Question

Even though the para position is one carbon farther from the carboxy group than the meta position, p-cyanobenzoic acid is more acidic than m-cyanobenzoic acid. Explain the differences in acidity between p-cyanobenzoic acid and m-cyanobenzoic acid.

Answer 1

Dissociation of O-H bond is highly favorable in p-cyanobenzoic acid resulting to higher acidity as compared to m-cyanobenzoic acid.

Step-by-step explanation:

In p-cyanobenzoic acid, adjacent carbon to carboxyl group (-COOH) gets a partial positive charge due to electron withdrawing resonating effect of carboxyl group as compared to m-cyanobenzoic acid.

Due to this partial positive charge on carbon atom in p-cyanobenzoic acid, O-H bond in -COOH group remains highly polarized towards oxygen atom. Hence, dissociation of O-H bond is highly favorable in p-cyanobenzoic acid resulting to higher acidity as compared to m-cyanobenzoic acid.

Resonance structures are given below.