Stereochemistry

stereochemistry, study of the three-dimensional configuration of the atoms that make up a molecule and the ways in which this arrangement affects the physical and chemical properties of the molecule. It is a third aspect of chemical analysis, the first being the determination of which atoms are present in a molecule and the second being the determination of the interconnections between those atoms by chemical bonds. Central to stereochemistry is the concept of isomerism. Isomers are sets of chemical compounds having identical atomic composition but different structural properties. With geometric isomers, the differences arise from the atoms being bonded in different sequences or patterns. An example is ortho- and para-chlorobenzene; the former has chlorine atoms replacing adjacent carbon atoms in a benzene ring while the latter has chlorine atoms replacing opposing carbon atoms. Optical isomers are pairs of molecules that differ in the same way that a lefthand and righthand screw differ; i.e., they are mirror images of each other. Such molecules with a "handedness" typically rotate the plane of polarization of light that passes through them, but in opposite directions. The sugars glucose and dextrose are a pair of optical isomers; glucose rotates the plane of polarization to the left and dextrose to the right. Stereochemistry is particularly important in biochemistry and molecular biology.

---

stereochemistry

Term originated c. 1878 by Viktor Meyer (1848–97) for the study of stereoisomers (see isomer). Louis Pasteur had shown in 1848 that tartaric acid has optical activity and that this depends on molecular asymmetry, and Jacobus H. van't Hoff and Joseph-Achille Le Bel (1847–1930) had independently explained in 1874 how a molecule with a carbon atom bonded to four different groups has two mirror-image forms. Stereochemistry deals with stereoisomers and with asymmetric synthesis. John Cornforth (b. 1917) and Vladimir Prelog (1906–98) shared a 1975 Nobel Prize for work on stereochemistry and stereoisomerism of alkaloids, enzymes, antibiotics, and other natural compounds.

---

stereochemistry [¦ster·ē·ə′kem·ə·strē]

(physical chemistry)

The study of the spatial arrangement of atoms in molecules and the chemical and physical consequences of such arrangement.

---

Stereochemistry 

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 (5_N1 mechanism). The phenomenon known as the Walden inversion is an indication of bimolecular nucleophilic substitution (5_N2 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.

REFERENCES

Eliel, E. Osnovy stereokhimii. Moscow, 1971. (Translated from English.)
Potapov, V. M. Stereokhimiia. Moscow, 1975.

V. M. POTAPOV