Carbon atoms chemical precursors

Butadiene, used in the chemical industiy as a precursor of synthetic mbber, is a hydrocarbon with the formula C4 Hg. (See Box 13-1 to learn more about the mbber industry.) The Lewis stmcture of butadiene contains two double bonds on sequential pairs of carbon atoms. The chemistry of butadiene, including its ability to form mbber, can be traced to the delocalized electrons in the tt system of the molecule. 

While evaporative methods of generating C atoms utilize bulk carbon and a great deal of thermal or photochemical energy, another approach is to use the exother-micity of the decomposition of a suitable precursor to produce C atoms. These methods yield what are called chemically generated carbon atoms and will be outlined below. 

Sol-gel processing almost exclusively involves metal alkoxides as precursor chemicals. An alkoxide is an alkane with an oxygen interposed between at least one of the carbon atoms and the metal—for example, (OC2H5)4Si, known as tetraethoxysilane (TEOS, a.k.a. tetratethyl orthosilicate). Other common alkoxide precursors are listed in Table 7.15. TEOS is used to produce the sol-gel-derived silica that generated the X-ray diffraction pattern in Figure 1.50. Let us use silica as an example to study the steps in the sol-gel process. 

Because of the importance of carbonyl groups to the mechanism of condensation reactions, much of the assembly of either straight-chain or branched-carbon skeletons takes place between compounds in which the average oxidation state of the carbon atoms is similar to that in carbohydrates (or in formaldehyde, H2CO). The diversity of chemical reactions possible with compounds at this state of oxidation is a maximum, a fact that may explain why carbohydrates and closely related substances are major biosynthetic precursors and why the average state of oxidation of the carbon in... 

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