Question
Why is phenol more acidic than ethanol, even though both have an –OH group?
Solution — Step by Step
Phenol phenoxide ion + H. Ethanol ethoxide ion + H. The acid is stronger if its conjugate base is more stable.
In phenoxide (CHO), the negative charge on O is delocalized into the benzene ring through resonance. We can draw four resonance structures: charge on O, on ortho-C, on para-C, and on the other ortho-C.
In ethoxide (CHCHO), the negative charge is localized entirely on the O. The alkyl group ( effect) actually pushes electron density toward the O, destabilizing the negative charge further.
Phenoxide’s resonance delocalization is a major stabilization that ethoxide lacks. Plus, phenol’s –OH O is -attached to the ring, which slightly increases its -character and helps it hold a positive partial charge near the H, weakening the O–H bond. Both effects make phenol the stronger acid.
p values: phenol , ethanol . Six orders of magnitude difference, all explained by resonance.
Final Answer: Phenol is more acidic because the phenoxide ion is resonance-stabilized, while the ethoxide ion has no comparable stabilization.
Why This Works
Resonance is the dominant effect controlling acidity in aromatic systems. Whenever the conjugate base of an acid can spread its negative charge across multiple atoms via resonance, that acid will be stronger than an analog where the charge stays on one atom.
The same logic explains why carboxylic acids (RCOOH) are far more acidic than alcohols: the carboxylate (RCOO) has its negative charge equally on two oxygens by resonance.
Alternative Method
Look at electron density on the O–H bond using the inductive effect alone: the phenyl group is (electron-withdrawing), so it pulls density away from the O–H bond and weakens it. This explains some of the difference but underestimates the magnitude. Resonance is the bigger factor.
Ranking phenol and an ortho-nitrophenol the same way without considering nitro’s effect. Electron-withdrawing groups (–NO, –Cl) on the ring further stabilize the phenoxide and increase acidity. Para- and ortho-nitrophenols are stronger acids than phenol itself.
JEE asks “rank by acidity” questions every single year. Recipe: (1) draw the conjugate base, (2) check for resonance and inductive effects, (3) more stable conjugate base → stronger acid. This three-step recipe handles any acidity ranking.