Alcohols, Phenols and Ethers
Domain 11: Alcohols, Phenols and Ethers
This domain tests structure–reactivity relationships: acidity, dehydration, oxidation, and the Williamson synthesis.
Core rules
- Alcohols are classified 1°, 2°, 3°. Lucas reagent (conc. HCl + anhydrous ZnCl2) gives immediate turbidity with 3°, slower with 2°, and no reaction at room temperature with 1°. Victor Meyer test also distinguishes them.
- Phenols are far more acidic than alcohols because the phenoxide ion is resonance-stabilised; electron-withdrawing groups such as NO2 increase acidity, electron-donating groups decrease it.
- Oxidation: 1° alcohol -> aldehyde -> carboxylic acid; 2° -> ketone; 3° resists oxidation under mild conditions. PCC stops at aldehyde; hot KMnO4/H+ goes to acid.
- Dehydration with conc. H2SO4 follows Saytzeff's rule and can rearrange via carbocations.
- Williamson synthesis: alkoxide + primary alkyl halide -> ether. Tertiary halides favour elimination, so they are poor substrates for ether formation.
- Phenol reactions include Reimer–Tiemann (CHCl3/NaOH), Kolbe (CO2/NaOH), and coupling with diazonium salts.
Worked example
Acidity order: p-nitrophenol > phenol > ethanol. The NO2 group stabilises the phenoxide ion by resonance and induction, so p-nitrophenol loses H+ most readily. Ethanol has no resonance stabilisation of its ethoxide ion, so it is least acidic. This order is not the same as solubility in water.
Traps
- Thinking all alcohols are acidic like phenols.
- Using the Lucas test for phenols.
- Oxidising a 3° alcohol as if it gave a ketone.
- Using a tertiary halide in Williamson synthesis and getting elimination.
- Forgetting that phenol is not an alcohol and does not undergo simple dehydration.