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(also ethene), H2C═CH2, an unsaturated hydrocarbon; the first member of the homologous series of olefins (alkenes).
Ethylene takes the form of a colorless gas with a weak ethereal odor. It has a melting point of –169.5°C, a boiling point of –103.8°C, and a density of 0.570 g/cm3 at –103.8°C. Virtually insoluble in water, it is poorly soluble in ethyl alcohol; it dissolves better in ether and acetone. Ethylene has a flash point of 540°C, and it burns with a slightly smoky flame. Mixtures of ethylene and air become explosive when the concentration of ethylene is 3–34 percent by volume.
Ethylene is highly reactive. Its most characteristic reaction is addition to the carbon-carbon double bond. For example, the catalytic hydrogenation of ethylene yields ethane according to the reaction
H2C═CH2 + H2 → H3C—CH3
Similarly, the chlorination of ethylene produces dichloroethane, as described by the equation
H2C═CH2 + C12 → C1H2C—CH2Cl
Hypochlorination (the addition of hypochlorous acid) yields ethylene chlorohydrin according to the reaction
H2C═CH2 + HOCl → HOH2C—CH2Cl
Many reactions of ethylene serve as a basis for industrial methods of producing important chemicals. For example, ethyl alcohol is produced by either the single-step or two-step hydration of ethylene (see), and ethylene oxide and acetaldehyde are produced by the oxidation of ethylene. The alkylation of benzene by ethylene gives ethylbenzene (seeFRIEDEL-CRAFTS REACTION), polymerization gives polyethylene (in the presence of Ziegler-Natta catalysts, for example), and oxidative chlorination yields vinylchloride. Vinyl acetate is produced by reacting ethylene with acetic acid; ethyl chloride, by adding hydrogen chloride; and mustard gas, by reacting ethylene with sulfur chlorides.
The principal industrial method for producing ethylene is the thermal cracking (at 700°-850°C) of liquid petroleum distillates and lower alkanes—mainly ethane and propane (seePETROLEUM REFINING GASES). The separation and purification of ethylene are performed by fractional distillation, fractional absorption, and deep cooling. In the laboratory, ethylene is prepared by the dehydration of ethyl alcohol; this process may be accomplished by various methods, including heating the alcohol with either sulfuric or phosphoric acid.
Ethylene in living organisms. Ethylene is formed in very small amounts in plant and animal tissues as a metabolic intermediate. In the fruits, flowers, leaves, stems, and roots of plants, it interferes with the activity and biosynthesis of a class of plant hormones known as auxins, which similarly inhibit the activity and biosynthesis of ethylene. When ethylene is predominant, it slows the growth of plants and accelerates the aging, ripening, and falling of fruits and the shedding of flowers (or their corollas), pericarp, and leaves; when auxins predominate, their effects include inhibition of the aging, ripening, and falling of fruit. The biosynthetic pathways of ethylene and its metabolism in plant tissues have not been definitively established.
Ethylene is used to accelerate the ripening of fruits (including tomatoes, melons, oranges, mandarins, lemons, and bananas), defoliate plants, reduce the falling of fruits before the harvest, and reduce the strength of the attachment of the fruit to the mother plant (thus facilitating mechanized harvesting). In high concentrations, ethylene has an anesthetic effect on humans and animals.
REFERENCESJensen, E. “Etilen i poliatsetileny.” Biokhimiia rastenii. Moscow, 1968. (Translated from English.)
“Stimuliatsiia i tormozhenie fiziologicheskikh protsessov u rastenii.” In the collection Istoriia i sovremennoe sostoianoe fiziologii rastenii. Moscow, 1967.
IU. V. RAKITIN