Explain why is ortho nitrophenol more acidic than ortho methoxyphenol ?

Explanation

Nitro `(NO_(2))` group is an electron withdrawing group while methoxy `(OCH_(3))` group is electron releasing in nature. The release of `H^(+)` ion is therefore, easier from o-nitrophenol while it is quite difficult from o-methoxyphenol. Apart form that, o-nitrophenoxide ion is stabilised due to resonance (for the resonating structures refer to Q. No. 8,. On the contary, o-methoxyphenoxide is destabilised since the electron density on the negatively charged oxygen tends to increase due to the electron releasing tendency of the methoxy `(OCH_(3))` group.

In the light of the above disscussion, we may conclude that o-nitrophenol is a stronger acid `(pK_(a)=7.23)` than o-methoxyphenol `(pK_(a) = 9.98)`.