13.4: Substituent Effects on Acidity of Carboxylic Acids

The acidity of substituted carboxylic acids varies depending on the stability of their conjugate base—carboxylate anions.

An electron-withdrawing substituent on the α carbon reduces the electron density to stabilize the carboxylate anion, and increases the strength of the acid.

The presence of multiple electron-withdrawing groups magnifies the inductive effect, further increasing the acidity.

Increased distance between the electron-withdrawing substituents and the –COOH group diminishes the inductive effect. Thus γ-substituted carboxylic acids are less acidic than α-substituted.

In contrast, an electron-donating substituent destabilizes the carboxylate anion, making them less acidic than the unsubstituted acids.

Aromatic carboxylic acids are more acidic than aliphatic carboxylic acids due to the electron-withdrawing effect of the aryl ring. Further, the presence of electron-withdrawing or electron-donating substituents alters their acidity.

In dicarboxylic acids, one –COOH group exhibits electron-withdrawing effect over the other, resulting in higher acidity of the first –COOH group compared to the second. This difference is reduced as the inductive effect decreases with the increased carbon chain.