magnesium salts and an extended alkyl compound. Elimination In dehydrohalogenation reactions, the halogen and an adjacent proton are removed from halocarbons, thus forming an alkene. For example, with bromoethane and sodium hydroxide (NaOH) in ethanol, the hydroxide ion HO− abstracts a hydrogen atom. Bromide ion is then lost, resulting in ethene, H2O and NaBr. Thus, haloalkanes can be converted to alkenes. Similarly, dihaloalkanes can be converted to alkynes. In related reactions, 1,2-dibromocompounds are debrominated by zinc dust to give alkenes and geminal dihalides can react with strong bases to give carbenes. Other Haloalkanes undergo free-radical reactions with elemental magnesium to give alkylmagnesium compounds: Grignard reagents. Haloalkanes also react with lithium metal to give organolithium compounds. Both Grignard reagents and organolithium compounds behave as the R− synthon. Alkali metals such as sodium and lithium are able to cause haloalkanes to couple in the Wurtz reaction, giving symmetrical alkanes. Haloalkanes, especially iodoalkanes, also undergo oxidative addition reactions to give organometallic compounds. Applications Chlorinated or fluorinated alkenes undergo polymerization. Important halogenated polymers include polyvinyl chloride (PVC), and polytetrafluoroethene (PTFE, or teflon). Alkyl fluorides An estimated one fifth of pharmaceuticals contain fluorine, including several of the top drugs. Most of these compounds are alkyl fluorides. Examples include 5-fluorouracil, flunitrazepam (Rohypnol), fluoxetine (Prozac), paroxetine (Paxil), ciprofloxacin (Cipro), mefloquine and fluconazole. Fluorine-substituted ethers are volatile anesthetics, including the commercial products methoxyflurane, enflurane, isoflurane, sevoflurane and desflurane. Alkyl chlorides Some low molecular weight chlorinated hydrocarbons such as chloroform, dichloromethane, dichloroethene, and trichloroethane are useful solvents. Several million tons of chlorinated methanes are produced annually. Chloromethane is a precursor to chlorosilanes and silicones. Chlorodifluoromethane (CHClF2) is used to make teflon. Alkyl bromides Large scale applications of alkyl bromides exploit their toxicity, which also limits their usefulness. Methyl bromide is also an effective fumigant, but its production and use are controversial. Alkyl iodides No large scale applications are known for alkyl iodides. Methyl iodide is a popular methylating agent in organic synthesis. Chlorofluorocarbons Chlorofluorocarbons were used almost universally as refrigerants and propellants due to their relatively low toxicity and high heat of vaporization. Starting in the 1980s, as their contribution to ozone depletion became known, their use was increasingly restricted, and they have now largely been replaced by HFCs. Environmental considerations As potential toxins, even carcinogens, pollution by chlorocarbons has attracted attention. Similarly, great interest has been shown in remediation of halocarbons. Volatile halocarbons degrade photochemically because the carbon-halogen