Kekulé Centennial

Kekulé's interest in architecture influenced his structural theory. His opinions are compared with those of his contemporaries—Frankland, Butlerov, Ladenburg, and Wislicenus. Evidence of Kekulé's concern for the spatial arrangement of atoms in molecules is obtained from his introspective comments, his use of tetrahedral models, and some arguments in support of his benzene structure. He did not explicitly anticipate the solutions proposed by van't Hoff and le Bel. Clarification of Kekulé's views is valuable in suggesting approaches to the teaching of organic structural theory. It is historically inaccurate and confusing to suggest that the Kekulé-Couper theory considered molecules as two-dimensional entities. The theory required no specific arrangement in space but did refer to chemical relations between atoms in three dimensions.

V. V. Markovnikov was a student of Butlerov and an ardent supporter of his master's claim to have originated the structural theory of organic chemistry. More aggressive than Butlerov, he took a leading part in the polemics against Kekulé and his supporters, which enlivened the decade from 1860 to 1870. At the same time he devoted himself to experimental studies on the reciprocal effects of atoms and groups in organic compounds and their effect on reaction mechanisms. His well known "rule" was one of the results of these studies, which foreshadowed the later development of the electron theory of organic reactions.

August Kekulé worked on his theories for a long time before publishing them, and when he did publish, he was almost too late for priority. However, his theories were so well founded that they have withstood the real critical examinations of the ensuing years. The benzene theory was his greatest achievement. After its publication, Kekulé wrote nothing more in support and explanation until four and seven years later. One group of his former students, assistants, and friends kept the benzene theory alive and developed it through their own structural concepts and elaborations; the other group, intent on building a synthetic dye industry, made good use of Kekulé's work to convert a struggling industry enmeshed in alchemical empiricism into a fantastically large and important industry.

Three-dimensional models of benzene correspond to the structural formulas with double, diagonal, and centric bonds. The most interesting is the formula of Sachse, which corresponds to the formula containing three-electron bonds. The recently synthesized three-dimensional isomers of benzene are not aromatic, and the less stable, the more double bonds they contain. Hence, the Kekulé formula with three double bonds can not express the benzene properties and is only a "Bildungsformel." It is accepted that hydrogen atoms are located in the same plane which contains carbon atoms, but some experimental data contradict this presumption.

The central problem in the study of unsaturated substances resolved itself into the question: what unique feature do these compounds possess, which allows some to behave as they do? A satisfactory answer was found between 1858 and 1870, growing out of the concept of carbon tetravalency and graphic representation of how bonded atoms are arranged in molecules. Kekulé and Couper provided the former simultaneously in 1858, and at the same time Couper suggested a method for graphic representation. Couper's suggestion was developed by Crum Brown's explaining isomerism, adapted by Erlenmeyer, and made generally applicable by Butlerov, who demonstrated that the assumption of multiple bonds was not only compatible with chemical behavior of unsaturated compounds but necessary to explain that behavior.

The molecular orbital calculations of E. Hückel on the relative electronic stability of different monocyclic, conjugated systems has led to the preparation of a number of new, non-benzenoid aromatic compounds. The preparation and properties are given for the more intensely studied of these monocyclic aromatic systems that obey Hückel's rule. A few of the related compounds that do not obey Hückel's rule and that do not display aromatic properties are discussed.

Geometry has played a variety of roles in explaining natural phenomena at different periods of history. The discovery of the limited number of regular solids by the Pythagoreans was followed soon by the linking of the five solids to Empedocles' four elements plus the fifth essence associated with the heavens. Plato's "Timaeus" develops a quantitative atomism based on these figures and their constituent triangles—an atomism in many respects closer to modern thought than that of Democritus. The regular solids reappear in Renaissance discussions of natural phenomena. With Dalton's atomism, spatial arrangements of the atoms were considered early, and eventually the regular solids were incorporated into modern structural theory.

In addition to accidental observations, analogies, and inferences by close reasoning, dreams and visions had an important part in the progress of chemistry. Four classes of progress can be distinguished: (1) symbolization and construction of models: Kekulé, van't Hoff, J. J. Thompson; (2) extrapolation in quantity: Wöhler, Sabatier, Kurnakow; (3) projection in time: Kuhlmann, Le Bon, Aston; (4) generalizations: Clausius, Le Chatelier, Ostwald. This list is incomplete and leaves out the failures. Not all those with dreams and visions were as careful as Kekulé was to check and test before publishing. The courage to persist must be combined with a critical evaluation of the facts, and this is especially necessary when solutions are achieved primarily in broad jumps rather than small steps.

To account for the apparent instability of rings with less than five carbon atoms, Baeyer suggested in 1885 a strain of the valencies of carbon away from the normal tetrahedral angle of 109° 28′. The concept proved valuable in interpreting structural problems connected with small rings, bridged rings, and structures found in natural products such as sterols and terpenes. Application to six-membered and larger rings caused problems until it was realized that such rings are strainless owing to their ability to take on a puckered conformation. Heats of combustion, dipole moments, spectra, as well as chemical evidence have generally been in accord with strain predictions based on examination of models.

The disposition of the fourth valence of the carbon atoms in benzene has caused extensive discussion and speculation. Following Kekulé's formula of 1865, a variety of formulas for benzene was proposed. Two trends were evident: the desire to arrive at the actual structure and the desire to devise formulas which were faithful to the functional behavior and broadly indicative of the structural relationship of the constituent elements. In 1869, Ladenburg criticized Kekulé's formula and suggested alternatives, one of which was the prism formula, which for a time was a serious rival of the hexagon. The strengths and weaknesses of the Ladenburg formula relative to the Kekulé formula are assessed. Recent laboratory studies by Viehe and co-workers have renewed interest in the prism structure.