Ethers

The essentials

Ethers have the general formula R–O–R′. Since they lack an O–H group, they cannot hydrogen bond with each other, so their boiling points are close to those of alkanes of similar mass and much lower than alcohols. However, they can accept hydrogen bonds from water, so lower ethers have solubility similar to alcohols of the same molar mass.

Key points

• Dehydration of alcohols: primary alcohols heated with conc. H₂SO₄ at 413 K give symmetrical ethers (ethanol gives ethoxyethane). At 443 K ethene forms instead. This method suits primary alcohols only.
• Williamson synthesis: an alkyl halide reacts with a sodium alkoxide or phenoxide, R–X + R′–ONa → R–O–R′ + NaX. It works best with primary alkyl halides (SN2). With tertiary halides, elimination gives an alkene instead.
• To make tert-butyl methyl ether, use sodium tert-butoxide with CH₃Br, not CH₃ONa with (CH₃)₃CBr.
• Cleavage by HI: ethers react with HI to give an alkyl iodide and an alcohol. For methyl alkyl ethers with primary groups, the iodide forms on the smaller (methyl) group by SN2. If one group is tertiary, the tertiary halide forms by SN1.
• Anisole + HI gives phenol and methyl iodide, because the aryl C–O bond has partial double-bond character.
• The –OCH₃ group in anisole activates the ring and directs ortho and para: bromination in ethanoic acid gives mainly p-bromoanisole; Friedel–Crafts and nitration also give o/p products.

Common mistakes

• Predicting iodobenzene from anisole and HI; phenol and CH₃I form instead.