LOUIS PASTEUR (1822 - 1895). Recherches sur les Relations qui peuvent Exister entre la Forme Cristalline, la Composition Chimique et le Sens de la Polarisation Rotatoire. (Annales de Chimie et de Physique184824, 442-459.)

This paper, which instantly brought Pasteur to the attention of the scientific community, grew out of the work he did for his doctoral dissertation at the École Normale in Paris. He was studying isomorphism, the phenomenon that two different substances having similar chemical compositions often form crystals of similar shape. He began by investigating some discrepancies in the isomorphism of tartrate and paratartrate crystals; such salts are present in wine and are a byproduct of fermentation. Pasteur confirmed previous observation that sodium ammonium paratartrate has the same chemical composition, the same crystalline form with the same angles as the corresponding tartrate. Surprisingly, however, the tartrate was optically active (i.e., its solutions rotated the plane of polarized light) whereas the paratartrate was inactive.

Pasteur then discovered what had previously been overlooked: that sodium ammonium tartrate formed hemihedral (dissymmetric) crystals but the corresponding paratartrate was also hemihedral but the hemihedral facets were turned sometimes one way and sometimes the other. In his 1848 paper, Pasteur describes his key experiment: "I carefully separated the crystal hemihedral to the right and the crystals hemihedral to the left; I observed separately their solutions in Biot's polarizing apparatus, and I saw with surprise and pleasure that the crystals hemihedral to the right deviated the plane of polarization to the right, those hemihedral to the left deviated it to the left."

Pasteur concluded that the optical inactivity of paratartaric acid (and its derivatives) resulted from its being a combination of two optically active acids that were mirror images of one another, the separate optical activities of which, in opposite directions, cancelled one another. Pasteur's discovery was the foundation of the new science of stereochemistry, the study of how the properties of a chemical compound depend not only on the connectivity of the atoms within the molecule, but also on the relative arrangement of the atoms in space.

Complete Dictionary of Scientific Biography. Detroit: Charles Scribner's Sons, 2008, v10, p350-416.

J. R. Partington, A History of Chemistry, New York: MacMillan, vol. 4, 1964, p750-752.

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