Ethers

Making ethers.

By dehydrating alcohols. With concentrated H₂SO₄ at 413 K, two molecules of a primary alcohol join and lose water: ethanol gives ethoxyethane. Temperature matters: at 443 K the same mixture gives ethene instead. Secondary and tertiary alcohols give alkenes, not ethers, so this method only suits primary alcohols and makes only symmetrical ethers.

Williamson synthesis, the most useful method. An alkoxide (R–O⁻Na⁺) reacts with an alkyl halide (R′–X): the Na⁺ and X⁻ leave as salt, and R–O–R′ is formed. It works for unsymmetrical ethers too. One rule: the alkyl halide should be primary. With a tertiary halide, the alkoxide (a strong base) pulls off an H instead and makes an alkene. So put the bulky group on the alkoxide side. Aryl halides do not react, so to make an aryl ether, use the phenoxide with an alkyl halide.

Physical properties. Ethers have no O–H, so they cannot hydrogen-bond to each other: their boiling points are close to those of alkanes of similar mass, and far below alcohols. Small ethers dissolve a little in water, because water can hydrogen-bond to the ether oxygen.

Breaking ethers with HX. Ethers are fairly unreactive, but hot concentrated HI (or HBr) splits the C–O bond, giving an alkyl halide and an alcohol. Which side gets the halogen?

• If both groups are primary or secondary, the reaction goes by SN2, so the halide ion attacks the smaller, less crowded group: CH₃–O–C₂H₅ with HI gives CH₃I and ethanol.
• If one group is tertiary, the reaction goes by SN1: the tertiary carbocation forms, and the halide ends up on the tertiary carbon.
• In an aryl alkyl ether such as anisole, the aryl-O bond never breaks (it has partial double-bond character), so you always get phenol and the alkyl halide.

Ring reactions of aryl ethers. The –OR group activates the benzene ring and directs ortho/para, just like –OH but a little less strongly. Anisole can be brominated, nitrated or put through Friedel-Crafts reactions, and the para product is the main one.