Arrange the following compounds in the increasing order of their acidic strength:
p-cresol, p-nitrophenol, phenol
Electron-withdrawing group ($$-NO_2$$) increases the acidic character of phenols. Electron withdrawing group withdraws the electrons, as a result, the electron density of phenol decreases. This weakens the O—H bond and thus the release of proton become easy which makes it more acidic. Hence the nitrophenol is a stronger acid than phenol.
Electron-donating group ($$=CH_3$$) decreases the acidic character of phenols. Electron donating group donate electrons to the phenol, as a result, the electron density of phenol increases. This strengthens the O—H bond and thus the release of proton become difficult which makes it less acidic. Hence the p-cresol (4-methylphenol) is less acidic than phenol and nitrophenol.
Thus, p-nitrophenol has maximum acidic strength and p-cresol has least acidic strength. The order is shown below. p-nitrophenol > phenol > p-cresol.