Describe two methods of preparing ethers, giving the mechanism of one. Why does anisole give phenol, and not iodobenzene, with HI?

1. 1 · Open with the methods. Ethers, R-O-R', are made by dehydrating alcohols or by Williamson synthesis.
2. 2 · Dehydration, with mechanism.
   • 2C₂H₅OH → C₂H₅OC₂H₅ + H₂O (conc H₂SO₄, 413 K)
   • Step 1: C₂H₅OH + H⁺ → C₂H₅O⁺H₂
   • Step 2: C₂H₅OH attacks C₂H₅O⁺H₂ (SN2); H₂O leaves
   • Step 3: loss of H⁺ gives C₂H₅OC₂H₅
3. 3 · Williamson synthesis. R-X + R'-ONa → R-O-R' + NaX, by SN2; e.g. (CH₃)₃CONa + CH₃Br → (CH₃)₃COCH₃ + NaBr.
4. 4 · Limitation. With a 3° halide, the alkoxide acts as a base: (CH₃)₃CBr + CH₃ONa → (CH₃)₂C=CH₂ + CH₃OH + NaBr (elimination).
5. 5 · Anisole and HI. C₆H₅OCH₃ + HI → C₆H₅OH + CH₃I. The O-C₆H₅ bond has partial double bond character, and an sp² carbon resists substitution.
6. 6 · Link to real life. Ethoxyethane was an early anaesthetic and is a common laboratory solvent.
7. 7 · Close in one line. So use a primary alkyl halide for Williamson synthesis, and remember that HI cuts the alkyl side of an aryl ether.

Scoring tip: Always write 'primary alkyl halide' in a Williamson answer.