phenol
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A phenol is an aromatic ring with an -OH group bonded directly to a ring carbon. The parent compound, also called phenol, is benzene wearing a single hydroxyl: C6H5-OH. At first glance it looks like an alcohol, but attaching the -OH to an aromatic ring rather than to an ordinary sp3 carbon changes its chemistry enough to make it its own class.
The headline difference is acidity. An ordinary alcohol has a pKa around 16; phenol's is about 10, making it roughly a million times more acidic. The reason is resonance: when phenol loses its O-H proton, the resulting negative charge does not sit trapped on the oxygen but spreads out into the aromatic ring, delocalised over several ring carbons. That extra stabilisation of the phenoxide ion makes the proton much easier to give up. Electron-withdrawing groups on the ring (like the nitro groups in picric acid) deepen the effect and can make a phenol as acidic as a carboxylic acid.
Phenols are everywhere in the real world. The natural antioxidant vitamin E, the antiseptic in old hospitals (carbolic acid is just phenol), the flavour molecules vanillin and eugenol, and the BHT preservative in foods are all phenols; their easily-given hydrogen lets them mop up destructive free radicals, which is why so many antioxidants are phenolic. Because they are more acidic than alcohols, phenols dissolve in dilute aqueous NaOH (forming the sodium phenoxide salt) while ordinary alcohols do not — a simple test that tells the two apart.
Phenol (C6H5-OH, pKa about 10) dissolves in aqueous NaOH because it is acidic enough to form the sodium phenoxide salt. Cyclohexanol (an ordinary alcohol, pKa about 16) does not — too weak an acid.
Resonance into the ring makes phenols far more acidic than alcohols.
Phenols are far more acidic than alcohols but still much weaker than carboxylic acids, so phenol dissolves in NaOH but not in the weaker base NaHCO3 — a test that separates phenols from carboxylic acids.