Which of the following compound has the highest acidic strength?

The correct answer is option 3.

Let us delve into the factors that contribute to the increased acidity of o-Nitrobenzoic acid compared to other compounds.

Ortho Effect:

The ortho effect refers to the phenomenon where substituents located at the ortho position (adjacent positions) relative to a functional group can significantly influence its properties. In the case of o-Nitrobenzoic acid:

1. Intramolecular Hydrogen Bonding: The nitro group \((-NO_2)\) at the ortho position to the carboxylic acid group can form strong intramolecular hydrogen bonds with the hydrogen atom of the carboxylic acid group. This intramolecular hydrogen bonding stabilizes the deprotonated form (conjugate base) of the acid, making it easier to lose a proton and increasing the acidity.

2. Resonance Stabilization: The presence of the nitro group at the ortho position allows for resonance stabilization of the conjugate base. The lone pair of electrons on the oxygen atom of the nitro group can delocalize into the benzene ring, leading to resonance structures where the negative charge is spread over the oxygen atom and the adjacent carbon atoms of the benzene ring. This resonance stabilization further stabilizes the conjugate base, enhancing the acidity.

Electron-Withdrawing Nature of the Nitro Group:

The nitro group \((-NO_2)\) is an electron-withdrawing group due to the presence of the strongly electronegative nitrogen and oxygen atoms. This electron-withdrawing nature of the nitro group pulls electron density away from the benzene ring and the carbonyl carbon of the carboxylic acid group. As a result:

• The electron density around the carbonyl carbon is reduced, making the proton more acidic and easier to deprotonate.
• The resonance stabilization mentioned earlier is enhanced due to the withdrawal of electron density by the nitro group, further stabilizing the conjugate base and increasing the acidity.

Overall:

The combined effects of the ortho effect of the nitro group and its electron-withdrawing nature significantly increase the acidity of o-Nitrobenzoic acid compared to other positional isomers, such as m-Nitrobenzoic acid or p-Nitrobenzoic acid. The intramolecular hydrogen bonding, resonance stabilization, and electron withdrawal collectively contribute to making o-Nitrobenzoic acid the most acidic among the isomers.

Therefore, o-Nitrobenzoic acid exhibits the highest acidic strength due to these factors.