13.6: Substituent Effects on Acidity of Carboxylic Acids
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.