In substituted phenols, the presence of electron withdrawing
groups such as nitro group, enhances the acidic strength of
phenol This effect is more pronounced when such a group is
present at ortho and para positions It is due to the effective
delocalisation of negative charge in phenoxide ion when
substituent is at ortho or para position On the other hand,
electron releasing groups, such as alkyl groups, in general, do
not favour the formation of phenoxide ion resulting in decrease
in acid strength