ESOC 2021

22nd European Symposium on Organic Chemistry

 Ghent, Belgium    July 11-15 2021

Aromaticity of the benzene ring

Friedrich August Kekulé was born in Darmstadt, Germany on September 7, 1829. In 1856 he started his career as private teacher in Heidelberg, Germany. In 1858 he was nominated professor at the University of Gent, Belgium.

Kekulé's most famous work was on the structure of benzene. In 1865 Kekulé published a paper in French (for he was then still in Belgium) suggesting that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. The following year he published a much longer paper in German on the same subject.

The empirical formula for benzene had been long known, but its highly unsaturated structure was a challenge to determine. Archibald Scott Couper in 1858 and Joseph Loschmidt in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its earliest years, and too little evidence was then available to help chemists decide on any particular structure.

More evidence was available by 1865, especially regarding the relationships of aromatic isomers. Kekulé argued for his proposed structure by considering the number of isomers observed for derivatives of benzene. For every monoderivative of benzene (C6H5X, where X = Cl, OH, CH3, NH2, etc.) only one isomer was ever found, implying that all six carbons are equivalent, so that substitution on any carbon gives only a single possible product. For diderivatives such as the toluidines, C6H4(NH2)(CH3), three isomers were observed, for which Kekulé proposed structures with the two substituted carbon atoms separated by one, two and three carbon-carbon bonds, later named ortho, meta, and para isomers respectively.

The counting of possible isomers for diderivatives was however criticized by Albert Ladenburg, a former student of Kekulé, who argued that Kekulé's 1865 structure implied two distinct "ortho" structures, depending on whether the substituted carbons are separated by a single or a double bond. Since ortho derivatives of benzene were never actually found in more than one isomeric form, Kekulé modified his proposal in 1872 and suggested that the benzene molecule oscillates between two equivalent structures, in such a way that the single and double bonds continually interchange positions. This implies that all six carbon-carbon bonds are equivalent, as each is single half the time and double half the time. A firmer theoretical basis for a similar idea was later proposed in 1928 by Linus Pauling, who replaced Kekulé's oscillation by the concept of resonance between quantum-mechanical structures.

Kekulé's dream

The new understanding of benzene, and hence of all aromatic compounds, proved to be so important for both pure and applied chemistry after 1865 that in 1890 the German Chemical Society organized an elaborate appreciation in Kekulé's honor, celebrating the twenty-fifth anniversary of his first benzene paper. Here Kekulé spoke of the creation of the theory. He said that he had discovered the ring shape of the benzene molecule after having a dream of a snake seizing its own tail (this is an ancient symbol known as the ouroboros).

This article uses material from the Wikipedia article "August Kekulé", which is released under the Creative Commons Attribution-Share-Alike License 3.0.