Which of the following is the strongest acid ?

Options

A. Phenol
B. `p`-Nitophenol
C. `m`-Nitrophenol
D. `o`-Nitophenol

Answer

B

Explanation

`NO_(2)` is electron withdrawing and therefore acid-strengthening by both induction `(-I_("effect"))` and resonance `(-R_("effect"))`.Its resonance effect is effective from only the ortho and para positions to about an equal extent but does not operate at the meta positions. It predominates over the inductive effect `(-R_("effect") gt -I_("effect"))` which operates from all positions but at decreasing effectiveness with increasing separation of `NO_(2)` and `OH`.Hence , all the nitrophenols are more acidic than phenol with `m`-nitrophenol being the weakest of the three on account of absence of absence of resonance effect.
Since the inductive effect from the closest ortho-position is the strongest , one excepts `o`-nitrophenol to be a stronger acid than `p`-nitrophenol. However, the intramolecular hydrogen bond in `o`-nitrophenol must be broken to release proton and this requires some energy.

Thus , the decreasing order is