Chemistry - GOC Question with Solution | TestHub

Look at the conjugate base of each. Removing a proton from the ethylidene methyl of (2) gives an anion that delocalises into the five-membered ring, which then holds two double bonds plus the lone pair - a 6 pi-electron aromatic cyclopentadienide system, enormously stabilised. In (1) the cross-conjugated exocyclic methylene prevents the ring from completing a 6 π circuit (the ring is left with only 4 π electrons), and in (3) one ring carbon interrupts the conjugation so the anion is only allylic, not aromatic. The aromatic conjugate base makes (2) by far the strongest acid.