Lead-based compounds

In addition to their antiknock properties, organic lead compounds possess bactericidal properties and motor fuels with lead are known to inhibit bacterial growth during storage in contact with water. With the disappearance of lead-based compounds, it is necessary to incorporate biocides from the cyclic imine family, (piperidine, pyrrolidine, hexamethyleneimine), alkylpropylene diamines or imidazolines (Figure 9.2). 

During the 1960s, Americans lived in a lead-drenched society. They fueled their cars with leaded, antiknock gasoline. They ate food and their babies drank milk from lead-soldered cans. They stored drinking water in lead-lined tanks and transported it through lead or lead-soldered pipes. They squeezed toothpaste from lead-lined tubes and poured wine from bottles sealed with lead-covered corks. They picked fruit sprayed with lead arsenate pesticide and served it on lead-glazed dishes in houses painted and puttied with lead-based compounds. 

The development and optimisation is described of a new curing system to replace lead-based compounds used in epichlorohydrin elastomers currently used in automotive applications. The system is based on 2,4,6-trimercapto-1,3,5-triazine and dialkyldithiophosphate, which is shown to produce a scorch-safe curing system and which confers excellent physical and ageing properties on epichlorohydrin rubbers. Trials are conducted in formulations for multilayer fuel hoses. 9 refs. 

Chariot), named after a lead-based compound that one places in the crucible as the upper and lower layers during the preparation of the elixirs. The Shiyao erya Ft (Synonymic Dictionary of Materia Medica) gives... 

It is known that, when certain metal compounds are present in air at concentrations above the levels of homeostatic regulation, they can act as a health hazard. The most common metals and metal compounds that can be found in the indoors atmosphere, are mainly those of antimony, lead, (methyl) mercury, and cadmium, which can exist in plastics as additives (mostly found and used as stabilisers). These are believed to disrupt the endocrine system by causing problems in steroid production. The fate of these metal and metal ions has been more extensively studied for the lead and lead-based compounds, however, other metals have been studied much less than others. 

A lead silicate sulfate chloride has also been identified (Corbeil et ah, 1996). Meggiolaro et al. (1997) have identified a yellow lead-based compound - Pb,Fe(Sb,Zn) arsenate - among pigments found in Corinth (mid-first century ad). A lead oxide hydroxide, 3Pb0.H20 has been identified as a lead corrosion product (Tetreault et ah, 1998). 

Owing to environmental regulations, the consumption of lead-based pigments, lead chromate [7738-97-6], in particular, has been decreasing steadily at an average aimual rate of about 5% since the early 1980s. In 1993 production was estimated to be around 20,000 t (see Lead compounds). 

Lead forms two types of chemical compounds lead (II), and lead (IV) compounds based on Pb24 and Pb4 ions, where those based on Pb2 ions are the more stable. The metal is oxidized even at room temperature to lead oxide (PbO) and also by water that contains oxygen and forms lead hydroxide (Pb(OH),). In the lead-acid battery, the (less stable) lead (IV) oxide (lead dioxide, Pb02), is of greatest importance. Beside these two, a number of oxides are observed in the battery that are mostly mixtures. A brief survey will now be given of those compounds that are of interest for lead-acid batteries. 

Three-way automotive catalysts based on palladium, rather than the more expensive metals platinum and rhodium, have long been desired. However, Pd is more sensitive than R to poisoning by lead (Pb) compounds [1-4], Consequently, widespread commercial use of Pd-based automotive three-way catalysts (TWC) was delayed in the U.S. until the early 1990s, by which time residual Pb concentrations in unleaded gasoline had decreased to negligible levels. The past five years have witnessed... 

Catalytic enantioselective crossed aldehyde-ketone benzoin cyclizations of ketoaldehydes, such as 13, readily obtained from an aryl nitrile oxide and a 1,3-diketone, were studied in order to perform the synthesis of complex molecules. Significant asymmetric induction was observed with chiral triazolium salts such as 14, in the presence of DBU as base, leading to compound 15 in high yield and with 99% ee in favor of the R enantiomer <06AG(E)3492>. 

Recent ab initio calculations delineate the remarkable thermodynamic destabilization of lead(IV) compounds by electronegative substituents182,183. Based on population analyses of the molecular wave functions it was proposed that electronegative substituents increase the charge of the metal and increase the difference in the radial extensions of the 6s and 6p orbitals. By increasing the differences in the radial extensions of the s and p orbitals, 6th-row relativistic effects also contribute to a destabilization of the higher valence state. 

The sequence of reactions leading to compound 83 by an elimination that liberates the aldehyde 82, which is immediately reduced to 83, is depicted. This result also explains the formation of 43 by Purdie-Irvine methylations of 4 (Ref. 29) and 33 (Ref. 24) in both, moist silver oxide is the base, and it initiates elimination followed... 

Parkes A process for removing silver from lead, based on the use of zinc, which forms in-termetallic compounds of lower melting point. Developed by A. Parkes in Birmingham, England, in the 1850s. Parkes also invented the first plastic (Parkesine), used for making billiard balls. 

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