STEREO-CHEMISTRY (Greek; stereos, "solid"). In the development of the science of chemistry, it was discovered that two or more compounds may have the same empirical for mula and yet differ from one another in chem ical and physical properties to a marked degree. Resorcin, pyrocatechin and hydroquinone, for example, all have the empirical formula C414.4(OH)7, and yet they are distinctly differ ent substances. Most cases of this kind are explainable by supposing that the constituent atoms are linked together in different ways in the several compounds; the compounds them selves being called °isomers," and the phenom enon 'isomerism.' (See ISOMERISM ; AROMATIC COMPOUNDS; FATTY COMPOUNDS). The arrange ment of the atoms in such compounds has been investigated in many of these cases, and °con stitutional formula" have been devised for the purpose of representing the internal structures of their molecules. Compounds have been found to exist, however, which apparently have the same constitutional formula and which resemble one another very closely indeed in most respects, but which nevertheless exhibit certain differences* in physical and chemical properties, so that they cannot be regarded as absolutely identical. It is the province of stereo-chemistry to investigate such bodies and to show how it is possible to reconcile their exceedingly close similarity with the existence of recognizable differences in certain respects. Isomerism of the kind here contemplated is commonly called 'optical isomerism," from the fact that the bodies that manifest it differ with respect to their action upon polarized light. It has long been known that certain crystals possess the power of rotating the plane of polarization of the light that they transmit. It is also known that the crystals of a sub stance that affects light in this manner may sometimes be obtained in two slightly different forms, which possess symmetry of the same kind, and whose corresponding angles are equal, but which differ in the same manner as a man's right hand differs from his left one, or as the image of an object as seen in a mirror differs from the object itself. A pair of crystals that differ only in this respect are said to be "enantiomorphous," and if one of them rotates the plane of polarization to the right, the other, when similarly placed, will rotate it to the left; the angle of rotation being the same in each case, if the crystals are of equal thick ness. If two such enantiomorphous crystals of the same substance are melted or dis solved, the respective fluids that are obtained from them are commonly found to be identical with each other in all respects, so that if they affect the plane of polarization at all, they rotate it in the same direction, and by the same amount. It is plain that in cases of this sort the differences between the original crystals are of a purely physical nature, depending only upon the way in which the molecules of the substance are grouped, and not at all upon the internal structure of those moleclues. In other
words, two enantiomorphous crystals are to be regarded as composed of identically the same chemical substance, if they differ only in their optical properties, and become identical in all respects upon being melted or dissolved. Stereo chemistry has nothing to do with substances of this sort, which are not to be regarded as isomeric in any sense. Stereo-chemistry deals entirely with substances whose optical differ ences are manifested even in the liquid state, and hence are to be attributed to internal differ ences in the molecules rather than to mere differences in the grouping of the molecules among themselves.
As a simple example of optical isomerism, the case of lactic acid and its isomers may be cited. Lactic acid (which occurs in sour milk, and which may be extracted in the form of a syrupy liquid) has no effect upon polarized light; but sarcolactic acid (which occurs in the juice of flesh) rotates the plane of polari zation to the right, and yet its chemical proper ties are practically identical with those of com mon lactic acid. A third acid, known as "lavo-rotatory lactic acid," has been prepared, which is almost indistinguishable from the two foregoing substances so far as its chem ical properties are concerned, but which rotates the plane of polarization to the left. The chem ical properties of all three agree with the con stitutional formula CILCH(OH).COOH; and yet they cannot be regarded as chemically iden tical. When polarized light is passed through equal thicknesses of equally concentrated solu tions of sarcolactic and lzvo-rotatory lactic acids, the plane of polarization is rotated in opposite directions, but by the same amount; and when equal quantities of sarcolactic and la:vo-rotatory lactic acids are mixed, the mix ture has no effect upon the plane of polariza tion, and (in fact) the mixture cannot be dis tinguished from ordinary lactic acid in any way. These general characteristics are manifested by all of the simpler optical isomers. They occur in pairs (or °twine)), one member of which rotates the plane of polarization to the right, while the other rotates it equally to the left. A third isomer also exists, which is composed of equal parts of the two optically active ones, and which is, therefore, itself inactive. The in active member of the group is often called the oracemicp modification, the name being derived from racemic acid, which is an optically inac tive mixture of dextro-rotatory and lzvo-rota tory tartaric acids. (This statement, it is to be observed, applies only to the simpler cases of optical isomerism. For a more general state ment, applicable in all cases, see below).