Which of the following compounds is most reactive towards nucleophilic addition reaction?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Ethanal (CH3CHO)
Reactivity towards nucleophilic addition depends on how positive and how exposed the carbonyl carbon is:
Electronic factor: alkyl groups release electrons (+I effect) and reduce the positive charge on the carbonyl carbon. Two alkyl groups (ketones) reduce it more than one (aldehydes).
Steric factor: two alkyl groups crowd the carbonyl carbon and hinder the approach of the nucleophile.
Resonance with a benzene ring: in aromatic aldehydes and ketones the ring releases electrons to the carbonyl group by resonance, so they are less reactive than their aliphatic analogues.
So aldehydes are more reactive than ketones, and aliphatic compounds more reactive than aromatic ones. Ethanal (\( \ce{CH3CHO} \)) is an aliphatic aldehyde with only a small methyl group, so it is the most reactive of the four.
Why the other options are incorrect
Option (2) Propanone — Incorrect. A ketone (\( \ce{CH3COCH3} \)); two methyl groups lower its reactivity below that of ethanal.
Option (3) Benzaldehyde — Incorrect. An aldehyde, but the benzene ring releases electrons to the carbonyl group by resonance, so its carbonyl carbon is less positive than that of ethanal.
Option (4) Acetophenone — Incorrect. A ketone (\( \ce{C6H5COCH3} \)) that also has the resonance effect of the benzene ring; it is the least reactive of the four.
NCERT (2026-27 edition):
Reactivity
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 10-06 S3 09-06 S3 07-06 S3 25-05 S3 | – |