Aryl ethers, uses and safety

Anisole on the ring

An aryl ether such as anisole, $\text{C}_{6}\text{H}_{5}\text{OCH}_{3}$, has an oxygen next to the ring. The oxygen has lone pairs, and one pair overlaps with the ring and pushes electron density into it. The alkoxy group is an activating, ortho, para directing group, like OH but a little weaker. The reactions run faster than for benzene, and the new group goes to the ortho or para carbon. The para isomer is in the larger amount, because the ortho position is crowded.

• Bromination. Bromine in ethanoic acid acts on anisole even without an iron(III) bromide catalyst. The para isomer, 4-bromoanisole, is the main product, with some ortho isomer.
• Friedel-Crafts alkylation. Chloromethane with anhydrous aluminium chloride gives a mixture of 2-methylanisole and 4-methylanisole.
• Friedel-Crafts acylation. Ethanoyl chloride with anhydrous aluminium chloride gives 4-methoxyacetophenone as the major product, with some 2-methoxyacetophenone.
• Nitration. A mixture of concentrated nitric acid and concentrated sulphuric acid gives 2-nitroanisole and 4-nitroanisole, mainly the para isomer.

What ethers are used for

• Solvent. Ethoxyethane (diethyl ether) dissolves fats, oils and many organic compounds, and boils so low that it is easy to remove. It is used to extract organic compounds from water. Tetrahydrofuran (THF) is a solvent for polymers and for hydroboration and Grignard reactions.
• Anaesthetic. Diethyl ether was one of the first general anaesthetics. It has been replaced by safer substances, because it is flammable and it makes the patient sick.
• Fuel additive. Methyl tert-butyl ether (MTBE) was added to petrol to raise its octane number. It is now banned or restricted in some countries because it pollutes ground water.
• Aerosol propellant and fuel. Methoxymethane (dimethyl ether) is a gas that liquefies under mild pressure.
• Epoxides. Ethylene oxide, a strained cyclic ether, makes ethane-1,2-diol and is used to sterilise medical equipment.
• Crown ethers. 18-Crown-6 is a ring of oxygen atoms and $\text{CH}_2\text{CH}_2$ units. The ring binds a potassium ion in its centre. The outside is hydrocarbon, so the complex dissolves in organic solvents, and it carries salts there.

Using ethers safely

Fire. Ethoxyethane has a flash point near $-45\ ^\circ\text{C}$. It catches fire from a spark at any ordinary temperature. Its vapour is about 2.6 times as dense as air, so it flows along a bench or a floor to a distant flame or hot plate. The vapour in air burns between about 1.9 and 36 percent by volume. Work in a fume cupboard, with no flame or hot surface nearby. A spark from static electricity is enough.

Peroxides. In air and light, an ether slowly forms a hydroperoxide at a C-H bond next to the oxygen. For diethyl ether the product is $\text{CH}_{3}\text{CH}(\text{OOH})\text{OC}_{2}\text{H}_{5}$. Peroxides are less volatile than the ether and collect in the bottle. When the ether is distilled or evaporated, they collect in the residue, and a shock or heat can set them off in an explosion.

• Buy small bottles, write the date of opening on each, and store them in the dark.
• Test an old bottle by shaking it with acidified potassium iodide: peroxide releases iodine, which is yellow-brown.
• Remove peroxide by shaking with iron(II) sulphate solution, or by passing the ether over activated alumina.
• Never distil an ether to dryness.

Spill 600 g of ether in a room of 35 m³. The air then holds about 0.57 percent, well below the lower limit of 1.9 percent. About 2 kg is needed to reach it. Raise the mass or shrink the room and the air crosses the limit.

Practise with new numbers

Try it in Python

The program uses the ideal gas law to find the concentration of ether vapour after a spill, and the mass that would reach the lower limit.

Common mistakes

• Expecting the Lewis acid to be needed for bromination of anisole. The ring is activated.
• Forgetting that the para isomer is the major product. The ortho position is crowded.
• Thinking the ether vapour rises away. It is heavier than air and flows down.
• Using old ether without a peroxide test. Never distil it to dryness.
• Saying ethers are unreactive and so harmless. They burn easily and form peroxides.

Remember these

• OR is activating and ortho, para directing. Para usually dominates.
• Anisole: bromine in ethanoic acid, Friedel-Crafts with $\text{AlCl}_{3}$, and nitration all work.
• Diethyl ether vapour is flammable (1.9 to 36 percent), heavy, and sparks can set it off.
• Old ether holds peroxides: test with potassium iodide, remove with iron(II) sulphate, never distil to dryness.

Check yourself

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

1. Give the main product when anisole is treated with bromine in ethanoic acid.

2. Why does anisole react faster than benzene?

3. Why should an old bottle of diethyl ether not be distilled to dryness?

4. Ether vapour from a spill collects at floor level. Why?

References

• NCERT Chemistry Part II (Class 12), Chapter 7, the sections on ethers.
• Morrison and Boyd, Organic Chemistry, the chapter on ethers.
• National Research Council, Prudent Practices in the Laboratory, the section on peroxide-forming chemicals.