Question
A student spills phenol on her hand and notices a sharp burn, while ethanol on the same hand only feels cool. Why is phenol corrosive while ethanol is mild? Explain in terms of molecular structure and acidity.
Solution — Step by Step
Both compounds have an O–H bond. In water:
Phenol is roughly times more acidic than ethanol.
The phenoxide ion delocalises its negative charge across the aromatic ring, into ortho and para positions. This stabilisation is impossible in ethoxide , where the charge sits on a single oxygen.
Phenol’s higher acidity means it can protonate skin proteins, denaturing them on contact. The phenoxide that forms is also reactive enough to disrupt cellular membranes. Ethanol, being far less acidic, mostly just dehydrates skin and evaporates.
Phenol was the first surgical antiseptic (Joseph Lister, 1860s) — its corrosiveness toward bacteria was exactly the desired effect. The same property burns human tissue.
Final answer: Phenol burns skin because resonance delocalisation makes phenoxide stable, raising phenol’s acidity by over ethanol. Higher acidity = stronger denaturant.
Why This Works
The acidity of an O–H compound is largely about how stable the conjugate base is. Resonance into an aromatic ring is the most powerful stabiliser available to oxy-anions — every Class 12 chemistry teacher beats this point in for good reason.
Substituents on the ring shift acidity further: nitro groups (electron-withdrawing) make phenols even more acidic; methyl groups (electron-donating) make them less so.
Alternative Method
Look at values directly: lower means stronger acid. Phenol’s versus ethanol’s tells the same story without invoking resonance pictures.
JEE Main 2023 asked: “Arrange in increasing acidity: phenol, p-nitrophenol, p-methylphenol.” Order: p-methylphenol < phenol < p-nitrophenol. Memorise the substituent-effect direction.
Common Mistake
Saying “phenol is more acidic because it has a benzene ring”. Vague. The real reason is resonance stabilisation of the phenoxide anion, not the ring itself. Aniline also has a benzene ring but is a weak base, not a strong acid.