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.
During nucleophilic addition reaction the carbonyl carbon undergoes a change in hybridization from.......
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(sp^2\) to \(sp^3\)
The carbonyl carbon of an aldehyde or ketone is \(sp^2\) hybridised: it forms three \(\sigma\) bonds in one plane (trigonal planar, bond angles about 120°) and a \(\pi\) bond with oxygen. In a nucleophilic addition the nucleophile attacks this carbon from above or below the plane. The \(\pi\) bond breaks, its electron pair moves onto oxygen, and the carbon now carries four \(\sigma\) bonds, so it becomes \(sp^3\) hybridised and tetrahedral. This gives the tetrahedral alkoxide intermediate, which then takes up a proton:
\( \displaystyle \ce{R2C=O + Nu^- -> R2C(Nu)-O^-} \xrightarrow{\ce{H^+}} \ce{R2C(Nu)-OH} \)
So during the reaction the carbonyl carbon changes from \(sp^2\) to \(sp^3\).
Why the other options are incorrect
Option (2) \(sp^3\) to \(sp^2\) — Incorrect. This is the reverse change. It happens when a tetrahedral carbon forms a double bond, for example when the addition product of an ammonia derivative loses water to give a C=N bond.
Option (3) \(sp\) to \(sp^2\) — Incorrect. An sp carbon has two \(\pi\) bonds, as in alkynes and nitriles; the carbonyl carbon is not sp hybridised to begin with.
Option (4) \(sp^2\) to \(sp\) — Incorrect. Addition increases the number of \(\sigma\) bonds on the carbon from three to four; it cannot lead to an sp carbon with only two.
NCERT (2026-27 edition):
Mechanism of Nucleophilic Addition Reactions
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 31-05 S2 18-05 S1 | 14-05 S1 | – | – | – |