The Discovery of Benzene

The Kekule Structure In 1866, Friedrich Kekuld proposed a cyclic structure for benzene with three double bonds. Considering that multiple bonds had been proposed only recently (1859), the cyclic structure with alternating single and double bonds was considered somewhat bizarre. 

The Kekule structure has its shortcomings, however. For example, it predicts two different 1,2-dichlorobenzenes, but only one is known to exist. Kekul6 suggested (incorrectly) that a fast equilibrium interconverts the two isomers of 1,2-dichlorobenzene. 

As time passed, chemists found or synthesized many compounds with benzene rings that had no odor, such as benzoic acid and acetylsalicylic acid (aspirin). 

In this chapter we shall discuss in detail the structural principles that underlie how the term aromatic is used today. We will also see how the stracture of benzene proved so elusive. Even though benzene was discovered in 1825, it was not until the development of quantum mechanics in the 1920s that a reasonably clear understanding of its structure emerged. 

The study of the class of compounds that organic chemists call aromatic compounds (Section 2. ID) began with the discovery in 1825 of a new hydrocarbon by the English chemist Michael Faraday (Royal Institution). Faraday called this new hydrocarbon bicar-buret of hydrogen we now call it benzene. Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil. 

In 1834 the German chemist Eilhardt Mitscherlich (University of Berlin) synthesized benzene by heating benzoic acid with calcium oxide. Using vapor density measurements, Mitscherlich further showed that benzene has the molecular formula CeHe  

One of the tt molecular orbitals of benzene, seen through a mesh representation of its electrostatic potential at its van der Waals surface. 

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Let s begin by tracing the history of benzene its origin and its structure Many of the terms we use including aromaticity itself are of historical origin We 11 begin with the discovery of benzene... 

The sudden appearance of 59 on the chemical scene is likely to produce a truly dramatic effect on the further development of organic chemistry, comparable in its significance with the discovery of benzene in 1825 by Michael Faraday. The essential difference between these two discoveries is that it took nine years to establish the molecular formula of benzene (Mitcherlich, 1834), an additional 31 years to understand its structure (Kekule, 1865), and several decades more to develop the chemistry in this area. The arsenal of modern science, on the other hand, made it possible to cover such a distance for the case of 59 and related compounds in a matter of just a few years. 

For an account of the discovery of benzene and of the many names applied to it, see Badger, G. M. Aromatic Character and Aromaticity Cambridge University Press Cambridge, 1969 p. 1 ff. Visitors to London may visit the Faraday Museum in the Royal Institution, which has displayed what was reported to be the very sample of benzene isolated by Faraday in 1825. Also see Berson, J. A. Chemical Discovery and the Logicians Program A Problematic Pairing, Wiley-Interscience Wemheim, 2003. 

Faraday s experience grew. He studied chlorine and its reactions and isolated the first two compounds of carbon and chlorine. While serving as an expert witness in a court case, he investigated the ignition point of heated oil vapor, which led to the discovery of benzene. When he was 30 years old he married Sarah Barnard, the sister of one of his friends at the City Philosophical Society when he was about 40 he began investigating connections between electricity and chemistry— the work that would be his greatest claim to chemical fame. 

The Discovery of Benzene 627 Nomenclature of Benzene Derivatives 628 Reactions of Benzene 630 The Kekule Structure for Benzene 631 The Thermodynamic Stability of Benzene 632 Modern Theories of the Structure of Benzene 634 Huckel s Rule The 4n +2 tt Electron Rule 637 Other Aromatic Compounds 645... 

Kolbe has now begun to use Odling s notation (see p. 462) of dashes to represent the atomicity of a radical. Gerhardt and Chancel had previously given a table of compounds in which replaced carbon monoxide C 0 , and announced the discovery of benzene sulphochloride, C6H5SO2CI, as chlorure phenylsulfureux , C H S 0 C1. 

The parent compound of the aromatics is benzene it was first discovered by Michael Faraday in 1825 in the condensed part of a lighting gas derived from whale oil and obtained some years later by Eilhard Mitscherlich by decarboxylation of benzoic acid (as calcium benzoate). The occurrence of benzene in coal tar was first described by August Wilhelm v. Hofmann in 1845. John Leigh had already demonstrated to the British Natural Research Conference in 1842, that benzene is present in coal tar this claim was not immediately published, however. Even before the discovery of benzene, Ferdinand Runge had found aniline and phenol in coal tar in 1834. 

To be sure, during most of the first century after the discovery of benzene (Faraday 1825) and Kekule s hexagon structure (1865) the terms aromatic chemistry and benzene chemistry were practically synonyms, especially if the enormous success of the coal tar and dyestuffs industry is taken into accotmt. It is a safe... 

Aromaticity is a qualitative phenomenon, which is of utmost importance in the wide realm of chemistry [1], Prior to the discovery of benzene by Michael Faraday as early as 1825 [2], the compounds with a distinctive smell or aroma were found to be quite stable and were usually known by the name aromatic compounds. Soon after the discovery of benzene, this phenomenon has gained a tranendous impetus. With the discovery of new molecules exhibiting aromatic or antiaromatic behavior, the impetus has gained a further momentum in research [3]. The concept of aromaticity has earlier been confined to hydrocarbon systems only however, the modem era has also witnessed its importance among the heteroatomic systems as well [4]. Several such systems with o-electronic framework have attracted much attention. Moreover, innovation of metal clusters with d-orbital aromaticity is also very... 

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