Structure and Nomenclature

Ethers are compounds in which an oxygen atom joins two alkyl or aryl groups, R–O–R′. This lesson explains their bent structure around an sp³ oxygen atom, their classification, their common and IUPAC names, and the isomerism they show.

In an ether the oxygen atom is sp³ hybridised, with two bond pairs and two lone pairs, so the C–O–C unit is bent; in methoxymethane the angle is 111.7°, slightly more than the tetrahedral angle because the two alkyl groups repel each other. Ethers are simple (symmetrical) when both groups are the same and mixed (unsymmetrical) when they differ, and aliphatic or aromatic depending on the groups. Common names list the two groups in alphabetical order followed by the word ether, as in ethyl methyl ether. IUPAC names treat ethers as alkoxyalkanes: the larger group is the parent and the smaller one, with the oxygen, is the alkoxy substituent, as in methoxyethane. Ethers are functional isomers of alcohols and also show metamerism.

What an ether is

An ether has the general formula R–O–R′, where R and R′ are alkyl or aryl groups. The functional group is the oxygen atom bonded to two carbon atoms, –O–, called the ether linkage. An ether can be thought of as water with both hydrogen atoms replaced by carbon groups, or as an alcohol whose –OH hydrogen has been replaced by a carbon group.

Structure

• The oxygen atom is sp³ hybridised: two of its four electron pairs form σ bonds to carbon, and two are lone pairs. The four pairs are arranged roughly tetrahedrally, so the C–O–C unit is bent.
• In methoxymethane the C–O–C bond angle is 111.7°, slightly greater than the tetrahedral angle of 109.5°, because the two bulky alkyl groups repel each other. Compare water (104.5°), where the small hydrogen atoms allow the lone pairs to squeeze the angle.
• The C–O bond length is about 141 pm, almost the same as in alcohols.
• Because the C–O bonds are polar and the molecule is bent, an ether has a small net dipole moment. It has no O–H bond, so ether molecules cannot form hydrogen bonds with one another.

Classification

• Simple or symmetrical ethers: both groups are the same, as in methoxymethane (CH₃OCH₃) and ethoxyethane (C₂H₅OC₂H₅).
• Mixed or unsymmetrical ethers: the groups differ, as in methoxyethane (CH₃OC₂H₅) and methoxybenzene (C₆H₅OCH₃).
• Aliphatic ethers have two alkyl groups; aromatic ethers have at least one aryl group, either alkyl aryl ethers such as anisole or diaryl ethers such as diphenyl ether.

Common names

Name the two groups as separate words in alphabetical order and add the word ether; if both groups are the same, use the prefix di-. Diethyl ether is often called simply ether.

IUPAC names

Ethers are named as alkoxyalkanes:

1. Choose the larger group as the parent hydrocarbon.
2. Name the smaller group, together with the oxygen, as an alkoxy substituent: CH₃O– is methoxy, C₂H₅O– is ethoxy and C₆H₅O– is phenoxy.
3. Number the parent chain so that the substituents, the alkoxy group among them, get the lowest possible locants, and list all substituents in alphabetical order.

Worked naming

• CH₃–O–CH₂–CH(CH₃)–CH₃: the larger group, 2-methylpropyl, gives the parent propane, numbered from the carbon bonded to oxygen. The methoxy group is on C-1 and a methyl group on C-2, so the name is 1-methoxy-2-methylpropane (methoxy comes before methyl in alphabetical order).
• CH₃–O–C(CH₃)₃: the parent is 2-methylpropane, with the methoxy group on C-2, giving 2-methoxy-2-methylpropane.
• C₂H₅–O–CH(CH₃)–CH₂CH₃: the larger group is a four-carbon chain with oxygen on its second carbon, giving 2-ethoxybutane.

Isomerism in ethers

• Functional isomerism: ethers and alcohols with the same number of carbon atoms share a molecular formula. C₂H₆O is both ethanol and methoxymethane; C₃H₈O is propan-1-ol, propan-2-ol and methoxyethane.
• Metamerism: ethers with the same formula can have the carbon atoms divided differently between the two sides of the oxygen. C₄H₁₀O has three ethers: ethoxyethane (2 + 2 carbons), 1-methoxypropane and 2-methoxypropane (1 + 3 carbons). Ethoxyethane is a metamer of the two methoxypropanes.
• Counting all structures, C₄H₁₀O has seven structural isomers: four alcohols (butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol) and three ethers.