page_title: Aromaticity
section_title: The structure of the benzene ring
hierarchical_section_title: Aromaticity / History / The structure of the benzene ring
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: In organic chemistry, aromaticity is a property of cyclic, planar structures with a ring of resonance bonds that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic molecules are very stable, and do not break apart easily to react with other substances. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have special stability.
  Since the most common aromatic compounds are derivatives of benzene, the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA and DNA. An aromatic functional group or other substituent is called an aryl group.
  The earliest use of the term aromatic was in an article by August Wilhelm Hofmann in 1855. Hofmann used the term for a class of benzene compounds, many of which have odors, unlike pure saturated hydrocarbons.
context_section_description: In the 19th century chemists found it puzzling that benzene could be so unreactive toward addition reactions, given its presumed high degree of unsaturation.  The cyclohexatriene structure for benzene was first proposed by August Kekulé in 1865. Most chemists were quick to accept this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry. The hexagonal structure explains why only one isomer of benzene exists and why disubstituted compounds have three isomers.
  Between 1897 and 1906, J. J. Thomson, the discoverer of the electron, proposed three equivalent electrons between each pair of carbon atoms in benzene. An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first (in 1925) to coin the term aromatic sextet as a group of six electrons that resists disruption.
  In fact, this concept can be traced further back, via Ernest Crocker in 1922, to  Henry Edward Armstrong,  who in 1890 wrote "the (six) centric affinities act within a cycle...benzene may be represented by a double ring (sic) ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition".
  Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the electron, which was to be discovered only seven years later by J. J. Thomson. Second, he is describing electrophilic aromatic substitution, proceeding (third) through a Wheland intermediate, in which (fourth) the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle, thus anticipating Erich Clar's notation. It is argued that he also anticipated the nature of wave mechanics, since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring (much as the distribution of the electric charge in a body is altered by bringing it near to another body).
  The quantum mechanical origins of this stability, or aromaticity, were first modelled by Hückel in 1931. He was the first to separate the bonding electrons into sigma and pi electrons.
  Aromaticity of an arbitrary aromatic compound can be measured quantitatively by the nucleus-independent chemical shift (NICS) computational method and aromaticity percentage methods.
caption: Historic benzene formulae as proposed by August Kekulé in 1865.[6]
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