then adds to the oxygen. How fast this happens depends on how easily the nucleophile can reach the carbon and how positive that carbon is.
(A) During nucleophilic addition, the hybridisation of the carbonyl carbon changes from sp2 to sp3 and a tetrahedral alkoxide intermediate is produced. — True. The carbon goes from three groups in a plane to four groups, so it becomes sp3 and tetrahedral; the oxygen carries the negative charge until it captures a proton.
(B) In nucleophilic addition, the nucleophile approaches the carbonyl carbon from a direction approximately parallel to the plane of its sp2 hybridised orbitals. — False. The attack comes from a direction approximately perpendicular to that plane, where the π electron cloud of the C=O bond lies.
(C) Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. — True. The two alkyl groups of a ketone hinder the approach of the nucleophile and also reduce the electrophilicity of the carbonyl carbon more than the single group of an aldehyde.
(D) The polarity of the carbonyl group is increased in benzaldehyde due to resonance, and hence benzaldehyde is more reactive than propanal in nucleophilic addition reactions. — False. Resonance with the benzene ring reduces the polarity of the carbonyl group, so the carbonyl carbon of benzaldehyde is less electrophilic and benzaldehyde is less reactive than propanal.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon (Fig. 8.2). The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced.”
“Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons.”
“…The polarity of the carbonyl group is reduced in benzaldehyde due to resonance as shown below and hence it is less reactive than propanal.”