In SN₁ reactions of optically active alkyl halides, racemisation occurs due to the planar (achiral) nature of the sp² hybridized carbocation intermediate. The nucleophile can attack from either side of the plane, resulting in a mixture of products with the same and opposite configurations.
How does racemisation occur in SN₁ reactions of optically active alkyl halides, and what is the role of the carbocation intermediate?
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In SN₁ (nucleophilic substitution unimolecular) reactions involving optically active alkyl halides, racemization occurs due to the formation of a planar, achiral carbocation intermediate. The leaving group departs, generating a carbocation that lacks stereochemistry. Nucleophilic attack can then occur from either face of the carbocation, leading to the formation of both enantiomers. Since the attack is equally probable from both sides, a racemic mixture of products is obtained. The carbocation’s inherent lack of stereochemistry contributes to the racemization phenomenon in SN₁ reactions, contrasting with SN₂ reactions where stereochemistry is directly inverted during the reaction.