Concept 05 / 06Alcohols, Phenols and Ethers
Ethers: Preparation, Properties and Reactions
Preparation of ethers by acid-catalysed dehydration of alcohols and by Williamson synthesis (choice of reactants), physical properties (boiling points, solubility, bond angle), cleavage of C-O bonds by hydrogen halides including the order of cleavage for mixed ethers, and electrophilic substitution in anisole.
Ethers have the general formula R-O-R' (symmetrical if R = R', unsymmetrical or mixed otherwise). Examples: methoxymethane , ethoxyethane (diethyl ether) , methoxybenzene (anisole) .
Preparation.
Dehydration of alcohols: . The mechanism is SN2: a protonated alcohol is attacked by a second alcohol molecule. This works only for primary alcohols (secondary and tertiary alcohols give alkenes, since their carbocations eliminate), and temperature must be controlled ( gives ethene).
Williamson synthesis: an alkyl halide reacts with a sodium alkoxide or phenoxide by SN2: . Good for symmetrical and unsymmetrical ethers. The alkyl halide should be primary: with secondary and especially tertiary halides, the alkoxide (a strong base) causes elimination instead (see figure). So to make tert-butyl methyl ether, use sodium tert-butoxide with methyl bromide, not sodium methoxide with tert-butyl bromide. Aryl ethers are made from phenoxides and alkyl halides: (the reverse, aryl halide + alkoxide, fails).
Physical properties. The C-O bonds are polar and the molecule is bent (C-O-C in methoxymethane), giving a small net dipole moment. Boiling points are similar to those of alkanes of comparable mass and much lower than those of alcohols, since ethers cannot hydrogen-bond with themselves (diethyl ether , n-pentane , n-butanol ). Their solubility in water, however, is similar to that of alcohols of the same mass, because the oxygen accepts hydrogen bonds from water.
Chemical reactions.
Cleavage of C-O by hydrogen halides: ethers are fairly unreactive, but concentrated HI (or HBr) at high temperature cleaves them: ; with excess HX, the alcohol also gives RX. Reactivity: HI > HBr > HCl. Mechanism: the ether is protonated; then attacks. For primary and secondary alkyl groups, attack is SN2 at the less hindered carbon, so the smaller alkyl group becomes the halide: . If one group is tertiary, the reaction goes by SN1 and the tertiary group becomes the halide: . Alkyl aryl ethers are cleaved at the alkyl-oxygen bond, giving phenol and alkyl halide: (the C-O bond has partial double-bond character, and phenol does not react further with HI).
Electrophilic substitution in anisole: the group activates the ring (ortho/para): bromination in ethanoic acid (even without catalyst) gives mainly p-bromoanisole; Friedel-Crafts alkylation and acylation () give 2- and 4-substituted products; nitration gives 2- and 4-nitroanisole.
Diethyl ether is widely used as a solvent and was once used as an anaesthetic; ethers form explosive peroxides on long standing in air.
Key Takeaways:
- Dehydration at for primary alcohols only; Williamson synthesis with a primary halide.
- Ethers boil like alkanes but dissolve in water like alcohols.
- HI cleavage: SN2 puts I on the smaller group; SN1 puts I on a tertiary group.
- Anisole + HI → phenol + ; is ortho/para directing.
- Pitfall: an aryl halide plus alkoxide does not give an aryl ether; use phenoxide plus alkyl halide.
Brain-Teaser 1: Give the best Williamson route to 2-ethoxy-2-methylpropane, .
Solution: Sodium tert-butoxide with bromoethane: . The alternative, sodium ethoxide with tert-butyl bromide, would give mostly 2-methylpropene by elimination.
Brain-Teaser 2: What are the products of heating with HI: (a) 1-propoxypropane, (b) methoxybenzene, (c) 2-methoxy-2-methylpropane?
Solution: (a) Propyl iodide and propan-1-ol (excess HI converts both to propyl iodide). (b) Phenol and iodomethane. (c) tert-Butyl iodide and methanol (SN1 via the tertiary carbocation).
Brain-Teaser 3: Why does diethyl ether (b.p. ) boil far lower than its isomer butan-1-ol (), though their water solubility is similar (about and per )?
Solution: Butanol molecules hydrogen-bond to one another, so much energy is needed to separate them; ether molecules lack an O-H and cannot. Yet both can accept hydrogen bonds from water through the oxygen lone pairs, so their solubilities are comparable.
Practice this concept
Free · no account required · instant solutions