Chemistry - GOC Question with Solution | TestHub

Removing ${\mathrm{H}}_1$ gives the indenyl anion :

Removing ${\mathrm{H}}_2$ gives a vinylic carbanion :

The indenyl anion has 10 π electrons delocalised over the fused five-membered ring - it is aromatic (Hückel 4n+2 with n = 2) and exceptionally stable (indene $\mathrm{p}{K}_{\mathrm{a}}$ ~ 20). The vinylic anion has no such stabilisation ($\mathrm{p}{K}_{\mathrm{a}}$ ~ 44). ${\mathrm{H}}_1$ is far more acidic.