Naming ethers and making them

The oxygen between two carbons

Alcohols have an O-H bond. Ethers have none. An ether has an oxygen atom bonded to two carbon groups, written $\text{R}-\text{O}-\text{R}'$. The two groups can be the same (a simple or symmetrical ether, such as ethoxyethane) or different (a mixed ether, such as methoxyethane).

The oxygen is $sp^3$. The C-O-C angle in methoxymethane is $111.7^\circ$, a little wider than the C-O-H angle of an alcohol, because the two bulky groups push each other apart. The C-O bond is about $141\ \text{pm}$ long. The molecule is bent, and the two C-O bond dipoles add up to a small net dipole.

Naming ethers

In the IUPAC name the shorter group is named as an alkoxy group, and the longer group is the parent chain.

• $\text{CH}_{3}\text{OCH}_{3}$: methoxymethane (dimethyl ether).
• $\text{CH}_{3}\text{OC}_{2}\text{H}_{5}$: methoxyethane (ethyl methyl ether).
• $\text{C}_{2}\text{H}_{5}\text{OC}_{2}\text{H}_{5}$: ethoxyethane (diethyl ether).
• $\text{CH}_{3}\text{OCH}(\text{CH}_{3})_{2}$: 2-methoxypropane (isopropyl methyl ether).
• $\text{C}_{6}\text{H}_{5}\text{OCH}_{3}$: methoxybenzene (anisole).
• $\text{C}_{6}\text{H}_{5}\text{OC}_{2}\text{H}_{5}$: ethoxybenzene (ethyl phenyl ether).

A ring with an oxygen atom is a cyclic ether: oxirane (ethylene oxide) and tetrahydrofuran (THF) are two examples.

Isomers of an ether

An ether and an alcohol with the same formula are functional isomers. Ethanol and methoxymethane both have the formula $\text{C}_{2}\text{H}_{6}\text{O}$. Two ethers with the same formula and different groups on the oxygen are metamers: methoxypropane, ethoxyethane and 2-methoxypropane all have the formula $\text{C}_{4}\text{H}_{10}\text{O}$. Together with the four butanols that makes seven isomers.

Making ethers

1. Dehydration of an alcohol. A primary alcohol is heated with concentrated sulphuric acid at $413\ \text{K}$ with the alcohol in excess.

One alcohol is protonated. A second alcohol molecule attacks the protonated one, and water leaves. At $443\ \text{K}$ with excess acid, ethene forms in place of the ether. A secondary or tertiary alcohol gives an alkene instead, because it loses a proton easily. Two different alcohols give a mixture of three ethers, so this method makes only simple ethers.

2. The Williamson synthesis. This is the more general route. A sodium alkoxide (or phenoxide) is made from an alcohol and sodium, and it is then heated with an alkyl halide.

The alkoxide ion attacks the carbon of the halide, and the halide leaves, in one step: an $S_N2$ reaction. This works best when the halide is a methyl or a primary alkyl halide, because the attack needs a carbon that is not crowded. A secondary or tertiary halide meets a strong base and loses HX to give an alkene instead.

So choose the route with care. To make tert-butyl methyl ether, use sodium tert-butoxide with iodomethane or bromomethane. Sodium methoxide with a tertiary halide would give 2-methylpropene.

Aryl ethers come the same way. Sodium phenoxide with iodomethane gives anisole: $\text{C}_{6}\text{H}_{5}\text{ONa} + \text{CH}_{3}\text{I} \to \text{C}_{6}\text{H}_{5}\text{OCH}_{3} + \text{NaI}$. A haloarene cannot be the halide, because an $S_N2$ attack on an $sp^2$ carbon of a ring does not happen.

At 70 percent yield, 20 g of ethanol makes about 14 g of ether.

Practise with new numbers

Try it in Python

The program counts the alcohols and the ethers for each formula $\text{C}_n\text{H}_{2n+2}\text{O}$ by building every alkyl group, then pairing them.

Common mistakes

• Using the tertiary halide in a Williamson synthesis. It gives an alkene. Put the bulky group on the alkoxide.
• Making a mixed ether by dehydrating two alcohols. The result is a mixture of three ethers.
• Naming from the longer group as the alkoxy group. The longer group is the parent.
• Dehydrating a tertiary alcohol to an ether. It gives an alkene.
• Using a haloarene in the Williamson synthesis. An aryl halide does not undergo $S_N2$.

Remember these

• An ether is R-O-R'. Name: alkoxy group (shorter) plus the parent alkane (longer).
• Ethers and alcohols of the same formula are functional isomers.
• Dehydration of a primary alcohol: concentrated $\text{H}_{2}\text{SO}_{4}$, 413 K, excess alcohol.
• Williamson: alkoxide plus methyl or primary halide ($S_N2$).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OCH}_{3}$ and give its common name.

2. How would you make ethoxypropane by a Williamson synthesis?

3. Why does the Williamson synthesis of 2-methoxy-2-methylpropane fail with sodium methoxide and 2-bromo-2-methylpropane?

4. What is formed when ethanol is heated with concentrated sulphuric acid at 413 K, and at 443 K?

References

• NCERT Chemistry Part II (Class 12), Chapter 7, the sections on nomenclature and preparation of ethers.
• Morrison and Boyd, Organic Chemistry, the chapter on ethers.
• OpenStax Chemistry 2e, Chapter 20, section 20.2 on alcohols and ethers.