Exams › JEE Main › Chemistry

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.

Related JEE Main Chemistry questions

• Lactic acid loses its optical activity when the hydroxyl group is replaced by hydrogen because
• Which is the most suitable technique for separating a mixture of sugars?
• Which statement correctly compares the staggered and eclipsed conformations of ethane?
• Which statement correctly describes a common feature shared by optical and geometrical isomerism?
• Which substituent lowers the reactivity of a benzene ring toward electrophilic substitution, yet still mainly directs a new group to the ortho and para positions?
• Among the following substances, which has the greatest affinity for a proton?

⚔️ Practice JEE Main Chemistry free + battle 1v1 →