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| Two representations ofchloroform. |
Organochlorine chemistry is concerned with the properties oforganochlorine compounds, ororganochlorides,organic compounds that contain one or morecarbon–chlorinebonds.[1] Thechloroalkane 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, withTCDD being one of the most notorious.[2]
Organochlorides such astrichloroethylene,tetrachloroethylene,dichloromethane andchloroform are commonly used as solvents and are referred to as "chlorinated solvents".[citation needed]
Chlorination modifies the physical properties of hydrocarbons in several ways. These compounds are typically denser thanwater 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.[citation needed]
Aliphatic organochlorides are oftenalkylating agents as chlorine can act as aleaving group, which can result in cellular damage.[citation needed]
Many organochlorine compounds have been isolated from natural sources ranging from bacteria to humans.[3][4] Chlorinated organic compounds are found in nearly every class of biomolecules andnatural products includingalkaloids,terpenes,amino acids,flavonoids,steroids, andfatty acids.[3][5]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.[6] In addition, a variety of simple chlorinated hydrocarbons includingdichloromethane,chloroform, andcarbon tetrachloride have been isolated from marine algae.[7] A majority of thechloromethane in the environment is produced naturally by biological decomposition, forest fires, and volcanoes.[8]
The natural organochlorideepibatidine, an alkaloid isolated from tree frogs, has potentanalgesic effects and has stimulated research into new pain medication. However, because of its unacceptabletherapeutic index, it is no longer a subject of research for potential therapeutic uses.[9] The frogs obtain epibatidine through their diet which is then sequestered into their skin. Likely dietary sources are beetles, ants, mites, and flies.[10]
Alkanes andaryl 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 theFriedel–Crafts reaction, using chlorine and aLewis acid catalyst.[2]
Thehaloform reaction, using chlorine andsodium hydroxide, is also able to generate alkyl halides from methyl ketones, and related compounds. Chloroform was formerly produced thus.[citation needed]
Chlorine adds to the multiple bonds on alkenes and alkynes as well, giving di- or tetra-chloro compounds.[citation needed]
Alkenes react withhydrogen chloride (HCl) to give alkyl chlorides. For example, the industrial production ofchloroethane proceeds by the reaction ofethylene with HCl:[citation needed]
Inoxychlorination, hydrogen chloride instead of the more expensive chlorine is used for the same purpose:
Secondary and tertiary alcohols react with hydrogen chloride to give the corresponding chlorides. In the laboratory, the related reaction involvingzinc chloride in concentratedhydrochloric acid:
Called theLucas reagent, this mixture was once used inqualitative organic analysis for classifying alcohols.
Alkyl chlorides are most easily prepared by treating alcohols withthionyl chloride (SOCl2) orphosphorus pentachloride (PCl5), but also commonly with sulfuryl chloride (SO2Cl2) andphosphorus trichloride (PCl3):
In the laboratory, thionyl chloride is especially convenient, because the byproducts are gaseous. Alternatively, theAppel reaction can be used:
Alkyl chlorides are versatile building blocks in organic chemistry. While alkyl bromides and iodides are more reactive, alkyl chlorides tend to be less expensive and more readily available. Alkyl chlorides readily undergo attack by nucleophiles.[citation needed]
Heating alkyl halides withsodium hydroxide orwater gives alcohols. Reaction withalkoxides oraryloxides giveethers in theWilliamson ether synthesis; reaction withthiols givethioethers. Alkyl chlorides readily react withamines to give substitutedamines. Alkyl chlorides are substituted by softer halides such as theiodide in theFinkelstein reaction. Reaction with otherpseudohalides such asazide,cyanide, andthiocyanate are possible as well. In the presence of a strong base, alkyl chlorides undergo dehydrohalogenation to givealkenes oralkynes.[citation needed]
Alkyl chlorides react withmagnesium to giveGrignard reagents, transforming anelectrophilic compound into anucleophilic compound. TheWurtz reaction reductively couples two alkyl halides to couple withsodium.[citation needed]
Some organochlorides (such asethyl chloride) may be used asalkylating agents.Tetraethyllead was produced fromethyl chloride and asodium–leadalloy:[11][12]
Reductive dechlorination is rarely useful in chemical synthesis, but is a key step in thebiodegradation of several organochlorinepersistent pollutants.[citation needed]
The largest application of organochlorine chemistry is the production ofvinyl chloride. The annual production in 1985 was around 13 million tons, almost all of which was converted intopolyvinylchloride (PVC).[citation needed]
Most low molecular weight and liquid chlorinated hydrocarbons such asdichloromethane,chloroform,carbon tetrachloride,dichloroethylene,trichloroethylene,tetrachloroethylene,1,2-Dichloroethane andhexachlorobutadiene are useful solvents. These solvents tend to be relativelynon-polar; they are therefore immiscible with water and effective in cleaning applications such asdegreasing anddry cleaning for their ability to dissolveoils andgrease. They are mostly nonflammable or have very low flammability.[citation needed]
Some, like carbon tetrachloride and1,1,1-Trichloroethane have been phased out due to their toxicity or negative environmental impact (ozone depletion by 1,1,1-Trichloroethane).[citation needed]
Several billion kilograms of chlorinated methanes are produced annually, mainly by chlorination of methane:
The most important is dichloromethane, which is mainly used as a solvent. Chloromethane is a precursor tochlorosilanes andsilicones. Historically significant (as an anaesthetic), but smaller in scale is chloroform, mainly a precursor tochlorodifluoromethane (CHClF2) andtetrafluoroethene which is used in the manufacture of Teflon.[2]
The two main groups of organochlorineinsecticides are theDDT-type compounds and the chlorinatedalicyclics.Their mechanism of action differs slightly.
Polychlorinated biphenyls (PCBs) were once commonly used electrical insulators and heat transfer agents. Their use has generally been phased out due to health concerns. PCBs were replaced bypolybrominated diphenyl ethers (PBDEs), which bring similar toxicity andbioaccumulation concerns.[citation needed]
Some types of organochlorides have significant toxicity to plants or animals, including humans. Dioxins, produced when organic matter is burned in the presence of chlorine, arepersistent organic pollutants which pose dangers when they are released into the environment, as are some insecticides (such asDDT). For example, DDT, which was widely used to control insects in the mid-20th century, also accumulates in food chains, as do its metabolitesDDE andDDD, and causes reproductive problems (e.g., eggshell thinning) in certain bird species.[15] DDT also posed further issues to the environment as it is extremely mobile, traces even being found in Antarctica despite the chemical never being used there. Some organochlorine compounds, such assulfur mustards,nitrogen mustards, andLewisite, are even used aschemical weapons due to their toxicity.[citation needed]
However, the presence of chlorine in an organic compound does not ensure toxicity. Some organochlorides are considered safe enough for consumption in foods and medicines. For example, peas and broad beans contain the natural chlorinated plant hormone4-chloroindole-3-acetic acid (4-Cl-IAA);[16][17] and the sweetenersucralose (Splenda) is widely used in diet products. As of 2004[update], at least 165 organochlorides had been approved worldwide for use as pharmaceutical drugs, including the natural antibioticvancomycin, the antihistamineloratadine (Claritin), the antidepressantsertraline (Zoloft), the anti-epilepticlamotrigine (Lamictal), and the inhalation anestheticisoflurane.[18]
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