Alcohols, Phenols and Ethers
Alcohols, Phenols and Ethers rewards precise knowledge of distinguishing tests, acidity comparisons and the named reactions of phenol.
Core rules
- Lucas test with conc. HCl/ZnCl₂: 3° alcohol turns cloudy at once, 2° in about five minutes, 1° only on heating; allylic and benzylic alcohols react quickly.
- Phenol is acidic (pKa ≈ 10) because phenoxide is resonance-stabilised, but it is too weak to liberate CO₂ from NaHCO₃ (carbonic acid, pKa ≈ 6.4).
- Acidity order: p-nitrophenol > phenol > p-cresol > ethanol. Electron-withdrawing groups stabilise phenoxide; electron-donating groups destabilise it.
- Phenol reactions: violet colour with neutral FeCl₃; bromine water gives 2,4,6-tribromophenol; Kolbe carboxylation (CO₂, NaOH, then H⁺) gives salicylic acid; Reimer–Tiemann (CHCl₃, NaOH) gives salicylaldehyde.
- Ethers: Williamson synthesis uses a primary halide with a sodium alkoxide; a 3° halide gives mainly elimination. Cleavage with cold HI gives the alcohol and the iodoalkane from the smaller alkyl group.
- Boiling points: alcohol > ether > alkane of comparable mass, because of hydrogen bonding.
Worked example
Anisole (C₆H₅OCH₃) with hot concentrated HI gives phenol and iodomethane. The methyl–oxygen bond breaks because the aryl carbon–oxygen bond has partial double-bond character from delocalisation into the ring and resists cleavage; iodide attacks the methyl carbon instead. Writing iodobenzene plus methanol as the product loses the mark.
Traps that cost marks
- Claiming phenol gives a positive Lucas test.
- Using NaHCO₃ to detect phenol - the test is used the other way round, to detect carboxylic acids.
- Assuming both C–O bonds of an ether break equally.
- Confusing Reimer–Tiemann (–CHO) with Kolbe (–COOH).
- Forgetting that phenol is not soluble in NaHCO₃ but dissolves in NaOH.