Chemistry - GOC Question with Solution | TestHub

In (b) the two bulky isopropyl groups flank the $\mathrm{C}{\mathrm{H}}_2^{+}$ centre and force its empty p-orbital out of the ring plane, so the charge cannot delocalise into the ring - steric inhibition of resonance. In (a) the empty p-orbital overlaps freely with the ring π system and the charge is delocalised. Resonance stabilisation wins, so (a) is more stable.