Friedrich Kekulé von Stradonitz

Friedrich August von Stradonitz, Kekulé (September 7th 1829 with Darmstadt - July 13rd 1896 with Bonn) is a German organic chemist famous for the discovery of the tetravalence of carbon and the developed formula of the Benzène.

Life and career

It was born in Darmstadt, wire of a civil servant. After the secondary school, in 1847 it is registered the University of Giessen to study architecture, but changes into chemistry after having listened to the courses of Justus von Liebig. After its studies with Giessen, it makes studies postdoctorales in Paris (1851-52), Coire (Swiss, 1852-53), and in London (1853-55) under the direction of Alexander Williamson.

In 1856 it becomes Privat-docent with the Université of Heidelberg. In 1858 it obtains a post of professor with the Université of Ghent (Belgium). It remains there up to 1867 when he becomes professor with the Université of Bonn.

Kekulé forever employed its first first name; it is known during its life like August Kekulé. In 1895, one year before its death, it is ennobli by the emperor Guillaume II and adopts the title von Stradonitz. This title derives from Stradonice (Bohemia) where its family had had grounds.

Tetravalence of carbon and chemical structure

In 1857-58 Kekulé the theory of the chemical structure, based on two concepts develops: the tetravalence of carbon, and capacity of the carbon atoms to form connections between them. The Scottish chemist Archibald Scott Couper finds the second concept independently, and provides the first diagrams to which the connections are represented by lines.

This theory of structure allows the comprehension of the organic molecules and their reactions, and led to a true explosion of research in chemical Synthèse of the organic compounds as from 1860.

Benzene

In 1865, he works since weeks on the developed formula of the benzene of which he has the empirical formula C₆H₆. None the formulas which it produced, linear or ramified, corresponds perfectly with the monovalence of the Hydrogène (H) and especially the tetravalence of the Carbone (C) which it has just discovered. He proposes finally a cyclic structure - the first in the history of chemistry - with a ring of six carbons bound by connections simple and double in alternation.

Kekulé supports this structure by considering the number of Isomère S of the molecules derived from benzene. For monosubstituted benzenes (C₆H₅X, where X = Cl, OH, CH₃, NH₂, etc), only one isomer is found, which implies that all six carbons are equivalent, so that substitution on each carbon forms the same product. For disubstituted benzenes such as the Toluidines C₆H₄ (NH₂) (CH₃), three isomers are observed. Kekulé proposes structures to them for which the two carbons substituted are separated by one, two, or three connections; called thereafter isomers ortho, méta and para respectively.

The counting of possible isomers of disubstituted benzenes is criticized however by Albert Ladenburg, former student of Kekulé. According to Ladenburg, the structure suggested by Kekulé in 1865 would imply two distinct isomers “ortho” because two substituted close carbons could be separated by a simple connection or by a double linking. Actually only one isomer ortho exists, and then Kekulé modifies its structure in 1872 and proposes an oscillation between two equivalent structures, so that the connections simple and double exchange their positions quickly. The six connections carbon-carbon are then equivalent, each one being simple connection half of time and connection doubles half of time. This equivalence receives a more solid theoretical base in 1928, when Linus Pauling replaces the oscillation of Kekulé by the superposition of two structures according to mechanics quantum, known as the resonance or the Mésomérie.

The proposal of Kekulé allows the development of a new branch of the organic chemistry, namely chemistry of the aromatic molecules which contain a benzene ring. In 1890 the German chemical Company celebrates the 25e birthday of its first article on benzene. Kekulé tells whereas its theory had been created when one night, he dreams of the Ouroboros (the gnostic snake which bites the tail). It failed in its dream to exclaim Eureka! in a flash, it comes to discover the developed formula of the benzene which it seeks since of the weeks! However the historians still discuss the truth of this history; some believe that Kekulé would have invented it when he told it in 1890.

Price and honors

Its research in Organic chemistry is worth to him the Médaille Copley in 1885. Among the first five Nobel Prize in chemistry, three are its former students: van' T Hoff in 1901, Fischer in 1902, and Baeyer in 1905.

References

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