JEE Chemistry Alcohols Phenols and Ethers Notes Set 11

Preparation of Phenol

1. Cumene process: cumene is oxidised by air to cumene hydroperoxide, which on acid hydrolysis gives phenol and acetone (industrial method).
2. Dow's process: chlorobenzene is heated with NaOH at about 350 °C under high pressure and then acidified.
3. Alkali fusion: benzenesulphonic acid (or its salt) is fused with NaOH at about 300-320 °C, then acidified.
4. Decarboxylation: salicylic acid or its sodium salt heated with soda lime gives phenol.
5. Diazonium route: aniline with NaNO₂/HCl at about 5 °C gives the diazonium salt, which on boiling with water gives phenol.

Reactions and Tests of Phenol

1. Br₂/H₂O (or Cl₂ in ethanol/water) gives the 2,4,6-trihalophenol (white precipitate for bromine). Br₂ in CS₂ or acetic acid at low temperature gives only the mono-bromo product.
2. Reimer-Tiemann: CHCl₃/NaOH then H₂O gives mainly salicylaldehyde (ortho); CCl₄/NaOH then H⁺ gives salicylic acid.
3. Kolbe-Schmidt: phenoxide ion + CO₂, then HCl, gives salicylic acid. Acetylation of salicylic acid gives aspirin.
4. Phenol with formaldehyde and a base condenses and polymerises to Bakelite.
5. Tests: phenol is weakly acidic (turns blue litmus red) and gives a violet colour with neutral FeCl₃.

Preparation of Alcohols from Alkenes

1. Hydroboration-oxidation (syn, anti-Markovnikov); oxymercuration-demercuration (Markovnikov, no rearrangement); acid-catalysed hydration (dilute H₂SO₄, Markovnikov).
2. Dihydroxylation: OsO₄/NaHSO₃ or cold dilute alkaline KMnO₄ (Baeyer's reagent) gives the cis-diol (syn addition); a peroxy acid followed by H₃O⁺ gives the trans-diol (anti addition).
3. Grignard reagents add to carbonyl compounds: HCHO gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols.

Reactions and Tests of Alcohols

1. With HX: reactivity 3° > 2° > 1°; ZnCl₂ is needed for 1° and 2° alcohols with HCl. PCl₃, PBr₃, PCl₅ and SOCl₂/pyridine also replace OH by halogen.
2. Dehydration with conc. H₂SO₄: ease 3° > 2° > 1°; the more substituted (Saytzeff) alkene is major. Ethanol gives the alkyl hydrogen sulphate at about 80 °C, diethyl ether at 140 °C (excess alcohol) and ethene at 170 °C.
3. Tests: ceric ammonium nitrate gives a red colour. Lucas reagent (conc. HCl + anhydrous ZnCl₂): tertiary alcohols give turbidity at once, secondary in about 5 minutes, primary only on heating. Victor Meyer test: red for 1°, blue for 2°, colourless for 3° alcohols.

Preparation of Ethers

1. Williamson synthesis: RX + R'ONa → R–O–R' + NaX. It works best with a primary alkyl halide; phenoxides give alkyl aryl ethers such as anisole.
2. Dehydration of alcohols with conc. H₂SO₄ at about 140 °C. A mixture of two different alcohols gives three ethers.