Question

In $C{{H}_{3}}COOH$ and ${{C}_{2}}{{H}_{5}}COOH$ which is more acidic?

Answer 1

Electron-donating substituents decrease the acidity of the benzoic acid because they decrease the stability of the carboxylate ion. Electron-withdrawing substituents increase the acidity of the benzoic acid because they increase the stability of the carboxylate ion.

Complete step by step answer:
When a carboxylic acid is substituted by electron-withdrawing groups like nitro ($N{{O}_{2}}$) the acidity increases.
When a carboxylic acid is substituted by electron-donating groups like hydroxyl ($OH$) the acidity decreases.
The displacement of $\sigma -electrons$ along the saturated carbon chain whenever an electron-withdrawing or electron-donating group is present at the end of the carbon chain is called inductive effect or I-effect.
When the substituent attached to the end of the carbon chain is electron-withdrawing, then the effect is called –I-Effect.
When the substituent attached to the end of the carbon chain is electron-donating, then the effect is +I-Effect.
+I-Effect intensifies the negative charge on the carboxylate ion thereby making the carboxylate ion less stable, which decreases the acidity of the carboxylic acid. –I-Effect disperse the negative charge on the carboxylate ion thereby making the carboxylate ion more stable, which increases the acidity of the carboxylic acid.
We know that the methyl ($C{{H}_{3}}$) group is an electron-donating group so it decreases the acidity of the carboxylic acid. The ethyl group (${{C}_{2}}{{H}_{5}}$) has more +I-Effect than the methyl group, so the propanoic acid (${{C}_{2}}{{H}_{5}}COOH$) is less acidic than ethanoic acid ($C{{H}_{3}}COOH$).

Therefore, $C{{H}_{3}}COOH$ is more acidic.

Note: The effect of substituents on the benzoic acid is the same as that of the effect of substituents on the carboxylic acid. As the number of halogens or electron-withdrawing groups increases on the molecule, the acidity increases, and as the number of a methyl group or electron-donating group increases on the molecule, the acidity decreases.