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Why is the carbocation formed in SN1 reactions racemised when starting from an optically active substrate?
ABecause optical activity is lost on heating
BThe carbocation is sp²-hybridised, planar, and achiral. The nucleophile attacks both faces with equal probability → 50:50 mixture of enantiomers
CBecause the solvent changes optical rotation
DBecause the temperature drops
Answer & Solution
Correct answer: B. The carbocation is sp²-hybridised, planar, and achiral. The nucleophile attacks both faces with equal probability → 50:50 mixture of enantiomers
Key SN1 stereochemistry: the carbocation intermediate is **sp²-hybridised → planar → achiral**. Nucleophile attacks either face with equal probability → equal amounts of the two enantiomers → **racemisation**. (Some inversion-preference can occur via ion-pair effects, but textbook SN1 ⇒ racemic.)
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