GEOMETRICAL ISOMERISM. It may be seen from the preceding paragraphs that the funda mental hypothesis of stereo-chemistry, viz. that the valencies of a carbon atom act symmetrically around it in space. while indispensable for cor relating optical isomers, also throws additional light on the phenomena of chemical isomerism proper. Take, for example, again the ease of methylene chloride, Were optical isomer ism unknown and the stereo-chemical hypothesis non-existent, the question as to why only one modification of this compound is possible would be answered by pointing out that the four carbon valencies are identical and that, hydrogen and chlorine atoms being univalent, only one mode of linking the atoms is possible. If it were fur ther asked, "But how do we know that two methylene chlorides might not exist, in whose molecules the atoms are linked similarly, but arranged differently ?"—chemists would answer: "Of course, such a state of things is not incon ceivable; only in all the innumerable cases thus far discovered. the doctrine of the linking of atoms is sufficient, the number of known isomeric compounds never exceeding the number of pos sible modes of linking; and so we deem it un necessary to enter into speculations as to the arrangement of atoms in space." From the standpoint of stereo-chemistry the answer is thorough: only one methylene chloride is pos sible. (I) because the four valencies of carbon are identical; (2) because only one mode of link ing the atoms is possible; (3) because only one arrangement of the atoms in space is possible, as may he readily demonstrated by the use of tetrahedron models.
It is. therefore, clear that stereo-chemistry not only explains. and hence permits of foreseeing. all possible eases of optical isomerism. hut also explains thoroughly why more cases of chemical isomerism than are explained and foretold by the doctrine of the linking of atoms are not pos sible. But stereo-chemistry has still another im portant application. Its fundamental hypothesis applied to the so-called 'unsaturated' compounds (see CARBON COMPOUNDS) leads to a clear ex planation of cases of isomerism which, again, cannot be explained by the doctrine of atomic linking alone. In the simplest class of un
saturated compounds one carbon atom is united by two valencies to another carbon atom. In stereo-chemistry this means that two carbon tetrahedra have an edge in common, the four atomic groups held by the two carbons being situ ated at the four free corners of the tetrahedra. Take, for example, the case of two carbon atoms united by a double bond and holding two hydrogen atoms and two carboxyl In this case the doctrine of the linking of atoms recognizes the possibility of two isomeric modifications, viz.: Stereo-chemical doctrine recognizes that with the second of these modes of linking two differ ent arrangements in space are possible, as may be seen from the accompanying diagram, and better still by the use of models. As a matter of fact, three unsaturated acids (COOH) 2 are known, viz. methylene-malonic acid char acterized by the first of the above modes of linking, and two acids—maleic and fumarie both characterized by the second mode of atomic linking. and differentiated from each other by the different configuration of their atoms. Opti cal activity is of course out of question, because none of the molecules under consideration con tains an asymmetric carbon atom. But corre sponding to the difference in configuration are certain differences in the chemical behavior of maleic and fumarie acids, although the close relationship between the two is indicated by the great readiness With which they are transformed into each other. The most important difference between the two consists in the fact that maleic acid readily forms an anhydride, its two car boxyl groups losing the elements of water and becoming joined together, thus: Fumaric acid, on the contrary, has no anhydride of its own. This indicates that in maleic acid the two carboxyl groups must be nearer together than in futnaric acid, and therefore the first of the above two stereo-chemical formulas is as signed to maleic, the second to fumarie acid. See FUM.XRIC AND MAI.Eic ACIDS.