Ethylene or ethene is the simplest alkene hydrocarbon, consisting of two carbon atoms and four hydrogens. There is a double bond between the two carbons. Because it contains a double bond, ethylene is called an unsaturated hydrocarbon or an olefin.
The molecule cannot twist around the double bond, and all six atoms lie in the same plane. The angle made by two carbon-hydrogen bonds in the molecule is 117°, very close to the 120° that would be predicted from ideal sp2 hybridization.
From 1795 on, ethylene was referred to as the olefiant gas (oil-making gas), because it combined with chlorine to produce the oil of the Dutch chemists (1,2-dichloroethane ), first synthesized in 1795 by a collaboration of four Dutch chemists.
In the mid-19th century, the suffix -ene (a Greek root added to the end of female names meaning "daughter of") was widely used to refer to a molecule or part thereof that contained one fewer hydrogen atoms than the word being modified. Thus, ethylene (C2H4) was the "daughter of ethyl" (C2H5). The name ethylene was used in this sense as early as 1852.
In 1866, the German chemist Augustus von Hofmann proposed a system of hydrocarbon nomenclature in which the suffixes -ane, -ene-, -ine, -one, and -une were used to denote the hydrocarbons with 0, 2, 4, 6, and 8 fewer hydogens than their parent alkane. In this system, ethylene became ethene. Hofmann's system eventually became the basis for the Geneva nomenclature approved by the International Congress of Chemists in 1892, which remains at the core of the IUPAC nomenclature. However, by that time, the name ethylene was deeply entrenched, and it remains in wide use today, especially in the chemical industry.
The double bond is a region of slightly higher electron density, and most of ethylene's chemistry involves other molecules reacting with and adding across its double bond. Ethylene can react with bromine, chlorine, and other halogens, to produce halogenated hydrocarbons. It can also react with water to produce ethanol but the rate at which this happens is very slow unless a suitable catalyst, such as phosphoric or sulfuric acid, is used. Under high pressure, and in the presence of a catalytic metal (platinum, rhodium, nickel), hydrogen will react with ethylene, saturating it.
Ethylene is produced in the petrochemical industry via steam cracking. In this process, gaseous or light liquid hydrocarbons are briefly heated to 750–950 °C, causing numerous free radical reactions to take place. Generally, in the course of these reactions, large hydrocarbons break down in to smaller ones and saturated hydrocarbons become unsaturated.
The result of this process is a complex mixture of hydrocarbons in which ethylene is one of the principal components. The mixture is separated by repeated compression and distillation.
Ethylene is used primarily as an intermediate in the manufacture of other chemicals, especially plastics. Ethylene may be polymerized directly to produce polyethylene (also called polyethene or polythene), the world's most widely used plastic. Ethylene can be chlorinated to produce 1,2-dichloroethane , a precursor to the plastic polyvinyl chloride, or combined with benzene to produce ethylbenzene, which is used in the manufacture of polystyrene, another important plastic.
Smaller amounts of ethylene are oxidized to produce chemicals including ethylene oxide, ethanol, and polyvinyl acetate.
Ethylene was once used as an inhaled anesthetic, but it has long since been replaced in this role by nonflammable gases.
Ethylene is a plant hormone. Small amounts of ethylene are used in agriculture to bring about the ripening of already-picked fruit. It also stimulates the opening of flowers, and the abscission of leaves. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate. See Ethylene (plant hormone)
Ethylene is also a widely used refrigerant in commercially ultra low temperature systems due to the low boiling point.