Resonance Effect On Acidity

The acidity of a phenol is governed by how well the resulting phenoxide ion can disperse its negative charge after losing the O-H proton. The nitro group is a powerful electron-withdrawing group exerting a strong -M (mesomeric) effect; when placed at the para position it withdraws negative charge from the phenoxide oxygen into the ring and onto its own oxygen atoms by resonance, greatly stabilising the conjugate base and increasing acidity. The option naming hyperconjugation is wrong because the nitro group has no alpha C-H bonds to provide that effect and its dominant influence is resonance. The option claiming electron donation is incorrect since the nitro group withdraws, not donates, electron density. The option of lost aromaticity is wrong because the ring stays aromatic and aromaticity is not the cause of the acidity change. This resonance stabilisation of para-nitrophenoxide is highlighted in NCERT, and the effect is strongest when the nitro group is ortho or para because only those positions allow direct conjugation with the oxygen-bearing carbon. A meta-nitro group, which cannot delocalise the charge by resonance, raises acidity far less and acts mainly through its inductive effect. This positional dependence is a favourite JEE discriminator. Plausibility check: extra delocalisation of negative charge always lowers the energy of the conjugate base, which must increase acid strength, confirming the resonance explanation.