To order the provided phenolic compounds by increasing pH, their acidic strengths must be evaluated. pH is inversely proportional to acidity; stronger acids exhibit lower pH values. The acidic strengths are assessed as follows:
p-Nitrophenol (A): Acidity is enhanced by the electron-withdrawing nitro group, which stabilizes the phenoxide ion via resonance and inductive effects.
m-Nitrophenol (C): The nitro group at the meta position is moderately electron-withdrawing. Its resonance stabilization of the phenoxide ion is less pronounced than when it is in the para position.
m-Cresol (B): The electron-donating methyl group reduces acidity relative to phenol, as it increases electron density, destabilizing the phenoxide ion.
Phenol (D): This is a weak acid; the hydroxyl group offers some acidity through resonance stabilization.
Based on these effects, the order of acidic strength from strongest to weakest is:
p-Nitrophenol (A): Strongest due to the electron-withdrawing nitro group, leading to the lowest pH.
m-Nitrophenol (C): Weaker than p-nitrophenol due to less effective resonance stabilization.
Phenol (D): Less acidic than nitrophenols but more acidic than m-cresol due to the minimal electron-donating effect.
m-Cresol (B): Weakest due to the electron-donating methyl group destabilizing the phenoxide ion, resulting in the highest pH.
Therefore, the compounds arranged in increasing order of their pH values are: