Organochlorine chemistry is concerned with the properties of organochlorine compounds, or organochlorides, organic compounds that contain one or more carbon–chlorine bonds. The chloroalkane class (alkanes with one or more hydrogens substituted by chlorine) includes common examples. The wide structural variety and divergent chemical properties of organochlorides lead to a broad range of names, applications, and properties. Organochlorine compounds have wide use in many applications, though some are of profound environmental concern, with DDT and TCDD being among the most notorious. Organochlorides such as trichloroethylene, tetrachloroethylene, dichloromethane and chloroform are commonly used as solvents and are referred to as "chlorinated solvents".
Physical and chemical properties Chlorination modifies the physical properties of hydrocarbons in several ways. These compounds are typically denser than water due to the higher atomic weight of chlorine versus hydrogen. They have higher boiling and melting points compared to related hydrocarbons. Flammability reduces with increased chlorine substitution in hydrocarbons. Aliphatic organochlorides are often alkylating agents as chlorine can act as a leaving group, which can result in cellular damage.
Natural occurrence Many organochlorine compounds have been isolated from natural sources ranging from bacteria to humans (e.g 3-chlorotyrosine). Chlorinated organic compounds are found in nearly every class of biomolecules and natural products including alkaloids, terpenes, amino acids, flavonoids, steroids, and fatty acids. Dioxins, which are of particular concern to human and environmental health, are produced in the high temperature environment of forest fires and have been found in the preserved ashes of lightning-ignited fires that predate synthetic dioxins. In addition, a variety of simple chlorinated hydrocarbons including dichloromethane, chloroform, and carbon tetrachloride have been isolated from macroalgae. A majority of the chloromethane in the environment is produced naturally by microbial communities, forest fires, and volcanoes. The natural organochloride epibatidine, an alkaloid isolated from tree frogs, has potent analgesic effects and has stimulated research into new pain medication. However, because of its unacceptable therapeutic index, it is no longer a subject of research for potential therapeutic uses. The frogs obtain epibatidine through their diet which is then sequestered into their skin. Likely dietary sources are beetles, ants, mites, and flies.
Preparation
From chlorine Alkanes and aryl alkanes may be chlorinated under free radical conditions, with UV light. However, the extent of chlorination is difficult to control. Aryl chlorides may be prepared by the Friedel–Crafts reaction, using chlorine and a Lewis acid catalyst. The haloform reaction, using chlorine and sodium hydroxide, is also able to generate alkyl halides from methyl ketones, and related compounds. Chloroform was formerly produced thus. Chlorine adds to the multiple bonds on alkenes and alkynes as well, giving di- or tetra-chloro compounds.
Reaction with hydrogen chloride Alkenes react with hydrogen chloride (HCl) to give alkyl chlorides. For example, the industrial production of chloroethane proceeds by the reaction of ethylene with HCl:
H2C=CH2 + HCl → CH3CH2Cl In oxychlorination, hydrogen chloride instead of the more expensive chlorine is used for the same purpose:
CH2=CH2 + 2 HCl + 1⁄2 O2 → ClCH2CH2Cl + H2O. Secondary and tertiary alcohols react with hydrogen chloride to give the corresponding chlorides. In the laboratory, the related reaction involving zinc chloride in concentrated hydrochloric acid:
R − OH + HCl → Δ ZnCl 2 R − Cl alkyl halide + H 2 O {\displaystyle {\ce {{R-OH}+HCl->[{\ce {ZnCl2}}][\Delta ]{\overset {alkyl\ halide}{R-Cl}}+H2O}}}
Called the Lucas reagent, this mixture was once used in qualitative organic analysis for classifying alcohols.
Other chlorinating agents Alkyl chlorides are most easily prepared by treating alcohols with thionyl chloride (SOCl2) or phosphorus pentachloride (PCl5), but also commonly with sulfuryl chloride (SO2Cl2) and phosphorus trichloride (PCl3):
ROH + SOCl2 → RCl + SO2 + HCl 3 ROH + PCl3 → 3 RCl + H3PO3 ROH + PCl5 → RCl + POCl3 + HCl In the laboratory, thionyl chloride is especially convenient, because the byproducts are gaseous. Alternatively, the Appel reaction can be used:
… excerpt ends here. Continue reading the full article.
