Question

In which pairs, the first ion is more stable than the second?

• A. Pair A: Cyclohexyl carbocation with OMe substituent vs cyclohexyl carbocation with Me substituent
• B. Pair B: Cyclohexyl carbanion with p-NO₂-phenyl substituent vs cyclohexyl carbocation with p-NO₂-phenyl substituent
• C. Pair C: Two primary cyclohexylmethyl carbocations in different conformations
• D. Pair D: t-Butyl carbocation vs t-Butyl carbocation bearing an OMe group

Answer 1

Answer: (B)

Pair B: Cyclohexyl carbanion with p-NO₂-phenyl substituent vs cyclohexyl carbocation with p-NO₂-phenyl substituent

Answer 2

Key concept: Nitro group (–NO₂) is a strong electron-withdrawing group by both inductive and resonance effects.

• In the first ion of Pair B, the nitro group stabilizes the carbanion by delocalizing and withdrawing negative charge (–I and –R effects).
• In the second ion, the nitro group destabilizes the carbocation (positive charge), since electron withdrawal from a positive center exacerbates instability.

Hence, the first ion is significantly more stable than the second in Pair B.