Ethylene

Ethylene (also called ethene) is a hydrocarbon with the formula C₂H₄ or H₂C=CH₂. It is a colourless, flammable gas that, in its pure form, has a faint sweet and musky smell. As the simplest alkene, it contains a carbon–carbon double bond.

In the chemical industry, ethylene is the most produced organic compound worldwide, with over 225 million tonnes manufactured in 2022. Most of this output goes into making polyethylene, a common plastic composed of long chains of ethylene units. This production process also releases significant greenhouse gases. Beyond industry, ethylene acts as a natural plant hormone and is used agriculturally to speed up fruit ripening. Its hydrate is ethanol.

The molecule consists of four hydrogen atoms bonded to two carbon atoms linked by a double bond. All six atoms lie in the same plane. The H–C–H bond angle is 117.4°, close to the 120° expected for sp² hybridized carbon. The double bond is relatively weak; rotating around the C–C bond requires only low energy, breaking the π-bond when heated to 50 °C. This π-bond gives ethylene its useful reactivity, as the double bond’s high electron density attracts electrophiles. Many reactions rely on transition metals that temporarily bind to ethylene using both its π and π* orbitals. Because it is a simple molecule, ethylene has a straightforward UV-vis spectrum, still used to test theoretical models.

Global ethylene production reached 107 million tonnes in 2005, 109 million in 2006, 138 million in 2010, and 141 million in 2011. By 2013, at least 117 companies in 32 countries produced it. To keep up with rising demand, new production facilities are built, especially in the Middle East and China. Production emits substantial carbon dioxide.

Industrially, ethylene is made through several methods. The main one is steam cracking, where hydrocarbons and steam are heated to 750–950 °C, breaking large hydrocarbons into smaller ones and introducing double bonds. Using ethane as feedstock yields ethylene, which is then separated from the mixture by repeated compression and distillation. In Europe and Asia, ethylene comes mainly from cracking naphtha, gasoil, and condensates, also producing propylene, C4 olefins, and aromatics. Other methods include Fischer–Tropsch synthesis and methanol-to-olefins (MTO).

In the lab, ethylene is rarely synthesized and is usually purchased. It can be made by dehydrating ethanol with sulfuric acid or, in the gas phase, with aluminium oxide or activated alumina.

Naturally, ethylene is produced from methionine, with 1-aminocyclopropane-1-carboxylic acid as its immediate precursor.

Major industrial reactions of ethylene, in order of scale, are: polymerization, oxidation, halogenation and hydrohalogenation, alkylation, hydration, oligomerization, and hydroformylation. In the US and Europe, about 90% of ethylene goes into making ethylene oxide, ethylene dichloride, ethylbenzene, and polyethylene. Most reactions involve electrophilic addition. Ethylene is also used to produce mustard gas by adding sulfur dichloride.

Polymerization consumes over half the world’s ethylene supply. Polyethylene, the most widely used plastic, is mainly turned into films for packaging, carrier bags, and trash liners. Linear alpha-olefins, made by oligomerization, serve as precursors, detergents, plasticisers, synthetic lubricants, additives, and co-monomers for polyethylenes.

Oxidation of ethylene yields ethylene oxide, a key raw material for surfactants and detergents via ethoxylation. Ethylene oxide is also hydrolyzed to ethylene glycol, used as automotive antifreeze, and to higher molecular weight glycols, glycol ethers, and polyethylene terephthalate. With a palladium catalyst, ethylene oxidation can form acetaldehyde—a major industrial process producing 10 million kg per year—through initial complexation of ethylene to a Pd(II) center.

Halogenation and hydrohalogenation produce intermediates like ethylene dichloride, ethyl chloride, and ethylene dibromide. Chlorine addition uses oxychlorination rather than chlorine directly. Derived products include polyvinyl chloride, trichloroethylene, perchloroethylene, methyl chloroform, polyvinylidene chloride and copolymers, and ethyl bromide.

Alkylation with ethylene gives ethylbenzene, the precursor to styrene. Styrene is mainly used in polystyrene for packaging and insulation, and in styrene-butadiene rubber for tires and footwear. Smaller-scale products include ethyltoluene, ethylanilines, 1,4-hexadiene, and aluminium alkyls, leading to polystyrene, unsaturated polyesters, and ethylene-propylene terpolymers.

Hydroformylation of ethylene yields propionaldehyde, a precursor to propionic acid and n-propyl alcohol.

Hydration has long made ethylene the main non-fermentative source of ethanol.