edlted by
JOHN H. WOTlZ Southern Illinois Universiw Carbondale, Illinois 82901
Why Did Adolf Baeyer Propose a Planar, Strained Cyclohexane Ring? 0. Bertrand Ramsay Eastern Michigan Uniuersity Ypsilanti, 48197 In 1885, Adolf Baeyer proposed that cyclic compounds smaller or larger than the cyclopentane ring would he "strained" (I).The strain theory was based on Baeyer's observation that the smaller rines could onlv be constructed with tetrahedral models if the wires joining t h i carhon atoms in the smaller rines were "bent." He then went on to calculate, for the 2- through &membered rings, the deviations fromthe tetrahedral angle of 109' 28' (Fig. 1). Many chemists have wondered bow it was possible for Baeyer to consider that the 6-memhered ring was planar, since when the ring is constructed with the usual "hall-and-stick" tetrahedral carhon models the ring ends u p in either a chair or boat conformation. I t has generally been assumed that Baeyer proposed a planar structure because of cyclohexane's derivative relationship to benzene. Historians, however, have not considered that the answer might he found in examining the particular kind of molecular models used by Baeyer.' In the early 1880's Baever oreoared some molecular models that im~rovedthe deskn dithe tetrahedral carbon models first usedby August KekulP in 186:. Srimtific Suoolv Com~aniesin Furow and America sold KekulB-von ~ a e ; e i models until well into the 1930's. A set of these models, that were purchased a t Eastern Michigan University in the 1920's or 1930's. serve to illustrate the structure of cyclohexane as i t would have appeared t o Baeyer (Fig. 2). Note that in these models the valency wires are connected by means of an adjustable joint. An examination of this model provides the answer to our question. First, the "strain" of the ring is related t o the angle between the wires, and furthermore in this case, the wires are directed inward (thus accounting for the "negative" strain). Secondly, it is also apparent that the 6 carhonatoms lie in one plane. That this model does correspond to that used by Baeyer is supported by later publications. In 1890, Baeyer pointed out that the two carhoxyl groups in trans-cyclohexane-1,2dicarboxylic acid form an [dihedral] angle of about log0, not 180° as is found in fumaric acid (5).The close proximity of the two carboxyl groups accounts for the ease with which a cyclic anhydride is formed. By the time experimental evidence had accumulated indicating the strainless character of the 6-membered ring, other models were used, and the models used by Baeyer had been forgotten. Literature Clted (1) Bseyer.A..Ber. 18.2277 (188S).Thetranslationofththkey pa%sqeiafffdinreffffffe ,I, \",.
(2) Leimster, H. M..and Kliehtein, H. S.,"A Source Bwk of Chcmiatrv: 15W-19W," MeGraw-Hill Bwk Co.. New York, 1952, p. 465. (3) Ramsay, 0.B.. Chemistry. 17 (no.11.6 (no.2),6(1974). (4) Ramsay, 0. B; "The van? Hof~LeBe1 Centennial? American Chemical Society,
Weshington,D.C.,1975.p.71.
(5) Bseyer, A..Ann.,258,145 (18%).
'For
further background, see references (3) and (4).
Figure 1. Baeyer's calculations of deviations from a tetrahedral angle.
Figure 2. Madel of cyclohexane as it appeared to Baeyer.
Louis Pasteur as Seen by His Nephew and Assistant Adrien Loir Marie-Louise Hemphill' I Place de VJagram 75017 Paris, France In the late 1870's, in order to teach themselves public soeakine. m e r classmen a t the ... it was customaw amona.. u .. I.vc&eof Lyons to give a talk on n topic of general interest LO the whole class. This thev ~ o m ~ o u s called lv "Conf6rence AmpBre". When his turn came, ~ d r i e n ~ o aged i r , 16, choose to speak about Louis Pasteur. His reasons were obvious since his mother and Madame Pasteur were sisters2 and the families often met though living in different towns. His father was professor of chemistry at the University a t Lyons whereas Pasteur was living in Paris. As a child the young Adrien had often watched his uncle and perhaps even helped him climb the steep steps leading t o his father's laboratories since a t that time Pasteur, a t the age of 46, was recovering from a stroke that left him partly paralyzed. Pasteur was alsoavoiding the long journey toParis which was much further from Alais where he was working on the diseases of the silkworm under rather nrimitive local conditions. Hewas pleased to find near a t hand a suitahle place for his experiments. This work was of great importance to France and i t was said later by Roux3 to be the model to his subsequent discoveries on human diseases. In 1878, a t Easter, Pasteur came t o relax a t Lyons. When told of the "lecture" Adrien was about to deliver he promptly asked the boy to read him the notes he had prepared. After hearing a few lines, he interrupted him, thought for a moment and said: "Take a pen and I'll dictate to you." This happened Volume 54, Number 9, September 1977 1 563
