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Q.
Among the following compounds, the most acidic is
Organic Chemistry – Some Basic Principles and Techniques
Solution:
A monosubstituted benzoic acid is stronger than a monosubstituted 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 decreases acid strength. It is due to very high stabilisation of conjugate base by intramolecular $H$-bond which outweight the electron donating resonance effect of $- OH$.
The overall order of acid-strength of given four acids is ortho-hydroxybenzoic acid $\left( pK _{ a }=\right.$ 2.98) $ >$ Toluic acid $\left.pk _{ a }=4.37\right) >$ $p$-hydroxybenzoic acid $\left(p k_a=4.58\right) >p-$ nitrophenol $\left(p k_a=7.15\right)$.
