phenol

Meaning: Phenol is an aromatic compound with a hydroxyl group on a ring carbon (C₆H₅OH), and the word also names that whole class. Its O–H is far more acidic than an aliphatic alcohol's because the negative charge on the conjugate base, phenoxide, delocalizes into the ring by resonance. Electron-withdrawing groups such as NO₂ at the ortho or para position stabilize phenoxide further and raise acidity. Electron-donating alkyl groups lower it slightly. The OH group is also a strongly activating, ortho/para-directing substituent in electrophilic aromatic substitution. The side chain of tyrosine is a phenol.

Equation: C₆H₅OH + OH⁻ ⇌ C₆H₅O⁻ + H₂O, with Ka = [C₆H₅O⁻][H₃O⁺] / [C₆H₅OH] and pKa = −log Ka. Concentrations are in mol/L, and Ka is conventionally written without units.

On the MCAT: Passages ask you to rank the acidity of substituted phenols, alcohols and carboxylic acids, or to explain the ranking by resonance and inductive effects. In extraction schemes, aqueous NaOH deprotonates phenol to a water-soluble phenoxide salt and pulls it into the aqueous layer. Sodium bicarbonate is a weaker base and generally does not deprotonate phenol to a significant extent, but it does deprotonate carboxylic acids. You may also be asked to predict the products of substitution on the ring.

Common confusion: Students treat phenol as an ordinary alcohol. The settling fact is the acidity order: carboxylic acid (pKa about 4 to 5) > phenol (about 10) > aliphatic alcohol (about 16 to 18).