In 1874 Le Bel and van't Hoff, simultane ously and independently, pointed out that all known compounds which manifest optical isomerism contain at least one uasymmetricalp (or unsymmetrical) central atom. In most of the known cases, this central, asymmetrical atom is a carbon atom; but a few cases are now known in which it is nitrogen. The chemistry of asymmetrical nitrogen compounds is stilt in its infancy, however, and hence in the present article attention will be confined solely to the asymmetrical carbon atoms; and we shall first treat of compounds in which only one such atom is present.
Carbon is a tetravalent substance, having four valencies (or bonds); and it has been well established (especially by the exhaustive re searches of L. Henry) that these four valencies of carbon are all alike, so that if one atom of hydrogen in methane (CH,), for example, be replaced by one atom of any other mono valent substance, such as chlorine, the resulting compound is precisely the same, whichever of the hydrogen atoms is so replaced. The funda mental discovery of Le Bel and van't Hoff was that the simplest class of the substances which exhibit optical isomerism may be regarded as derived from methane, by substituting, for at least three of the hydrogen atoms that it con tains, an equal number of monovalent atoms or radicals, no two of which are alike; so that the resulting compound consists of a central nucleus of carbon, each of whose valencies is satisfied by a different atom or radical. A car bon atom whose valencies are satisfied in this way is called an uasymmetncalp carbon atom.
In the case of the various lactic acids which have already been cited, one of the fundamen tal hydrogen atoms of the methane remains undisturbed, while the other three are respec tively replaced by OH, CH,. and COOH. The structural formula of any one of these acids may, therefore, be written OH COOH.
H The central carbon atom is here the atom, since its four bonds are satisfied by four unlike univalent radicals or atoms. From the admitted equality and similarity of the four carbon bonds, it appears to be inadmis sible to suppose that the nature of the com pound that is represented above can depend (for example) upon whether the COOH group is opposite the OH group, or adjacent to it. Le Bel and van't Hoff, however, suggested that we should think of the four carbon bonds, not as four lines radiating out from the carbon atom in some one plane, but as lines radiating out from the carbon atoms in space, in four sym metrically related directions. The carbon atom being represented by a given point in space, for example, we are to think of the four bonds that it has as corresponding to the four vertices of an equilateral tetrahedron, having the carbon atom at its centre. For the sake of further illustration, let us suppose that we are looking down upon one of the vertices of such an equi lateral tetrahedron, as suggested in Figs. 1 and 2; and let us represent the four dissimilar radi cals (or atoms) by which the four bonds of the carbon are satisfied, by the letters A, B, D and E. We may suppose that in one of the optically active isomers these four radicals are disposed as in Fig. 1, and in the other as rep resented in Fig. 2. It is evident that two sub stances having such similar constitutions would be chemically so similar that it would he diffi cult to distinguish them by any ordinary test; and yet it is evident that they are not identical, since it will be found to be impossible to super pose either of these tetrahedra upon the other one, in such a way as to bring into coincidence all the vertices that carry similar radicals (or atoms). In fact, these two tetrahedra' resemble each other enantiomorphically, just as the image of an object in a mirror resembles the 'object itself ; and superposition is, therefore, impossible. The constitution of sarcolactic acid,
according to this general scheme, might be rep resented as in Fig. 3, that of bevo-rotatory lactic acid being then represented as in Fig. 4. The central dot by which we have here repre sented the asymmetric carbon atom in each of these compounds is usually omitted from diagrams of this sort, the presence of this atom being sufficiently represented by the tetrahedron itself.
The name 'stereo-chemistry° has reference to the use of a solid diagram for representing the constitutions of the compounds with which it is concerned; and the ideas which have been presented above are capable of being expanded into a general theory of the constitution of carbon compountls. The first successful attempt at the formulation of a theory of this sort was Made by van't Hoff, in 1878. In this the ory (which includes the explanation of isomer ism of all kinds by the geometric relations of the solid diagrams that are used, and which is, therefore, often called the theory of 'geometri cal isomerism"), each carbon atom in a given compound is supposed to be represented by a separate tetrahedron; though in the practical applications of the theory it is customary to omit all the tetrahedra except those that relate to the central Carbon atom, or to those which are of special or fundamental importance. When two carbon atoms are united by a single bond, their corresponding tetrahedra are sup posed to be united in such a way that they are symmetrically situated, with one vertex in com mon; when they are united by two bonds, they are supposed to be symmetrically situated, with one edge in common; and when they are united by three bonds, they are supposed to have one entire face in common. In the case of the triple bond, the theory has no special interest; for in this case theory does not indicate any possibility of isomerism, nor has experiment revealed any case of this kind in which isomer ism exists. The use of a solid diagram for illustrating the isomerism of compounds con taining two carbon atoms united by a double bond will be understood from Figs. 5 and 6, which correspond, respectively, to maleic and fumaric acids. (See MALEIC Am). There is no optical isomerism in the case of these com pounds; this circumstance being explained, ac cording to the theory of geometrical isomerism, by the fact that the points of attachment of the hydrogen atoms and the COOH groups are in each case all in the same plane. Moreover there is no asymmetric carbon atom in either case, be cause if the tetrahedra were separated, they could be turned so as to be perfectly superpos able. The relative distances between the points of attachment of corresponding radicals (or atoms) is not the same in Fig, 5 as in Fig. 6, and hence it is to be expected that the isomers that these diagrams represent will behave differ ently with respect to solubility, melting point, boiling point and in other respects also, includ ing the facility with which they will react with other substances. It may he noted, in passing, that the atoms and radicals shown in Figs. 5 and 6 admit of 'still another arrangement, in which both of the hydrogen atoms are attached to one of the tetrahedra, and both of the carboxyl groups (COOH) are attached to the other one. The substance having this structure is actually known, and is called "methylene-malonic acid. Its properties are quite different, however, from those of maleic and fumaric acids, as might be inferred from the markedly different character of its structural diagram.