Overview

The acidity of carboxylic acids is influenced by the nature of the substituents bounded to the functional group. The acid strength is determined by the stability of the carboxylate anion—the conjugate base formed by dissociating the corresponding carboxylic acid.

Suppose the carboxylic acid bears an electron-withdrawing substituent. In that case, it stabilizes the conjugate base through the electron-withdrawing inductive effect, thereby decreasing the electron density on the carboxylate anion and enhancing the carboxylic acid's acidity. The presence of an electron-donating substituent destabilizes the conjugate carboxylate anion via the electron-donating inductive effect and, thus, decreases the acidity of the carboxylic acid.

The acidity of carboxylic acids also depends on the electronegativity of the substituent, the number of substituents bonded, and the proximity of the substituent from the –COOH group. Thus, fluoroacetic acid is more acidic than chloroacetic acid, as fluorine is more electronegative than chlorine. Trichloroacetic acid is more acidic than di- and monochloro acetic acid.

Procedure

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.