than phenols. Effect of substituents on the acidity of carboxylic acids: Substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base.
Electron withdrawing group (EWG) stabilises the carboxylate anion and strengthens the acid Electron donating group (EDG) destabilises the carboxylate anion and weakens the acid For the above reaction: The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO < CF Thus, the following acids are arranged in order of increasing acidity (based on p K a values): CF COOH > CCl COOH > CHCl COOH > NO CH COOH > NC-CH COOH > FCH COOH > ClCH COOH > BrCH COOH > HCOOH > ClCH CH COOH > (continue) C H COOH > C H CH COOH > CH COOH > CH CH COOH (continue ) Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid, contrary to the decrease expected due to resonance effect shown below: This is because of greater electronegativity of sp hybridised carbon to which carboxyl carbon is attached. The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity. COOH OCH COOH COOH NO -Methoxy benzoic acid (p = . ) K a Benzoic acid (p = .
) K a -Nitrobenzoic acid (p = . ) K a . Formation of anhydride Carboxylic acids on heating with mineral acids such as H SO or with P O give corresponding anhydride. .
Esterification Carboxylic acids are esterified with alcohols or phenols in the presence of a