ABSTRACT
The first half of the nineteenth century was the great age of geometrical optics. Several French scientists studied diffraction, interference, and polarization of light. In particular, linear polarization of light and rotation of the plane of polarization very quickly attracted attention because of the possible relationship between these phenomena and the structure of matter. Optical activity, the ability of a substance to rotate the plane of polarization of light, was discovered in 1815 at the College de France by the physicist Jean-Baptiste Biot. In 1848 at the Ecole Normale in Paris, Louis Pasteur made a set of observations that led him a few years later to make this proposal, which is the foundation of stereochemistry: Optical activity of organic solutions is determined by molecular asymmetry, which produces nonsuperimposable mirror-image structures. A logical extension of this idea occurred in 1874 when a theory of organic structure in three dimensions was advanced independently and almost simultaneously by Jacobus Henricus van’t Hoff in Holland and Joseph Achille Le Bel in France. By this time it was known from the work of Kekule in 1858 that carbon is tetravalent (links up with four other groups or atoms). van’t Hoff and Le Bel proposed that the four valances of the carbon atom were not planar, but directed into threedimensional space. van’t Hoff specifically proposed that the spatial arrangement was tetrahedral. A compound containing a carbon substituted with four different groups, which van’t Hoff defined as an asymmetric carbon (asymmetrisch koolstof-atoom), would therefore be capable of existing in two distinctly different nonsuperimposable forms. The asymmetric carbon atom, they proposed, was the cause of molecular asymmetry and therefore optical activity.
