STEREO-CHEMISTRY. 555 STEREO-CHEMISTRY. vtetric' carhon atoms, i.e. carbon atoms directly linked to four atoms or groups of atoms differ- ent from one another. The followino; formulas, containing asymmetric carbon atoms (denoted by an italic C), represent optically active com- pounds : CH, OH H CH.COOH / / a / / H COOH OH OOOH Lactic acids Malic acids H I n— c-01 I ci H ci-c— CI k CeHs OH / C / H COOH Mandelic acids On the contrary, formulas like that of propionic acid, CH3 H / C / H COOH which contains no asymmetric carbon atom (i.e. no atom combined with four dilJercnt atoms or groups ) , have in no case been found to represent optically active compounds. The relation be- tween the optic rotatory power and the asym- metric carbon atom must therefore form the cornerstone of anj' explanation of optical isomer- ism that might be advanced. Altogether three different explanations suggest themselves as pos- sible: (1) the assumption that the four valencies of the carbon atom ai'e unequal and that eases of isomerism may result from different distribu- tions of the four atoms or groups combined with the carbon atom, among its different valencies; (2) the assumption that the atoms or groups combined with the carbon atom are in constant motion around it and that the molecules of opti- cally different compounds may be made up ot the same atoms or groups in different motions around the carbon atom; and (3) the assumption that the molecules of different compounds may be made up of the same atoms or groups, similar- ly linked to the carbon atom, but differently ar- ranged in space around it. The first of these assumptions compels us to recognize the possible existence of isomers that are positively known to be impossible. For instance, if the assump- tion were correct, two different nitro-methanes would be possible; yet a systematic experimental investigation carried out by Henry showed that nitro-methane can exist in one form only. The second of the above assumptions is alto- gether incapable of either purely theoretical development or systematic correlation with facts, and must therefore be abandoned. The third as- sumption, viz. that the difference between opti- cal isomers is due to differences in the configura- tion of the atoms in their molecules, presents it- self, therefore, as the only possible explanation. Furthermore, even this explanation must first of all discard the hypothesis of the arrangement of atoms in a plane. Indeed, were the plane graphical formulas, as we ordinarilj' write them, true to reality, then, for instance, two methylene chlorides would be possible, the configurations of whose atoms would be represented by the follow- ing formulas: Vol. XVIII.— 36. Yet only one methylene chloride is possible. If the valencies of the carbon atom were assumed to be distributed, in the plane, not at equal angles, but, say, as in the symbol / C / then the number of theoretically possible yet really non-existing isomers would be even greater. Considerations of this nature have led to the conclusion that if optical isomerism is to be ex- plained, it is necessary to assume that the four atoms or groups held by an asymmetric carbon atom are situated at equal distances from the carbon atom and at equal distances from one an- other — which is equivalent to assuming that thg as)immct7'ic carhon atom is situated nt the een- tre of a regular tetrahedron, and the four differ- ent atoms or groupsi held by it arc situated at the four corners of the tetrahedron. This, then, is the fundamental hypothesis of stereo-chemistry. Its application to the study of isomerism is very difficult to explain or to grasp without tlie use of models. A crude but sufficient form of model may be readily made in a few minutes from pasteboard, by cutting out four equal equi- lateral triangles and fastening them together to form a regular tetrahedron, by means of strips of gummed paper. Small slips of paper of differ- ent colors may be used to represent the four dif- ferent atoms or groups and may be fastened to the corners by means of pins. With the aid of two such models it is easy to demonstrate the following cases: (1) With three slips of paper of one color and one of another color, only one ar- rangement is possible, which corresponds, for instance, to the fact that only one nitro-methane, CH3(N0j), can be obtained. ' (2) With two slips of paper of one color and two of another color, again only one arrangement is possible, which corresponds, for instance, to the fact that only one methylene chloride, CH.Ch, can be obtained. (3) With two slips of paper of one color, one slip of another color, and one of a third color, again only one arrangement is possible, which corresponds, for instance, to the fact that only one propionic acid. CHnjCHJ (COOH) , can be obtained. (4) With four slips of different colors two different arrangements arc possible, which corresponds to the fact that only molecules con- taining an asymmetric carbon atom, i.e. one linked to four different atoms or groups, can be different in spite of Ijeing made up of the same atoms linked together in the same manner. The two models will, in this case, be like an object and its image in a mirror, or like the right hand and the left hand ; turn both hands palm down- ward and it is impossible to superpose them so that the corresponding fingers touch — the thumb of th(! richt hand will touch, not the thumb, but the little finger of the left hand, etc. Thus the models will illustrate the dilTerence between the two optically active lactic acids, malic acids, or mandelic acids (see above) ; or, in general, any pair of optical isomers whose molecules contain one asymmetric carbon atom. As an instructive example it may be demonstrated, by means of the
