Epoxides

Introduction

Fig.1. Structure of epoxides

Epoxides are a class of cyclic ether compounds with three-atom rings which approximate an equilateral triangle, which make them strained and hence more highly reactive than other ethers. The fundamental structure of epoxides contains two carbon atoms of a hydrocarbon attached to an oxygen atom. Epoxides are electrophilic by virtue of the strained three-membered ring system; nucleophilic attack at carbon releases the ring strain.

A compound containing the epoxide functional group can be called an epoxy, epoxide, oxirane, and ethoxyline. Thus, the epoxide of ethylene (C₂H₄) is ethylene oxide (C₂H₄O). Some names emphasize the presence of the epoxide functional group, as in the compound 1,2-epoxyheptane, which can also be called 1,2-heptene oxide. The dominant epoxides industrially are ethylene oxide and propylene oxide, which are produced on the scales of approximately 15 and 3 million tonnes/year respectively. In general, low molecular weight epoxides are colourless and nonpolar, and often volatile.

Synthesis

• Oxidation of alkenes with peroxycarboxylic acids: One of the best laboratory preparations of epoxides involves the direct oxidation of alkenes with peroxycarboxylic acids which are a class of carboxylic acids that contain hydroperoxy groups instead of hydroxyl groups (Fig 2).

Fig.2. Oxidation of alkenes with peroxycarboxylic acids

• Olefin oxidation using metal catalysts: Metal complexes are useful catalysts for epoxidations, for example, modified heterogeneous silver catalysts are typically employed.

• Intramolecular S_N2 substitution: This method involves the process of dehydrohalogenation, so it can be regard as a variant of the Williamson ether synthesis. In this case, an alkoxide ion intramolecularly displaces chloride. Starting with propylene chlorohydrin, most of the world's supply of propylene oxide arises via this route.

Fig.3. Intramolecular S_N2 substitution

• Biosynthesis: Epoxides are uncommon in nature and they can be obtained usually via oxygenation of alkenes by the action of cytochrome P450.

Reactions

Epoxides are easily opened, under acidic or basic conditions (Fig 4), to give a variety of products with useful functional groups. Ring-opening reactions dominate the reactivity of epoxides, so they are potent electrophiles. Alcohols, water, amines, thiols and many other reagents can serve as the nucleophile for this reaction. This reaction is the basis of the formation of epoxy glues and the production of glycols.

Fig.4. Ring-opening reactions of epoxides

Applications

Epoxides are produced on a large scale for many applications. First of all, Epoxides are important organic intermediates and are widely used in the field of organic synthesis. In addition, epoxides can be used to assemble various polymers known as epoxies, which are excellent adhesives and useful surface coatings. Furthermore, epoxides can also be applied to generate detergents and surfactants.