JEE MainChemistryHaloalkanes and Haloarenes
(S)-2-Bromopentane is subjected to two different reaction conditions: Condition I: Treatment with a large excess of NaSH in acetone. Condition II: Heating in pure methanol. Which of the following correctly describes the predominant mechanism and the stereochemical outcome for Condition I and Condition II, respectively?
Options
- ACondition I: S _ N 1 , Racemization; Condition II: S _ N 2 , Inversion
- BCondition I: S _ N 2 , Inversion; Condition II: S _ N 1 , Racemization
- CCondition I: S _ N 2 , Retention; Condition II: S _ N 1 , Racemization
- DCondition I: S _ N 2 , Inversion; Condition II: S _ N 2 , Inversion
Correct answer
B. Condition I: S _ N 2 , Inversion; Condition II: S _ N 1 , Racemization
Step-by-step solution
For Condition I: The substrate is a secondary alkyl halide. NaSH provides a strong nucleophile ( SH ⁻ ) and acetone is a polar aprotic solvent. A high concentration of a strong nucleophile in a polar aprotic solvent strongly favours the S _ N 2 mechanism. The S _ N 2 reaction proceeds with complete inversion of configuration. For Condition II: Heating the secondary alkyl halide in pure methanol (a polar protic solvent) without any added strong nucleophile corresponds to solvolysis. Under these conditions, the weak