Oxirane Functional Group
The oxirane functional group is a subset of organic cyclic ether compounds. "Cyclic" refers to the fact that the oxirane atoms--two carbons and an oxygen in a three-membered ring--and "ether" that indicates that there is a carbon-oxygen-carbon bonding arrangement. More commonly known as epoxides, the oxirane group is an essential component of many industrial products and processes and is a common functional group introduced during the metabolism of many compounds.
What makes epoxides so useful in synthetic chemistry is the high strain of the ring structure. In this functional group, there is an oxygen atom tenuously connected to two neighboring carbon atoms by single bonds, making a saturated, three-sided ring. The resulting strain on the bonds weakens the carbon-oxygen bond, allowing the epoxide to readily break open the three-membered ring.
Epoxidation, the conversion of an olefin (an unsaturated alkene hydrocarbon) to a cyclic ether through reaction with hydrogen peroxide, a peracid, or oxygen, is an important chemical technique. The process produces such key industrial chemicals as ethylene oxide, the simplest epoxide. Ethylene oxide is a disinfectant, fumigant, and sterilizer, and its derivatives have a wide range of industrial and commercial uses. For instance, ethylene glycol (formed by adding water to, or hydrolyzing, ethylene oxide) is used for antifreeze formulations and making polyester fibers; polyethylene glycol goes into cosmetics, lubricants, paints, and drugs; ethylene glycol ethers are useful for brake fluids and solvents; and ethanolamine (formed by adding ammonia, or ammonolyzing, ethylene oxide) is often an ingredient in detergents and soaps.
One of the biggest uses of epoxides is in the manufacture of epoxy resins. These high-strength polymers are created by the reaction of less-rigid, one-dimensional polymers with epoxides to produce a much more rigid three-dimensional polymer. These resins all contain the characteristic three-sided ring of the oxirane functional group.
Epoxy resins, or epoxies, are ubiquitous in such industries as electronics, where they are often used to encapsulate or embed components; boating and automotive manufacture, where they serve as protective coatings and sealants; and construction, where they form many kinds of tough, durable structures. Epoxy resins are popular in general because of their low shrinkage rate, strength, excellent adhesion, and resistance to chemicals.
Epoxides are so useful and sought after commercially that chemists have invented a number of processes to create them. For many years, chlorohydrins treated with alkalies were the standard source, but demand for epoxides has increased to keep pace with the development of many new industrial applications. This demand has led to significant developments in oxidation techniques, although direct oxidation using oxygen is commercially viable only for forming ethylene oxide from ethylene. Epichlorohydrin is an important epoxide whose creation comes mainly from hydrochlorination of allyl chloride. Another major epoxide, propylene oxide, comes from propene reacting with organic hydroperoxides (more reactive equivalents of hydrogen peroxides). Many epoxides still come from hydrogen peroxide because it is inexpensive and a good donor of oxygen atoms, but most often epoxides are made by adding an oxygen atom to the double bond of an olefin.
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