Tetravalent Organic Lead Compounds as Gasoline Additives

Whereas the compounds of Pb(Il) are ionic, most Pb(IV) compounds are covalent molecules rather than ionic compounds of Pb4+. In this respect, tetravalent lead is similar to the corresponding form of the other elements (C, Si, Ge, Sn) in its group of the periodic table.

Commercially and environmentally, the most important covalent compounds of lead (IV) are tetraalkyl compounds, PbR4, especially those formed with the methyl group, CH3, and the ethyl group, CH2CH3—namely tetra-methyllead, Pb(CH3)4, and tetraethyllead, Pb(C2H5)4. In the past, both compounds found widespread use as additives to gasoline—about a gram per liter—to produce leaded gasoline. As discussed in Chapter 7, this practice has now been phased out in North America and in many other developed countries, except in some types of aviation fuel, for which no acceptable substitute for lead has yet been found.

Since tetraalkyl lead compounds are volatile, they evaporate to some extent from gasoline and enter the environment in gaseous form. They are not water-soluble, but they are readily absorbed through the skin. In the human liver, FbR4 molecules are converted into the more toxic compounds of PbR3+, which are neurotoxins because they can cross the blood-brain barrier. In substantial doses, these organic compounds of lead cause symptoms that mimic psychosis. It is not clear what the effects may be, if any, of chronic low-level exposure to them. At very high exposures, tetraalkyllead compounds are fatal, as was discovered many years ago when several employees of the companies that originally produced these compounds died. In contrast to mercury, little or no methylation of inorganic lead occurs in nature. Thus almost all the tetraalkylated lead in the environment probably originated from leaded gasoline.

Continue reading here: Environmental Lead from Leaded Gasoline

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