CCl 2F 2 and other chlorofluorocarbons) used as refrigeration gases and fire extinguishing agents. Other organic halogen compounds that have been implicated in environmental damage include the polychloro- and polybromo-biphenyls (PCBs and PBBs), used as heat transfer fluids and fire retardants and freons (e.g. 2,4,5-T and 2,4-D are common herbicides that are sold by most garden stores. Because DDT is a cheap and effective mosquito control agent, underdeveloped countries in Africa and Latin America have experienced a dramatic increase in malaria deaths following its removal, and arguments are made for returning it to limited use. have been used as pesticides, but their persistence in the environment, once applied, has led to restrictions, including banning, of their use in developed countries. Some halogen compounds, shown in the box. Many of these have proven useful as intermediates in traditional synthetic processes. Synthetic organic halogen compounds are readily available by direct halogenation of hydrocarbons and by addition reactions to alkenes and alkynes. Many subsequent chemical and biological processes produce poly-halogenated methanes. Furthermore, the ocean is also estimated to supply 10-20% of atmospheric methyl chloride, with other significant contributions coming from biomass burning, salt marshes and wood-rotting fungi. The ocean is the largest known source for atmospheric methyl bromide and methyl iodide. However, the halogen rich environment of the ocean has produced many interesting natural products incorporating large amounts of halogen. The thyroid hormones T 3 and T 4 are exceptions as is fluoroacetate, the toxic agent in the South African shrub Dichapetalum cymosum, known as "gifblaar". Halogen containing organic compounds are relatively rare in terrestrial plants and animals.
0 kommentar(er)
