Frank Brescla and Pietro Manglaracina CtvCollege of New York, CUNY
New York. 10031
II
Conformational Analysis, 1869
The historical accounts (1) of the concept of the tetrahedral carbon atom generally ignore the 1869 paper (2) of Emmanuele Paternb, U. of Palermo, while most, if not all, textbook authors (3) credit Joseph A. Le Be1 (1874) (4) and Jacobus H. van't Hoff (1874) (5)for the tetrahedral model. The development of a chemical theory may he effectively used to motivate and excite student interest. In fact, the use of historical material in the teaching of chemistry was the subject of a Discussion Session a t the 1974 Conference on Chemical Education (6).The study of the Patemb paper, the relevant parts of which are translated below (with author's comments in parentheses), the Butlerov, Le Bel, and van't Hoff papers may he used as a study in scientific methodology. An alarm nevertheless sounds the warning that (7) the way scientists hehnvr (according to hirturinn4 might not be a gvod model for students. There writings du videnee to the pnv fessiunal ideal and puhl~eimage of sricntislr as rntronal, openminded investigators, proceeding methodically, grounded ineontrovertibly in the outcome of controlled experiments, and seeking objectively for the truth, let the chips fall where they may.
The Paternb Paper The theory of the structure of organic compounds is based on the atomicity of the elements and, in particular, on the concept of a tetravalent carbon. However, the principle that the four ualences of carbon ham identical chemical properties is also fundamentol to the the0ry.l (Our italics since Patern8 insisted on this hypothesis as an essential part of chemical structural theory.) It then follows that there can he only one compound corresponding to the composition of methyl chloride, methyl alcohol, and so forth.2 On the other hand, i t is impossible to explain the existence of three isomers having the formula C2HCls unless the principle of the equivalence of the four carbon valences is denied. (If the valences of carbon are assumed to be non-equivalent, then CzHCls should possess three isomers.) But this is the only known example that contradicts the theory of the equivalence of the four carbon valences. However, three isomers of CsHdBra, if they actually exist, (Paterni, picked C2H4Br2BS B typical chemical combination that can yield three isomers on the assumption of valence equivalence) are easily explained without assuming, as Butler09 believes, that differences occur among the four valences of the carbon atom when they are directed towards the corners of a regular tetrahedron. One isomer would have two atoms of bromine (or any other monovalent group) on the same carbon atom. In the other two isomers, each of the carbon atoms would bond one bromine atom; the difference between these two isomers results when in one isomer the two bromine atoms are symmetrically arranged (eclipsed arrangement), but not in the other (staggered arrangement). The figure in which the bromine atoms are labelled a and b makes this clear.
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Butlerov was the first to assume a quadrivalent tetrahedral carbon in which each of the four faces is capable of bonding a monovalent atom hut the attraction (bond energy, bond length) is different for each face (8). This, of course, yields an irregular carbon tetrahedron or an asymmetric carbon. Nevertheless, Butlerov is credited as the first who regarded such molecules as actual repre.entariuni of alums in inolerulrs. Hutleruv (An, prrdirtcd four isomers for CHrCI and CHCI. Rutlrrov r X h , nssumrd thnt the "bonding ctrengtha uf polwa. lent atoms like carbon must be different" toexplain the existence of three isomers of C2H4Br2,synthesis credited to W"rtz and Caventou, and three isomers of C2H3C13,synthesis credited to Regnault and Hiihner.
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32 1 Journal of Chemical Education
The three predicted isomers of C2HlBr2.
These ideas require experimental proof. The described experiments (inaccurate data led M. V. Regnault ( 9 ) t o the canclusion that two isomers of C2HC15 existed but Patern8 showed that C2HC15prepared by different methods possesses identical properties) refute the only example that makes it necessary to deny the equivalence of the four carbon valences. I believe that it should be possible to demonstrate the equivalence of the four carbon valences by the proper choice of experiments. (19th century instrumentation could not detect conformational isomers. We could ask our students for their opinions regarding the faith of the atomic theory if available equipment in Dalton's time detected that some geological specimens, for example, of calcium have an atomic weight significantly different from 40.08 (10) and in Cannizzaro's time detected rotational isomers.)
Paternb's Contribution
Thus, Regnault's inaccurate data appeared to show the existence of two C2HCls isomers leading Paternb to theorize that if this cot&sion were true, thkn the valences of the carbon atom must he nonequivalent, in which model CzHCls should have three isomers, not two. The proponents stressed the non-equivalence feature rather than the symmetrical geometry of molecules. Paternb however insisted on the equivalence of the valences in a tetrahedral carbon atom, proceeded to demonstrate experimentally that only one compound of CzHCls exists and, finally, theorized that his model nredicts C?H
