Compound	pKa	Structural Note
p-Nitrophenol	7.2	The nitro group is strongly electron-withdrawing and conjugated directly into the ring
Phenol	10.0	Reference value

Why phenols are more acidic than alcohols (compare phenol, pKa 10, to ethanol, pKa 16): the phenoxide ion delocalizes its negative charge into the aromatic ring by resonance, spreading it onto the ortho and para carbons. An alkoxide has no comparable delocalization pathway. This is the same resonance argument used for carboxylate, applied to a different scaffold — see Appendix A’s entry on phenols and Chapter 17 for how ring substituents shift this value further.

Why electron-withdrawing ring substituents increase acidity further: a para-nitro group can accept negative charge directly by resonance (the charge can be drawn all the way onto the nitro oxygens), which is why p-nitrophenol is nearly 1,000 times more acidic than phenol itself. Electron-donating substituents (e.g., para-methoxy) have the opposite effect.

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