Question

The weakest acid among the following is

• A. Benzoic acid
• B. o-nitrobenzoic acid
• C. Cyclohexanecarboxylic acid
• D. p-nitrobenzoic acid

Answer 1

The correct option is C Cyclohexanecarboxylic acid

Cyclohexane carboxylic acid is the weakest as it doesn't have any pi-pi conjugation for stablisation of carboxylate ion.
Both o-nitrobenzoic and p-nitrobenzoic acid are more acidic than benzoic acid due to presence of electron withdrawing group which increases the acidity of acid.
o-nitrobenzoic acid is more acidic compared to p-nitrobenzoic acid. This is because of two reasons:

1. Ortho Effect
2. Inductive and Resonance Effect

The first one, due to presence of nitro group on ortho position, the $-COOH$ group bends out of the plane of the ring due to steric hindrance. As a result, $H^{+}$ ion can be easily removed because it has come out of the plane of ring.

Nitro group is electron withdrawing group. Hence, it increases the acidity of benzoic acid by removing electron density from carboxylate ion. This effect is maximum when nitro group is in ortho position. the effect is known as ortho effect.

The nitro group $(-N{O}_2)$ is always electron withdrawing either by resonance (ortho or para position) or inductive (ortho or meta positions) and will stabilize formation of the carboxylate and $H^{+}$ ion.
When $H^{+}$ ion is liberated, the negative charge on oxygen is reduced a little due to withdrawing effect of nitro group. The near the group, more is the inductive effect. Hence, nitro group on ortho position has higher inductive effect compared to the nitro group on para position. Hence, o-nitrobenzoic acid is more acidic compared to p-nitrobenzoic acid.