Ethylene

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Ethylene
General
Systematic name	Ethene
Molecular formula	C2H4
SMILES	C=C
Molar mass	28.05 g/mol
Appearance	colourless gas
CAS number	[74-85-1]
Properties
Density and phase	1.178 g/l at 15 °C, gas
Solubility of gas in water	25 mL/100 mL (0 °C) 12 mL/100 mL (25 °C)[1]
Melting point	−169.1 °C
Boiling point	−103.7 °C
Structure
Molecular shape	planar
Dipole moment	zero
Symmetry group	D2h
Thermodynamic data
Std enthalpy of formation ΔfH°gas	+52.47 kJ/mol
Standard molar entropy S°gas	219.32 J·K−1·mol−1
Hazards
MSDS	External MSDS
EU classification	Extremely flammable (F+)
NFPA 704	4 1 2
R-phrases	R12, R67
S-phrases	S2, S9, S16, S33, S46
Flash point	Flammable gas
Explosive limits	2.7–36.0%
Autoignition temperature	490 °C
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other alkenes	Propene Butene
Related compounds	Ethane Acetylene
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Ethylene (or IUPAC name ethene) is the chemical compound with the formula C₂H₄. It is the simplest alkene. Because it contains a double bond, ethylene is called an unsaturated hydrocarbon or an olefin. It is exceedingly important in industry and even has a role in biology as a hormone.^[2] Ethylene is the organic compound produced on the largest scale: global production of ethylene exceeded 75 million metric tonnes per year in 2005.^[3]

Contents 1 Structure 2 Nomenclature 3 Production 4 Theoretical considerations 5 Chemical reactions 5.1 Additions to double bond 5.2 Oxidation 5.3 In the synthesis of fine chemicals 5.4 Polymerization 5.5 Miscellaneous 6 Ethylene as a plant hormone 6.1 Location, characteristics, and occasions for synthesis induction 6.2 Effects 7 Effects upon humans 8 References 9 External links

[edit] Structure

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°, 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, while retaining the σ-bond between the carbon atoms.

The double bond is a region of high electron density, and most reactions occur at this double bond.

[edit] Nomenclature

From 1795 on, ethylene was referred to as the olefiant gas (oil-making gas), because it combined with chlorine to produce the oil of the Dutch chemists (1,2-dichloroethane), first synthesized in 1795 by a collaboration of four Dutch chemists.

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 Augustus 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[1]. 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.

IUPAC nomenclature rules make an exception for retaining the non-systematic "ethylene" name. ^[4]

[edit] Production

Ethylene is produced in the petrochemical industry by steam cracking. In this process, gaseous or light liquid hydrocarbons are briefly heated to 750–950 °C, inducing numerous free radical reactions. Generally, in these reactions, large hydrocarbons break down into smaller ones and saturated hydrocarbons become unsaturated. The result of this process is a complex mixture of hydrocarbons in which ethylene is one of the principal components. The mixture is separated by repeated compression and distillation.

In another process used in oil refineries, large hydrocarbon molecules are "cracked" into smaller ones. Zeolite catalyst allows the cracking to be achieved at a lower temperature.

[edit] Theoretical considerations

Although ethylene is a relatively simple molecule, its spectrum^[5] is considered to be one of the most difficult to explain adequately from both a theoretical and practical perspective. For this reason, it is often used as a test case in computational chemistry. Of particular note is the difficulty in characterizing the ultraviolet absorption of the molecule. Interest in the subtleties and details of the ethylene spectrum can be dated back to at least the 1950s.

[edit] Chemical reactions

[edit] Additions to double bond

Like most alkenes, ethylene reacts with halogens to produce halogenated hydrocarbons1,2-C₂H₄X₂. It can also react with water to produce ethanol, but the rate at which this happens is very slow unless a suitable catalyst, such as phosphoric or sulfuric acid, is used. Under high pressure, and, in the presence of a catalytic metal (platinum, rhodium, nickel), hydrogen will react with ethylene.

Ethylene is used primarily as an intermediate in the manufacture of other chemicals used in the synthesis of monomers. Ethylene can be chlorinated to produce 1,2-dichloroethane (ethylene dichloride), which can be converted to vinyl chloride, the monomer precursor to plastic polyvinyl chloride, or combined with benzene to produce ethylbenzene, which is used in the manufacture of polystyrene, another important plastic.

Ethylene is more reactive than any alkanes because of two reasons:

1. It has a double bond, one called the π-bond(pi) and one called the σ-bond (sigma). Where π-bond is weak and σ-bond is strong. The presence of the π-bond makes it a high energy molecule. Thus bromine water decolourises readily.

2. High electron density at the double bond makes it react readily. It is broken in an addition reaction to produce many useful products.

[edit] Oxidation

Ethylene is oxidized to produce ethylene oxide, which is hydrolysed to ethylene glycol. It is also a precursor to vinyl acetate.

Main article: Wacker process

Ethylene undergoes oxidation by palladium to give acetaldehyde. This conversion was at one time a major industrial process.^[6] The process proceeds via the initial complexation of ethylene to a Pd(II) center.

[edit] In the synthesis of fine chemicals

Ethylene is useful in organic synthesis.^[7] Representative reactions include Diels-alder additions, ene reaction, and arene alkylation.

[edit] Polymerization

Main article: Ziegler-Natta catalyst

Ethylene polymerizes to produce polyethylene, also called polyethene or polythene, the world's most widely-used plastic.

[edit] Miscellaneous

Ethylene was once used as a general anesthetic applicable via inhalation, but it has long since been replaced.

It has also been hypothesized that ethylene was the catalyst for utterances of the oracle at Delphi in ancient Greece.Template:See Broad, Wm. J. (2006) The Oracle. New York ISBN 1-59420-081-5

It is also found in many lip gloss products.

Production of Ethylene in mineral oil filled transformers is a key indicator of severe localized overheating (>750 degrees C.)

[edit] Ethylene as a plant hormone

Ethylene acts physiologically as a hormone in plants.^[8][9] It stimulates the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate.

"Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening. 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 (the triple response)[see plant senescence](Arteca, 1996; Salisbury and Ross, 1992). In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene (Neljubow, 1901). Doubt discovered that ethylene stimulated abscission in 1917 (Doubt, 1917). It wasn't until 1934 that Gane reported that plants synthesize ethylene (Gane, 1934). In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as inhibition of vegetative tissues (Crocker, 1935).

Because Nicotiana benthamiana leaves are susceptible to injuries, they are used in plant physiology practicals to study ethylene secretion.

[edit] Location, characteristics, and occasions for synthesis induction

The biosysthesis of the hormone can be induced by ethylene itself, auxin and cytokinin; it is inhibited by abscisic acid.

Environmental cues can induce the biosynthesis of the plant hormone. Flooding, drought, chilling, wounding, and pathogen attack can induce the ethylene formation in the plant. In flooding, root suffers from anoxia, leading to the synthesis of the 1-Aminocyclopropane-1-carboxylic acid(ACC). As it lacks of oxygen, ACC is transported upwards in the plant and then oxidized in leaves. The product, the ethylene causes the epinasty of the leaves.

The internal development of the plant can also enhance the biosynthesis of the plant hormones. Germination can induce the synthesis of the plant hormone; it is to enhance the formation of active gibberellin for the stimulation of the enzymes in germination. Ethylene is synthesized in the ripening fruits and the senescent leaf and flowers. For the climactic fruit, the respiration rate and also the ethylene formation are sharply increased. In pollination, when the pollen reaches the stigma, the precusor of the ethylene, ACC, is secreted to the petal, the ACC releases ethylene with ACC oxidase.

• Synthesized in nodes of stems
• Rapidly diffuses
• Inhibiting effects of ethylene on shoot growth (more specifically on stem elongation) reduced in the presence of light. Also ethylene levels are decreased by light

[edit] Effects

• Stimulates leaf and flower senescence
• Induces leaf abscission mainly in older leaves.
• Induces seed germination
• Induces root hair growth – this increases the efficiency of water and mineral absorption
• Stimulates epinasty – leaf petiole grows out, leaf hangs down and curls into itself
• Stimulates fruit ripening
• Induces the growth of adventitious roots during flooding
• Affects neighboring individuals
• Disease/wounding resistance
• Triple response when applied to seedlings – root ? and shoot growth inhibition and pronounced hypocotyl hook bending
• Inhibits stem swelling or Stimulates cell broadening and lateral root growth (some sources are in disagreement)
• Interference with auxin transport (with high auxin concentrations)
• Induces flowering in pineapples
• In food production, some plants are considered ethylene producers, while others are considered ethylene sensitive.

[edit] Effects upon humans

Ethylene is colorless, has a pleasant sweet faint odor, and has a slightly sweet taste, and as it enhances fruit ripening, assists in the development of odour-active aroma volatiles (especially esters), which are responsible for the specific smell of each kind of flower or fruit. In high concentrations it can cause nausea. Its use in the food industry to induce ripening of fruit and vegetables, can lead to accumulation in refrigerator crispers, accelerating spoilage of these foods when compared with naturally ripened products.

Ethylene has long been in use as an inhalatory anaesthetic. It shows little or no carcinogenic or mutagenic properties, and although there may be moderate hyperglycemia, post operative nausea, whilst higher than nitrous oxide is less than in the use of cyclopropane. During the induction and early phases, blood pressure may rise a little, but this effect may be due to patient anxiety, as blood pressure quickly returns to normal. Cardiac arrythmias are infrequent and cardio-vascular effects are benign. Exposure at 37.5% for 15 minutes may result in marked memory disturbances. Humans exposed to as much as 50% ethylene in air, whereby the oxygen availability is decreased to 10%, experience a complete loss of consciousness and may subsequently die. Effects of exposure seem related to the issue of oxygen deprivation.

In mild doses, ethylene produces states of euphoria, associated with stimulus to the pleasure centres of the human brain. It has been hypothesised that human liking for the odours of flowers is due in part to a mild action of ethylene associated with the plant. Many geologists and scholars believe that the famous Greek Oracle at Delphi (the Pythia) went into her trance-like state as an affect of ethylene rising from ground faults.^[10]

STAGE 1) INDIFFERENCE

• Percent of O₂ Saturation at 90%
• Night vision decreased
• Mild euphoria reported.

STAGE 2) COMPENSATION

• Percent of O₂ Saturation at 82 to 90%
• Respiratory rate has compensatory increase
• Pulse, also a compensatory increase
• Night vision is decreased further, focus is simplified
• Performance ability is somewhat reduced, mild distortion to speech, utterances increasingly ambiguous.
• General Alertness level is somewhat reduced to anything but central concerns
• Symptoms may begin in those patients with pre-existing significant cardiac, pulmonary, or hematologic diseases.
• Euphoria

STAGE 3) DISTURBANCE

• Percent of O₂ Saturation at 64 to 82%
• Compensatory mechanisms increasingly become inadequate
• Air hunger, gasping for breath
• Fatigue, lassitude, inability to maintain balance
• Tunnel Vision, out-of-body experiences
• Dizziness
• Mild to Persistent Headache
• Belligerence, certainty of truth
• Extreme Euphoria, belief in capacities of the self enhanced
• Visual acuity is reduced, dreamlike seeing of visions
• Numbness and tingling of extremities
• Hyperventilation
• Distortions of judgement, abnormal or illogical inferences drawn
• Memory loss after event
• Increased Cyanosis
• Decreased ability for escape from toxic environment

STAGE 4) CRITICAL DISTURBANCE

• Percent of O₂ Saturation at 60 to 70% or less
• Further deterioration in judgement and coordination may occur in 3 to 5 minutes or less
• Total incapacitation and unconsciousness follow rapidly

In air, ethylene acts primarily as an asphyxiant. Concentrations of ethylene required to produce any marked physiological effect will reduce the oxygen content to such a low level that life cannot be supported. For example, air containing 50% of ethylene will contain only about 10% oxygen.

Loss of consciousness results when the air contains about 11% of oxygen. Death occurs quickly when the oxygen content falls to 8% or less. There is no evidence to indicate that prolonged exposure to low concentrations of ethylene can result in chronic effects. Prolonged exposure to high concentrations may cause permanent effects because of oxygen deprivation.

Ethylene has a very low order of systemic toxicity. When used as a surgical anaesthetic, it is always administered with oxygen with an increased risk of fire. In such cases, however, it acts as a simple, rapid anaesthetic having a quick recovery. Prolonged inhalation of about 85% in oxygen is slightly toxic, resulting in a slow fall in the blood pressure; at about 94% in oxygen, ethylene is acutely fatal.

[edit] References

1. ^ The Merck Index" 13th Edition, Merck & Co, Whitehouse Station, NJ. 2001. ISBN 0-911910-13-1
2. ^ Wang K, Li H, Ecker J. "Ethylene biosynthesis and signaling networks.". Plant Cell 14 Suppl: S131-51. PMID 12045274. 
3. ^ “Production: Growth is the Norm” Chemical and Engineering News, July 1 0, 2006, p. 59.
4. ^ IUPAC nomenclature rule A-3.1 The following non-systematic names are retained: Ethylene H₂C=CH₂
5. ^ Ethylene:UV/Visible Spectrum. NIST Webbook. Retrieved on 2006-09-27.
6. ^ Elschenbroich, C.;Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (2006) Wiley-VCH: Weinheim. ISBN 3-527-28165-7
7. ^ Crimmins, M. T.; Kim-Meade, A. S. "Ethylene" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
8. ^ Chow B, McCourt P (2006). "Plant hormone receptors: perception is everything.". Genes Dev 20 (15): 1998-2008. PMID 16882977. 
9. ^ De Paepe A, Van der Straeten D (2005). "Ethylene biosynthesis and signaling: an overview.". Vitam Horm 72: 399-430. PMID 16492477. 
10. ^ John Roach. "Delphic Oracle's Lips May Have Been Loosened by Gas Vapors", National Geographic, August 14, 2001. Retrieved on March 8, 2007

[edit] External links

• International Chemical Safety Card 0475
• European Chemicals Bureau
• Links to external chemical sources