Question

Which of the following is not correct for \( \mathrm{S_N1} \) reaction ?

• A. Follows first order kinetics
• B. Favoured by polar protic solvents
• C. Proceeds through the formation of carbocation
• D. Results in inversion of configuration

Hint:

SN1 goes through a flat carbocation, so it gives racemisation. Inversion of configuration belongs to SN2.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
\( \mathrm{S_N1} \) means substitution, nucleophilic, unimolecular.
The question asks for the statement that is NOT true for it.

Step 2: Mechanism of \( \mathrm{S_N1} \):
The reaction runs in two steps.
Step one is slow: the C-X bond breaks and a planar carbocation forms.
Step two is fast: the nucleophile attacks the carbocation.
\[ \mathrm{R{-}X} \xrightarrow{\text{slow}} \mathrm{R^+ + X^-} \xrightarrow{\mathrm{Nu^-}} \mathrm{R{-}Nu} \]

Step 3: Check option (A).
The slow step involves only the alkyl halide. So rate = \( k[\mathrm{RX}] \), which is first order. So (A) is TRUE.

Step 4: Check option (B).
Polar protic solvents such as water and alcohols stabilise the carbocation and the halide ion by solvation. This speeds up ionisation. So (B) is TRUE.

Step 5: Check option (C).
The carbocation is the intermediate formed in the slow step. So (C) is TRUE.

Step 6: Check option (D).
The carbocation is planar. The nucleophile can attack from either face with nearly equal ease. This gives a mixture of retention and inversion, called racemisation. Complete inversion is the feature of \( \mathrm{S_N2} \), not \( \mathrm{S_N1} \). So (D) is NOT correct.

Final Answer:
The statement that is not correct for \( \mathrm{S_N1} \) is option (D).\[ \boxed{\text{(D) Results in inversion of configuration}} \]