E. Campaigne
Indiana University Bloomington
I
The Contributions of Fritz Arndt Resonance Theory
Keku16's ideas on tetravalence and selflinking of carbon atoms, which were so important in the development of structural theory, were described in 1858, just one hundred years ago. Only seven years later, in 1865, Kekul6 (1) showed how these ideas could explain the composition and anomalous behavior of the "aromatic" substances, by assuming the structure of benzene as a nucleus of six carbon atoms in a ring, with alternating single and double bonds. Kekul6 believed, but was unable to prove, that the six combining positions of benzene were identical. The Kekul6 structure has been called "the most fruitful single suggestion in the history of organic chemistryn(%). During the next decade. Kekul6's first ~ostulatethat the six combining valences of benzene were equivalent was quickly demonstrated by Ladenburg and Wroblewsky and then by Kiirner, with the first applications of what was later to he called the "absolute" method. Thus within ten years the structure of benzene as a symmetrical hexagon with six equivalent hydrogen atoms was pretty well established. At the same time, a much more difficult problem was presented, that of the actual location and function of the six carbon valences of benzene which are not needed to hold the atoms in place. Kekul6 himself soon suggested (3) that oscillation occurring between the two possible arrangements of the double bonds (I) would account for the non-existence of certain isomers of disubstituted benzenes, and be more consistent with the apparent lack of Presented as part of the Kekul&Couper Centennial Symposium on the Development of Theoretical Orgenic Chemistry before the Division of History of Chemistry at the 134th Meeting of the American Chemical Society, Chicago, September, 1958. Contribution No. 877 from the Chemistry Laboratories of Indiana University.
336
/
Journal of Chemical Education
unsaturation in aromatic compounds.
However, the concept of moving bonds in molecules was never quite satisfactory, and several other suggestions to account for the unused carbon valences in a manner consistent with the "aromatic" chemical properties of benzene were put forward. Among these were the Clans para-bond formula, 11, and the Arrnstrong-
v
v
(11)
(111)
Baeyer centric formula 111. Perhaps the best of these suggestions was advanced by Thiele (4), and was based on his ideas of partial valence used to explain the a,y-reactions observed in olefins c o n t a i ~ n galternating single and double bonds. This he called "conjugation," and he pointed out that in benzene the conjugation was cyclic and endless, and therefore unreactive. This suggestion was valuable in that it related "aromatic" character to the behavior of certain aliphatic compounds, but, like previous suggestions, it depended on unorthodox valence concepts. Regarding these suggestions, F. G. Arndt states (6) : Kekulh's idea of the oscillating double bonds in benzene was the el~st,.itilppn,ximsrion to rhn rcPonancc intrrprctntiun of benrmr and similar molrrul+r uclt~r~vcd duringall rlw rimc from Krkuhl up to lb"1 I tlrink t l h i i so hrw~urel
