Increasing order of acid strength among p-methoxyphenol, p-methylphenol and p-nitrophenol is

p-methoxyphenol, p-methylphenol, p-nitrophenol

The correct answer is option 4. p-methoxyphenol, p-methylphenol, p-nitrophenol.

The acid strength of substituted phenols is influenced by the nature of the substituents on the benzene ring. Electron-withdrawing groups tend to increase the acidity of phenols, while electron-donating groups tend to decrease it.

Substituent Effects:

p-Nitrophenol:

Substituent: Nitro group (\(-NO_2\))

Effect: Strong electron-withdrawing group (via both inductive and resonance effects), which stabilizes the phenoxide ion, making the phenol more acidic.

p-Methylphenol (p-Cresol):

Substituent: Methyl group (\(-CH_3\))

Effect: Electron-donating group (via inductive and hyperconjugation effects), which destabilizes the phenoxide ion, making the phenol less acidic.

p-Methoxyphenol (p-Anisole):

Substituent: Methoxy group (\(-OCH_3\))

Effect: Electron-donating group (via resonance and inductive effects), which destabilizes the phenoxide ion even more than the methyl group, making the phenol even less acidic.

Order of Acid Strength:

Based on the effects described:

p-Nitrophenol: Most acidic due to the strong electron-withdrawing effect of the nitro group.

p-Methylphenol (p-Cresol): Less acidic than p-nitrophenol but more acidic than p-methoxyphenol due to the weaker electron-donating effect of the methyl group.

p-Methoxyphenol (p-Anisole): Least acidic due to the strong electron-donating effect of the methoxy group.

Thus, the increasing order of acid strength is: p-methoxyphenol, p-methylphenol, p-nitrophenol