Aldehydes and ketones are two important classes of organic compounds which are widely used in the synthesis of a variety of other organic compounds. Due to the polarity of the carbonyl group, they undergo nucleophilic addition reactions. Broadly speaking, these addition reactions are of two types- one in which simply the addition of the nucleophilic reagent occurs across \(>C=O\) bond and the other in which addition is followed by the elimination of a molecule of water. A number of ammonia derivatives such as hydroxylamine, hydrazine, phenylhydrazine, 2,4-dinitrophenylhydrazine, and semicarbazide belong to the second category of reactions. These derivatives are used for the identification and characterization of aldehydes/ketones. Both types of nucleophilic addition reactions are influenced by steric and electronic factors. In general, aliphatic aldehydes are more reactive than aromatic aldehydes. The aliphatic aldehydes reduce both Fehling's solution and Tollen's reagent but aromatic aldehydes reduce only Tollen's reagent. Like aldehydes, aromatic ketones are less reactive than aliphatic ketones towards nucleophilic addition reactions.
Phenols and carboxylic acids are acidic in nature. Both dissolve in \(NaOH\) solution and turn blue litmus red. However carboxylic acids are much stronger acids than phenols and decompose \(NaHCO_3\) with the evolution of \(CO_2\) but phenols do not. Both electron-donating and electron-withdrawing substituents influence the acid strength of aliphatic as well as aromatic acids. Unlike all other aliphatic acids, formic acid has reducing properties and also does not show reactions of the alkyl group.
Which of the following compounds most readily undergoes nucleophilic addition reaction?
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. So aldehydes are more reactive than ketones, and the reactivity falls as the groups on the carbonyl carbon get larger or more numerous.
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:
\( \displaystyle \text{HCHO} \gt \ce{CH3CHO} \gt \ce{CH3COCH3} \gt \ce{(CH3)3C-CO-C(CH3)3} \)
Why the other options are incorrect
Option (2) Acetone — Incorrect. It is a ketone; its two methyl groups hinder the nucleophile and lower the positive charge on the carbonyl carbon, so it is less reactive than both aldehydes.
Option (3) 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 (4) Di-tert-butyl ketone — Incorrect. Two very bulky tert-butyl groups crowd the carbonyl carbon and push electrons towards it, so 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 |
|---|---|---|---|---|
| – | 14-05 S1 | – | 02-06 S3 31-05 S1 | – |