JEE Main · 2025 · 22 Jan 2025 · Shift 2 · General Organic Chemistry

Stability of a benzylic carbocation is governed mainly by resonance donation from ring substituents, with the substituent's strength and position deciding how effectively the positive charge is delocalized.

A $-\mathrm{OC}{\mathrm{H}}_3$ group is a strong $+M$ (resonance electron-donating) substituent: from the para position, its lone pair conjugates all the way through the ring into the exocyclic $\mathrm{C}{\mathrm{H}}_2^{+}$ center, placing part of the positive charge on oxygen itself (an oxocarbenium-like resonance structure) and giving very effective stabilization.

Placed at the meta position, $-\mathrm{OC}{\mathrm{H}}_3$ cannot conjugate with the benzylic cation through the ring — meta positions are cross-conjugated, not directly resonance-linked to the exocyclic center — so its effect there is far weaker, acting mainly through a weak inductive pull.

A $-\mathrm{C}{\mathrm{H}}_3$ group at the para position stabilizes only through hyperconjugation and a mild $+I$ effect, much less effective than direct resonance donation.

The unsubstituted benzyl cation has only the ring's own resonance to fall back on, with no additional donor group.

The para-methoxybenzyl cation, stabilized by direct $+M$ donation from oxygen into the cationic center, is the most stable carbocation of the four.