Alcohols, Phenols and Ethers
Alcohols, Phenols and Ethers tests classification, preparation, acidity, characteristic reactions and the Williamson synthesis.
Core rules
- Alcohols: Lucas test (conc. HCl + ZnCl2) gives immediate turbidity for 3°, slow for 2°, and needs heating for 1°.
- Oxidation: 1° alcohol to aldehyde then acid; 2° to ketone; 3° resistant under mild conditions.
- Phenols are more acidic than alcohols because the phenoxide ion is resonance-stabilised; electron-withdrawing groups increase acidity.
- Phenol gives violet colour with neutral FeCl3 and undergoes electrophilic substitution readily.
- Williamson synthesis: R-X + R'ONa → R-O-R' + NaX; primary halides give the best yields.
Worked example
Compare the acidity of ethanol, phenol and p-nitrophenol. Ethanol has no resonance stabilisation of its alkoxide. Phenol stabilises the phenoxide by delocalisation over the ring. The -NO2 group withdraws electron density and further stabilises the phenoxide, so acidity is p-nitrophenol > phenol > ethanol. Reasoning: acidity depends on the stability of the conjugate base.
Traps
Saying phenol gives a positive Lucas test, using tertiary halides in Williamson synthesis (elimination competes), and writing oxidation of a 3° alcohol to a ketone. Phenol is not oxidised to benzoic acid under mild conditions.