Which compound has the greatest rate of hydrolysis at pH=3.5:benzamide, o-carboxybenzamide, o-formylbenzamide, or o-hydroxybenzamide?

In order to hydrolyse an amide, the amine group in the tetrahedral intermediate has to leave in preference to the less basic OH group. This can happen if the amine group is protonated because ammonium ion is a weaker base and therefore easier to eliminate than OH, of the four compounds, two ( ortho-carboxybenzamide and ortho-hydroxybenzamide) have substituents that can protonate the amine group by acting as acid catalyst in a hydrolysis reaction carried out at pH=3.5.

The structure of the given compounds respectively is shown below.

The carboxy group withdraws electrons from the ring by resonance and the OH group donates electrons to the ring by resonance. Formation of the tetrahedral intermediate will be faster for the carboxy substituted compound.

The carboxy group is a stronger acid than the OH group. Therefore, a larger fraction of the tetrahedral intermediate will be protonated than in the case of the hydroxyl-substituted compound. So collapse of the tetrahedral intermediate will be faster. Therefore, the ortho-carboxybenzamide has the faster rate of hydrolysis.