11.8 : Structure and Nomenclature of Epoxides

Cyclic analogs of ethers with an oxygen atom incorporated in the ring are called cyclic ethers. The general formula for cyclic ethers is (CH₂)_nO, where n represents the number of carbon atoms in the ring. Accordingly, cyclic ethers can be three-membered, four-membered, five-membered, or six-membered ring systems.

The three-membered cyclic ethers are called epoxides. Their basic structure consists of one oxygen atom and two carbon atoms.

The three-membered ring system approximates an equilateral triangle and is highly strained.

The strain arises due to the deviation of the interior bond angles from the ideal tetrahedral angle, that is, 109.5°, leading to the compression of bond angles. This ring strain makes epoxides highly reactive as compared to other ethers.

Epoxides derive their common names from the name of the parent alkene from which they are formed, followed by the word “oxide”. For example, ethylene oxide, which is the simplest epoxide, derives its name from its parent alkene — ethylene.

Other examples include cis-2-butene oxide, and cyclopentene oxide, formally derived from cis-2-butene and cyclopentene, respectively.

In the IUPAC nomenclature, the simplest epoxide, with no substitutions on the carbon atoms, is named oxirane. Substituted epoxides can have up to four R groups and are named as derivatives of oxiranes.

For example, (2R,3S)-2,3-dimethyloxirane and (2R,3S)-2,3-diethyl-2,3-dimethyloxirane have two and four substituents, respectively. The numbering of the oxirane starts from the oxygen atom, and the substituents are listed alphabetically, followed by the word oxirane.

In cases where the epoxide is present as a part of another hydrocarbon system, the prefix “epoxy” is added before the parent’s name.

For example, 1,2-epoxycyclopentane has the epoxide ring as part of another ring system. Similarly, 2,3-dimethyl-2,3-epoxy pentane has the epoxide as part of a straight hydrocarbon chain.

The parent chain is numbered in such a way that epoxy carbon atoms receive the lowest possible numbers.