Chemistry - GOC Question with Solution | TestHub

In (a) the $\mathrm{C}=\mathrm{O}$ polarises to ${\mathrm{C}}^{+}-{\mathrm{O}}^{-}$, and the three-membered ring then becomes the aromatic cyclopropenyl cation (2π). This dipolar form contributes strongly, so (a) has the larger dipole moment. In (b) acetone has no such extra stabilisation of the charge-separated form.