Phenol () is a much stronger acid than ethanol () but weaker than carboxylic acids. Which of the fol — Aromatic Hydrocarbons Chemistry Question
Question
Phenol ($pK_a \approx 10$) is a much stronger acid than ethanol ($pK_a \approx 16$) but weaker than carboxylic acids. Which of the following substituents on the phenol ring would increase its acidity significantly, yielding a $pK_a$ lower than that of standard unsubstituted phenol?
💡 Solution & Explanation
The acidity of phenol depends heavily on the stability of the resulting phenoxide conjugate base. Electron-withdrawing groups (like $-NO_2$) stabilize the phenoxide anion through synergistic $-I$ (inductive) and $-M$ (resonance) effects, particularly when located at the ortho and para positions where the negative charge is delocalized. Alkyl ($-CH_3$) and alkoxy ($-OCH_3$) groups are electron-donating and destabilize the anion, reducing acidity.