Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of \(CO_2\) with water in the presence of sunlight and chlorophyll. Commercially, formaldehyde is obtained by the controlled oxidation of methane in the presence of molybdenum oxide \((MoO)\) acting as a catalyst. Formaldehyde is a constituent of a number of polymers also called resins. These are non volatile solids or semi solids which are obtained directly from certain plants and can be synthesized in the laboratory.
Benzaldehyde is less reactive towards nucleophilic addition than formaldehyde because of:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → −R effect of carbonyl group and steric factor due to phenyl group
Nucleophilic addition depends on how electrophilic the carbonyl carbon is and how easily the nucleophile can reach it.
Electronic factor: in benzaldehyde the C=O group is conjugated with the benzene ring. The carbonyl group withdraws π-electrons from the ring by resonance (its −R effect), so the ring’s electrons are delocalised onto the C=O group. This lowers the positive charge on the carbonyl carbon, and the polarity of the C=O group is reduced.
Steric factor: the bulky phenyl group hinders the approach of the nucleophile, whereas formaldehyde (HCHO) has only two small H atoms on the carbonyl carbon.
So benzaldehyde is less reactive than formaldehyde towards nucleophilic addition.
Why the other options are incorrect
Option (1) +R effect of carbonyl group — Incorrect. The carbonyl group is electron-withdrawing; it does not donate electrons by resonance, so it has no +R effect.
Option (3) −I effect of carbonyl group — Incorrect. An inductive pull would make the carbonyl carbon more positive, not less; the lowering of reactivity comes from conjugation (resonance), not from the inductive effect.
Option (4) +I effect of carbonyl group — Incorrect. The carbonyl group does not release electrons inductively; the oxygen makes it electron-withdrawing.
The book describes this −R effect as the lowering of the polarity of the carbonyl group by resonance with the ring.
NCERT (2026-27 edition):
Reactivity – “Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.”
Nucleophilic Addition Reactions – “… The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance as shown below and hence it is less reactive than propanal.”