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A benzylic carbocation bearing a para-CH3 group (I) is more stable than the same carbocation bearing a para-CD3 group (II). The reason is:

1. -CD3 has a greater +I effect than -CH3
2. -CH3 has a greater +I effect than -CD3
3. -CH3 has a greater +H (hyperconjugation) effect than -CD3
4. -CD3 has a greater +H (hyperconjugation) effect than -CH3

Correct answer: -CH3 has a greater +H (hyperconjugation) effect than -CD3

Solution

The stabilization here arises from hyperconjugation (+H effect) of the methyl C-H bonds into the cationic system. Because C-H bonds are more easily polarizable/donating than the stronger C-D bonds (secondary isotope effect), -CH3 provides greater hyperconjugative electron release than -CD3. Hence (I) with -CH3 is more stable.

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