Which type of reactions are favoured by carbonyl compounds?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Nucleophilic addition reactions
In the carbonyl group, \( \ce{C=O} \), oxygen is more electronegative than carbon, so the bond is polarised and the carbonyl carbon is electrophilic (it carries a partial positive charge).
A nucleophile attacks this carbon and the hybridisation of carbon changes from sp2 to sp3, giving a tetrahedral alkoxide intermediate. This intermediate takes up a proton, so the net change is addition of Nu− and H+ across the C=O bond: aldehydes and ketones favour nucleophilic addition.
Why the other options are incorrect
Option (1) Nucleophilic substitution reactions — Incorrect.
Substitution needs a leaving group on the attacked carbon, as the halogen in a haloalkane.
The H or alkyl groups on the carbonyl carbon of an aldehyde or ketone do not leave, so the nucleophile adds instead.
Option (3) Electrophilic addition reactions — Incorrect. Electrophilic addition is typical of alkenes, whose C=C bond is electron rich. The C=O carbon is electron poor, so it is attacked by a nucleophile, not an electrophile.
Option (4) Electrophilic substitution reactions — Incorrect. Electrophilic substitution takes place on an aromatic ring, not at the carbonyl group; in aromatic aldehydes and ketones the carbonyl group even deactivates the ring.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “Contrary to electrophilic addition reactions observed in alkenes, the aldehydes and ketones undergo nucleophilic addition reactions.”
(i) Mechanism of nucleophilic addition 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 …. The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced.”