Due to strong -R and -I effect of the `(---NO_(2))` group, electron density in the `(O---H)` bond decreases and hence the loss of a proton becomes easy.
Further, after the loss of a proton, the o-nitrophenoxide ion left behind is stabilised by reasonance.
o-Nitrophenoxide ion is stabilised by resonance, thereby making o-nitrophenol a stronger acid. In contrast, due to `+R` effect to the `(OCH_(3))` group, it increases the electron-density in the `(O---H)` bond, thereby making the loss of proton difficult.
Furthermore, the o-methoxyphenoxide ion left after the loss of a proton is destabilised by resonance.
The two negative charges repel each other, thereby destabilising the o-methoxyphenoxide ion. Thus, o-nitropenol is more acidic than o-methoxy-phenol.
