Ethers: Preparation, Reactions and Uses: Quick Revision

A brief note on Ethers: Preparation, Reactions and Uses. Read it before a test. Try each question at the end before you open its answer.

Structure and names

• An ether is $\text{R}-\text{O}-\text{R}'$. The shorter group is named as alkoxy: methoxyethane, ethoxyethane, methoxybenzene (anisole).
• Ethers and alcohols of the same formula are functional isomers. $\text{C}_{4}\text{H}_{10}\text{O}$ has 3 ethers and 4 alcohols.
• No O-H bond, so no hydrogen bond between ether molecules. Boiling points are close to those of alkanes of equal molar mass.

Preparation

• Dehydration of a primary alcohol: concentrated $\text{H}_{2}\text{SO}_{4}$, 413 K, excess alcohol. At 443 K the product is the alkene.
• Williamson synthesis: alkoxide plus a methyl or primary halide ($S_N2$). A tertiary halide gives an alkene. Put the bulky group on the alkoxide.

Reactions

• Cleavage by HI: iodide attacks the less crowded carbon ($S_N2$), or the tertiary carbon ($S_N1$). Aryl alkyl ethers give a phenol and an alkyl iodide. Excess HI gives two alkyl iodides from an alkyl ether.
• Aryl ethers: OR activates the ring and directs to ortho and para. Bromine in ethanoic acid needs no catalyst. Friedel-Crafts and nitration both work, with the para isomer in the larger amount.

Uses and safety

• Solvents (ethoxyethane, THF), fuel additive (MTBE), epoxides, crown ethers that bind metal ions.
• Ether vapour is flammable (1.9 to 36 percent by volume) and about 2.6 times as dense as air. Old ether holds peroxides: test with acidified potassium iodide, remove with iron(II) sulphate, and never distil to dryness.

A worked example

Find the products when $\text{CH}_{3}\text{CH}_{2}\text{OC}(\text{CH}_{3})_{3}$ is heated with hydrogen iodide.

1. The groups. One is primary (ethyl) and one is tertiary.
2. The rule. A tertiary group takes the iodide, by $S_N1$.
3. The products. 2-Iodo-2-methylpropane and ethanol. With excess HI, the ethanol is converted to iodoethane.

Practice with new numbers

A program to try

The program says which group of each ether becomes the iodide with hydrogen iodide.

Mistakes to avoid

• Using a tertiary halide in the Williamson synthesis.
• Writing iodobenzene as a product of the cleavage of anisole.
• Forgetting that old ether holds peroxides.
• Making a mixed ether by dehydrating two alcohols.
• Forgetting that aryl ethers are substituted at ortho and para positions.

Check yourself

1. How is methoxyethane made by the Williamson synthesis?

2. What do HI and anisole give?

3. Why is ether vapour a hazard at the floor of a laboratory?

4. Which isomer of bromoanisole is the main product of the bromination of anisole?

Sources

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