Ethylene

| Section2 = {{Chembox Properties | Formula = C₂H₄ | MolarMass = 28.05 g/mol | Appearance = colorless gas | Density = 1.178 kg/m³ at 15 °C, gas | MeltingPt = −169.2 °C (104.0 K, -272.6 °F) | BoilingPt = −103.7 °C (169.5 K, -154.7 °F) | Solubility = 3.5 mg/100 mL (17 °C) ; 2.9 mg/L | Solubility1 = 4.22 mg/L Ethylene is the most produced organic compound in the world; global production of ethylene exceeded 107 million tonnes in 2005. To meet the ever increasing demand for ethylene, sharp increases in production facilities are added globally, particularly in the Persian Gulf countries and in China.

Structure and bonding

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 119°, close to the 120° for ideal sp² hybridized carbon. The molecule is also relatively rigid: rotation about the C-C bond is a high energy process that requires breaking the π-bond.

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.

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. Ethylene can also be hydrated to give ethanol, but this method is rarely used industrially. In the United States and Europe, approximately 90% of ethylene is used to produce three chemical compounds—ethylene oxide, ethylene dichloride, and ethylbenzene—and a variety of kinds of polyethylene. , precursor to ethylene glycol, to ethylbenzene, precursor to styrene, to various kinds of polyethylene, to ethylene dichloride, precursor to vinyl chloride.]]

Polymerization

Polyethylenes of various types consume more than half of world ethylene supply. Polyethylene, also called polythene, is the world's most widely-used plastic, being primarily used to make films used in packaging, carrier bags and trash liners. Linear alpha-olefins, produced by oligomerization (formation of short polymers) are used as precursors, detergents, plasticisers, synthetic lubricants, additives, and also as co-monomers in the production of polyethylenes. The process proceeds via the initial complexation of ethylene to a Pd(II) center.

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, polyvinylidiene chloride and copolymers, and ethyl bromide. It can also be used to hasten fruit ripening, as well as a welding gas.

Production

In 2006, global ethylene production was 109 million tonnes. By 2010 ethylene was produced by at least 117 companies in 55 countries.

The areas of an ethylene plant are: # steam cracking furnaces: # primary and secondary heat recovery with quench; # a dilution steam recycle system between the furnaces and the quench system; # primary compression of the cracked gas (3 stages of compression); # hydrogen sulfide and carbon dioxide removal (acid gas removal); # secondary compression (1 or 2 stages); # drying of the cracked gas; # cryogenic treatment; # all of the cold cracked gas stream goes to the demethanizer tower. The overhead stream from the demethanizer tower consists of all the hydrogen and methane that was in the cracked gas stream. Cryogenically () treating this overhead stream separates hydrogen from methane. Methane recovery is critical to the economical operation of an ethylene plant. # the bottom stream from the demethanizer tower goes to the deethanizer tower. The overhead stream from the deethanizer tower consists of all the C_2,'s that were in the cracked gas stream. The C₂ stream contains acetylene, which is explosive above 200 kPa (29 psi). If the partial pressure of acetylene is expected to exceed these values, the C₂ stream is partially hydrogenated. The C₂'s then proceed to a C₂ splitter. The product ethylene is taken from the overhead of the tower and the ethane coming from the bottom of the splitter is recycled to the furnaces to be cracked again; # the bottom stream from the de-ethanizer tower goes to the depropanizer tower. The overhead stream from the depropanizer tower consists of all the C₃'s that were in the cracked gas stream. Before feeding the C₃'s to the C₃ splitter, the stream is hydrogenated to convert the methylacetylene and propadiene (allene) mix. This stream is then sent to the C₃ splitter. The overhead stream from the C₃ splitter is product propylene and the bottom stream is propane which is sent back to the furnaces for cracking or used as fuel. # The bottom stream from the depropanizer tower is fed to the debutanizer tower. The overhead stream from the debutanizer is all of the C₄'s that were in the cracked gas stream. The bottom stream from the debutanizer (light pyrolysis gasoline) consists of everything in the cracked gas stream that is C₅ or heavier. Interestingly for such a useful compound, ethylene is rarely used in organic synthesis in the laboratory.

Being a simple molecule, ethylene is spectroscopically simple. Its UV-vis spectrum is still used as a test of theoretical methods.

Ethylene as a plant hormone

Ethylene serves as a hormone in plants. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves. Commercial ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 ppm to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (68F) has been seen to produce CO2 levels of 10% in 24 hours.

History of ethylene in plant biology

Ethylene has been used in practice since the ancient Egyptians, who would gash figs in order to stimulate ripening (wounding stimulates ethylene production by plant tissues). The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears. In 1864, it was discovered that gas leaks from street lights led to stunting of growth, twisting of plants, and abnormal thickening of stems. Doubt discovered that ethylene stimulated abscission in 1917. It wasn't until 1934 that Gane reported that plants synthesize ethylene. In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as senescence of vegetative tissues.

Ethylene biosynthesis in plants

Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seedlings.

"Ethylene production is regulated by a variety of developmental and environmental factors. During the life of the plant, ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers. Ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators".

The biosynthesis of the hormone starts with conversion of the amino acid methionine to S-adenosyl-L-methionine (SAM, also called Adomet) by the enzyme Met Adenosyltransferase. SAM is then converted to 1-aminocyclopropane-1-carboxylic-acid (ACC) by the enzyme ACC synthase (ACS); the activity of ACS determines the rate of ethylene production, therefore regulation of this enzyme is key for the ethylene biosynthesis. The final step requires oxygen and involves the action of the enzyme ACC-oxidase (ACO), formerly known as the Ethylene Forming Enzyme (EFE). Ethylene biosynthesis can be induced by endogenous or exogenous ethylene. ACC synthesis increases with high levels of auxins, especially Indole acetic acid (IAA), and cytokinins. ACC synthase is inhibited by abscisic acid.

Ethylene perception in plants

Ethylene could be perceived by a transmembrane protein dimer complex. The gene encoding an ethylene receptor has been cloned in Arabidopsis thaliana and then in tomato. Ethylene receptors are encoded by multiple genes in the Arabidopsis and tomato genomes. The gene family comprises five receptors in Arabidopsis and at least six in tomato, most of which have been shown to bind ethylene. DNA sequences for ethylene receptors have also been identified in many other plant species and an ethylene binding protein has even been identified in Cyanobacteria. and Chrysanthemum morifolium

Commercial issues

Ethylene shortens the shelf life of many fruits by hastening fruit ripening and floral senescence. Ethylene will shorten the shelf life of cut flowers and potted plants by accelerating floral senescence and floral abscission. Flowers and plants which are subjected to stress during shipping, handling, or storage produce ethylene causing a significant reduction in floral display. Flowers affected by ethylene include carnation, geranium, petunia, rose, and many others.

Ethylene can cause significant economic losses for florists, markets, suppliers, and growers. Researchers have developed several ways to inhibit ethylene, including inhibiting ethylene synthesis and inhibiting ethylene perception. Aminoethoxyvinylglycine (AVG), Aminooxyacetic acid (AOA), and silver ions are ethylene inhibitors. Inhibiting ethylene synthesis is less effective for reducing post-harvest losses since ethylene from other sources can still have an effect. By inhibiting ethylene perception, fruits, plants and flowers don't respond to ethylene produced endogenously or from exogenous sources. Inhibitors of ethylene perception include compounds that have a similar shape to ethylene, but do not elicit the ethylene response. One example of an ethylene perception inhibitor is 1-methylcyclopropene (1-MCP).

Commercial growers of bromeliads, including pineapple plants, use ethylene to induce flowering. Plants can be induced to flower either by treatment with the gas in a chamber, or by placing a banana peel next to the plant in an enclosed area.

Historical significance

Many 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.

History

Ethylene appears to have been discovered by Johann Joachim Becher, who obtained it by heating ethanol with sulfuric acid; he mentioned the gas in his Physica Subterranea (1669). 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. 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. This group also discovered that ethylene could be combined with chlorine to produce the oil of the Dutch chemists, 1,2-dichloroethane; this discovery gave ethylene the name used for it at that time, olefiant gas (oil-making gas.)

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 (C₂H₄) was the "daughter of ethyl" (C₂H₅). The name ethylene was used in this sense as early as 1852.

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. 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.

Nomenclature

The 1979 IUPAC nomenclature rules made an exception for retaining the non-systematic name ethylene, however, this decision was reversed in the 1993 rules so the IUPAC name is now ethene.

Safety

Like all hydrocarbons, ethylene is an asphyxiant and combustible. It has been used as an anesthetic.

References

Appendix: ethylene production facilities

Alberta, Canada

Joffre, Joffre, (Dow Chemical and Nova Chemicals), 2165 kta ethylene

Ontario, Canada

Sarnia, Sarnia, (Nova Chemicals),

Louisiana, United States

Hahnville, Hahnville, (Dow Chemical),

Lake Charles, Lake Charles, (Westlake Chemical),

Norco, Norco, (Shell Chemical), 1500 kta ethylene

Texas, United States

Alvin, Alvin, (Ineos),

Corpus Christi, Corpus Christi, (LyondellBasell),

Deer Park, Deer Park, (Shell Chemical), 1200 kta ethylene

Orange, Orange, (DuPont),

Port Arthur, Port Arthur, (BASF and Total Petrochemical),

Nanhai, China

Nanhai, Nanhai, (Shell Chemical), not available kta ethylene

Ras Laffan Industrial City, Qatar

Ras Laffan Industrial City, Ras Laffan Industrial City, (QAPCO), 1300 kta ethylene

Texas, United States

Channelview, Channelview, (LyondellBasell), 1450 kta ethylene

National Iranian Petrochemical Company, 1320 kta ethylene

Baytown, (Chevron Chemicals Company), Cedar Bayou Plant, not available kta ethylene

Illinois, United States

Morris, Illinois, (LyondellBasell)

Further reading

Ethylene hormone receptor action in Arabidopsis. Chang C, Stadler R. Bioessays. 2001 Jul;23(7):619-27. Review. PMID 11462215

Differential petiole growth in Arabidopsis thaliana: photocontrol and hormonal regulation. Millenaar FF, van Zanten M, Cox MC, Pierik R, Voesenek LA, Peeters AJ. New Phytol. 2009 Jun 24. [Epub ahead of print] PMID 19558423

External links

International Chemical Safety Card 0475

European Chemicals Bureau Datasheet

Speculations Towards a General Plant Hormone Theory

MSDS

Category:Alkenes Category:Plant hormones Category:Monomers Category:General anesthetics