Which of the following is the weakest acid?

Options

A. `p`-Chlorophenol
B. Phenol
C. `m`-Chlorophenol
D. `o`-Chlorophenol

Answer

B

Explanation

Although halogen is electron donating by resonance, its electron withdrawing inductive effect that decreases with increasing separation of `X` and `OH` predominates,making all the halophenols more acidic than phenol.The decreasing order of acid strength is

`pK_(a) = -log K_(a) =log 1//Ka`
Thus
lower `pK_(a) rarr` higher `K_(a) rarr` stronger acid
Notice that fluorophenols are relatively weaker acids than the corresponding chlorophenols. This is due to the fact that `+R_("effect")` of `F`(on account of smaller orbitals) is stronger than `+R("effect") of Cl`.