Carbon fixed valence

Le Bel published his stereochemical ideas two months later, in November 1874, under the title, The relations that exist between the atomic formulas of organic compounds and the rotatory power of their solutions" (14). An English translation is presented in Le Bel (15). Le Bel approached the problem from a different direction from van t Hoff. His hypothesis was based on neither the tetrahedral model of the carbon atom nor the concept of fixed valences between the atoms. He proceeded purely from symmetry arguments he spoke of the asymmetry, not of individual atoms, but of the entire molecule, so that his views would nowadays be classed under the heading of molecular asymmetry. Only once does he mention the tetrahedral carbon atom, which he regarded as not a general principle but a special case. Today, substituted allenes, spiranes, and biphenyls are but a few examples of asymmetric molecules that do not contain any asymmetric carbons, thus confirming Le Bel s views on molecular asymmetry. The reason for the different approaches by van t Hoff and le Bel is easy to understand, van t Hoff came from the camp of structural chemists, and he... 

While the freely jointed chain is a simple model from which to begin predictions of chain dimensions, it is physically unrealistic. Since each carbon atom in a real polymer chain is tetrahedral with fixed valence bond angles of 109.5°, the links are subject to bond angle restrictions. Moreover, the links do not rotate freely because, as we have seen earlier, there are energy differences between diflferent conformations (cf. Fig. 2.3). Both of these effects cause to be larger than that predicted by the freely jointed... 

As a simple example of a flexible chain polymer, we may consider a high-molecular-weight saturated hydrocarbon, or polymethylene chain. The spatial configuration of such a singly bonded carbon chain is shown in Fig. 10. The bonds of fixed length are connected to each other at a fixed valence angle 6. Three carbon atoms, Ci, Cj, Cs define a plane with respect to which carbon atom C4 may lie any place on the circle which is the base of the cone described by the rotation of bond 3. The position of atom C4 is... 

Briefly summarize how the ideas of (a) chains of carbon atoms, 6) single fixed valences for all atoms, and (c) groups ( radicals ) of atoms also of fixed combining capacity led to disappointing results in picturing the bonding in coordination compounds. 

Radicals from Ring Systems. Univalent substituent groups derived from polycyclic hydrocarbons are named by changing the final e of the hydrocarbon name to -yl. The carbon atoms having free valences are given locants as low as possible consistent with the fixed numbering of the... 

So far, we have considered valence bonds only as fixed in number for a given element and, as Van t Hoff had shown for carbon, as having particular directions. Werner, like some but by no means all other chemists, rejected both of these notions, productive though they had proved. As far as direction was concerned, he regarded valence as an attractive force acting uniformly from the center of the atom over its entire surface. He was convinced that valence bonds could move they did not have fixed directions. 

Intramolecular forces must dominate the correlation, and this notion provides the basis for a theoretical discussion of g,--values. Intermolecular forces may be taken into account as a secondary perturbation. An important case concerns the arrangement of segmental dipoles, aligned perpendicularly with respect to the chain contour, along a polymeric molecule which has a backbone of carbon atoms. Part of the dipolar correlation is fixed, of course, by the tetrahedral nature of the carbon valence, but part depends on the possible rotation about the C-C bonds of the chain. For completely free rotation it may be shown that gT = 11/12. 

A general equation can be derived that describes the variation in direction of the valence electron density about the nucleus. The distortion from sphericity caused by valence electrons and lone-pair electrons is approximated by this equation, which includes a population parameter, a radial size function, and a spherical harmonic function, equivalent to various lobes (multipoles). In the analysis the core electron density of each atom is assigned a fixed quantity. For example, carbon has 2 core electrons and 4 valence electrons. Hydrogen has no core electrons but 1 valence electron. Experimental X-ray diffraction data are used to deri e the parameters that correspond to this function. The model is now more complicated, but gives a better representation of the true electron density (or so we would like to think). This method is useful for showing lone pair directionalities, and bent bonds in strained molecules. Since a larger number of diffraction data are included, the geometry of the molecular structure is probably better determined. 

From the foregoing discussion it is evident that the selection of certain fixed values for the bond energies of carbon bonds, even in compounds for which only one valence formula can be written, cannot be made to within a few kcals., and that the conclusions with respect... 

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