The correct statement regarding nucleophilic substitution reaction in a chiral alkyl halide is:
- Retention occurs in \(SN_1\) reaction and inversion occurs in \( SN_2 \) reaction.
- Racemisation occurs in \( SN_1 \) reaction and retention occurs in \( SN_2 \) reaction.
- Racemisation occurs in both \( SN_1 \) and \( SN_2 \) reactions.
- Racemisation occurs in \( S_N1 \) reaction and inversion occurs in \( S_N2 \) reaction.
The Correct Option is D
Approach Solution - 1
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:
- Retention occurs in \( SN_1 \) reaction and inversion occurs in \( SN_2 \) reaction.
- False, because \( SN_1 \) reactions lead to racemization, not retention.
- Racemisation occurs in \( SN_1 \) reaction and retention occurs in \( SN_2 \) reaction.
- False, because \( SN_2 \) reactions lead to inversion, not retention.
- Racemisation occurs in both \( SN_1 \) and \( SN_2 \) reactions.
- False, because \( SN_2 \) reactions cause inversion, not racemization.
- 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.
Approach Solution -2
- SN1 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.
- SN2 Reaction: This reaction involves a backside attack, resulting in inversion of configuration (Walden inversion) if the carbon is chiral.
Thus, racemisation occurs in SN1 reactions and inversion occurs in SN2 reactions.
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