The Discovery of Tetrahedral Carbon: Contributions of Paterno' and Cannizzaro

1 Gazz. Chim. It. 127,837 (1997) THE DISCOVERY OF TETRAHEDRAL CARBON: CONTRIBUTIONS OF PATERNO' AND CANNIZZARO. * Giorgio Montaudo, Dipartimento di Scienze Chimiche, Università di Catania. Viale A. Doria, 6 - 95125 Catania, Italy. ABSTRACT Although the discovery of stereoisomerism is due to Van't Hoff and Le Bel (1875), the hypothesis of tetrahedral carbon, on which is actually based the stereoisomerism, is older. The latter is due to Kekulé (1862), who then built the tetrahedral carbon molecular models, and applied them to solve the problem of the structure of benzene. The molecular models of Kekulé were used by the most talented pupils of his school (Dewar, Koerner, Van't Hoff), and were applied to diverse structural problems. Among these contributions, of great relevance is the work of Emanuele Paternò, who grew up in the laboratory of S.Cannizzaro in Palermo, and who came also in contact with the school of Kekulé. Paternò, using the molecular models of Kekulé, in 1869 applied for the first time the theory of the tetrahedral carbon to aliphatic organic compounds, and discussed the conformational isomerism of these compounds. Only a few years later, Van't Hoff and Le Bel succeeded in applying the hypothesis of Kekulé to the problem of optical antipodes, and discovered stereoisomerism. ........................ * Plenary Lecture presented at the Meeting of the Italian Chemical Society, Sicilian Section, held in Catania 19-20 December 1996 2 INTRODUCTION The discovery of Van't Hoff and Le Bel (1865) is a landmark, one of those events that mark the beginning of a new era. The theory of stereoisomerism, that explicitly affirmed the tetrahedral structure of carbon, and the subsequent rationalization of the whole organic chemistry, ought to be numbered among the greatest scientific events of the XIX century. However, alike to nearly all the discoveries, also this one has an antefact, and the events connected to it are more complex than what the official storiography, always inclining to simplify, has handed down to us. In fact, the clamorous controversy provoqued by the proposal of Van't Hoff and Le Bel, appears to have covered the importance of previous contributions. Some salient points of the history preceeding the discovery of Van't Hoff and Le Bel have remained in the shadow, almost forgotten. However, the latters are important contributions, perhaps essential, bound to the name of the great Kekulé, and they include also first rate italian scholars such as Paternò and Cannizzaro. This fact makes even more interesting for us the attempt of reconstructing the variegated scientific scenery existing before the discovery. INSTRUMENTALISM AND CHEMICAL THEORY The first atomic and structural hypotheses, those on which was based the "chemical theory" at the beginning of 1800, were accepted only at the "instrumental level" by the chemists of the XIX century, as well described in this fragment from Berzelius: "The theory is only a way to imagine the phenomena. Although in a certain historical period of the scientific development it serves entirely as a true theory, with the accumulation of knowledge during the centuries, the way of imagining the phenomena of science shall change, perhaps without ever reaching the truth. Sometimes it happens that two different explanations are both possible: it is then necessary to study them both. If we change theory, the new one must explain better the facts." BERZELIUS 1819. In another example, the atomic theory appeared a valid hypothesis to explain the observed facts, but the real existence of atoms was seriously doubted: "The principle that the atoms are indivisible is an hypothesis indifferent, and therefore only a convention. In fact, what difference it makes for the facts of chemistry if the elemental chemical masses would be susceptible of being cut indefinitely by means of forces independent from chemistry?" DUMAS 1836. The corpuscolar nature of the electricity and of the electron remained in the shadow for all the XIX century, and it was necessary to wait (1898) to gather experimental evidence on the existence of the electron in order to start wondering about its actual function inside the atom. Without understanding the role of the electron it was'nt possible to understand the nature of the chemical bond, and consequently the problem of valence remained for long time undefined, together with that of the existence of the atoms. This created big problems, especially in the field of organic chemistry, that started developing from 1828, when Wholer opened the way to organic syntheses. Within a short time this discipline became a crucial test for the further development of the chemical theory. In fact, the chemistry of carbon compounds soon showed enormous structural problems, such as the explanation of the widespread phenomenon of isomerism. The existence of isomerism in organic compounds might be accounted for by assuming a different order of the atoms inside these compounds, but serious doubts were risen in this respect: 3 "A popular prejudice is that it is possible to express by chemical formulas the molecular disposition of the compounds, that is, the real disposition of their atoms". "The chemical formulas are not meant to represent the disposition of the atoms, but have only the purpose to express in the most simple and precise way the ratios of the chemical bodies and their transformations". GERHARDT 1844. To face these objections, it became imperative to elaborate a new chemical formalism capable of explaining the complex differences among the varios compounds. The evolution of chemical symbols, that had been very slow starting from the ancient greek and alchemic symbols up to Lavoisier and Dalton, underwent a defined acceleration with Berzelius (1814), and from that point the symbolism used in the chemical formulas evolved up to the times of Kekulé. The 40 years from 1820 to about 1860, were marked by numerous and controversial attempts to establish a structural theory of organic chemistry, and culminated in the first KekulŠ formulation. Concluding a complex series of researches that brought him to the definition of the structural formulas of aliphatic homologous series, Kekulé (1858) affirmed that the carbon atom is always tetravalent, and also theoretized the existence of double bonds in unsaturated compounds. Of course, Kekulé was not alone in this enterprise, and a long series of great masters must also to be credited. Among many others, quite remarkable appears the contribution of Butlerov who, in the framework of the concepts emerging from the rationalization of the phenomena of isomery and homology, introduced the idea of chemical structure, defined as the way of reciprocal binding of the atoms inside to a compound: "I do not believe, at difference from Kolbe, that once ascertained the existence of the atoms, we shall not be capable of determining their position in the space." BUTLEROV 1863. "It is not impossible to represent in the plane the spatial position of atoms". "This is possible by means of mathematical formulas, and it is likely that the laws ruling the formation of the chemical molecules shall find, in due time, their mathematical formulation". BUTLEROV 1863. THE TETRAHEDRAL CARBON Already in 1858 Kekulé began to propose a cyclic strucure for benzene, but it was only in 1865 that he proposed the hexagonal formula with alternated double bonds. Actually, the benzene formula posed formidable structural problems at that time, and there were several plausible models. Kekulé started to think at the tetrahedral carbon already in 1862, when he formulated the hypothesis that "the valences of carbon are oriented in the direction of hesahedral axes terminating on the faces of a tetrahedron inscribed in a sphere". Due to the relevance and international prestige of Kekulé, whose laboratory was attended by the most promising european chemists, it is likely that the hypothesis of the tetrahedral carbon had wide circulation among the researchers in Europe in the '60 decade. Kekulé had immediate need to use this hypothesis in order to better define his benzene structural formula. Therefore, he built in his Gent laboratory molecular models based on tetrahedral carbon, of which we have notice starting from 1867. These models, later known as Kekulé-Baeyer models, were commonly used to represent the structure of organic compounds till about 1930, when they were substituted by the molecular models based on the X-ray diffraction data and on the orbital hybridization concepts proposed by Pauling (1931). 4 However, Kekulé judged unwise to consider those models as real objects. Since the existence of atoms was not yet proven, one ought to be cautious. His position was instrumental, and he tended to stress the utility of the molecular models in suggesting possibilities of advancement od the chemical science, rather then insisting on their reality: "The question if the atoms exist or not belongs to the methaphysics. We need only to decide if the assumption of the atoms is a hypothesis able to explain the chemical phenomena, and if a further development of the atomic hypothesis promises an advancement of our knowledge about the mechanism of the chemical phenomena". KEKULE' 1867. and more: "I am inclined to think that some day we shall find, for what we now call atoms, a mechano- mathematical explanation which shall account for the atomic weight, atomicity, and for numerous other properties of the so-called atoms". KEKULE' 1867. Given the great importance of the molecular models that he had invented, one may wonder why Kekulé never published an official scientific report about his tetrahedral models. This fact, which at first may sound mysterios, is instead quite obvious. Due to his instrumental position (see above), it appears simply logic that Kekulé was not eager to write a paper on his molecular models. Writing such a report would have involved the problem of discussing the concepts used in their construction, and Kekulé knew that he had no solid ground to defend his molecular models, based on the double assumption of atoms and of tetrahedral valences.

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