Question

Which has the lower value of pka ?? (a) 3-methyl-2-butanol (b) p-hydroxybenzaldehyde; I need some explanation for my thesis work.

Answer 1

Clearly, the benzaldehyde derivative is the strongest acid, and thus should have the lower `pK_a`.

Explanation:

3-methyl-2-butanol would be exceptionally difficult to deprotonate, and would never act as an acid in water. On the other hand, p-hydroxybenzaldehyde has an anion which is resonance stabilized, and the formyl group (para to hydroxy) could help stabilize the negative charge of the anion by resonance. The benzene derivative is thus the strongest acid. But you'll need to find some `pK_a` estimates in an organic text.