Answer the question on basis of passage given below:
Organic compounds containing carbon-oxygen double bonds (C=O) are called carbonyl compounds. These are highly polar molecules. Hence, their boiling points are higher than hydrocarbons and ethers. Aldehydes can be prepared from dehydrogenation or oxidation of alcohols and by reduction of acyl halides. Ketones are prepared by oxidation of secondary alcohols or by hydration of alkynes etc. Aldehydes and ketones undergo nucleophilic addition reaction with a number of nucleophiles such as HCN, NaHSO3, ammonia derivatives. Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes and alkenes, by hydrolysis of nitriles. Carboxylic acids are more acidic than alcohols and phenols.
Which of the following is most reactive towards nucleophilic addition reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Ethanal
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.
So aldehydes are more reactive than ketones, and among aldehydes the one with the smallest alkyl group is the most reactive. Ethanal (\( \ce{CH3CHO} \)) has only a methyl group, so it is the most reactive of the four:
Ethanal > Propanal > Propanone > Butanone
Why the other options are incorrect
Option (1) Butanone — Incorrect. A ketone (\( \ce{CH3COC2H5} \)) with a methyl and an ethyl group on the carbonyl carbon; it is the least reactive of the four.
Option (2) Propanone — Incorrect. A ketone (\( \ce{CH3COCH3} \)); two methyl groups lower its reactivity below that of the aldehydes.
Option (4) Propanal — Incorrect. An aldehyde, but its ethyl group releases more electrons and is bulkier than the methyl group of ethanal, so it is slightly less reactive.
Of the two aldehydes, ethanal has the smaller alkyl group (weaker +I effect, less steric hindrance) than propanal, so it is the most reactive.
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 | – |