Question

Why value of pka of p-hydroxylbenzaldehyde is lower than 3-methyl-2butanol ? Please explain , thanks

Answer 1

Acidity of the benzene derivative may be rationalized on the basis of aromatic stability of the anion.

Explanation:

`HO-C_6H_4-C(=O)H` versus `H_3C-CH(CH)_3CH(OH)CH_3`. Both `O-H` are bonds are strong, but it is the stability of the anion that is important. The phenoxide has an aryl ring to stabilize (to delocalize) the negative charge; note that also the carbonyl group is available to delocalize the negative charge. Since this anion is more stabilized, less charge dense, it is entropically favoured. On the other hand, the negative charge of the alkoxide is localized to a single oxygen centre.