Carbon tetravalent nature

Shortly after the tetravalent nature of carbon was proposed, extensions to the Kekule-Couper theory were made w7hen the possibility of multiple bonding between atoms was suggested. Emil Erlenmeyer proposed a carbon-carbon triple bond for acetylene, and Alexander Crum Brown proposed a carbon-carbon double bond for ethylene. In 1865, Kekule provided another major advance when he suggested that carbon chains can double back on themselves to form rings of atoms. 

Although Kekule and Couper were correct in describing the tetravalent nature of carbon, chemistry was still viewed in a two-dimensional way until 1874. In that year, Jacobus van t Hoff and Joseph Le Bel added a third dimension to our ideas about organic compounds when they proposed that the four bonds of carbon are not oriented randomly but have specific spatial directions. Van t Hoff went even further and suggested that the four atoms to... 

In 1859 Kekule started to use graphical representations of organic molecules, in part to emphasize the tetravalent nature of carbon atoms and their ability to form chains. He then turned his attention to the structure of benzene (CsHg), a compound with unusual properties that could not be explained by any theories of the day. 

Although these two papers contain no theoretical statements other than those just cited, their content is consistent with the idea that Kekule was deliberately building the factual groundwork for a future more general statement about how one might use the substitution-values of atoms schematically to build up proposed molecular constitutions. It is even possible that Kekule may have chosen precisely the fulminate series for his efforts because it so clearly illustrated the "tet-ratomic" (i.e., tetravalent) nature of carbon. ... 

The formation of carbosilanes in the gas phase pyrolysis of methylsilanes can be understood from an appraisal of the bond energies of possible reaction products. It is worth considering also the formation of molecules with Si—Si groups, as well as compounds with larger C—C units, as a result of the tetravalent nature of silicon and carbon. However, these were not observed only molecular structures with an Si—C—Si skeleton were found. 

Carbosilanes are compounds in which the elements silicon and carbon alternate in the molecular skeleton [1]. Just as the alkanes are formally derived from the diamond lattice and the aromatics from the graphite lattice, the carbosilanes are structurally derived from silicon carbide. Because of the tetravalent nature of silicon and carbon we can expect stable linear, cyclic and polycyclic compounds to occur. However, carbosilanes do not exist in nature. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

A chemical reaction Is the result of competition It Is a race that Is won by the fastest runner. A collection of molecules tend to do, by and large, what Is easiest for them. An alkyl halide with p-hydrogen atoms when reacted with a base or a nucleophile has two competing routes substitution (Sj,jl and Sj,j2) and elimination. Which route will be taken up depends upon the nature of alkyl halide, strength and size of base/nucleophile and reaction conditions. Thus, a bulkier nucleophile will prefer to act as a base and abstracts a proton rather than approach a tetravalent carbon atom (steric reasons) and vice versa. 

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

Also present in many natural waters are humic/fulvic acid, citric acid, and the like. These organics also can complex actinides. In Figure 15.18, we show the relative stability constants for the first complexation reaction of various ligands with actinides of different oxidation states. Clearly, the carbonate and humate ions along with hydrolysis dominate the chemistry. The tetravalent actinide ions will tend toward hydrolysis reactions or carbonate complexation rather than humate/fulvate formation. 

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