E. Paterṇ - On the Action of Phosphorus Pentachloride on Chloral
Giornale di Scienze Naturali ed Economiche di Palermo, 5, 117-122 (1869)
This result [existence of a single isomer of pentachloroethane, three had been reported previously] is not without a certain importance. Indeed one of the fundamental principles of the theory of the constitution of organic compounds, based on the atomicity [valence] of the elements and in particular on the notion of the tetraatomicity of carbon, is that the four valences of carbon are chemically identical, which is not possible unless there exists only one isomer of methyl chloride, a single methanol, etc. Now one could not explain the existence of isomers for a compound of the formula C2HCl5 without renouncing the concept of the equivalence of the four affinities of the carbon atom. This would be the only exception now known to this generally held concept; as for the three isomers of C2H4Br2, assuming that they in fact exist, they can be explained easily without the need of assuming a difference among the four affinities of the carbon atom, as Butleroff does, by supposing that the four atomic valences of this element point toward the four vertices of a regular tetrahedron; then the first modification would have the two bromine atoms (or any other monovalent group) connected to the same carbon atom; while in the other two modifications each of the two bromine atoms would be attached to a different carbon atom, with the only difference that in one case the two bromines would be arranged symmetrically and not in the other. This is made clearer in the following figure, in which the bromine atoms are indicated by a and b:
It is superfluous to say that this is only a way of representing the facts, and that this whole concept needs experimental proof.
I believe that with a choice of suitable facts one could succeed in demonstrating that the four affinities of the carbon atom are equivalent.
This work was suported by a special grant from the
ministry of public instruction to Prof. Cannizzaro to promote
scientific research in the laboraory of the University of Palermo.
Would you give Paterṇ the priority he kept insisting on for proposing tetrahedral carbon 6 years before van't Hoff?