Q37 / 150 · MHT CET 2026 (11 April Shift 1) · Chemistry
Which one of the following factors proceeds more rapidly, unimolecular nucleophilic substitution reaction?
- AAprotic solvent
- BStronger nucleophile
- CWeaker nucleophile
- Correct answerDPolar protic solvent
Solution
Unimolecular nucleophilic substitution ($S_{N}1$) reactions proceed via the formation of a carbocation intermediate in the rate-determining step.
Polar protic solvents have high dielectric constants and are capable of hydrogen bonding. They stabilize both the carbocation intermediate and the leaving group anion through solvation. This stabilization lowers the activation energy required for the cleavage of the carbon-halogen bond, thereby increasing the rate of the $S_{N}1$ reaction.
The strength of the nucleophile does not affect the rate of an $S_{N}1$ reaction because the nucleophile is not involved in the rate-determining step. Aprotic solvents generally favor $S_{N}2$ reactions.
Answer: Polar protic solvent
Polar protic solvents have high dielectric constants and are capable of hydrogen bonding. They stabilize both the carbocation intermediate and the leaving group anion through solvation. This stabilization lowers the activation energy required for the cleavage of the carbon-halogen bond, thereby increasing the rate of the $S_{N}1$ reaction.
The strength of the nucleophile does not affect the rate of an $S_{N}1$ reaction because the nucleophile is not involved in the rate-determining step. Aprotic solvents generally favor $S_{N}2$ reactions.
Answer: Polar protic solvent
Hence, the correct answer is option: 'D'.