p-Nitrophenol (4-nitrophenol) is a stronger acid than phenol due to the presence of a nitro group (-NO2) in the para position (i.e., at the fourth carbon atom) on the phenolic ring. This nitro group introduces several factors that enhance the acidity of p-nitrophenol compared to phenol:

1. Electron-withdrawing effect: The nitro group is highly electron-withdrawing due to the presence of electronegative nitrogen and oxygen atoms and the delocalized pi electrons in the nitro group. This strong electron-withdrawing effect destabilizes the electron density around the oxygen atom in the -OH group of p-nitrophenol, making it more prone to losing a proton (H+). In contrast, phenol lacks this electron-withdrawing group, and the oxygen atom in its -OH group retains more electron density.

2. Stabilization of the phenoxide ion: When p-nitrophenol donates a proton to a base, it forms the p-nitrophenoxide ion. The presence of the nitro group enhances the stability of the negative charge on the oxygen atom in the phenoxide ion through resonance. The nitro group can delocalize the negative charge, spreading it over the nitrogen and oxygen atoms and the aromatic ring. This resonance stabilization makes the phenoxide ion of p-nitrophenol more stable than the phenoxide ion of phenol.

3. Greater acidity: Because p-nitrophenol is more acidic, it readily donates a proton in the presence of a base, forming the phenoxide ion. This enhanced acidity is quantified by its lower pKa value (a measure of acidity), indicating a stronger tendency to lose a proton compared to phenol. The pKa of p-nitrophenol is around 7.15, while the pKa of phenol is around 10.0.

In summary, the electron-withdrawing nitro group in p-nitrophenol significantly increases its acidity by destabilizing the electron density around the -OH group, enhancing the stability of the resulting phenoxide ion, and lowering its pKa value. These factors make p-nitrophenol a stronger acid than phenol.