Ortho nitrophenol is more acidic than ortho-methoxyphenol. This is because the nitro-group is an electron-withdrawing group. The presence of this group in the ortho position in ortho nitro phenol decreases the electron density in the O−H bond. As a result, it is easier to lose a proton.Also, the o-nitrophenoxide ion formed after the loss of protons is stabilized by resonance. Hence, ortho nitrophenol is a stronger acid.
In the case of ortho methoxyphenol; methoxy group is an electron-releasing group. Thus, it increases the electron density in the O−H bond and hence, the proton cannot be given out easily. Hence, ortho-nitrophenol is more acidic than ortho-methoxyphenol.
