Organoleads Subject

As shown in Table 19, amounts of organolead were frequently below the detection limits and levels of both organic and inorganic lead were found to be subject to wide fluctuations which did not correspond closely to the water body. 

The major organolead compounds found in the environment are the tetraalkyllead compounds and their di- and trialkyl decomposition products. Elevated levels of tetraalkyl-leads have two possible sources either (i) anthropogenic leaded petroleum inputs or (ii) environmental methylation of natural lead compounds. While the former is well established, the latter is the subject of some controversy in the literature. Interest in the environmental methylation process derives from the increased toxicity of methyllead compounds compared to their inorganic analogs. 

The physical properties of the various types of organolead compounds have not been investigated as systematically as those of the other metals. Most of the physical properties measurements have been devoted to the tetraorganolead compounds, although in the past few years some interest has been generated in the organolead halides and carboxylates. A detailed discussion of the physical properties is beyond the scope of this article. This subject has been reviewed recently by Shapiro and Frey 289>. 

Another advantage of lead compounds is their ability to tolerate a variety of functional groups. This is noticeable from their usefulness in modern syntheses of complex natural products, one of the main subjects of this chapter. In early studies, for example, Pb(OAc)4 found particular application in alkaloid synthesis [28]. Donnelly and Finet have focused on the compact array of aromatic rings found in several members of the flavonoid family [29] by use of organolead species. A review of this new approach to arylation reactions with aryllead compounds can also be found in the total synthesis of natural products [30]. 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

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