Valencies of carbon

One of the cornerstones of the chemistry of carbon compounds (organic chemistry) is Kekule s concept, proposed in 1858, of the tetra-valence of carbon. It was independently proposed in the same year by Couper who, however, got little recognition (vide infra). Kekule realized that carbon can bind at the same time to not more than four other atoms or groups. It can, however, at the same time use one or more of its valences to form bonds to another carbon atom. In this way carbon can form chains or rings, as well as multiple-bonded compounds. 

In this equation nf is the bond order it is equal to the bond number n for n = 1, 2, and 3, but has a different interpretation for fractional values. Whereas we have taken the bond number for the carbon-carbon bonds in benzene to be 1, so that the valence of carbon remains equal to the sum of the bond numbers of the bonds formed by the atom, the bond order is taken to be somewhat larger, reflecting the extra resonance energy of the molecule. The value of n calculated... 

A brilliant solution to the problem posed in the preceding section came in 1874 when J. H. van t Hoff proposed that all four valences of carbon are equivalent and directed to the corners of a regular tetrahedron.3 If we redraw the structures for C2H5Br as 1, we see that there is only one possible arrangement and, contrary to the impression we got from our earlier structural formulas, the bromine is equivalently located with respect to each of the hydrogens on the same carbon. 

In the transition from sp3 to sp2 and to sp hybridization of the valency of carbon, i.e. from alkanes to alkenes, and alkynes, the polarity of the CH bond is increased and the mobility of the hydrogen becomes greater. Electrons belonging to multiple bonds may take part in the formation of donor-acceptor complexes, and unsaturated hydrocarbons are stronger bases than saturated hydrocarbons. 

The four bridgehead positions of adamantane are formally analogous to the four tetrahedral valences of carbon. Adamantanes with four different bridgehead substituents are, therefore, chiral. 

Pasteur showed that optical activity was related to molecular right- or left-handedness (chirality). Later, van t Hoff and LeBel proposed that the four valences of carbon are directed toward the corners of a tetrahedron. If the four attached groups are different, two arrangements are possible and are related as an object and its nonsuperimposable mirror image. Enantiomers differ only in chiral (or handed) properties, such as the direction of rotation of plane-polarized light. They have identical achiral properties, such as melting and boiling points. 

This statement is hardly more informative than any popular interpretation (.e.g. [84]) of van t Hoff s original proposal that the valences of carbon are directed towards the corners of a regular tetrahedron. 

In the above examples CO2- and SO2- are ionic compounds. Their ionic weights are the algebraic sum of the atomic weights of the atoms involved the ionic weight of CO2- is 12 +(3 x 16) = 60, and the ionic weight of SO2- is 32 +(4 x 16) = 96. The valences of ionic compounds are the algebraic sum of the valences of the atoms involved. Thus the valence of carbonate is... 

You draw alkenes using the same method you learned for drawing alkanes. There is only one difference you have to place the double bond in the main chain. Remember the valence of carbon, and be careful to count to four for each carbon atom on the structure. (Figure 13.22 gives an example of another alkene, 2-methyl-2-butene.) Be especially careful with the carbon atoms on each side of the double bond. The double bond is worth two for each carbon Now complete the Practice Problems to reinforce what you have learned about naming and drawing alkenes. 

Recent investigations and theories that have to do with exceptions to the unvarying tetra-valence of carbon will not be considered in this book as they pertain to a more advanced study than is contemplated. 

The group (—COOH) is known as the carboxyl group. The general formula for acids of this class is then R—COOH or (C H2n+i)—COOH which agrees with our conception of the valence of carbon. The reaction between acetic acid and phosphorus pentachloride and that between the acid and sodium may then be written as follows ... 

Hexagon Theory and Tetra-valence of Carbon.—One point, how ever, and that a fundamental one, we have not yet considered. Does... 

Claus, Armstrong and Baeyer.—Still other formulas have been suggested on account of this difficulty, each endeavoring to avoid the necessity of oscillation in order to satisfy the four valencies of carbon. One of these was suggested by Claus and called the diagonal formula and another by Armstrong and Baeyer and known as the centric formula. 

The relationship between a chiral object and its mirror-related object is called enantiomerism. A knowledge of the existence of enantiomers was one of the reasons that van t Hoff and Le Bel proposed, as described in Chapter 1, that the four valences of carbon are spatially directed to the corners of a regular tetrahedron.The only difference between a pair of enantiomers is that, if one can be described as a left-handed form, the other will be a right-handed form they have identical chemical formulae. The major physical property that allows one to distinguish between enantiomers is the direction in which they, or their solutions, rotate the plane of polarized light, that is, when they are studied in the chiral environment provided by the polarized light. It is important to note that a molecule is not necessarily chiral just because it contains an asymmetric center, or that it is necessarily achiral because it lacks such an asymmetric center. These are not the criteria for molecular chirality. The test for chirality in a molecule is the nonsuperimposability of the object on its mirror image. 

The first proposal that the valences of carbon were arranged tetrahedrally was made by Aleksandr Mikhailovich Butlerov (1828-1886) in 1862. In an attempt to explain the isomerism (now known to be illusory) of C2H5.H and CH3.CH3, he proposed as a model a tetrahedral carbon atom, each face of which was capable of attaching a univalent atom or group. He proceeded to calculate the number of isomers to be expected in the case of methane and its substitution products if two, three, or four of the valences of carbon (even if all bonded to hydrogen) were different in character. By assuming differences in carbon affinities he was able to explain the isomerism between methyl and ethyl hydride mentioned above. 

The beginnings of stereochemistry were hardly more auspicious. The tetrahedral distribution of the valences of carbon was a necessary assumption to explain isomerism. Though there was some logic to the correlation of a tetrahedron with the number four, no successful general correlation between valence and stereochemistry was forthcoming for over half a century. Chemistry became empirical. Success could be achieved without asking the question, Why Synthetic organic chemistry flourished, while theoretical chemistry waned. 

---