Stereochemistry

STEREOCHEMISTRY, the doctrine of the spatial arrangement of atoms in molecules of chemical compounds; S. is that part of theoretical chemistry which studies spatial isomerism or stereoisomerism (see). By the second half of the 19th century, a significant number of facts had accumulated in organic chemistry that did not fit within the framework of the theory of the structure of matter, such as the presence of optically active isomers of lactic acid, tartaric acid, and others. The works of Wislicenus (1869) on the isomers of lactic acid served as the impetus for the creation by van't Hoff (1874) of a brilliant theory about the arrangement of atoms in space, a theory that resolved the question of the cause of optical isomerism and forms the basis of modern views in S. Independently of van't Hoff, the French scientist Le Bel arrived at similar views, which he published in the same year as the works of van't Hoff, so it is quite fair that both scientists are considered the founders of stereochemistry. The basic concept of S. is the tetrahedral model of the carbon atom, which allows for the existence of asymmetric carbon (see), which is the cause of the appearance of two different isomers in space. One isomer is a mirror image of the other, hence the name "mirror isomerism." Both isomers will rotate the plane of polarized light in opposite directions (see Asymmetric carbon) by the same amount, which is what we have in optical isomers (antipodes), for example, right- and left-rotating lactic acid. Optically inactive forms of organic substances having an asymmetric carbon atom represent, from this point of view, unstable compounds of right- and left-rotating molecules in equivalent quantities (so-called racemic compounds), which can be proven by separating the inactive form into optically active components (the classical works of Pasteur). In addition to optical isomerism, there is so-called geometric isomerism, caused by the different arrangement of groups relative to the plane formed by a particular cycle. In the simplest case, we encounter geometric isomerism in unsaturated compounds of the type: C(ab)=C(ad). In the case where groups b and d are located on the same side of the plane formed by the double bond, we have the so-called cis-isomer. In the case of the arrangement of b and d on different sides of this plane, we have the trans-isomer. In projection on a plane, cis- and trans-isomerism is depicted as follows: a-c-b, a-c-b II, ., II a-c-d, d-c-a cis-form, trans-form. A similar case of isomerism we observe in more complex compounds, such as glycosides: here the presence of α- and β-forms is also explained by the different arrangement of the alcohol residue relative to the plane formed by the glucose cycle: CH₂OH-CHOH-CHOH-CHOH-CHOH-:H^H°-OH CH₂OH-CHOH-CHOH-CHOH-CHOH-CH₂OH, -OH, -OH, -CH₂OH. Spatial isomerism occurs not only with the carbon atom but also with other multivalent atoms. Trivalent nitrogen gives isomers analogous to cis- and trans-forms, for example, in oximes R-C=N, R-C=N I, I N-OH, HO-N trans-form, cis-form (the works of Beckmann, Meyer, Hantzsch, and Werner). In compounds with pentavalent nitrogen, optical isomers are possible. The works of E. Wedekind, Meisenheimer, Kipping, and others (Wedekind, Meisenheimer, Kipping) showed the presence of spatial isomers in compounds of sulfur, selenium, silicon, phosphorus, and others. X-ray studies of recent years have confirmed the main conclusions of the theory of the spatial arrangement of atoms of van't Hoff and Le Bel. The stoichiometric concepts that arose in the study of organic chemistry became a powerful stimulus for a number of fruitful works both in inorganic chemistry and in a number of other disciplines, including in medicine in those departments that study the transformation of substances in a living organism and the connection between the structure of a substance and its effect on the organism (the works of Ehrlich and others). See also Optical activity.

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