Carbon Tetra-valence

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. 

This has been looked for previously but without success. The structure of the diamond cannot be explained on the hypothesis that the field of force around the carbon atom is the same in all directions or in other words, that the force between the two atoms can be expressed simply by a function of the distance between the centres. If this were so, the sphere, which would then represent the carbon atoms appropriately, would adopt the closed-packed arrangement. As a matter of fact, each atom is surrounded by four neighbours only. It is necessary, therefore, to suppose that the attachment of one atom to the next is due to some directed property, and the carbon atom has four such special directions as indeed the tetra-valency of the atom might suggest. In that case the properties of the atom in diamond are based upon a tetrahedral not a spherical form. The tetrahedra point away from any (111) plane in case of half the atoms in diamond and towards it in case of the other half. Consecutive (111) sheets are not exactly of the same nature and it might reasonably be expected that they would not entirely destroy each other s effects in the second order reflection from the tetrahedral plane. It is this effect which is now found to be quite distinct, though small. ... 

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. 

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

Complete these structural formulas by adding enough hydrogens to complete the tetra-valence of each carbon. Then write the molecular formula of each compound. 

All this could be taken as evidence for the tt bonding in d tetrahedral configuration. On the other hand, the force constant of Ni-C in tetra-carbonylnickel (2.1-2.5 mdyne/A.) Ill, 179) was found to be similar to the M-C force constant in Ge(CH3)4, Sn(CH3)4, and Zn(CH3)2, which is about 2.5 mdyne/A. 36). Taking three as the bond order in pure carbon monoxide, the C-0 bond order in tetracarbonylnickel is 2.64 and that of Ni-C bond only 1.33—much less than it is usually considered 179). The thermochemical data show too, apparently, that the M-C bond order in Ni(CO)4 is less than in Fe(CO)5 and Cr(CO)e. In fact, the bond energies, calculated with respect to the spin-coupled valence states, are CrC 55, FeC 58, and NiC 46 kcal./mole 177). 

Ethane may be considered then as di-methyl, or as methyl methane, i.e.y methane in which a methyl radical (CH3-) has been substituted for one hydrogen atom. In it three of the valencies of each carbon atom are satisfied by hydrogen atoms while the fourth valencies of the two carbon atoms mutually satisfy each other. The two carbon atoms thus become directly linked together. In such a compound both of the carbon atoms have all four of their valencies satisfied, and the compound is, therefore, saturated. This formula then agrees both with the fact that ethane acts as a saturated compound and with the theory that carbon is tetra-valent and, furthermore, it is the logical explanation of the reaction by which it is formed from methyl iodide. 

The most basic notion of organic chemistry is probably the quadri-valency of carbon, which was very clearly formulated by K cule in 1858 3>. Olefinic compounds like ethylene suggested that the carbon atom could exhibit the valence three, but these molecules were finally formulated with a double bond, according to Erlenmeyer s proposition 4>. Kekule s benzene formula 5> completed this classic period of valence theory. About 1875, Le Bel 6> and Van t Hoff 7> introduced the theory of steric valency, where the double bonds between carbon atoms were looked at from a new point of view Van t Hoff proposed his famous model, where the tetra-hedra of doubly-bonded carbon atoms were supposed to have an edge in common and those of triply-bonded carbon atoms a face in common. This picture was quite satisfactory for isolated double bonds, but the peculiar properties of conjugated and aromatic systems could be understood only by imagining that different double bonds in a molecule can interact in a way not possible for single bonds. 

We have already assumed that electron pairs, whether in bonds or as nonbonding pairs, repel other electron pairs. This is manifested in the tetrahedral and trigonal geometry of tetravalent and trivalent carbon compounds. These geometries correspond to maximum separation of the electron-pair bonds. Part of this repulsion is electrostatic, but there is another important factor. The Pauli exclusion principle states that only two electrons can occupy the same point in space and that they must have opposite spin quantum numbers. Equivalent orbitals therefore maintain maximum separation, as found in the sp, sjf, and sp hybridization for tetra-, tri-, and divalent compounds of the second-row elements. The combination of Pauli exclusion and electrostatic repulsion leads to the valence shell electron-pair repulsion rule (VSEPR), which states that bonds and unshared electron pairs assume the orientation that permits maximum separation. 

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