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:
Rationalised 2023-24
251
Aldehydes, Ketones and Carboxylic Acids
The effect of the following groups in increasing acidity order is
Ph < I < Br < Cl < F < CN < NO2 < CF3
Thus, the following acids are arranged in order of increasing acidity
(based on pKa values):
CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH >
FCH2COOH > ClCH2COOH > BrCH2COOH > HCOOH >  ClCH2CH2COOH >
 (continue)
C6H5COOH > C6H5CH2COOH > CH3COOH > CH3CH2COOH
 (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
2 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
OCH3
COOH
COOH
NO2
4-Methoxy
benzoic acid
(p
= 4