Kekule—Couper theory

Shortly after the tetravalent nature of carbon was proposed, extensions to the Kekule-Couper theory were made w7hen the possibility of multiple bonding between atoms was suggested. Emil Erlenmeyer proposed a carbon-carbon triple bond for acetylene, and Alexander Crum Brown proposed a carbon-carbon double bond for ethylene. In 1865, Kekule provided another major advance when he suggested that carbon chains can double back on themselves to form rings of atoms. 

The backstory of this "just-puhlished paper on lactic acid" (CsHgOa) is important. We have already encountered lactic acid in the third section of chapter 4. Its constitution—formulable according to the dictates of the Kekul -Couper theory as either methyl glycolic acid or, what is the same thing, hydroxy propionic acid—was understood by... 

The development of valency arose from Berzelius theory of chemical combination which stressed [17, 18] the electronegative and electropositive character of combining atoms. In the mid-nineteenth century, Frankland, Kekule, Couper, Butlerov and Kolbe [19-26], building on the theory of radicals, developed the theory of valency in which elements in compounds were joined by an attraction of positive and negative poles. The concept of valency preceded the discovery of the electron and the planetary view of the atom and may be traced to the 1850 paper by Frankland [19, 24]. He combined the older theories of free radicals and type theory and demonstrated that elements have the tendency to combine with other elements to form compounds containing an integer number of attached elements, e.g. in the three attached atoms M/j, NI3, four attached atoms in CH4 and five attached atoms in PCls. Based on these examples and postulates, Frankland articulated the truism ... 

From the concept of isomerism we can trace the origins of the structural theory—the idea that a precise arrangement of atoms uniquely defines a substance Ammonium cyanate and urea are different compounds because they have different structures To some degree the structural theory was an idea whose time had come Three scientists stand out however for independently proposing the elements of the structural theory August Kekule Archibald S Couper and Alexander M Butlerov... 

Shortly thereafter but independently of Kekule Archibald S Couper a Scot working m the laboratory of Charles Adolphe Wurtz at the Ecole de Medicine m Pans and Alexan der Butlerov a Russian chemist at the University of Kazan proposed similar theories... 

To Avogadro and Cannizzaro, as to Couper and Kekule, the molecules and atoms considered in this great theory were real objects they were thought of the same way as one thinks of tables and chairs. 7... 

In England, in 1856, only two years before Kekule and Couper laid the foundations of the stmctural theory, W.H. Perkin [6], acting upon the advice of his mentor A.W. Hofmann, attempted to synthesise quinine by the oxidative dimerization of allyltoluidine following the reaction ... 

This paper (2), Sur une nouvelle theorie chimique, was eventually presented before the Academie des Sciences by Dumas, and published in the Comptes rendus on June 14, 1858, six weeks after Kekule s paper appeared. Wurtz was not a member of the Academie at that time, and therefore could not present the paper himself without the sponsorship of a member (usually Balard). Whatever its cause, Wurtz delay in finding a sponsor brought Couper to a fmy, with far-reaching consequences. 

It may also have been unfortunate for Couper that he was an excellent experimentalist, because his greater skill in the laboratory unwittingly gave ammunition to his opponents. In his early work that culminated in stmctural theory, he reported the formation of a cyclic compound from the reaction between phosphoms pentachloride and salicylic acid (57). Both Kekule (56) and Kolbe (57) attempted to reproduce his reaction without success, and used their failure to duplicate his results to cast doubt on his other claims. It was not until a better experimentalist, Richard Anschutz, repeated Couper s work exactly as he had set out in his paper that his skill as an experimentalist was confirmed, and his... 

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