Which of the following statement(s) is (are) not associated with \(S_N1\) reaction:
(A) Rearrangement is possible.
(B) The rate is affected by solvent polarity.
(C) The strength of nucleophile is involved in rate determining step.
(D) The reaction order of alkyl halides is tertiary > secondary > primary.
(E) Proceeds with complete inversion of configuration.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (E) Only
An SN1 reaction takes place in two steps. First the C–X bond breaks slowly to give a carbocation; then the nucleophile attacks the carbocation quickly. For tert-butyl bromide and hydroxide ion:
\( \ce{(CH3)3C-Br} \xrightarrow{\text{slow}} \ce{(CH3)3C^+ + Br^-} \)
\( \ce{(CH3)3C^+ + OH^-} \xrightarrow{\text{fast}} \ce{(CH3)3C-OH} \)
The rate depends only on the slow step: Rate = k[R–X].
(A) Rearrangement is possible. — Associated with SN1. A carbocation can rearrange (by a hydride or alkyl shift) to a more stable carbocation before the nucleophile attacks.
(B) The rate is affected by solvent polarity. — Associated with SN1. Polar protic solvents (water, alcohol) solvate the carbocation and the halide ion and so speed up the slow ionisation step.
(C) The strength of nucleophile is involved in rate determining step. — Not associated with SN1. The nucleophile enters only in the fast second step, so it does not appear in the rate law and its strength does not affect the rate.
(D) The reaction order of alkyl halides is tertiary > secondary > primary. — Associated with SN1. A tertiary carbocation is the most stable, so tertiary halides ionise fastest.
(E) Proceeds with complete inversion of configuration. — Not associated with SN1. The carbocation is planar (sp2) and is attacked from both faces, so an optically active halide gives racemisation. Complete inversion is the mark of SN2.
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1)
Stereochemical Aspects of Nucleophilic Substitution Reactions – “… In case of optically active alkyl halides, SN1 reactions are accompanied by racemisation. … Actually the carbocation formed in the slow step being sp2 hybridised is planar (achiral).”