Phenols - Acidity Comparison

Acidity of a phenol is governed by how well the phenoxide ion delocalises its negative charge: any group that withdraws electrons stabilises the anion and increases acidity, while electron-donating groups destabilise it and lower acidity. In p-nitrophenol the nitro group is a strong electron-withdrawing substituent that delocalises the phenoxide charge onto its oxygen atoms through resonance, making this the most acidic species listed. Phenol itself is the unsubstituted reference and is therefore less acidic. p-cresol carries a methyl group that donates electron density by hyperconjugation and induction, destabilising the anion, so it is weaker than phenol. p-methoxyphenol has a methoxy group whose lone pair donates into the ring by resonance, raising electron density and making it the weakest acid here. This trend follows the NCERT discussion of substituent effects on phenol acidity and is a classic JEE Advanced ranking question. A useful generalisation is that ortho and para electron-withdrawing groups influence acidity most strongly because they share direct resonance communication with the phenoxide oxygen, whereas meta groups act mainly by the weaker inductive effect. The nitro group at the para position is special because its empty pi system overlaps with the ring to delocalise the anionic charge right onto the nitro oxygens. As a sanity check, the para-nitro group is known to lower pKa from about 10 for phenol to roughly 7.1, confirming it as strongest.