Alcohols and Ethers

Lessons

Senior 5 · Chemistry

Alcohols and Ethers

5 lessons

Lesson 5 of 5

Ethers

22 min

The operation that did not hurt

On 16 October 1846, in the operating theatre of Massachusetts General Hospital in Boston, the dentist William Morton held a glass flask of ether vapour to the face of a young man, Gilbert Abbott. When Abbott was unconscious, the surgeon John Collins Warren removed a tumour from his neck, and Abbott later said he had felt no pain. The room is still called the Ether Dome. Within months, ethoxyethane ("ether") was used for surgery around the world. It has since been replaced by safer anaesthetics, many of them fluorinated ethers, but ethers remain some of the most useful solvents in chemistry.

Definition

Ether: a compound in which an oxygen atom is bonded to two alkyl (or aryl) groups, R–O–R'. Ethers have the general formula $\mathrm{C_nH_{2n+2}O}$ , the same as alcohols. Example: ethoxyethane, $\mathrm{C_2H_5OC_2H_5}$ .

Alkoxy group: an alkyl group joined through oxygen, RO–, named from the alkyl group: $\mathrm{CH_3O{-}}$ methoxy, $\mathrm{C_2H_5O{-}}$ ethoxy.

Metamerism: structural isomerism in which the functional group stays the same but the alkyl groups on either side of it differ. Example: 1-methoxypropane and ethoxyethane.

Structure and names

Figure 5.1: Ethers. The C–O–C unit is bent, like the H–O–H of water, and the oxygen has two lone pairs but no O–H bond.

The oxygen is $s p^{3}$ hybridised: two bonds and two lone pairs, so the C–O–C unit is bent, with an angle close to 110°. To name an ether, take the larger alkyl group as the parent alkane and name the smaller part, with its oxygen, as an alkoxy prefix: $\mathrm{CH_3OCH_2CH_3}$ is methoxyethane; $\mathrm{CH_3OCH(CH_3)_2}$ is 2-methoxypropane. Traditional names list the two alkyl groups: diethyl ether, ethyl methyl ether. In a ring, oxygen-containing ethers have their own names: oxolane (tetrahydrofuran, THF) and epoxyethane (oxirane).

Isomerism. Ethers show metamerism and position isomerism among themselves, and are functional-group isomers of alcohols: $\mathrm{C_4H_{10}O}$ has three ethers (ethoxyethane, 1-methoxypropane, 2-methoxypropane) and four alcohols.

Physical properties

Compound	$M_{r}$	Boiling point / °C	Solubility in water
pentane	72	36	insoluble
ethoxyethane	74	35	about 7 g per 100 g
butan-1-ol	74	118	about 8 g per 100 g

Ether molecules have no O–H group, so they cannot hydrogen-bond to each other: their boiling points are close to those of alkanes of the same mass. But the oxygen lone pairs can accept hydrogen bonds from water, so small ethers are slightly soluble. Ethers dissolve a wide range of organic compounds, react with very little, and are less dense than water, which makes them ideal solvents for extracting compounds from water.

Preparation

Dehydration of alcohols: excess alcohol with concentrated sulfuric acid at about 140 °C (Lesson 3): $\mathrm{2C_2H_5OH \rightarrow C_2H_5OC_2H_5 + H_2O}$ . This gives only symmetrical ethers; at 170 °C the alkene forms instead.
Williamson synthesis: a sodium alkoxide, made from the alcohol and sodium, is warmed with a halogenoalkane. The alkoxide ion is the nucleophile in an SN2 reaction (Unit 4): $\mathrm{C_2H_5O^-Na^+ + CH_3I \rightarrow C_2H_5OCH_3 + NaI}$ . Any ether, symmetrical or not, can be made this way, as long as the halogenoalkane is methyl or primary; a tertiary halide is attacked by the strongly basic alkoxide as a base, and an alkene forms.

Figure 5.2: Planning a Williamson synthesis. The halide must be one that undergoes SN2.

Chemical properties

Low reactivity. With no O–H bond, ethers do not react with sodium or with phosphorus(V) chloride, and are not oxidised by acidified dichromate. These tests tell an ether from its alcohol isomers.
Combustion. Ethoxyethane is extremely flammable: its vapour is heavier than air, can travel along a bench to a flame, and ignites easily. $\mathrm{C_2H_5OC_2H_5(l) + 6O_2(g) \rightarrow 4CO_2(g) + 5H_2O(l)}$ .
Basic lone pairs. Ethers dissolve in cold concentrated sulfuric acid by accepting $\mathrm{H^+}$ on the oxygen, which alkanes cannot do.
Cleavage by hot concentrated HI (or HBr): $\mathrm{C_2H_5OC_2H_5 + HI \rightarrow C_2H_5OH + C_2H_5I}$ ; with excess HI the alcohol is also converted, giving two molecules of iodoethane.
Peroxides. On standing in air and light, ethers slowly form explosive peroxides. Old bottles must be tested, and an ether must never be distilled to dryness.

Uses

Solvents for extraction and for making Grignard reagents (which need a dry, unreactive solvent); fluorinated ethers such as sevoflurane as modern anaesthetics; 2-methoxy-2-methylpropane (MTBE) as a petrol additive that helps fuel burn cleanly, now limited in some countries because leaks pollute groundwater; methoxymethane as an aerosol propellant and as a clean-burning fuel.

Worked example

Problem. Give the names of the three ethers $\mathrm{C_4H_{10}O}$ , and describe one chemical test that distinguishes ethoxyethane from its isomer butan-1-ol.

Solution. Ethoxyethane, $\mathrm{C_2H_5OC_2H_5}$ ; 1-methoxypropane, $\mathrm{CH_3OCH_2CH_2CH_3}$ ; 2-methoxypropane, $\mathrm{CH_3OCH(CH_3)_2}$ . Test: add a small piece of sodium to each dry liquid. Butan-1-ol fizzes, giving hydrogen; ethoxyethane does not react, because it has no O–H bond. (Alternatively, $\mathrm{PCl_5}$ gives steamy HCl fumes only with the alcohol, and acidified dichromate turns green only with the alcohol.)

Worked example

Problem. A student wants to make 2-methoxy-2-methylpropane (MTBE) by a Williamson synthesis. Evaluate the two possible pairs of reagents (Figure 5.2).

Solution. The ether can be split either side of the oxygen. Route A, sodium 2-methylpropan-2-oxide with iodomethane, puts the halogen on a methyl carbon, which is easily attacked from the back by SN2: the ether forms. Route B, sodium methoxide with 2-bromo-2-methylpropane, puts the halogen on a tertiary carbon, which is blocked to SN2; the methoxide acts as a base instead, removing $\mathrm{H^+}$ and giving 2-methylpropene by elimination. Route A should be chosen.

Warning

Safety. Ethoxyethane boils at 35 °C and its vapour ignites very easily: use it only in a fume cupboard with no flames or hot plates nearby, and keep bottles tightly closed, cool and dark. Breathing the vapour causes drowsiness and unconsciousness, which is why it was once an anaesthetic.

Tip

Alcohol or ether? Look for O–H. Alcohols have it: hydrogen bonding, high boiling point, reaction with Na and $\mathrm{PCl_5}$ . Ethers do not: low boiling point and no reaction.

Professor Dave Explains: Williamson Ether Synthesis (8 min). Watch how an alkoxide attacks a halogenoalkane by SN2, and why the halide should be methyl or primary.

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Practice

Name: (a) $\mathrm{CH_3OCH_2CH_2CH_3}$ ; (b) $\mathrm{CH_3CH_2OCH(CH_3)_2}$ ; (c) $\mathrm{CH_3OCH_3}$ .
Explain why ethoxyethane boils at 35 °C while its isomer butan-1-ol boils at 118 °C.
Explain why ethoxyethane is slightly soluble in water although it cannot hydrogen-bond to itself.
Write an equation for the Williamson synthesis of methoxyethane from sodium ethoxide.
Why does dehydration of a mixture of methanol and ethanol give a poor yield of methoxyethane?
Write equations for the reaction of ethoxyethane with (a) one mole of hot concentrated HI; (b) excess HI.
Give two hazards of ethoxyethane and one precaution for each.

Show answersHide answers

(a) 1-methoxypropane; (b) 2-ethoxypropane; (c) methoxymethane.
Butan-1-ol molecules form hydrogen bonds (O–H···O); ether molecules have no O–H, so only weaker dipole–dipole and London forces must be overcome.
The oxygen lone pairs accept hydrogen bonds from water molecules.
$\mathrm{C_2H_5O^-Na^+ + CH_3I \rightarrow C_2H_5OCH_3 + NaI}$ (or with bromomethane).
Three ethers form: methoxymethane, methoxyethane and ethoxyethane, as each alcohol can combine with itself or with the other.
(a) $\mathrm{C_2H_5OC_2H_5 + HI \rightarrow C_2H_5OH + C_2H_5I}$ ; (b) $\mathrm{C_2H_5OC_2H_5 + 2HI \rightarrow 2C_2H_5I + H_2O}$ .
Extremely flammable (no flames, fume cupboard); forms explosive peroxides (keep dark and cool, never distil to dryness); anaesthetic vapour (good ventilation).

1The operation that did not hurt

2Structure and names

3Physical properties

4Preparation

5Chemical properties