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Ethylene (or IUPAC name ethene) is the chemical compound with the formula CH2CH2. 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.• Ethylene is the organic compound produced on the largest scale: global production of ethylene exceeded 75,000,000 metric tonnes per year in 2005.
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. 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 (ethylene dichloride), first synthesized in 1795 by a collaboration of four Dutch chemists. In the mid-19th century, the suffix -ene (a 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 word 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*. 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. 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 in to 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. Theoretical considerations 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. 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 ethylene dichloride (1,2-dichloroethane), 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. Oxidation Ethylene are oxidized to produce ethylene oxide, which is hydrolysed to ethylene glycol. It is also a precursor to vinyl acetate. Ethylene undergoes oxidation by palladium to give acetaldehyde. This conversion was at one time a major industrial process. The process proceeds via the initial complexation of ethylene to a Pd(II) center. In the synthesis of fine chemicals Ethylene is useful in organic synthesis. Representative reactions include Diels-alder additions, ene reaction, and arene alkylation. Polymerization Ethylene polymerizes to produce polyethylene, also called polyethene or polythene, the world's most widely-used plastic. Miscellaneous Ethylene was once used as a local 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. 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.) Ethylene as a plant hormone Ethylene acts physiologically as a hormone in plants.•• 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. It was in 1864, gas leaks from street lights showed 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). Ethylene is now known to have many other functions as well." - from (plant-hormones.info) Since Nicotiana benthamiana leaves are susceptible to injuries, they are used in plant physiology practicals to study ethylene secretion. Location, characteristics, and occasions for synthesis induction Effects 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. STAGE 1) INDIFFERENCE STAGE 2) COMPENSATION STAGE 3) DISTURBANCE STAGE 4) CRITICAL DISTURBANCE 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. | |||||||||||||||||||||
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