VINYL CHLORIDE

also called chloroethylene a colourless, flammable, toxic gas belonging to the family of organic halogen compounds, and used principally in making polyvinyl chloride (q.v.), an important synthetic resin. Vinyl chloride was first prepared in 1835 by the reaction of ethylene chloride with caustic potash; its slow transformation into a flaky solid when exposed to sunlight also was noted but not recognized as polymerization until many years later. An industrial process for making vinyl chloride from acetylene and hydrogen chloride in the presence of mercury(II) chloride was discovered early in the 20th century, but commercial production was not begun until World War II, when substitutes for natural rubber were required. The process based on acetylene has remained important in Europe, but in the United States vinyl chloride is commonly manufactured from ethylene chloride, either by heating it to temperatures of 480510 C (900950 F) or by treating it with a dilute solution of caustic soda. Its chemical properties are similar to those of olefins, but, apart from its conversion to polymers, its only important reaction is that with hypochlorous acid, forming chloroacetaldehyde, which is used in making pharmaceuticals such as sulfathiazole. Prolonged exposure of humans and laboratory animals to vinyl chloride vapour has been linked to several forms of cancer.