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

$H O - {C}_{6} {H}_{4} - C \left(= O\right) H$ versus ${H}_{3} C - C H {\left(C H\right)}_{3} C H \left(O H\right) C {H}_{3}$. 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.