Question

Which of the following represent correct stability order:

• A. p-nitrobenzyl carbocation < p-chlorobenzyl carbocation < benzyl carbocation < p-methylbenzyl carbocation
• B. p-hydroxybenzyl carbocation < p-methoxybenzyl carbocation < benzyl carbocation < p-methylbenzyl carbocation
• C. p-methylbenzyl carbocation < benzyl carbocation < p-chlorobenzyl carbocation < p-nitrobenzyl carbocation
• D. p-isopropylbenzyl carbocation < p-isopropylbenzyl carbocation < p-t-butylbenzyl carbocation < p-methylbenzyl carbocation

Answer 1

Answer: (A)

p-nitrobenzyl carbocation < p-chlorobenzyl carbocation < benzyl carbocation < p-methylbenzyl carbocation

Answer 2

The stability of carbocations is influenced by electronic effects. Electron-donating groups stabilize carbocations, while electron-withdrawing groups destabilize them.

• p-nitrobenzyl carbocation: The nitro group (-NO2) is a strong electron-withdrawing group (EWG) via inductive (-I) and resonance (-M) effects, significantly destabilizing the carbocation.
• p-chlorobenzyl carbocation: The chlorine atom (-Cl) is an EWG due to its stronger inductive effect (-I) compared to its resonance donating effect (+M).
• Benzyl carbocation: Stabilized by resonance delocalization of the positive charge into the benzene ring.
• p-methylbenzyl carbocation: The methyl group (-CH3) is an electron-donating group (EDG) via inductive (+I) and hyperconjugation effects, providing stabilization.

The order of stability based on these effects is: Strong EWG (-NO2) < Weak EWG (-Cl) < No significant effect (H in benzyl) < EDG (-CH3). Therefore, the correct stability order is: p-nitrobenzyl carbocation < p-chlorobenzyl carbocation < benzyl carbocation < p-methylbenzyl carbocation.