Question

p-Nitrophenol is a stronger acid than phenol while p-cresol is a weaker acid. This can be explained as:
A. $- C{H_3}$ group decreases the electron density on the oxygen $O - H$ group making cresol a weaker acid.
B. $- N{O_2}$ group decreases electron density on the oxygen of $O - H$ group making nitrophenol a stronger acid.
C. $- C{H_3}$ group increases the electron density on the oxygen of $O - H$ release of ${H^ + }$ easier.
D. $- N{O_2}$ group increases the electron density on oxygen of $O - H$ group making release of ${H^ + }$ easier.

Answer 1

Methyl group is an electron releasing group. So, it increases the electron density on the oxygen of the O-H bond. Therefore, the proton cannot be given out easily. The nitro group is an electron withdrawing group. The para position in this group is electron density on the benzene ring. This is defined by the nature of the nitro group and resonance stabilization of the group.

Complete step by step answer:
A strong acid is formed by the liberated protons and forms a stable compound. The more stable compound which forms after losing protons, the more is the acidic in nature.
Now, as we know that $- N{O_2}$ group is electron withdrawing group as well as $- C{H_3}$ is electron acceptor group.
The group withdrawing an electron decreases electron density on oxygen of $O - H$ group. Thus making release of ${H^ + }$ easier and increasing acidity of phenol.
But methyl groups increase the electron density on oxygen atoms thus decreasing acidity of phenol.

Thus, the correct answer is option (B).

Note:
When the group acts as electron withdrawing ,it affects an electron density and makes release ${H^ + }$.
If the decrease in electron density of the $O - H$ bond of p-nitrophenol, the polarity of $O - H$ bond decreases and in turn makes it more acidic than phenol.