Q. Among the following compounds, the most acidic is
IIT JEEIIT JEE 2011Aldehydes Ketones and Carboxylic Acids
Solution:
A mono-substituted benzoic acid is stronger than a
mono-substituted phenol as former being a carboxylic acid.
Among the given substituted benzoic acid, ortho-hydroxy acid
is strongest acid although -OH causes electron donation by
resonance effect which tends to decrease acid strength.
It is due to a very high stabilisation of conjugate base by
intramolecular H-bond which outweigh the electron donating
resonance effect of -OH.
The overall order of acid-strength of given four acids is
ortho--hydroxybenzoic acid $(pK_\alpha = 2.98) >$ Toluic acid
$(pK_\alpha = 4.37) >$ p-hydroxybenzoic acid $(pK_\alpha = 4.58) >$
p-nitrophenol $(pK_\alpha = 7.15)$
