Chemistry - GOC Question with Solution | TestHub

Removing H_a (gamma to the carbonyl) gives a carbanion at the end of the conjugated O=C-C=C system : the charge delocalises through the diene unit onto the carbonyl oxygen (a vinylogous enolate, charge finishing on O). H_b is an ordinary α proton whose anion delocalises over only one carbonyl, H_c is a plain beta C-H with no resonance stabilisation, and H_d is vinylic and very hard to remove. The most stabilised conjugate base comes from H_a, so H_a is the most acidic.