Alcohols are compounds in which a hydroxyl group (–OH) is bonded to an sp³-hybridized carbon atom. They are classified as primary (1°, –OH on a carbon bonded to one other carbon), secondary (2°, bonded to two carbons), or tertiary (3°, bonded to three carbons). Phenols have the –OH group directly attached to an aromatic ring. This seemingly small difference results in dramatically different acidity, reactivity, and physical properties between the two classes.
IUPAC nomenclature for alcohols replaces the final -e of the parent alkane with -ol (e.g., methane → methanol). The position of the –OH group is indicated by the lowest possible number. Polyols use suffixes such as -diol, -triol. Common names (isopropyl alcohol, tert-butyl alcohol) remain widely used. For phenols, the parent name is phenol, with substituents named as prefixes (e.g., 2-methylphenol, commonly called o-cresol).
The physical properties of alcohols are dominated by hydrogen bonding between the –OH group and water or other alcohol molecules. Boiling points are significantly higher than those of corresponding alkanes and ethers. Methanol (b.p. 65 °C) versus ethane (b.p. –89 °C) illustrates the dramatic effect. Lower alcohols (methanol, ethanol, propanol, isopropanol) are miscible with water; solubility decreases as the hydrophobic alkyl chain lengthens. Phenols are partially water-soluble but less so than analogous alcohols due to the reduced hydrogen bond accepting ability of the aromatic –OH.
The acidity of the hydroxyl proton differs markedly between alcohols and phenols. Simple alcohols have pKa ≈ 16–18, comparable to water (pKa 15.7), and require strong bases (NaH, NaNH₂) for deprotonation. Phenols are substantially more acidic (pKa ≈ 10) due to resonance stabilization of the phenoxide conjugate base: the negative charge is delocalized into the aromatic ring, spreading the charge density over ortho and para positions. Electron-withdrawing substituents (e.g., –NO₂, –CN) further increase phenol acidity, most dramatically in picric acid (2,4,6-trinitrophenol, pKa 0.25).
Alcohols are synthesized by several routes. Hydration of alkenes (H₂O/H⁺) gives Markovnikov alcohols. Reduction of carbonyl compounds — aldehydes to primary alcohols, ketones to secondary alcohols (NaBH₄, LiAlH₄) — is a common laboratory method. Grignard reactions (RMgX + carbonyl → alcohol) enable carbon-carbon bond formation, producing primary, secondary, or tertiary alcohols depending on the carbonyl substrate. Fermentation remains the major industrial route to ethanol.
Alcohol oxidation is a key transformation. Primary alcohols oxidize to aldehydes (with PCC, pyridinium chlorochromate) or further to carboxylic acids (with Jones reagent, CrO₃/H₂SO₄, or KMnO₄). Secondary alcohols oxidize to ketones; tertiary alcohols resist oxidation under mild conditions. Dehydration of alcohols (H₂SO₄ or Al₂O₃, heat) produces alkenes via an E1 or E2 elimination mechanism; the Saytzeff product predominates. Esterification with carboxylic acids (Fischer esterification, H⁺ catalyst) yields esters. Phenols undergo electrophilic aromatic substitution readily; the –OH group is strongly activating and ortho/para-directing. The Kolbe-Schmitt reaction (phenol + CO₂/NaOH → salicylic acid) is industrially important for aspirin synthesis.
