122 



NA TURE 



iDec. 8, 1887 



permitting the geometrical properties of paper (or black- 

 boards) to influence his conceptions unduly. 



In the great majority of cases the ordinary graphic 

 formulae fulfilled the purpose for which they were 

 primarily devised : they enabled chemists to predict the 

 number and constitution of the isomerides possible for 

 any given combination of atoms. But there were cases, 

 on the other hand, in which the number of isomerides 

 discovered by experiment exceeded that predicted by 

 the theory. This was especially noticeable in the case of 

 those compounds which in the liquid form or in solution 

 produce rotation of the plane of polarized light. Thus, of 

 three compounds to which chemists, from a study of their 

 modes of formation and decomposition, were obliged to 

 ascribe identical atomic linkage, one would be found to 

 cause rotation of the polarized ray to tha right, another 

 to the left, whilst a third was optically inactive. As this 

 optical difference was frequently the only difference dis- 

 coverable, the phenomenon was described as "physical " 

 isomerism — a name which appeared to suggest that the 

 investigation of it lay outside the province of chemistry. 

 Wislicenus, however, in 1873, in discussing an isomerism 

 of the foregoing type occurring in the case of fermenta- 

 tion-lactic acid and paralactic acid, suggested that this 

 was really a "geometrical" isomerism ; that, although the 

 mode of linkage of the atoms was undoubtedly the same 

 in the two compounds, the positions of the atoms in space 

 were different. 



The demand thus made for a system of tridimensional 

 formulas was speedily responded to. In the following 

 year. Van 't Hoff, in Holland, and Le Bel, in France, 

 independently, and almost simultaneously,^ suggested a 

 very simple hypothesis with regard to the distribution of 

 the four affinities of the atom of carbon. From this 

 hypothesis they developed a system of formulas under 

 which not only the old anomalies of isomerism disappeared, 

 but new lines of experiment in the preparation of unknown 

 isomerides were indicated. 



Van 't Hoff and Le Bel called attention to the hitherto un- 

 noticed fact that all organic compounds which in the liquid 

 state or in solution exhibit optical activity, contain in their 

 constitutional formulse at least one carbon atom, the four 

 affinities of which are satisfied by four different atoms or 

 groups. Such a carbon atom they termed, for reasons to 

 be explained presently, "asymmetric" (Van 't Hoff), or 

 "dissymmetric" (Le Bel). 



With regard to the distribution of the affinities of an 

 atom of carbon. Van 't Hoff and Le Bel made the assump- 

 tion that the four monad atoms or groups satisfying the 

 four affinities of s.ich a tetrad atom are situated at the 

 solid angles of a tetrahedron, the centre of which is occu- 

 pied by the carbon atom. If, now, the four monad atoms 

 or groups are dissimilar, as in the case of optically active 

 compounds, it is possible to arrange them about the angles 

 of the tetrahedron in two different ways, so as to produce 

 two asymmetric tetrahedra (considered with regard to the 

 positions of these atoms or groups) —two non-superposable 

 tetrahedra, one of which is the mirror-imige of the other. 



Van 't Hoff 's views were first published in a pamphlet in the Dutch 

 language, in September 1874. L-; Bel's original me noir appeared in the 

 Bulletin de la Societe Ckimir/ue, in Novembe-of the sane year. In M ly 

 1875, Van 't Hoff published his pamphlet, "La Chimie dans TEspace." 

 which, ho ivever, dd not attract much njtice un'.il the appearance of the 

 German translation by Herrmann in 1877. 



A continuous curve, passing through the four atoms or 

 groups in the same order, will in the one case describe a 

 right-handed, in the other a left-handed, screw-line. Two 

 compounds thus differing in atomic structure only as 

 regards conditions of symmetry might be expected to 

 possess the same chemical and physical properties, save 

 where dissymmetry or polarity is concerned. As a fact, 

 this is found to be the case with optically active com- 

 pounds. When a compound contains an asymmetric 

 carbon atom, this compound, provided that it has been 

 adequately investigated, can always be shown to exist in 

 two modifications, possessing the same chemical proper- 

 ties and displaying the same chemical reactions, and, as 

 regards physical properties, agreeing in melting-point, 

 boiling-point, solubility, specific gravity, and all other 

 properties not involving the operation of polar forces. 

 But let dissymmetry or polarity in any form intervene, 

 and the non-identity of the two compounds is at once 

 manifested. Thus, as regards the action of the com- 

 pounds upon polarized light, the one compound turns the 

 polarized ray through a given angle to the right, the other 

 through the same angle to the left. Again, if the two 

 compounds crystallize, although they do so in forms 

 belonging to the same system and having the same 

 angles, yet the crystals exhibit hemihedral faces which 

 are situated to the right in the one case, and in the other 

 to the left. The one crystal is thus the mirror-image of 

 the other — a relation corresponding with that which is 

 supposed to prevail between the asymmetric carbon atoms 

 themselves within the molecule. These two hemihedral 

 crystals also display opposite pyro-electricity. Even the 

 otherwise identical chemical action of the two compounds 

 may be modified by the dissymmetry of a third com- 

 pound with which they combine; thus, for example, a 

 dextro-rotatory and a laevo-rotatory acid differ in the 

 degree of their affinity for a dextro-rotatory base, and the 

 two resulting salts are generally quite distinct in their 

 properties. 



In all artificial syntheses of compounds containing an 

 asymmetric carbon atom the substance obtained is 

 optically inactive. This was to be expected. The 

 chances in favour of the formation of each of the two 

 modifications of opposite rotatory power are equal : both 

 are therefore formed in equal quantity ; and the resulting 

 mixture is inactive. There are three known methods of 

 separating the optically active constituents of such a 

 mixture. These methods, due to M. Pasteur, were dis- 

 covered, it should.be mentioned, many years ago, before 

 the Van 't Hoff-Le Bel hypothesis was put forward. The 

 separation is effected: (i) by the greater ease with which 

 one of the two modifications is attacked by some particular 

 micro organism, it being thus possible to destroy thej 

 whole of one modification leaving the other almost intact, 

 and by properly selecting the organism even to destroj 

 at will either the dextro-rotatory or the laevo-rotatory| 

 modification ; (2) by the different degree of affinity whicl 

 the two modifications exhibit towards some other opticallyl 

 active compound ; and (3) by means of the factthat under ; 

 certain conditions of temperature and concentration it isJ 

 sometimes possible to separate the two modifications by[ 

 ordinary crystallization. By these means Pasteur suc- 

 ceeded in breaking up racemic acid into dextro-tartaricl 

 and laevo-tartaric acids. 



