THE STEREOCHEMISTRY OF OPEN-CHAIN COMPOUNDS Mirror-image Isomerism.—As regards their isomerism, open-chain compounds may be considered according to the number of carbon atoms that their molecules contain. Dissymmetric compounds with one or two carbon atoms in the molecule suitable for resolution are not easy to obtain, and it was not until 1913 that a one-carbon compound—bromo iodomethanesulphonic acid, satisfactorily re solved (Pope and Read). It proved to be optically stable. Among the compounds with three carbon atoms in the molecule are the biochemically important substances, lactic acid and the a-amino acids, alanine, serine and cystine. The configuration of the group = OH or seems to be the same in the naturally occurring optically active forms of these sub stances. The compounds containing four carbon atoms in the molecule include the tartaric acids which played so important a part in the establishment of the fundamental principles of stereochemistry. Their structure having been fixed as dihydroxy-succinic acids, the tetrahedral theory enabled a precise form to be given to their molecular dissymmetry envisaged by Pasteur; d- and /-tartaric acids will have the enantiomorphous con figurations shown in fig. 7 which can also be conveniently indicated by the projec tion formulae shown on the right. It will probably only be possible to determine which of these configurations corresponds with which antimer when a method has been discovered of calculating the optical rotation of a dissymmetric compound from some measurable property, such as the refractivity, of the radicals a, b, c, d, associated with the asymmetric carbon atom.
Besides the two configurations of the tartaric acid molecule shown above, a third, represented in fig. 8, is also possible. This configuration has a plane of symmetry, as indicated, and is therefore superposable upon its mirror-image and is incapable of optical activity. This inactive, non-resolvable form of tartaric acid was discovered by Pasteur, who obtained it by the action of heat on salts of tartaric acid, and is known as mesotartaric acid. The existence of this form is determined by the structural identity of the two halves of the tartaric acid molecule. The acid contains two equivalent asymmetric carbon atoms, and the relation be tween its d-, 1-, and meso-modification can be represented by the symbols +A+A, —A—A, and +A—A. Every compound con taining two equivalent asymmetric carbon atoms must be capable of existence in three such forms and generally in a racemic form.
In the five-carbon series a new stereochemical feature is exhibited in trihydroxyglutaric acid (IV.–VII.).
This compound contains two equivalent asymmetric carbon atoms and thus occurs in two optically active antimeric forms, IV. and V., but instead of having only one meso-form like tartaric, it possesses two such inactive non-resolvable modifications as shown by the projection formulae VI. and VII. If the enantio morphous configurations of the group are indi cated by +R and —R, the re lation between these two non resolvable forms of the acid can also be represented as in fig. 9.
The six-carbon series contains the sugars glucose and fructose and their stereoisomers, and it is in this group of compounds that the usefulness of the conception of the asymmetric carbon atoms has been most strikingly demonstrated. The hydroxy-aldehydic formula for glucose contains four non-equivalent asymmetric carbon atoms. Since the introduction of a new asymmetric carbon atom into a com pound doubles the number of stereoisomerides previously pos sible, a compound containing n non-equivalent asymmetric carbon atoms should exist in 2't forms. The number of possible stereo isomers of the same structure as glucose is therefore sixteen ; in other words, there should be eight stereoisomeric aldohexoses, each existing in two antimeric forms. All the eight are known, many of them in both antimeric forms, and their configurations have all been determined. (See CARBOHYDRATES.) It is unneces sary to proceed further with the consideration of open-chain compounds as the study of the more complex series has not brought new principles to light.
Van't Hoff pointed out in his original brochure that this view of the ethylenic linking leads to two conclusions: (r) That there should be no free rotation about an ethylenic linking; the mole cule should possess two equilibrium configurations as indicated by figs. r r and 12. Ethylenic compounds should therefore exist in stereoisomeric forms, except when the radicals a and b (or c and d) are identical; (2) That the doubly linked carbon atoms and the four radicals a, b, c, d all lie in one plane. This plane is then a plane of symmetry, and thus ethylenic compounds, even when of the type ,C:C, , should be incapable of exhibiting b' optical activity.
These conclusions are fully confirmed by experiment. A large number of ethylenic compounds of the requisite structural type have been found to exist in two isomeric forms, and no adequate explanation has been found for this isomerism except the cis-trans isomerism determined by this conception of the ethylenic linking; also no case of optical activity determined by the non-planar configuration of the complex has been b' established. The best known example of cis-trans isomerism is that presented by fumaric and maleic acids. These acids are structurally identical; each of them is an ethylenedicarboxylic acid of the structure The great difference in properties which they exhibit can only be due to difference in configuration. Maleic acid, which readily forms a cyclic anhy dride X., and is regenerated from it by addition of water, must have the cis-configuration IX. Fumaric acid consequently has the trans-configuration VIII. Other examples of cis-trans isomers are cinnamic, XI., and allocinnamic acids, XII., oleic and elaidic acids, XIII., and stilbene and isostilbene. XIV.