Question

The correct statement regarding nucleophilic substitution reaction in a chiral alkyl halide is:

• A. Retention occurs in \(SN_1\) reaction and inversion occurs in \( SN_2 \) reaction.
• B. Racemisation occurs in \( SN_1 \) reaction and retention occurs in \( SN_2 \) reaction.
• C. Racemisation occurs in both \( SN_1 \) and \( SN_2 \) reactions.
• D. Racemisation occurs in \( S_N1 \) reaction and inversion occurs in \( S_N2 \) reaction.

Answer 1

The Correct Option is D

To answer this question about nucleophilic substitution reactions in chiral alkyl halides, let's first understand the two types of nucleophilic substitution reactions: \( SN_1 \) and \( SN_2 \).

•  \( SN_1 \) Reaction:
  • It is a two-step reaction mechanism.
  • The first step involves the formation of a carbocation intermediate, which is a planar (sp² hybridized) structure.
  • Because the carbocation is planar, the nucleophile can attack from either side, leading to the possibility of forming both enantiomers, resulting in racemization.
• \( SN_2 \) Reaction:
  • It is a single-step bimolecular reaction mechanism where the nucleophile attacks the substrate from the opposite side of the leaving group.
  • This backside attack leads to the inversion of configuration at the chiral center. This process is often compared to an umbrella turning inside out.

Now, let's evaluate the given options:

1. Retention occurs in \( SN_1 \) reaction and inversion occurs in \( SN_2 \) reaction.
   • False, because \( SN_1 \) reactions lead to racemization, not retention.
2. Racemisation occurs in \( SN_1 \) reaction and retention occurs in \( SN_2 \) reaction.
   • False, because \( SN_2 \) reactions lead to inversion, not retention.
3. Racemisation occurs in both \( SN_1 \) and \( SN_2 \) reactions.
   • False, because \( SN_2 \) reactions cause inversion, not racemization.
4. Racemisation occurs in \( S_N1 \) reaction and inversion occurs in \( S_N2 \) reaction.
   • True, as explained above. This option correctly states the outcomes of both \( SN_1 \) and \( SN_2 \) reactions.

Thus, the correct statement is: Racemisation occurs in \( S_N1 \) reaction and inversion occurs in \( S_N2 \) reaction.

Answer 2

- S_N1 Reaction: This reaction proceeds via a carbocation intermediate, leading to a racemic mixture if the carbon is chiral, due to the planar nature of the carbocation.

- S_N2 Reaction: This reaction involves a backside attack, resulting in inversion of configuration (Walden inversion) if the carbon is chiral.

Thus, racemisation occurs in S_N1 reactions and inversion occurs in S_N2 reactions.