Tetrahedral structure, of carbon

Figure 11.12 The three-dimensional tetrahedral structure of carbon (e.g., in methane, CH4), with an angle between the bonds of 109.5°. The simple straight lines are in the plane of the paper, the solid tapered line points towards the observer and the dashed line is into the paper.

Some illustrative examples are shown above. Despite his disclaimer Kekule s structural formulae are clearly the harbingers of their modern equivalents. The proposed tetrahedral structure of carbon, which followed, ignored the good advice and amounts to no more than a geometrical rearrangement of Kekule s diagrams, as shown here to represent the actual size and shape of carbon in methane. 

In 1891, only 10 years after the tetrahedral structure of carbon was proposed, Fischer determined the relative configuration of the four stereogenic centers in naturally occurring (-t)-glucose. This body of work is called the Fischer proof of the structure of glucose. 

Pasteur s discovery of enantiomerism and his demonstration that the optical activity of the two forms of tartaric acid was a property of the molecules themselves led, in 1874, to the proposal of the tetrahedral structure of carbon by van t Hoff and Le Bel. 

Emil Fischer began his work on the stereochemistry of (+)-glucose in 1888, only 12 years after van t Hoff and Le Bel had made their proposal concerning the tetrahedral structure of carbon. Only a small body of data was available to Fischer at the beginning. 

Tetrahedral structure of carbon, (a) A regular tetrahedron (b) a carbon atom within a reguiar tetrahedron (c) a methane moiecuie, CH4 (d) spacefiiiing modei of methane. CH4. 

The axes of the sp orbitals point toward the corners of a tetrahedron Therefore sp hybridization of carbon is consistent with the tetrahedral structure of methane Each C—H bond is a ct bond m which a half filled Is orbital of hydrogen over laps with a half filled sp orbital of carbon along a line drawn between them... 

At this stage, it looks as though electron promotion should result in two different types of bonds in methane, one bond from the overlap of a hydrogen ls-orbital and a carbon 2s-orbital, and three more bonds from the overlap of hydrogen Is-orbitals with each of the three carbon 2/ -orbitals. The overlap with the 2p-orbitals should result in three cr-bonds at 90° to one another. However, this arrangement is inconsistent with the known tetrahedral structure of methane with four equivalent bonds. 

The preservative powers of salt stem from its chemistry and its interaction with water. The H2O molecule is a tetrahedral structure. It does not look like a tetrahedron because two of the positions are occupied not by atoms but by electron pairs. Another molecule with a tetrahedral structure is carbon tetrachloride. The difference between the structures of the two molecules is that carbon tetrachloride has no unbonded electron pairs (Figure 8.1). 

H. van t Hoff (Dutch scientist) proposed a tetrahedral structure for carbon atom in September of 1874. J. A. Le Bel (French scientist) published the same idea independently in November of 1874. 

The landmark in the history of conformation started in 1874 when van t Hoff and Le Bel gave the tetrahedral structure of the carbon atom in which the angle between the valency bonds was... 

C (graph ite), hP4, structural type. In comparison with the tetrahedral structure of C diamond a very different structure is adopted by carbon in graphite. 

The final molecule of this series is methane, the tetrahedral structure of which follows if a fourth unit positive charge is removed from the nucleus in the ammonia lone-pair direction. There are now four equivalent bonding orbitals, which may be represented approximately as linear combinations of carbon s-p hybrid and hydrogen Is functions. The transformation from molecular orbitals into equivalent orbitals or vice versa is exactly the same as for the neon atom. 

The classical valency concept of the tetrahedral carbon atom has been more than fully verified by its success in explaining the chemistry of countless thousands of organic compounds. The first direct physical confirmation of the tetrahedral distribution of carbon valency bonds, however, came with the elucidation of the structure of diamond by W. H. and W. L. Bragg (1913) using the newly discovered method of X-ray diffraction. 

The most immediate roots of structural chemistry are in the well-known works and discoveries of Pasteur, van t Hoff, and others [78], The four valences in a tetrahedral arrangement of carbon, however, were first described by Emanuele Paterno [79] in an obscure journal. Since he did not develop all the consequences of his hypothesis, the credit, justifiably, belongs to van t Hoff (and Le Bel). Yet it is worthwhile to quote Paterno (after [20]) because there is even direct reference in his work to what would be called conformational isomers today ... 

Ihe tetrahedral geometry -of carbon were barely a decade oM. modem methods of product purification were unknown, and modem spectroscopic techniques of structure determination were undreamed of. Despite these obstaclee, Emil Fischer published in 1891 what remains today perhaps the finest use of chemical logic over recorded —a structure proof of the steren-chemistry of natural ly occurring (+ >g1ucose. Let s follow f> ischcr s logic and see how he arrived at his conclusions,... 

The structure of LiIn(CH3)4 (56) (76) is a three-dimensional network. Each lithium atom is surrounded by a tetrahedral array of carbon atoms. The structure of LiB(CH3)4 (57) (79) consists of planar sheets of lithium atoms bridged by tetramethylboron groups. A unique feature is the presence of both linear and bent Li—C—B units. A neutron diffraction study of LiB(CH3)4 (79) illustrates the Li—H—C interactions present in... 

The tetrahedral set of four sp3 orbitals on the carbon atom is used to share electron pairs with the four is orbitals of the hydrogen atoms to form the four equivalent C—H bonds. This accounts for the known tetrahedral structure of the CH4 molecule. 

FIGURE 7-6 The Structure of Diamond, (a) Subdivision of the unit cell, with atoms in alternating smaller cubes, (b) The tetrahedral coordination of carbon is shown for the four interior atoms. 

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