a branch of chemistry concerned with the spatial arrangements of atoms and groups in molecules and the effect of these arrangements on the molecules’ physical properties (static stereochemistry) and the direction and rate of reactions (dynamic stereochemistry). It is primarily organic compounds that are studied in stereochemistry; of the inorganic compounds, complexes and chelates are investigated.
The basis of stereochemistry was provided by L. Pasteur in his study of the isomerism of tartaric acids in 1848 and by J. van’t Hoff and J. le Bel, who simultaneously in 1874 and working independently, proposed the fundamental stereochemical concept that the four valence bonds of a saturated carbon atom are directed to the corners of a tetrahedron. The tetrahedral model subsequently received direct confirmation in studies of molecules by physical methods.
An important area of modern stereochemistry is conformational analysis, which studies the spatial shape of molecules (conformations). Stereochemistry also involves the study of spatial isomerism (stereoisomerism). Stereoisomers are isomers in which the molecular composition and chemical structure are identical but the arrangement of atoms in space is different. Stereoisomerism is divided into optical (mirror-image) isomerism, which is demonstrated by the existence of optical antipodes, and diastereoisomerism, in which the spatial isomers do not exhibit a mirror-image relationship. A special case of diastereoisomerism is geometric isomerism (cis-trans isomerism), which is seen in ethylene and nonaromatic cyclic compounds. One of the tasks of stereochemistry is to prepare and to determine the configuration and study the properties of individual compounds.
Physical and physicochemical methods are widely used in modern stereochemistry. Through the techniques of X-ray and electron diffraction, interatomic distances and bond angles can be determined and a picture of the arrangement of atoms in a molecule can be obtained. Stereochemical information can also be obtained from the measurements of dipole moments and from the spectra of nuclear magnetic resonance, the data of infrared and ultraviolet spectroscopy, and the measurements of optical activity. The spatial arrangement of atoms in molecules can be predicted by the calculations of quantum chemistry.
While classical stereochemistry was only an abstract theoretical branch of science, modern stereochemistry has acquired considerable practical significance. Thus, it has been found that the properties of polymers depend to a great extent on the spatial arrangement of atoms in molecules. This dependence also holds for such synthetic polymers as polystyrene, polypropylene, and butadiene and isoprene rubbers and for such natural high-molecular-weight compounds as polysaccharides, proteins, nucleic acids, and natural rubber. The spatial arrangement of atoms also markedly influences the physiological properties of substances and explains the effect of many pharmaceuticals. Thus, stereochemistry has great importance in the chemistry and production of polymers, as well as in biochemistry, molecular biology, medicine, and pharmacology.
Stereochemistry is also used in solving problems in theoretical inorganic and organic chemistry, for example, with regard to the mechanisms of organic reactions. Thus, the loss of optical rotation (racemization) upon substitution at an asymmetric atom serves as an indication of unimolecular nucleophilic substitution (5N1 mechanism). The phenomenon known as the Walden inversion is an indication of bimolecular nucleophilic substitution (5N2 mechanism).
The measurement of optical activity is an important method for the quantitative determination of optically active substances in the sugar industry (saccharimetry) and in the production of pharmaceuticals and perfumes.
V. M. POTAPOV