Chemistry - GOC Question with Solution | TestHub

In (a) the two bulky tert-butyl groups flank the CH₂⁺ 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 (b) the empty p-orbital overlaps freely with the ring π system and the charge is delocalised. Resonance stabilisation wins, so (b) is more stable.