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A benzyl cation (para-CH2+ substituted benzene) carries an alkyl group at the para position. Which para substituent makes the cation most stable?

1. para CH3 group
2. para CH2CH3 (ethyl) group
3. para CH(CH3)2 (isopropyl) group
4. para C(CH3)3 (tert-butyl) group

Correct answer: para CH3 group

Solution

The cationic centre is conjugated to the ring, so stabilization from the para alkyl group is dominated by hyperconjugation, which depends on the number of alpha C-H bonds on that group. CH3 has 3 alpha C-H, ethyl has 2, isopropyl has 1, and tert-butyl has 0. Therefore the methyl group, with the maximum number of hyperconjugating C-H bonds, gives the most stable cation.

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