page_title: August Kekulé
section_title: Benzene
hierarchical_section_title: August Kekulé / Benzene
caption_attribution_description: Deutsch: August Kekulés Vorschlag für die Benzol-Strukturformel im Original, entnommen aus: English: August Kekulé's proposal for the benzene structure in its original, taken from: August Kekulé&#32;(1872).&#32;"Ueber einige Condensationsproducte des Aldehyds". Liebigs Ann. Chem.&#32;162&#32;(1): 77–124. DOI:10.1002/jlac.18721620110.&#32;"[…] man sieht daher, dass jedes Kohlenstoffatom mit den beiden anderen, mit welchen es zusammenstösst, gleich oft zusammenprallt, also zu seinen beiden Nachbarn genau in derselben Beziehung steht. Die gewöhnliche Benzolformel drückt natürlich nur die in Einer Zeiteinheit erfolgenden Stösse […] aus, und so ist man zu der Ansicht verleitet worden, Biderivate mit den Stellungen 1,2 und 1,6 müssten nothwendig verschieden sein. Wenn die eben mitgetheilte Vorstellung oder eine ihr ähnliche für richtig gehalten werden darf, so folgt daraus, dass diese Verschiedenheit nur eine scheinbare, aber keine wirkliche ist."
context_page_description: Friedrich August Kekulé, later Friedrich August Kekule von Stradonitz, was a German organic chemist. From the 1850s until his death, Kekulé was one of the most prominent chemists in Europe, especially in theoretical chemistry. He was the principal founder of the theory of chemical structure and in particular the Kekulé structure of benzene.
context_section_description: 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.
caption: Kekulé structure of benzene with alternating double bonds
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