Question: Rank the carboxylic acids in order of increasing acidity.

Electron-withdrawing substituents attached to a carboxylic acid increase their acidity, while electron-donating substituents decrease their acidity.

Electron-withdrawing groups stabilize the conjugate bases of acids and thereby increase their strength.

The skeletal structures of the given acids are as follows:

Structure of the given carboxylic acids

The carboxylic acid-containing electron-withdrawing substituents are more acidic than normal carboxylic acids and acids containing electron-donating substituents.

Therefore, the benzoic acid-containing ${\mathrm{CF}}_3$group (electron-withdrawing due to the electronegativity of the fluorine atom) is the most acidic, followed by the benzoic acid.

The benzoic acid-containing ${\mathrm{CH}}_3$ (electron-donating methyl group) group is the least acidic.

Decreasing order of acidity of the given acids