Ethers: properties and reactions

The C–O bonds are polar, so ethers have a net dipole moment, but this weak polarity barely raises the boiling point: it stays close to that of an alkane of similar mass and far below that of an alcohol.

Boiling points: n-pentane 309.1 K, ethoxyethane 307.6 K, butan-1-ol 390 K. Alcohols are linked by hydrogen bonds; ether molecules are not.

Ethers of low mass dissolve in water about as well as alcohols of similar mass, because ether oxygen can accept hydrogen bonds from water. Ethoxyethane and butan-1-ol dissolve to 7.5 g and 9 g per 100 mL of water; pentane hardly dissolves.

Ethers are among the least reactive functional groups. The C–O bond is cleaved only under drastic conditions with an excess of a hydrogen halide; the reactivity order is HI > HBr > HCl.

A dialkyl ether with HX gives two alkyl halides. The ether oxygen is first protonated; the halide ion then attacks by SN2 at the less substituted carbon, so the smaller alkyl group ends up as the halide.

When one group is tertiary the cleavage goes by SN1 through the stable tertiary carbocation, and the tertiary group becomes the halide.

Anisole with HI gives phenol and methyl iodide: the O–C₆H₅ bond has partial double-bond character and does not break, and phenyl halides never form this way.

The alkoxy group activates the ring and directs to ortho and para. Anisole brominates in ethanoic acid even without an iron(III) bromide catalyst, the para isomer forming in 90% yield.

Anisole undergoes Friedel–Crafts alkylation and acylation with AlCl₃, and a mixture of concentrated sulphuric and nitric acids gives ortho- and para-nitroanisole.