Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and " odour when pure.[ It is the simplest alkene (a hydrocarbon with carbon–carbon ).
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Ethylene is widely used in the chemical industry, and its worldwide production (over 150 million in 2016
Ethylene is also an important natural plant hormone and is used in agriculture to induce ripening of .
Structure and properties
This hydrocarbon has four hydrogen bound to a pair of carbon atoms that are connected by a double bond. All six atoms that comprise ethylene are coplanar. The H-C-H angle is 117.4°, close to the 120° for ideal sp² hybridized carbon. The molecule is also relatively weak: rotation about the C-C bond is a very low energy process that requires breaking the Pi bond by supplying heat at 50 °C.
The Pi bond in the ethylene molecule is responsible for its useful reactivity. The double bond is a region of high electron density, thus it is susceptible to attack by electrophiles. Many reactions of ethylene are catalyzed by transition metals, which bind transiently to the ethylene using both the π and π* orbitals.
Being a simple molecule, ethylene is spectroscopically simple. Its UV-vis Spectroscopy is still used as a test of theoretical methods.
Uses
Major industrial reactions of ethylene include in order of scale: 1) polymerization, 2) oxidation, 3) halogenation and hydrohalogenation, 4) alkylation, 5) hydration, 6) oligomerization, and 7) hydroformylation. In the United States and Europe, approximately 90% of ethylene is used to produce ethylene oxide, ethylene dichloride, ethylbenzene and polyethylene.
Polymerization
Polyethylene production uses more than half of the world's ethylene supply. Polyethylene, also called polyethene and polythene, is the world's most widely used plastic. It is primarily used to make films in packaging, carrier bags and trash bin bag. Linear alpha-olefins, produced by oligomerization (formation of short-chain molecules) are used as precursors, detergents, plasticisers, synthetic lubricants, additives, and also as co-monomers in the production of polyethylenes.
Oxidation
Ethylene is oxidation to produce ethylene oxide, a key raw material in the production of and by ethoxylation. Ethylene oxide is also hydrolyzed to produce ethylene glycol, widely used as an automotive antifreeze as well as higher molecular weight glycols, glycol ethers, and polyethylene terephthalate.
Ethylene oxidation in the presence of a palladium catalyst can form acetaldehyde. This conversion remains a major industrial process (10M kg/y).
Halogenation and hydrohalogenation
Major intermediates from the halogenation and hydrohalogenation of ethylene include ethylene dichloride, ethyl chloride, and ethylene dibromide. The addition of chlorine entails "oxychlorination", i.e. chlorine itself is not used. Some products derived from this group are polyvinyl chloride, trichloroethylene, perchloroethylene, methyl chloroform, polyvinylidene chloride and , and ethyl bromide.[
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Alkylation
Major chemical intermediates from the alkylation with ethylene is ethylbenzene, precursor to styrene. Styrene is used principally in polystyrene for packaging and insulation, as well as in styrene-butadiene rubber for tires and footwear. On a smaller scale, ethyltoluene, ethylanilines, 1,4-hexadiene, and aluminium alkyls. Products of these intermediates include polystyrene, unsaturated and ethylene-propylene copolymer.[
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Oxo reaction
The hydroformylation (oxo reaction) of ethylene results in propionaldehyde, a precursor to propionic acid and n-propyl alcohol.[
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Hydration
Ethylene has long represented the major nonfermentative precursor to ethanol. The original method entailed its conversion to diethyl sulfate, followed by hydrolysis. The main method practiced since the mid-1990s is the direct hydration of ethylene catalyzed by solid acid catalysts:
- C2H4 + H2O → CH3CH2OH
Dimerization to butenes
Ethylene is dimerized by hydrovinylation to give n-butenes using processes licensed by Lummus or IFP. The Lummus process produces mixed n-butenes (primarily 2-butenes) while the IFP process produces 1-butene. 1-Butene is used as a comonomer in the production of certain kinds of polyethylene.
Fruit and flowering
Ethylene is a hormone that affects the ripening and flowering of many plants. It is widely used to control freshness in horticulture and .
Niche uses
An example of a niche use is as an anesthesiology (in an 85% ethylene/15% oxygen ratio).
Production
Global ethylene production was 107 million tonnes in 2005,[Nattrass, L and Higson, A (22 July 2010) NNFCC Renewable Chemicals Factsheet: Ethanol . National Non-Food Crops Centre] 138 million tonnes in 2010, and 141 million tonnes in 2011.
Industrial process
Ethylene is produced by several methods in the petrochemical industry. A primary method is steam cracking (SC) where hydrocarbons and steam are heated to 750–950 °C. This process converts large hydrocarbons into smaller ones and introduces unsaturation. When ethane is the feedstock, ethylene is the product. Ethylene is separated from the resulting mixture by repeated compression and distillation.
Laboratory synthesis
Although of great value industrially, ethylene is rarely synthesized in the laboratory and is ordinarily purchased.
Biosynthesis
Ethylene is produced from methionine in nature. The immediate precursor is 1-aminocyclopropane-1-carboxylic acid.
Ligand
Ethylene is a fundamental ligand in transition metal alkene complexes. One of the first organometallic compounds, Zeise's salt is a complex of ethylene. Useful reagents containing ethylene include Pt(PPh3)2(C2H4) and Rh2Cl2(C2H4)4. The Rh-catalysed hydroformylation of ethylene is conducted on an industrial scale to provide propionaldehyde.
History
Some geologists and scholars believe that the famous Greek Oracle at Delphi (the Pythia) went into her trance-like state as an effect of ethylene rising from ground faults.
Ethylene appears to have been discovered by Johann Joachim Becher, who obtained it by heating ethanol with sulfuric acid;[Appendix, §VIII, pp. 474 ff., Experiments and observations relating to the various branches of natural philosophy: with a continuation of the observations on air, Joseph Priestley, London: printed for J. Johnson, 1779, vol. 1.] The properties of ethylene were studied in 1795 by four Netherlands chemists, Johann Rudolph Deimann, Adrien Paets van Troostwyck, Anthoni Lauwerenburgh and Nicolas Bondt, who found that it differed from hydrogen gas and that it contained both carbon and hydrogen.
In the mid-19th century, the suffix -ene (an Ancient 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 molecule being modified. Thus, ethylene () was the "daughter of ethyl group" (). The name ethylene was used in this sense as early as 1852.
In 1866, the Germany chemist August Wilhelm 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 hydrogens than their parent alkane.
Following experimentation by Luckhardt, Crocker, and Carter at the University of Chicago,
Nomenclature
The 1979 IUPAC nomenclature rules made an exception for retaining the non-systematic name ethylene;[ IUPAC nomenclature rule A-3.1 (1979) . Acdlabs.com. Retrieved on 2016-04-24.] however, this decision was reversed in the 1993 rules,[ Footnote to IUPAC nomenclature rule R-9.1, table 19(b) . Acdlabs.com. Retrieved on 2016-04-24.] and it remains unchanged in the newest 2013 recommendations,
Greenhouse gas emissions
"A key factor affecting petrochemicals life-cycle emissions is the methane intensity of feedstocks, especially in the production segment."
Safety
Like all hydrocarbons, ethylene is a combustible Asphyxiant gas. It is listed as an IARC group 3 agent, since there is no current evidence that it causes cancer in humans.
See also
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RediRipe, an ethylene detector for fruits.
External links
