If electron-withdrawing groups are present on the benzene ring of benzoic acid, the benzoate ion stabilises and acidity thus increases. This is attributed to inductive and resonance effects. In case of p-nitrobenzoic acid and its m-isomer, based on the inductive effect alone the m-isomer would be assumed to be stronger than the p-isomer but the reverse is also true. No direct resonance of the carboxylate ion with the ring exists; but the resonance structures for each isomer shown below can be considered:

Apparently the resonance in p-isomer places the positive charge on the carbon atom carrying the −COO^– ion. Therefore, the ion stabilises due to strong inductive effect. In m-isomer, the positive charge is on the carbon atom adjacent to the carbon carrying the −COO^– ion. The inductive stabilisation of this ion is less. Therefore, acidity of p-nitrobenzoic acid is more than that of the m-isomer.

For p-chlorobenzoic acid isomers, similar arguments can be used. The resonance structures shown depict that the p-isomer is a stronger acid than the m-isomer because the negative charge is positioned ortho to the −COO^– ion in the former.

On the other hand, the effect of electron-donating groups is just the opposite as they destabilise the ion and place a negative charge on the −COO^– ion. This increases the ion’s ability to regain the proton. Table lists the pK_a values of some substituted benzoic acids. Lower the pK_a value, stronger is the acid.

The values in the table state that an alkyl group produces a little effect on the strength of benzoic acids. But the electron-withdrawing groups increase the acidity. These groups when present in o^– and p^– positions produce a more prominent effect. The electron withdrawing effect of o^–NO₂ group is significant due to the short distance over which the powerful inductive effect along with the resonance effect of this group is operating.

Presence of groups like −OCH₃,−OH, or halogens (electron-withdrawing groups inductively but electron-donating mesomerically) in o^– and p^– positions weakens the p^–  substituted acids than the m-substituted acids.

Sometimes it is also observed that some o^– substituted benzoic acids behave strangely and their acidity is higher than expected. This is because the group interacts with the adjacent carboxyl group, forming intermolecular hydrogen bond.