Ethylene
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Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula Template:Chem2 or Template:Chem2. It is a colourless, flammable gas with a faint "sweet and musky" odour when pure.<ref name=UllmannEthylene/> It is the simplest alkene (a hydrocarbon with carbon–carbon double bonds).
Ethylene is widely used in the chemical industry, and its worldwide production (over 150 million tonnes in 2016<ref>Template:Cite web</ref>) exceeds that of any other organic compound.<ref name="cenews">Template:Cite journal</ref><ref name="Technology Economics Program">Template:Cite book</ref> Much of this production goes toward creating polythene, which is a widely used plastic containing polymer chains of ethylene units in various chain lengths. Production emits greenhouse gases, including methane from feedstock production and carbon dioxide from any non-sustainable energy used.
Ethylene is also an important natural plant hormone and is used in agriculture to induce ripening of fruits.<ref name="Wang_2002">Template:Cite journal</ref> The hydrate of ethylene is ethanol.
Structure and properties
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This hydrocarbon has four hydrogen atoms 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 π-bond by supplying heat at 50 °C.Template:Citation needed
The π-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.Template:Citation needed
Being a simple molecule, ethylene is spectroscopically simple. Its UV-vis spectrum is still used as a test of theoretical methods.<ref name=NIST_Webbook>Template:Cite web</ref>
Uses
[edit]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.<ref name="inchem">Template:Cite web</ref> Most of the reactions with ethylene are electrophilic addition.Template:Citation needed
Polymerization
[edit]Template:See also 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 liners. 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.<ref name="inchem" />
Oxidation
[edit]Ethylene is oxidized to produce ethylene oxide, a key raw material in the production of surfactants and detergents 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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
Template:Main Ethylene oxidation in the presence of a palladium catalyst can form acetaldehyde. This conversion remains a major industrial process (10M kg/y).<ref>Template:Cite book</ref> The process proceeds via the initial complexation of ethylene to a Pd(II) center.Template:Citation needed
Halogenation and hydrohalogenation
[edit]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 copolymers, and ethyl bromide.<ref name=Keystone/>
Alkylation
[edit]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 polyesters and ethylene-propylene terpolymers.<ref name=Keystone/>
Oxo reaction
[edit]The hydroformylation (oxo reaction) of ethylene results in propionaldehyde, a precursor to propionic acid and n-propyl alcohol.<ref name=Keystone/>
Hydration
[edit]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:<ref>Template:Cite book</ref>
- C2H4 + H2O → CH3CH2OH
Dimerization to butenes
[edit]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.<ref>Template:Cite web</ref>
Fruit and flowering
[edit]Template:Main Ethylene is a hormone that affects the ripening and flowering of many plants. It is widely used to control freshness in horticulture and fruits.<ref>Template:Cite book</ref> The scrubbing of naturally occurring ethylene delays ripening.<ref>Template:Cite book</ref> Adsorption of ethylene by nets coated in titanium dioxide gel has also been shown to be effective.<ref>Template:Cite journal</ref>
Niche uses
[edit]An example of a niche use is as an anesthetic agent (in an 85% ethylene/15% oxygen ratio).<ref>Template:Cite journal</ref> It is also used as a refrigerant gas for low temperature applications under the name R-1150.<ref>Template:Cite web</ref>
Production
[edit]Global ethylene production was 107 million tonnes in 2005,<ref name="cenews"/> 109 million tonnes in 2006,<ref>Nattrass, L and Higson, A (22 July 2010) NNFCC Renewable Chemicals Factsheet: Ethanol Template:Webarchive. National Non-Food Crops Centre</ref> 138 million tonnes in 2010, and 141 million tonnes in 2011.<ref>Template:Cite journal</ref> By 2013, ethylene was produced by at least 117 companies in 32 countries. To meet the ever-increasing demand for ethylene, sharp increases in production facilities are added globally, particularly in the Mideast and in China.<ref name="Ceresana">Template:Cite web</ref> Production emits greenhouse gas, namely significant amounts of carbon dioxide.<ref>Template:Cite journal</ref>
Industrial process
[edit]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.<ref name=Keystone>Template:Cite book</ref> In Europe and Asia, ethylene is obtained mainly from cracking naphtha, gasoil and condensates with the coproduction of propylene, C4 olefins and aromatics (pyrolysis gasoline).<ref>Template:Cite web</ref> Other procedures employed for the production of ethylene include Fischer-Tropsch synthesis and methanol-to-olefins (MTO).<ref>Template:Cite journal</ref>
Laboratory synthesis
[edit]Although of great value industrially, ethylene is rarely synthesized in the laboratory and is ordinarily purchased.<ref>Template:Cite book</ref> It can be produced via dehydration of ethanol with sulfuric acid or in the gas phase with aluminium oxide or activated alumina.<ref>Template:Cite book</ref>
Biosynthesis
[edit]Ethylene is produced from methionine in nature. The immediate precursor is 1-aminocyclopropane-1-carboxylic acid.<ref name=Yang_1984>Template:Cite journal</ref>
Ligand
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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.<ref>Template:Cite book</ref>
History
[edit]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.<ref name=Roach>Template:Cite magazine</ref>
Ethylene appears to have been discovered by Johann Joachim Becher, who obtained it by heating ethanol with sulfuric acid;<ref>Template:Cite book</ref> he mentioned the gas in his Physica Subterranea (1669).<ref>Template:Cite book</ref> Joseph Priestley also mentions the gas in his Experiments and observations relating to the various branches of natural philosophy: with a continuation of the observations on air (1779), where he reports that Jan Ingenhousz saw ethylene synthesized in the same way by a Mr. Enée in Amsterdam in 1777 and that Ingenhousz subsequently produced the gas himself.<ref>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.</ref> The properties of ethylene were studied in 1795 by four Dutch 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.<ref>Template:Harvnb</ref> This group also discovered that ethylene could be combined with chlorine to produce the Dutch oil, 1,2-dichloroethane; this discovery gave ethylene the name used for it at that time, olefiant gas (oil-making gas.)<ref>Template:Harvnb
Template:Cite book</ref> The term olefiant gas is in turn the etymological origin of the modern word "olefin", the class of hydrocarbons in which ethylene is the first member.Template:Citation needed
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 (Template:Chem) was the "daughter of ethyl" (Template:Chem). The name ethylene was used in this sense as early as 1852.<ref>Template:Cite web</ref>
In 1866, the German 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.<ref>Template:Cite web</ref> 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.
Following experimentation by Luckhardt, Crocker, and Carter at the University of Chicago,<ref>Template:Cite journal</ref> ethylene was used as an anesthetic.<ref>Template:Cite journal</ref><ref name=UllmannEthylene>Template:Cite book</ref> It remained in use through the 1940s, even while chloroform was being phased out. Its pungent odor and its explosive nature limit its use today.<ref>Template:Cite journal</ref>
Nomenclature
[edit]The 1979 IUPAC nomenclature rules made an exception for retaining the non-systematic name ethylene;<ref>IUPAC nomenclature rule A-3.1 (1979) Template:Webarchive. Acdlabs.com. Retrieved on 2016-04-24.</ref> however, this decision was reversed in the 1993 rules,<ref>Footnote to IUPAC nomenclature rule R-9.1, table 19(b) Template:Webarchive. Acdlabs.com. Retrieved on 2016-04-24.</ref> and it remains unchanged in the newest 2013 recommendations,<ref>Template:Cite book</ref> so the IUPAC name is now ethene. In the IUPAC system, the name ethylene is reserved for the divalent group -CH2CH2-. Hence, names like ethylene oxide and ethylene dibromide are permitted, but the use of the name ethylene for the two-carbon alkene is not. Nevertheless, use of the name ethylene for H2C=CH2 (and propylene for H2C=CHCH3) is still prevalent among chemists in North America.<ref>Template:Cite book</ref>
Greenhouse gas emissions
[edit]"A key factor affecting petrochemicals life-cycle emissions is the methane intensity of feedstocks, especially in the production segment."<ref>Template:Cite web</ref> Emissions from cracking of naptha and natural gas (common in the US as gas is cheap there) depend a lot on the source of energy (for example gas burnt to provide high temperatures<ref>Template:Cite web</ref>) but that from naptha is certainly more per kg of feedstock.<ref name="COMET">Template:Cite web</ref> Both steam cracking and production from natural gas via ethane are estimated to emit 1.8 to 2kg of CO2 per kg ethylene produced,<ref>Template:Cite journal</ref> totalling over 260 million tonnes a year.<ref>Template:Cite web</ref> This is more than all other manufactured chemicals except cement and ammonia.<ref>Template:Cite web</ref> According to a 2022 report using renewable or nuclear energy could cut emissions by almost half.<ref name="COMET"/>
Safety
[edit]Like all hydrocarbons, ethylene is a combustible asphyxiant. It is listed as an IARC group 3 agent, since there is no current evidence that it causes cancer in humans.<ref>Template:Cite web</ref>
See also
[edit]- RediRipe, an ethylene detector for fruits.
References
[edit]External links
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