Ethylene

Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula C
2H
4 or H2C=CH2. It is a colorless flammable gas with a faint "sweet and musky" odour when pure.[5] It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).

Not to be confused with ethane or ethyne.
Ethylene
Names
IUPAC name
Ethene
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.000.742
EC Number
  • 200-815-3
KEGG
UNII
CompTox Dashboard (EPA)
Properties
C
2H
4
Molar mass 28.054 g·mol−1
Appearance colorless gas
Density 1.178 kg/m3 at 15 °C, gas[1]
Melting point −169.2 °C (−272.6 °F; 104.0 K)
Boiling point −103.7 °C (−154.7 °F; 169.5 K)
0.131 mg/mL (25 °C); 2.9 mg/L[2]
Solubility in ethanol 4.22 mg/L[2]
Solubility in diethyl ether good[2]
Acidity (pKa) 44
Conjugate acid Ethenium
-15.30·10−6 cm3/mol
Viscosity 10.28 μPa·s[3]
Structure
Molecular shape
 D2h
zero
Thermochemistry
Std molar
entropy (So298)
 219.32 J·K−1·mol−1
+52.47 kJ/mol
Hazards
Safety data sheet See: data page
ICSC 0475
EU classification (DSD) (outdated)
 F+
R-phrases (outdated) R12 R67
S-phrases (outdated) (S2) S9 S16 S33 S46
NFPA 704 (fire diamond)
4
2
2
Flash point −136 °C (−213 °F; 137 K)
Autoignition
temperature
 542.8 °C (1,009.0 °F; 815.9 K)
Related compounds
Related compounds
 Ethane
Acetylene
Propene
Supplementary data page
Structure and
properties
 Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
Spectral data
 UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references

Ethylene is widely used in the chemical industry, and its worldwide production (over 150 million tonnes in 2016[6]) exceeds that of any other organic compound.[7][8] Much of this production goes toward polyethylene, a widely used plastic containing polymer chains of ethylene units in various chain lengths. Ethylene is also an important natural plant hormone and is used in agriculture to force the ripening of fruits.[9] Ethylene's hydrate is ethanol.

Structure and properties
Orbital description of bonding between ethylene and a transition metal.

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 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.

Being a simple molecule, ethylene is spectroscopically simple. Its UV-vis spectrum is still used as a test of theoretical methods.[10]

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.[11] Most of the reactions with ethylene are electrophilic addition.

Main industrial uses of ethylene. Clockwise from the upper right: its conversions to ethylene oxide, precursor to ethylene glycol; to ethylbenzene, precursor to styrene; to various kinds of polyethylene; to ethylene dichloride, precursor to vinyl chloride.

Polymerization
See also: Ziegler–Natta catalyst and Polyethylene

Polyethylene consumes more than half of the world's ethylene supply. Polyethylene, also called polyethene, 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 polymers) are used as precursors, detergents, plasticisers, synthetic lubricants, additives, and also as co-monomers in the production of polyethylenes.[11]

Oxidation

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.

Ethylene undergoes oxidation by palladium to give acetaldehyde. This conversion remains a major industrial process (10M kg/y).[12] 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, polyvinylidene chloride and copolymers, and ethyl bromide.[13]

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 polyesters and ethylene-propylene terpolymers.[13]

Oxo reaction

The hydroformylation (oxo reaction) of ethylene results in propionaldehyde, a precursor to propionic acid and n-propyl alcohol.[13]

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:[14]

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.

Niche uses

An example of a niche use is as an anesthetic agent (in an 85% ethylene/15% oxygen ratio).[15] It can also be used to hasten fruit ripening, as well as a welding gas.[11][16]

Production

Global ethylene production was 107 million tonnes in 2005,[7] 109 million tonnes in 2006,[17] 138 million tonnes in 2010 and 141 million tonnes in 2011.[18] 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.[19]

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.[13] 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). [20] Other technologies employed for the production of ethylene include oxidative coupling of methane, Fischer-Tropsch synthesis, methanol-to-olefins (MTO), and catalytic dehydrogenation.[21]

Laboratory synthesis

Although of great value industrially, ethylene is rarely synthesized in the laboratory and is ordinarily purchased.[22] It can be produced via dehydration of ethanol with sulfuric acid or in the gas phase with aluminium oxide.[23]

Ethylene as a plant hormone
An ethylene signal transduction pathway. Ethylene permeates the membrane and binds to a receptor on the endoplasmic reticulum. The receptor releases the repressed EIN2. This then activates a signal transduction pathway which activates regulatory genes that eventually trigger an Ethylene response. The activated DNA is transcribed into mRNA which is then translated into a functional enzyme that is used for ethylene biosynthesis.

Ethylene serves as a hormone in plants.[24] 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 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 (20 °C; 68 °F) has been seen to produce CO2 levels of 10% in 24 hours.[25]

History of ethylene in plant biology

Ethylene has been used 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.[24] In 1874 it was discovered that smoke caused pineapple fields to bloom. Smoke contains ethylene, and once this was realized the smoke was replaced with ethephon or naphthalene acetic acid, which induce ethylene production.[26] In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene.[27] Sarah Doubt discovered that ethylene stimulated abscission in 1917.[28] Farmers in Florida would commonly get their crops to ripen in sheds by lighting kerosene lamps, which was originally thought to induce ripening from the heat. In 1924, Frank E. Denny discovered that it was the molecule ethylene emitted by the kerosene lamps that induced the ripening.[29] It was not until 1934 that Gane reported that plants synthesize ethylene.[30] In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as senescence of vegetative tissues.[31]

Ethylene biosynthesis in plants
The Yang cycle

Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds. 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.[32] The pathway for ethylene biosynthesis is named the Yang cycle after the scientist Shang Fa Yang who made key contributions to elucidating this pathway.

Ethylene is biosynthesized from 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.

Ethylene perception in plants

Ethylene is perceived by a family of five transmembrane protein dimers such as the ETR1 protein in Arabidopsis. The genes encoding ethylene receptors have been cloned in the reference plant Arabidopsis thaliana and many other plants. Ethylene receptors are encoded by multiple genes in plant genomes. Dominant missense mutations in any of the gene family, which comprises five receptors in Arabidopsis and at least six in tomato, can confer insensitivity to ethylene.[33] Loss-of-function mutations in multiple members of the ethylene-receptor family result in a plant that exhibits constitutive ethylene responses.[34] 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.[24]

Ethylene response to salt stress

A large portion of the soil has been affected by over salinity and it has been known to limit the growth of many plants. Globally, the total area of saline soil was 397,000,000 ha and in continents like Africa, it makes up 2 percent of the soil.[35] The amount of soil salinization has reached 19.5% of the irrigated land and 2.1% of the dry-land agriculture around the world.[36] Soil salinization affects the plants using osmotic potential by net solute accumulation. The osmotic pressure in the plant is what maintains water uptake and cell turgor to help with stomatal function and other cellular mechanisms.[36] Over generations, many plant genes have adapted, allowing plants’ phenotypes to change and built distinct mechanisms to counter salinity effects.

The plant hormone ethylene is a combatant for salinity in most plants. Ethylene is known for regulating plant growth and development and adapted to stress conditions. Central membrane proteins in plants, such as ETO2, ERS1 and EIN2, are used for ethylene signaling in many plant growth processes. ETO2, Ethylene overproducer 2, is a protein that when mutated it will gain a function to continually produce ethylene even when there is no stress condition, causing the plant to grow short and stumpy. ERS1, Ethylene response sensor 1, is activated when ethylene is present in the signaling pathway and when mutated, it loses a function and cannot bind to ethylene. This means a response is never activated and the plant will not be able to cope with the abiotic stress. EIN2, Ethylene insensitive 2, is a protein that activates the pathway and when there is a mutation here the EIN2 will block ethylene stimulation and an ethylene response gene will not be activated. Mutations in these proteins can lead to heightened salt sensitivity and limit plant growth. The effects of salinity have been studied on Arabidopsis plants that have mutated ERS1 and EIN4 proteins.[37] These proteins are used for ethylene signaling again certain stress conditions, such as salt and the ethylene precursor ACC is allowing suppress of any sensitivity to the salt stress.[37] Mutations in these pathways can cause lack of ethylene signaling, causing stunt in plant growth and development.

Environmental and biological triggers of ethylene

Environmental cues such as flooding, drought, chilling, wounding, and pathogen attack can induce ethylene formation in plants. In flooding, roots suffer from lack of oxygen, or anoxia, which leads to the synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC). ACC is transported upwards in the plant and then oxidized in leaves. The ethylene produced causes nastic movements (epinasty) of the leaves, perhaps helping the plant to lose water.[38]

Corolla senescence

The corolla of a plant refers to its set of petals. Corolla development in plants is broken into phases from anthesis to corolla wilting. The development of the corolla is directed in part by ethylene, though its concentration is highest when the plant is fertilized and no longer requires the production or maintenance of structures and compounds that attract pollinators.[39][40] The role of ethylene in the developmental cycle is as a hormonal director of senescence in corolla tissue. This is evident as ethylene production and emission are maximized in developmental phases post-pollination, until corolla wilting.[39] Ethylene-directed senescence of corolla tissue can be observed as color change in the corolla or the wilting/ death of corolla tissue. At the chemical level, ethylene mediates the reduction in the amount of fragrance volatiles produced. Fragrance volatiles act mostly by attracting pollinators. Ethylene's role in this developmental scenario is to move the plant away from a state of attracting pollinators, so it also aids in decreasing the production of these volatiles.

Ethylene production in corolla tissue does not directly cause the senescence of corolla tissue, but acts by releasing secondary products that are consistent with tissue aging. While the mechanism of ethylene-mediated senescence are unclear, its role as a senescence-directing hormone can be confirmed by ethylene-sensitive petunia response to ethylene knockdown. Knockdown of ethylene biosynthesis genes was consistent with increased corolla longevity; inversely, up-regulation of ethylene biosynthesis gene transcription factors were consistent with a more rapid senescence of the corolla.[39]

List of plant responses to ethylene
  • Seedling triple response, thickening and shortening of hypocotyl with pronounced apical hook.
  • Stimulation of Arabidopsis hypocotyl elongation [41]
  • In pollination, when the pollen reaches the stigma, the precursor of the ethene, ACC, is secreted to the petal, the ACC releases ethylene with ACC oxidase.
  • Stimulates leaf senescence
  • Stimulates senescence of mature xylem cells in preparation for plant use
  • Induces leaf abscission
  • Induces seed germination
  • Induces root hair growth — increasing the efficiency of water and mineral absorption
  • Induces the growth of adventitious roots during flooding
  • Stimulates survival under low-oxygen conditions (hypoxia) in submerged plant tissues [42] [43] [44]
  • Stimulates epinasty — leaf petiole grows out, leaf hangs down and curls into itself
  • Stimulates fruit ripening
  • Induces a climacteric rise in respiration in some fruit which causes a release of additional ethylene.
  • Affects gravitropism
  • Stimulates nutational bending
  • Inhibits stem growth and stimulates stem and cell broadening and lateral branch growth outside of seedling stage (see Hyponastic response)
  • Interference with auxin transport (with high auxin concentrations)
  • Inhibits shoot growth and stomatal closing except in some water plants or habitually flooded ones such as some rice varieties, where the opposite occurs (conserving CO
    2     and O
    2    )
  • Induces flowering in pineapples
  • Inhibits short day induced flower initiation in Pharbitus nil[45] and Chrysanthemum morifolium[46]

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.[47]

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 salts are ethylene inhibitors.[48][49] 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.

Chrysanthemum flowering is delayed by ethylene gas[50] and growers have found that carbon dioxide 'burners' and the exhaust fumes from inefficient glasshouse heaters can raise the ethylene concentration to 0.05 ppmv causing delay in flowering of commercial crops.

Ligand

Ethylene is a ligand in organometallic chemistry. 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 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.[51]

Ethylene appears to have been discovered by Johann Joachim Becher, who obtained it by heating ethanol with sulfuric acid;[52] he mentioned the gas in his Physica Subterranea (1669).[53] 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.[54] 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.[55] 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.)[56] 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.

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 (C
2H
4) was the "daughter of ethyl" (C
2H
5). 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.[57] 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,[58] ethylene was used as an anesthetic.[59][5] It remained in use through the 1940s use even while chloroform was being phased out. Its pungent odor and its explosive nature limit its use today.[60]

Nomenclature

The 1979 IUPAC nomenclature rules made an exception for retaining the non-systematic name ethylene,[61] however, this decision was reversed in the 1993 rules[62] and remains unchanged in the newest 2013 recommendations[63] so the IUPAC name is now ethene. Note that 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 is still prevalent among chemists in North America.

Safety

Like all hydrocarbons, ethylene is an asphyxiant and combustible. It is listed as an IARC class 3 carcinogen as there is no evidence at present that it causes cancer in humans.[64]

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