Question

Among the following compounds, the most acidic is
A.p-nitrophenol
B.p-hydroxybenzoic acid
C.o-hydroxybenzoic acid
D.none

Answer 1

To answer this question, you must be familiar with factors that affect the acidity of an organic compound. The factors are, the strength of the $H - A$ bond, the electronegativity of A and the factors stabilizing the conjugate base formed after the loss of proton.

Complete step by step answer:
Presence of an electron withdrawing group on phenol increases its acidic strength.
In case of nitro substituent, we know that there is an electron withdrawing mesomeric effect when the nitro group is at ortho or para position, and this increases the acidic strength of nitrophenols because of more resonance stabilization of their conjugate base.
However, phenols are comparatively less acidic than carboxylic acids because although its conjugate base is stabilized by delocalization of negative charge, the extent of delocalization is less due to non-equivalent less stable contributing structure.
Among the ortho and para hydroxy benzoic acid, we can clearly say that ortho hydroxybenzoic acid is the stronger acid due to the stabilization of the conjugate base by intermolecular H-bonding which outweighs the electron donating resonance effect of the hydroxyl group.
Thus, among the given compounds, the strongest acid is ortho hydroxybenzoic acid.

Therefore, the correct option is C.

Note: It is a common observation that generally ortho substituted benzoic acid is more acidic than isomeric meta or para substituted benzoic acid. This effect is irrespective of whether the ortho substituent is electron withdrawing or donating. This effect is known as the ortho effect.
Ortho effect is a cumulative effect of various acidic strength enhancing factors such as steric inhibition of resonance, strong inductive effect from nearest position, intramolecular H-bonding in conjugate base etc.