Question

Which of the following fact(s) explain(s) as to why p-nitrophenol is more acidic than phenol?
I.$- {\text{I}}$ effect of nitro group.
II.Greater resonance effect of p-nitrophenoxy group
III.Steric effect of bulky nitro group
A.I and II
B.I and III
C.II and III
D.II alone

Answer 1

Acidity of compounds depends upon the stability of the conjugate base it forms by deprotonation. The more the negative charge of the base is dispersed in the molecules, more will be its stability.

Complete step by step answer:
If any group or an atom donates electron then it is said to be the $+ {\text{M}}$ group and if any group withdraw electron through resonance then it is known as $- {\text{M}}$ group. The mesomeric effect is generally due to the presence of a functional group. $+ {\text{M}}$ effect increases the electron density at Ortho and para position, whereas $- {\text{M}}$ effect decreases electron density at ortho and para position. They both have no impact on Meta position. Phenols are acidic in nature. Substituted phenol in which one phenol is substituted by the electron withdrawing group such as nitro group enhances the acidic strength of phenol. The effect is more pronounced when the group is present at Ortho and para position it is due to the extra delocalisation of electrons the compound the more will be more stable.
Acidic strength is determined by the stability of negative ions formed when the hydrogen gets removed. The electron withdrawing group takes an electron density and hence stabilizes the negative charge and this makes the para nitrophenol more acidic than phenol because in phenol there is no such electron withdrawing group present. And $+ {\text{M}}$ effect is a type of resonance only so the statement I and II is correct.

So, the correct option is option A.

Note:
The para nitrophenol is the most acidic among the ortho and meta nitrophenol. $- {\text{M}}$ Effect is same as in case of para and ortho nitrophenol but due to steric hindrance ortho nitrophenol is less acidic than para nitrophenol and Meta nitrophenol is least acidic because there is no $+ {\text{M}}$ group on meta position.