Why is acetic acid (vinegar) a stronger acid than phenol or ethanol?
When a carboxylic acid loses its O–H proton, the negative charge on the carboxylate is shared equally over two oxygen atoms by resonance, giving a very stable ion — so the acid readily gives up its proton.
When a carboxylic acid loses its O–H proton, the negative charge on the carboxylate is shared equally over two oxygen atoms by resonance, giving a very stable ion — so the acid readily gives up its proton. Phenoxide spreads charge into a ring (less effective, and onto carbon), and an alkoxide has no resonance at all. So the acidity order is carboxylic acid > phenol > water > alcohol.
Key point
Carboxylate resonance over two equivalent oxygens makes carboxylic acids the strongest of these acids.
Common mistake
assuming phenol, being aromatic, must be the most acidic.
Memory tip
two-oxygen charge spread beats ring or none: acid > phenol > alcohol.
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