Chemistry - GOC Question with Solution | TestHub

Removing $H^{+}$ from a $C{H}_3$ of the isopropylidene group of (II) gives $C{H}_2$=C($C{H}_3$)-(ring carbon with negative charge), which is a cyclopentadienyl anion (6π, aromatic). In (I), (III) and (IV) no deprotonation gives an aromatic cyclopentadienyl anion (in (IV) the ring still has an $s{p}^3$ $C{H}_2$ and two exocyclic C=C bonds), so (II) is much more acidic.