Jacobus H. van 't Hoff

Stereochemistry . In 1873 the German chemist Wislicenus published an article on lactic acids, in which he reiterated the view that the only difference between the two optically active forms of the acid must be in the spatial arrangements of the atoms. After van’t Hoff had studied this theory, he published a twelve–page pamphlet, Voorstel tot uitbreiding der tegenwoordig in de scheikunde gebruikte structuur–formules in de ruimte, which included a page of diagrams. Van’t Hoff’s name appeared only at the end of the paper, which was dated 5 September 1874.
At the suggestion of Buys Ballot, professor of physics at Utrecht, the paper was soon translated into French, and the following year van’t Hoff published his views in extended form as la chimie dans I’espace. His revolutionary ideas on the theory of the asymmetric carbon atom did not attract the attention of chemists, however, until Wislicenus asked van’t Hoff’s permission for a German translation by one of his pupils, Felix Herrmann. The translation was published in 1877 as Die Lagerung der Atome im Raume. An English translation by J. E. Marsh appeared in 1891 as Chemistry in space. Then, in 1887, van’t Hoff published Dix années dans I’histoire d’une théorie, in which he pointed out that Le Bel had independently arrived at the same idea, although in a more abstract form.
Both van’t Hoff and Le Bel showed that arrangements of four different univalent groups at the corners of a regular tetrahedron (which van’t Hoff defined as an asymmetric carbon atom) will produce two structures, one of which is the mirror-image of the other. The latter is a condition for the existence of optical isomers, already realized in 1860 by Pasteur, who found that optical rotation arises from asymmetry in the molecules themselves. Van’t Hoff stated that when the four affinities of one carbon atom are represented by four mutually perpendicular directions lying in the same plane, then we may expect two isomeric forms from derivatives of methane of the type CH₂(R₁)₂. Beacuse such isomertic types do not occur in nature, van’t Hoff supposed that the affinities of the carbon atom are directed to the corners of a tetrahedron and that the carbon atom is at the center. In such a tetrahedron a compound of the type CH₂(R₁)₂ cannot exist in two isomeric forms, but for compounds of the type CR₁R₂R₃R₄ it is possible to construct two spatial models that are nonsuperimposable images of one another. In this case there is no center or plane of symmetry for the tetrahedron.
In the first part of the Voorstel, van’t Hoff discussed the relationship between the asymmetric carbon atom and optical activity. Drawing on several examples he showed that all the compounds of carbon that, in solution, rotate the plane of polarized light possess an asymmetric carbon atom (for example, tartaric acid, maleic acid, sugars, and camphor). Then van’t Hoff showed that while the derivatives of optically active compounds lose their rotatory power when the asymmetry of all of the carbon atoms disappears, in the contrary case they usually do not lose this power. Finally he showed that if one makes a list of compounds that contain an asymmetric carbon atom it appears that in many cases the reverse of his first statement is not true, that is, not every compound with such an atom has an influence upon polarized light.
Van’t Hoff’s concepts of the asymmetric carbon atom explained the occurrence of many cases of isomerism not explicable in terms of the structural formulas then current. Moreover, it pointed out the existence of a link between optical activity and the presence of an asymmetric carbon atom. Van’t Hoff also discussed the relationship between the asymmetric carbon atom and the number of isomers. In La chimie dans l’espace he showed that the number of possible isomers of a compound with n inequivalent asymmetric carbon atoms is 2ⁿ, and he indicated how the number of isomers decreased if one or more of the asymmetric carbon atoms is equivalent.
Having introduced the concept of the tetrahedral carbon atom to explain the optical isomerism of a number of organic compounds, van’t Hoff turned in the second and third part of Voorstel to another type of isomerism, which also appeared to be a consequence of the tetrahedral atom, namely, compounds containing doubly and triply linked carbon atoms. A carbon-carbon double bond of the type R₁R₂C=CR₁R₂ is represented by two tetrahedrons with one edge in common, as in the case of maleic and fumaric acids, bromomaleic and bromoisomaleic acids, citraconic and mesaconic acids, crotonic and isocrotonic acids, and chlorocrotonic and chloroisocrotonic acids. Van’t Hoff pointed out that when two tetrahedrons are joined on one edge and R₁, R₂, R₂, and R₄ represent the univalent groups that saturate the remaining free affinities of the carbon atoms, possibilities for isomerism occur when R₁ differs from R₂ and when R₃ differs from R₄. This form of isomerism is now called geometric or cis-trans isomerism. In cases when optical activity was found but the formula was symmetrical, van’t Hoff postulated (usually correctly) either an error in the formula or the presence of an optically active impurity. In 1894 he ventured the opinion, later confirmed, that the occurrence of optically active substances in nature might be the consequence of the action of circularly polarized light in the atmosphere on optically inactive substances.
Although van’t Hoff and Le Bel shared certain views concerning the carbon atom, van’t Hoff was more imaginative and broader in his conceptions and thus incurred harsher criticism, especially from Kolbe, who saw in van’t Hoff’s work a regression of German chemical research to the speculative aspects of Naturphilosophie:
A Dr. J. H. van’t Hoff, of the veterinary school at Utrecht, has as it seems, no taste for exact chemical investigation. He has thought it more convenient to mount Pegasus (obviously loaned by the veterinary school) and to proclaim in his La chimie dans l’espace how during his bold flight to the top of the chemical Parnassus, the atoms appeared to him to have grouped themselves throughout universal space [“Zeichen der Zeit,“in Journal für praktische Chemie, 15 (1877), 473].
He was also criticized by Fittig, Adolf Claus, Wilhelm Lossen, and Friedrich Hinrichsen on the basis that his theories were incompatible with physical laws. Although Wurtz, Spring, and Louis Henry wrote warm acknowledgments, they made no attempt to discuss or criticize his theory. The first to give serious attention to van’t Hoff’s theory was Buys Ballot, who in the journal Maandblad voor natuurwetenschappen (1875) published an open letter to van’t Hoff. His reply, in the same journal, discusses a number of interesting points raised in the letter and includes diagrams of the configurations of the ten isomeric saccharic acids.
In volume I of Ansichten über die organische Chemie van’t Hoff systematically examined the physical and chemical properties of organic substances regarded and classified as derivaties of methane. In volume II he discussed the general relation between the constitution and fundamental properties of organic substances. Especially interested in their physical properties, he attempted to relate stability and reactivity to thermodynamic data, reaction velocities, and chemical equilibriums. Remarkably, van’t Hoff made little use here of his stereochemical ideas.