ethylene n : a flammable colorless gaseous alkene; obtained from petroleum and natural gas and used in manufacturing many other chemicals; sometimes used as an anesthetic [syn: ethene]

English

Pronunciation

• a UK: /ˈɛθəˌliːn/

Noun

1. The common name for the organic chemical compound ethene. The simplest alkene, a colorless gaseous (at room temperature and pressure) hydrocarbon with the chemical formula C2H4.
2. The divalent radical derived from ethane.

Synonyms

• IUPAC name: ethene

Derived terms

• polyethylene
• ethylene glycol
• ethylene oxide
• polyethylene terephthalate (ester)

Translations

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 extremely important in industry and even has a role in biology as a hormone. Ethylene is the most produced organic compound in the world; global production of ethylene exceeded 75 million metric tonnes per year in 2005. To meet the ever increasing demand for ethylene, sharp increases in production facilities have been added globally, particularly in the Gulf countries.

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 position.

History

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 (1,2-dichloroethane).Ethylene was 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 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.

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 correct name is now ethene.

Uses

Approximately 80% of ethylene used in the United States and Europe is used to create ethylene oxide, ethylene dichloride, and polyethylene. In smaller quantities, ethylene is used as an anesthetic agent (in an 85% ethylene/15% oxygen ratio), to hasten fruit ripening, as well as a welding gas.

The areas of an ethylene plant are:

1. steam cracking furnaces;
2. primary and secondary heat recovery with quench;
3. a dilution steam recycle system between the furnaces and the quench system;
4. primary compression of the cracked gas (3 stages of compression);
5. hydrogen sulfide and carbon dioxide removal (acid gas removal);
6. secondary compression (1 or 2 stages);
7. drying of the cracked gas;
8. cryogenic treatment;
9. 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. Different methods of cryogenically treating this overhead stream results in the separation of the hydrogen and the methane. This usually involves liquid methane at a temperature around -250 degrees F. Complete recovery of all the methane is critical to the economical operation of an ethylene plant. Often one or two Turboexpanders are used for Methane recovery from the demethanizer overhead stream.
10. the bottom stream from the demethanizer tower goes to the deethanizer tower. The overhead stream from the deethanizer tower consists of all the C2,'s that were in the cracked gas stream. The C2's then go 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;
11. the bottom stream from the deethanizer 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. Prior to sending the C3's to the C3 splitter this stream is hydrogenated in order to react out the methylacetylene and propadiene. Then this stream is sent to the C3 splitter. The overhead stream from the C3 splitter is product propylene and the bottom stream from the C3 splitter is propane which can be sent back to the furnaces for cracking or used as fuel.
12. 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 was in the cracked gas stream. The bottom stream from the debutanizer consists of everything in the cracked gas stream that is C5 or heavier. This could be called a light pyrolysis gasoline.

Peculiarity of spectrum

Although ethylene is a relatively simple molecule, its spectrum 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.

Chemical reactions

Ethylene is an extremely important building block in the petrochemical industry. It can undergo many types of reactions which leads to a plethora of major chemical products. A list of some major types of reactions includes, 1) Polymerization, 2) Oxidation, 3) Halogenation and Hydrohalogenation, 4) Alkylation, 5) Hydration, 6) Oligomerization, 7) Oxo-reaction, and 8) a ripening agent for fruits and vegetables (see Physiological responses of plants). The process proceeds via the initial complexation of ethylene to a Pd(II) center.

Major intermediates of the oxidation of Ethylene are ethylene oxide, acetaldehyde, vinyl acetate and ethylene glycol. The list of products made from these intermediates is long. Some of them are: polyesters, polyurethane, morpholine, ethanolamines, aspirin and glycol ethers. Representative reactions include Diels-Alder additions, ene reaction, and arene alkylation.

Miscellaneous

Ethylene is 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).

Ethylene as a plant hormone

Ethylene acts physiologically as a hormone in plants. It exists as a gas and 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. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate.

History of ethylene in plant biology

Ethylene has been used in practice since the ancient Africans, 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 (Arteca, 1996; Salisbury and Ross, 1992). In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene . 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, such as the dropping of leaves (Crocker, 1935).

Ethylene biosynthesis in plants

Environmental and biological triggers of ethylene

Environmental cues can induce the biosynthesis of the plant hormone. Flooding, drought, chilling, wounding, and pathogen attack can induce ethylene formation in the plant.

In flooding, root suffers 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 product, the ethylene causes epinasty of the leaves.

One speculation recently put forth for epinasty is the downard pointing leaves may act as pump handles in the wind. The ethylene may or may not additionally induce the growth of a valve in the xylem, but the idea would be that the plant would harness the power of the wind to pump out more water from the roots of the plants than would normally happen with transpiration.

Physiological responses of plants

Like the other plant hormones, ethylene is considered to have pleiotropic effects. This essentially means that it is thought that at least some of the effects of the hormone are unrelated. What is actually caused by the gas may depend on the tissue affected as well as environmental conditions. In the evolution of plants, ethylene would simply be a message that was coopted for unrelated uses by plants during different periods of the evolutionary development.

List of Plant Responses to Ethylene

• Seedling triple response, thickening and shortening of hypocotyl with pronounced apical hook. This is thought to be a seedling's reaction to an obstacle in the soil such a stone, allowing it to push past the obstruction.
• In pollination, when the pollen reaches the stigma, the precursor of the ethylene, ACC, is secreted to the petal, the ACC releases ethylene with ACC oxidase.
• Stimulates leaf and flower senescence
• Stimulates senescence of mature xylem cells in preparation for plant use
• Inhibits shoot growth except in some habitually flooded plants like rice
• 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 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. This can be the one bad apple in a barrel spoiling the rest phenomenon.
• Affects neighboring individuals
• Disease/wounding resistance
• Triple response when applied to seedlings – stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally. This strategy is thought to allow a seedling grow around an obstacle
• Inhibits stem growth outside of seedling stage
• Stimulates stem and cell broadening and lateral branch growth also outside of seedling stage
• Synthesis is stimulated by auxin and maybe cytokinin as well
• Ethylene levels are decreased by light
• The flooding of roots stimulates the production of ACC which travels through the xylem to the stem and leaves where it is converted to the gas
• Interference with auxin transport (with high auxin concentrations)
• Inhibits stomatal closing except in some water plants or habitually flooded ones such as some rice varieties, where the opposite occurs (conserving CO2 and O2)
• Where ethylene induces stomatal closing, it also induces stem elongation
• Induces flowering in pineapples

Commercial Issues

Ethylene shortens the shelf life of many fruits by hastening fruit ripening and floral senescence. Tomatoes, bananas, and apples will ripen faster in the presence of ethylene. Bananas placed next to other fruits will produce enough ethylene to cause accelerated fruit ripening. 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 come up with several ways to inhibit ethylene, including inhibiting ethylene synthesis and inhibiting ethylene perception. 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. An 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 be treated with the gas in a chamber or by placing a banana peel next to the plant in an enclosed area.

Effects upon humans

Depending on the concentration, ethylene gas can cause a pleasant odor, euphoria, nausea, hyperglycemia, a variety of psychological effects, blood pressure changes, hypoxia, loss of consciousness, or death.

Symptoms

Ethylene 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 mild doses, ethylene produces states of euphoria, associated with stimulus to the pleasure centers of the human brain.

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 due to hypoxia.

Symptoms of ethylene exposure include the following.

Mild exposure in air

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

Moderate exposure in air

• 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

High concentration in air

• 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 judgment, abnormal or illogical inferences drawn
• Memory loss after event
• Increased cyanosis
• Decreased ability for escape from toxic environment

Very high concentration in air

• Percent of O2 saturation at 60 to 70% or less
• Further deterioration in judgment and coordination may occur in 3 to 5 minutes or less

Severe oxygen deprivation

• Loss of consciousness results when the air contains about 11% of oxygen.
• Death occurs quickly when the oxygen content falls to 8% or less.

Very high concentrations in oxygen

• Prolonged inhalation of about 85% in oxygen is slightly toxic, resulting in a slow fall in blood pressure.
• At about 94% in oxygen, ethylene is acutely fatal.

Medical and historical use

Ethylene has long been in use as an inhalatory anaesthetic. 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.

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.

Safety

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. Prolonged inhalation of about 85% in oxygen (a relatively high concentration) is also slightly toxic, resulting in a slow fall in blood pressure. At about 94% in oxygen, ethylene is acutely fatal.

It shows little or no carcinogenic or mutagenic properties. Although there may be moderate hyperglycemia, post operative nausea - while 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.

See also

References

External links

• International Chemical Safety Card 0475
• European Chemicals Bureau
• Speculations Towards a General Plant Hormone Theory