Read the Passage carefully and answer the questions.
The carbonyl group of aldehydes and ketones is highly polar. The positively charged carbon is readily attacked by electron-rich nucleophile and the negatively charged oxygen atom is attacked by an electro-deficient electrophile. Aldehydes and ketones undergo nucleophilic addition reactions in the presence of acid or a base. The mechanism involves attack of a nucleophile on the electron deficient carbonyl carbon to form a tetrahedral intermediate. A new bond is formed between the carbon and oxygen. Protonation of the anion furnished by the reaction medium yields a neutral addition product. The addition product is same whether the reaction is catalyzed by acid or a base. For example in the presence of base aldehydes and ketones react with hydrogen cyanide to form cyanohydrin.
Which of the following compounds will undergo nucleophilic addition readily?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Formaldehyde
The ease of nucleophilic addition depends on two things. Steric factor: bulky groups on the carbonyl carbon hinder the approach of the nucleophile. Electronic factor: alkyl groups push electrons towards the carbonyl carbon (+I effect) and lower its positive charge, and an aryl ring lowers it further by resonance. So aldehydes are more reactive than ketones, and among aldehydes the reactivity falls as the group on the carbonyl carbon gets larger or donates more electrons.
Formaldehyde, HCHO, has two hydrogen atoms and no alkyl group on the carbonyl carbon. It is the least hindered and its carbonyl carbon is the most electrophilic, so it undergoes nucleophilic addition most readily:
\( \displaystyle \text{HCHO} \gt \ce{CH3CHO} \gt \ce{C6H5CHO} \gt \ce{CH3COC2H5} \)
Why the other options are incorrect
Option (2) Acetaldehyde — Incorrect. Its methyl group adds some bulk and lowers the positive charge on the carbonyl carbon by its +I effect, so it is less reactive than formaldehyde.
Option (3) Benzaldehyde — Incorrect. The benzene ring is conjugated with the C=O group; resonance reduces the polarity of the carbonyl group, so benzaldehyde is even less reactive than aliphatic aldehydes such as propanal.
Option (4) Butanone — Incorrect. It is a ketone with a methyl and an ethyl group on the carbonyl carbon; two alkyl groups hinder the nucleophile and reduce the electrophilicity most, so it is the least reactive of the four.
NCERT (2026-27 edition):
Reactivity
Reactivity – “… 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.”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | 14-05 S1 | – | 02-06 S3 31-05 S1 | – |