In the given reaction
(A) B is intermediate
(B) B is transition state
(C) B is formed in a single step
(D) B is stable
(E) B is unstable
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B, C and E only
The reaction shown is an SN2 substitution of chloromethane by hydroxide ion. It takes place in one step: \( \ce{OH^{-}} \) attacks the carbon from the side opposite to Cl while the C–Cl bond is breaking. Halfway along, carbon is partly bonded to both OH and Cl, with the three H atoms in one plane. This arrangement, B in the scheme, is the transition state at the top of the energy curve; it is not a compound and cannot be isolated. The product C (methanol) has an inverted configuration.
\( \ce{HO^- + CH3Cl -> CH3OH + Cl^-} \) (one step, through the transition state B)
(A) B is intermediate — Incorrect. An intermediate sits in an energy minimum and has a definite lifetime (like the carbocation of SN1); B is an energy maximum with partial bonds.
(B) B is transition state — Correct. Carbon is bonded to five groups at once, two of them partially.
(C) B is formed in a single step — Correct. SN2 is a concerted one-step reaction; bond making and bond breaking happen together.
(D) B is stable — Incorrect. A transition state is the highest-energy point of the reaction and passes on at once to products.
(E) B is unstable — Correct. Such structures are unstable and cannot be isolated.
NCERT (2026-27 edition):
Substitution Nucleophilic Bimolecular (SN2) – “… In the transition state, the carbon atom is simultaneously bonded to incoming nucleophile and the outgoing leaving group. Such structures are unstable and cannot be isolated. Thus, in the transition state, carbon is simultaneously bonded to five atoms.”
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 05-06 S3 | – |