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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Benzene. History and essence of scientific discovery
Directory / The most important scientific discoveries The study of aromatic compounds began to develop only after the basic principles of the theory of chemical structure were recognized by organic chemists. In the mid-nineteenth century, in the field of aromatic compounds, most chemists considered a group of six carbon atoms as a whole, without even commenting on its chemical structure. For aromatic compounds, the presence of a special carbon group of six atoms, for example, in benzene, was considered characteristic. As for benzene, it was mistakenly assumed that there were two varieties of it: ordinary with a boiling point of 80 degrees Celsius and parabenzene with a boiling point of 97 degrees. Such a theory made it even more difficult to answer the question of how many isomers can be obtained by substituting one hydrogen in the benzene ring. "Mr. Kolbe believed that, in addition to benzoic acid, there is an isomeric acid - salic acid," writes G.V. Bykov. . A. M. Butlerov in 1864 limited himself to the assumption that in benzene "and its derivatives at least some of the carbon shares are connected to each other by a greater amount of affinity than in the hydrocarbon C6H14 ..." Similarly Kekule in the same 1864, he referred aromatic compounds and naphthalene to compounds in which the carbon atoms are supposedly connected by "two or perhaps three units of affinity." In the first half of the sixties, new interesting facts began to appear, especially those concerning the number of deputies. In 1864, the identity of methylphenyl with toluene was shown, which already indicated the equivalence of six carbon atoms in benzene. Information was accumulating on the structure of disubstituted benzene derivatives: in 1863, K. Zaitsev obtained the third hydroxybenzoic acid; in the same year, G. Fischer isolated the third nitrobenzoic acid; in 1864, G. Glazivets and L. Barth synthesized resorcinol, the third representative of dihydric aromatic alcohols, etc. Based on the study of the properties of hydroxybenzoic acids, AM Butlerov concluded that "their chemical structure differs only in the different placement of the alcoholic water residue relative to carbon phenyl group". Thus, in the phenyl radical connected to a carboxyl group, he distinguished three hydrogen atoms; when each of them is replaced by hydroxyl, three different hydroxybenzoic acids are obtained. Thus, the ground has already been prepared for a successful generalization of the available material. In 1865, A. Kekule made just such a generalization, assuming that the carbon atoms in the benzene nucleus form a closed chain, connecting with each other alternately: either with the help of one pair, then two pairs of affinity units ... " August Kekule (1829–1896) was born in Germany. The boy was amazingly gifted. Even at school, he could speak four languages fluently, had literary abilities. According to the project of the high school student Kekule, three houses were built! After leaving school, August went to Giessen to study at the university. At the university, August first heard the name of Justus Liebig. Kekule decided to attend the famous scientist's lectures, although he was not interested in chemistry. Kekule's first scientific work on amylsulfuric acid was highly appreciated by Professor Bill. For her, in June 1852, the Academic Council of the University awarded him a doctorate in chemistry. After graduating from the university, the young scientist worked for some time in Switzerland with Adolf von Plant, and then moved to London, where he was recommended the laboratory of John Stenhouse. The question of valence was extremely interesting to Kekule, and he gradually matured the ideas of experimental verification of certain theoretical propositions, which he decided to present in his article. In it, Kekule made an attempt to generalize and expand the theory of types developed by Gerard. In the spring of 1855, Kekule left England and returned to Darmstadt. He visited the universities of Berlin, Giessen, Göttingen and Heidelberg, but there were no vacancies. Then he decided to ask permission to be appointed as Privatdozent in Heidelberg. Kekule devoted all his free time to research work. He concentrated his attention on explosive acid and its salts, the structure of which was still not clear. He managed to expand and supplement the theory of types. Kekule added one more to the main ones - a type of methane. He outlined his conclusions in the article "On the constitution of mercury fulminate." In the article "On the Theory of Polyatomic Radicals" Kekule formulated the main provisions of his theory of valency. He generalized the conclusions of Frankland, Williamson, Odling and developed the question of the connecting ability of atoms. In the article "On the Composition and Transformations of Chemical Compounds and on the Chemical Nature of Carbon", Kekule substantiated the tetravalence of carbon in organic compounds. He also noted that Gerard's attempt to bring all chemical reactions under one general principle - double exchange - is not justified, since there are reactions of direct combination of several molecules into one. Kekule came up with completely new ideas, ideas about carbon chains. It was a revolution in the theory of organic compounds. These were the first steps in the theory of the structure of organic compounds. At the end of 1858, Kekule left for Ghent, where he continued his research work. "...Kekule began to study the structure of benzene and its derivatives, which required, first of all, finding suitable means for presenting educational material in the section of aromatic compounds," writes K. Manolov. "He knew Loschmidt's book, published in 1861, which for the first time the formulas of organic compounds were presented according to the atomic theory... He also knew Butlerov's theory, which he still did not fully accept, but could not reject ... The atoms in the molecule mutually influence each other, and the properties of the molecule depend on the arrangement of the atoms. Kekule imagined carbon chains in the form of snakes.They wriggled, took on a variety of positions, gave or added atoms, turning into new compounds.Kekule had a great gift of imagination, and, closing his eyes, he really imagined a picture of miraculous transformations of one molecule into another.And yet he had not yet been able to imagine the structure of benzene.How are the six carbon and six hydrogen atoms arranged in its molecule? I did not make dozens of assumptions, but, on reflection, discarded. Tired of his work, Kekule put down the sheets of paper he had written and moved an armchair to the fireplace. Pleasant warmth gradually enveloped the body, and the scientist fell asleep. And again, six carbon atoms appeared in his mind, forming bizarre shapes. The six-atomic "snake" continuously "wriggled" and suddenly, as if angry with something, it began to bite its tail with bitterness, then firmly grabbed its tip and so froze. No, not a snake, this is the ring of Countess Görlitz, which was handed to Kekule by Justus Liebig. Yes, on his palm lies a ring - a platinum snake intertwined with gold. Kekule shuddered and woke up. What a strange dream! And it only lasted for a moment. But atoms and molecules did not disappear before his eyes, he continued to remember in reality the order of arrangement of atoms in a molecule, seen in a dream. Maybe this is the solution? Kekule hurriedly sketched a new chain shape on a piece of paper. The first ring formula of benzene... The idea of a benzene ring gave a new impetus to experimental and theoretical research. Kekule sent the article "On the Structure of Aromatic Compounds" to Wurtz, who presented it to the Paris Academy of Sciences. The article was published in the Bulletin of the Academy in January 1865. Science has been enriched by yet another new, exceptionally fruitful theory of the structure of aromatic compounds. Further research in this area led to the discovery of various isomeric compounds, many scientists began to conduct experiments to elucidate the structure of aromatic substances, proposed other formulas of benzene ... But Kekule's theory turned out to be the most legitimate and soon established itself everywhere. Based on his theory, Kekule predicted the possibility of the existence of three isomeric compounds (ortho, meta and para) in the presence of two substituents in the benzene ring. Another field of activity opened before scientists, the possibility of synthesizing new substances appeared. In Germany, Hoffmann worked on this, Bayer, in France - Wurtz, in Italy - Cannizzaro, in Russia - Butlerov and others. Kekule's formula for benzene also raised numerous objections. As G.V. Bykov: "A. Klaus in 1867 drew attention to the fact that benzene is dissimilar in its properties to ethylene, which it would have to resemble judging by the Kekule formula, and proposed his formulas with crossed bonds. A. Ladenburg in 1869 noted that, according to the Kekule formula, there should be two isomers for substitution products at neighboring carbons, and proposed his own, prismatic, formula. As far back as 1869, A. Kekule wrote that he considered these objections "not too weighty", and cited a number of reactions that are well explained by his formula, which seems to him, moreover, "more elegant and symmetrical" than others. In 1872, he tried to remove the objections put forward altogether by proposing the so-called oscillation hypothesis, according to which a carbon atom at some moment collides once with one and twice with another neighboring atom, and at the next moment - vice versa. These impacts, according to Kekule, correspond to single and double bonds. The discussion about the structure of the benzene ring continued for many more years. The prismatic formula of A. Ladenburg was experimentally refuted, the well-known formulas of G. Armstrong and A. Bayer were put forward, the physical meaning of which was even less clear, etc. But this was not essential for establishing the structure of the vast majority of aromatic compounds; only the following provisions were important: carbon atoms are located symmetrically (at the corners of a regular hexagon), and all of them are equivalent to each other. Author: Samin D.K.
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