(also epoxyethane or oxiran), the simplest epoxide.

A cyclic ether with an a-epoxide three-membered ring, ethylene oxide has the structural formula

It takes the form of a colorless gas with an ethereal odor, and it has a melting point of – 111.3°C, a boiling point of 10.7°C, and a density of 0.891 g/cm³ at 4°C. Ethylene oxide is readily soluble in water and such organic solvents as ethanol and ether. Highly flammable, it forms explosive mixtures with air when found in concentrations of 3 to 80 percent by volume.

The chemical properties of ethylene oxide are determined by the epoxide ring, which is strained and therefore relatively easy to open by the action of high temperatures or various chemical reagents. Thus, upon heating to 400°C (or 150°-300°C in the presence of Al₂O₃), ethylene oxide isomerizes into acetaldehyde. The hydrogenation of ethylene oxide over nickel at 80°C yields ethyl alcohol according to the reaction

Hydrohalogenation produces the corresponding ethylene halohydrin, as shown in the equation

In these and many analogous reactions, ethylene oxide acts as an alkylating agent for the introduction of the β-hydroxyethyl group, HOCH₂CH₂—. Because of this property, it is commonly used in industry and laboratory practice for the production of such valuable chemicals as ethylene cyanohydrin (by reaction with hydrocyanic acid); ethylene glycol and its monoethers, which are known as cellosolves (by hydration and alcoholysis); ethanolamines (by reaction with ammonia); β-mercaptoethanol (HSCH₂CH₂OH; by reaction with hydrogen sulfide); and β-phenylethyl alcohol (by the Friedel-Crafts reaction with benzene).

Polymerization, which also proceeds by means of the cleavage of a C—O bond, is also characteristic of ethylene oxide. Thus, passage of ethylene oxide vapor over NaHSO₄ at 110°–160°C yields the dimer dioxane. Ethylene oxide is readily polymerized—sometimes explosively—at ordinary temperatures by the catalytic action of tertiary amines or stannic chloride. Polyethylene oxides, [—CH₂—CH₂—O—]_n, have molecular weights ranging from 10² to 10⁷. A distinction is made between the polymers of low molecular weight (polyethylene glycols, which have a molecular weight of up to 40,000) and the polymers of high molecular weight (with a weight of 0.5–10 million). Those of low molecular weight are liquids or waxy products and are obtained by the catalytic polymerization of ethylene oxide at 100°–150°C. They are used in the textile industry as wetting agents, softeners, and anticlinging agents; they are also used in cosmetics and as detergent components. The polymers of high molecular weight are produced in industry by catalytic suspension polymerization at 20°–50°C. These polymers are solids with good thermoplastic and mechanical properties and some solubility in water. They are used as thickening agents in the textile industry and as flocculating agents (to reduce hydrodynamic resistance to water flow).

Upon condensation with higher alcohols (including oleyl, lauryl, and stearyl alcohol), alkylphenols (such as isooctylphenol), and fatty carboxylic acids, ethylene oxide yields polyethylene glycol ethers of the type RO(CH₂CH₂O)_n—H, where R is an organic radical and n is an integer with a value between 6 and 30, inclusive. These ethers are used as emulsifiers and detergent components. Crown ethers (cyclic ethers that are obtained from ethylene oxide) are commonly used in organic synthesis for separating the ion pairs of various salts in aprotic bipolar solvents; crown 18–6, for example, is used for complexing the potassium ion.

The principal industrial methods for the production of ethylene oxide are the dehydrochlorination of ethylene chlorohydrin and the catalytic oxidation of ethylene by atmospheric oxygen at 200°–300°C over a catalyst containing metallic silver. Ethylene oxide is an important reagent in fine organic synthesis; for example, the reaction of ethylene oxide with acetoacetic ester yields acetobutyrolactone, which is used in the production of vitamin B, and is an intermediate in the preparation of antimalarial agents.

Ethylene oxide is toxic. In small doses, it has a narcotic effect, and in large quantities it irritates the mucous membranes and causes asphyxiation and emphysema. The maximum permissible concentration of ethylene oxide in the air is 0.001 mg per liter.