Explain why is ortho nitrophenol more acidic than ortho methoxyphenol?
Ortho-nitrophenol is more acidic than ortho-methoxyphenol due to the nature of substituents attached to the benzene ring.
The nitro group is an electron withdrawing group. When present at the ortho position, it causes effective delocalisation of the negative charge in the phenoxide ion. This stabilizes the phenoxide ion and favours the ionisation of phenol, thereby enhancing the acidic strength.
On the other hand, the methoxy group is an electron releasing group. Such groups do not favour the formation of phenoxide ion and result in a decrease in acid strength. Therefore, ortho-methoxyphenol is less acidic than ortho-nitrophenol.
Explanation
The textbook explains that electron withdrawing groups like nitro group enhance acidic strength of phenol, especially at ortho and para positions, due to effective delocalisation of negative charge in phenoxide ion. Conversely, electron releasing groups decrease acid strength as they do not favour phenoxide ion formation. The pKa table confirms o-Nitrophenol (pKa 7.2) is much more acidic than phenol (pKa 10.0).