Aromatization
Aromatization is a chemical reaction in which an aromatic system is formed. It can also refer to the production of a new aromatic moiety in a molecule which is already aromatic. Theoretically, this can be achieved by dehydrogenation of existing cyclic compounds (such as in converting cyclohexane into benzene) or by formation of new cyclic system (such as in the cyclotrimerization of acetylene to benzene); practically, other moieties are typically required to carry out such conversion, and other approaches like applying condensation reactions are possible. Aromatization includes the formation of any aromatic system (including heterocyclic systems), and is not restricted to benzene and its derivatives.
The substance now called benzene, C6H6, was known as a component of Southeast Asian aromatic resins used in perfumery since the 15th century. It was first isolated and identified by Michael Faraday in 1825 who named it bicarburet of hydrogen. In 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar and began the industrial-scale production based on this method four years later. Over time, consensus developed that other substances were chemically related to benzene, comprising a diverse chemical family; Hofmann used the word "aromatic" for this family for the first time in 1856.
Historic interest in benzene arose from the uncertainty over its structure and reactivity. It was well established that it had a carbon to hydrogen ratio of 1:1 which suggested the presence of double or triple bond carbon-to-carbon bonds, yet such unsaturated compounds typically undergo addition reactions, which benzene does not. Numerous potential structures were proposed, including by Claus,Dewar,Ladenburg,Armstrong, and Thiele. The most influential proposal was that of Kekulé who in 1865 suggested a cyclic structure of six carbon atoms with alternating single and double bonds Kekulé based his argument on the now well-known observations of arene substitution patterns, namely that there are always only one isomer of any monoderivative of benzene and three isomers of every disubstituted derivative. Critics noted that Kekulé's proposal implied that there should be two distinguishable isomers for an ortho-substitution (a 1,2-disubstituion) depending on whether a single or double bond joined these two carbons. Kekulé suggested in reply that benzene had two complementary structures and that these forms rapidly interconverted, which would also explain the lack of addition reactions (as the valency of each carbon atom is not that expected of a double bond). Though this interconversion is incorrect —in fact, they are both resonance contributors to the actual structure (which is closest to Thiele's structure)— the geometry and structure proposed by Kekulé are correct. Benzene is the prototypical example of an aromatic compound, a hitherto unknown class of compounds, and its synthesis was the first example of aromatization, the process and chemical reactions whereby an aromatic system is formed. The new understanding of benzene and all aromatic compounds was so important for both pure and applied chemistry 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; its 150th anniversary was also marked. The cyclic nature of benzene was confirmed crystallographically by Kathleen Lonsdale in 1929.
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