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Tetrahedral carbon atom.

Kekule's way of writing the formulas of organic molecules (see 1858), now fifteen years old, was essentially two-dimensional. The four valence bonds of the carbon atom were directed toward the four angles of a square. That was inadequate for some purposes. For instance, some organic molecules, in solution, rotated the plane of polarized light. That meant the molecules must be asymmetric in one way or another, but the asymmetry wasn't visible in the Kekule formulas.

In 1874 a Dutch physical chemist, Jacobus Hendricus van't Hoff (1852-1911), advanced a three-dimensional representation of organic molecules. In his system, the four bonds of the carbon atom pointed toward the vertices of a tetrahedron, so that the atom could rest on three of the bonds as though it were a splayed-out three-legged stool, with the fourth bond pointing straight upward, each bond equidistant from the other three.

This tetrahedral carbon atom produced the necessary asymmetry. If four different groups were attached to the four valence bonds of a particular carbon atom, two distinct molecules could be formed, one being the mirror image of the other.

If one of these compounds twisted the plane of polarization clockwise, the other would twist it counterclockwise. Indeed, any compound shown by the tetrahedral carbon atom to be asymmetric was in fact optically active when tested, twisting the plane of polarized light in one direction or the other.

Any compound not shown to be asymmetric was not.

The way the tetrahedral carbon atom explained optical activity was so useful that the new outlook was adopted quickly. Van't Hoff's way of looking at formulas was considered stereochemistry, from the Greek meaning "solid chemistry," since the molecules were pictured in three dimensions.

A French chemist, Joseph-Achille Le Bel (1847-1930), advanced the tetrahedral carbon atom at about the same time, independently.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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