Why Diethyl Ether Boils Far Lower Than Butanol Despite the Same Formula

Ethers have an oxygen atom bridging two carbon groups, R-O-R'. Diethyl ether is a volatile laboratory solvent, anisole has a pleasant aniseed-like smell, and ether linkages appear in many medicines and flavours. Their chemistry is fairly quiet, but strong acids can still split them apart.

This lesson covers naming ethers and preparing them by Williamson's synthesis, and the physical properties and reactions of ethers.

Ethers are named in IUPAC as alkoxy-substituted hydrocarbons, and Williamson's synthesis prepares them by reacting a sodium alkoxide or phenoxide with a primary alkyl halide in an SN2 reaction.

Naming:

- Common names list the two groups alphabetically, then add ether: $\mathrm{C}{\mathrm{H}}_3\mathrm{O}{\mathrm{C}}_2{\mathrm{H}}_5$ is ethyl methyl ether
- IUPAC names treat the smaller group as an alkoxy substituent: $\mathrm{C}{\mathrm{H}}_3\mathrm{O}{\mathrm{C}}_2{\mathrm{H}}_5$ is methoxyethane
- ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}{\mathrm{C}}_2{\mathrm{H}}_5$ is ethoxyethane, and ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OC}{\mathrm{H}}_3$ is methoxybenzene, or anisole
- Symmetrical ethers have identical groups; unsymmetrical ethers have different groups

Williamson's synthesis:

$$\mathrm{R}-\mathrm{X}+{\mathrm{R}}^{\prime }-{\mathrm{O}}^{-}\mathrm{N}{\mathrm{a}}^{+}\to \mathrm{R}-\mathrm{O}-{\mathrm{R}}^{\prime }+\mathrm{NaX}$$

- The alkoxide ion is a strong nucleophile that attacks the alkyl halide by SN2
- It makes both symmetrical and unsymmetrical ethers

Worked example 1. Methyl bromide with sodium ethoxide gives methoxyethane: $\mathrm{C}{\mathrm{H}}_3\mathrm{Br}+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{ONa}\to \mathrm{C}{\mathrm{H}}_3\mathrm{O}{\mathrm{C}}_2{\mathrm{H}}_5+\mathrm{NaBr}$.

Worked example 2 — choosing the right pair for tert-butyl methyl ether:

- Correct: sodium tert-butoxide with methyl bromide, $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{CONa}+\mathrm{C}{\mathrm{H}}_3\mathrm{Br}\to (\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{COC}{\mathrm{H}}_3+\mathrm{NaBr}$
- Wrong: tert-butyl bromide with sodium methoxide, which gives mainly 2-methylpropene, because the strongly basic methoxide causes elimination at the crowded carbon

Aryl ethers. Sodium phenoxide with an alkyl halide gives an alkyl aryl ether, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{ONa}+\mathrm{C}{\mathrm{H}}_3\mathrm{I}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OC}{\mathrm{H}}_3+\mathrm{NaI}$; aryl halides cannot be used, because they resist nucleophilic substitution.

An everyday example. Anaesthetists in Indian hospitals use halogenated ethers such as isoflurane, whose ether backbone keeps them volatile and fairly unreactive in the body.

The substance. In Williamson's synthesis the halide must be primary — secondary and tertiary halides undergo elimination with strongly basic alkoxides instead of substitution.

Ethers are polar but cannot hydrogen bond with each other, so they boil far lower than alcohols of similar mass while dissolving in water about as well; chemically they are fairly unreactive, but hydrogen iodide cleaves their C-O bond, and aromatic ethers undergo electrophilic substitution.

Physical properties:

- The C-O-C bond angle is about 111.7° in dimethyl ether, giving a small dipole moment
- Boiling points are close to those of alkanes of similar mass and far below alcohols: ethoxyethane boils at about 308 K, while butan-1-ol, with the same formula, boils at about 390 K
- Solubility in water resembles that of alcohols, because water can hydrogen bond to the ether oxygen

Cleavage by hydrogen halides:

$$\mathrm{R}-\mathrm{O}-\mathrm{R}+\mathrm{HX}\to \mathrm{R}-\mathrm{X}+\mathrm{R}-\mathrm{OH}$$

- Reactivity: HI > HBr > HCl
- With a methyl and a primary or secondary group, iodide attacks the less hindered methyl carbon by SN2, giving methyl iodide
- With a tertiary group, the reaction goes by SN1 and gives the tertiary iodide
- Alkyl aryl ethers give phenol and an alkyl halide, because resonance strengthens the aryl C-O bond: ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OC}{\mathrm{H}}_3+\mathrm{HI}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}+\mathrm{C}{\mathrm{H}}_3\mathrm{I}$

Worked example. Methoxyethane with HI gives methyl iodide and ethanol, while $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{COC}{\mathrm{H}}_3$ gives 2-iodo-2-methylpropane and methanol, because its tertiary carbocation forms easily.

Electrophilic substitution in anisole. The $-\mathrm{OC}{\mathrm{H}}_3$ group activates the ring and directs to ortho and para positions: bromine in ethanoic acid gives mainly 4-bromoanisole without a catalyst, and nitration and Friedel-Crafts reactions give 2- and 4-substituted products.

An everyday example. Bottles of diethyl ether in college laboratories are kept well away from burners, because the liquid is so volatile and flammable that its vapour can drift along a bench to a flame.

The substance. Ethers slowly form explosive peroxides on standing in air — which is why old bottles must be tested before they are distilled.