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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Stereochemistry. History and essence of scientific discovery
Directory / The most important scientific discoveries “Ideas regarding the spatial arrangement of the smallest particles of matter began to be expressed since the very idea of molecules and their constituent atoms appeared in science,” writes V.M. Potapov. J. Dalton at the beginning of the XNUMXth century, he spoke about possible spherical, tetrahedral, hexahedral forms in atomism. At about the same time, Wollaston drew attention to the need to consider the arrangement of atoms in space and pointed out that "stable equilibrium" when two types of atoms are combined in a ratio of 1:4 is achieved with their tetrahedral arrangement. However, Wollaston was pessimistic about the possibility of knowing the "geometric arrangement of primary particles". Thoughts about the possibility of a different arrangement of atoms in molecules were repeatedly expressed at the beginning of the XNUMXth century by a number of scientists in connection with the discussion of the problems of isomerism... So, in 1831, J. Berzelius wrote that "there are bodies composed of the same number of atoms of the same elements, but located in an unequal way and therefore having unequal chemical properties and an unequal crystalline form." Already at the end of the forties, L. Gmelin noted: “Atoms are not located, as expressed by the formula, in one row ... but approach, on the basis of affinity, as close as possible to each other, as a result of which they form more or less regular figures. Therefore, it is extremely important to determine this arrangement of atoms ... because from this, perhaps, more light will be shed on the crystalline form, isomerism ... on the constitution of organic compounds. famous Russian chemist A. M. Butlerov in a number of his early works, he also expressed interesting thoughts about the spatial structure of molecules: "... I do not believe that it is impossible, as he thinks Kekule, represent on the plane the position of atoms in space". This is a statement of 1864, and two years earlier Butlerov spoke about the tetrahedral arrangement of substituents around a carbon atom: "... let's take a rough example and, assuming that all 4 units of affinity are different for a four-atom carbon unit, imagine it as a tetrahedron, in which each of the 4 planes is capable of binding 1 share of hydrogen ... "Nevertheless, there is no reason to rank Butlerov among the founders of stereochemistry. P.I. Walden argues: “Why, one wonders, did it take another 25 years for stereochemistry to arise only in 1874? .. The answer can be easily given: the idea appeared before the facts! as needed, depending on the accumulation of facts, the idea is transformed. The phenomena that directly served as an impetus for the birth of stereochemistry were discovered in one of the border areas of physics and chemistry in the study of the interaction of light and matter. First, polarized light was discovered. His further studies were carried out by the French scientist and politician Dominique Francois Arago (1786–1853). In 1811, he managed to discover that quartz has the ability to rotate the plane of polarization of light. Arago called this phenomenon optical activity. It became increasingly clear that this ability was related to the crystalline state. After all, it is worth dissolving quartz, and it loses optical activity. Four years later, the next step was taken by J. B. Biot, who established that a number of organic liquids also have optical activity. It is clear that here the explanation had to be sought no longer in the features of the crystal, but in the properties of the substance itself. Further progress is related to the work Louis Pasteur. The starting point of Pasteur's stereochemical work was crystallographic studies of salts of tartaric acid. V.M. Potapov describes this process as follows: “At the first stage of research on optically active substances, it was believed that their crystals are always hemihedral, that is, they can exist in two forms that relate to each other as an object to its mirror image. The only apparent exception to this rule was crystals of dextrorotatory tartaric acid, which, according to the German chemist E. Mitscherlich, turned out to be non-hemihedral, completely coinciding in shape with crystals of the optically inactive isomer - tartaric acid. In 1848, L. Pasteur repeated the experiment of E. Mitcherlich and discovered hemiedria in crystals of the sodium ammonium salt of grape (optically inactive) acid. At the same time, it turned out that crystals of two mirror forms meet simultaneously. Separating them with tweezers from each other and separately dissolving in water, Pasteur found that both solutions are optically active, with one rotating the plane of polarization to the right, like natural tartaric acid, and the other to the left. Thus, it was shown for the first time that an optically inactive substance - tartaric acid - is a mixture of two optically active components: dextrorotatory and levorotatory tartaric acid. All the above achievements prepared the triumph of Jacob Henry van't Hoff (1852–1911). He was born in Holland in Rotterdam in the family of a doctor. After graduating from school, Henry entered the Polytechnic Institute in Delft at the age of seventeen. At the end of the second year, he takes the exams for the third. van't Hoff believes that higher education is not enough and decides to work on his doctoral dissertation. To do this, he decides to continue his education at the University of Leiden. However, he decidedly did not like it there, and Henry goes to Bonn to the famous chemist Kekule. After the discovery of propionic acid by young scientists, Kekule recommended his student to go to Paris to Professor Wurtz, a specialist in organic synthesis. In Paris, Henry became close to the French industrial chemist Joseph Achille Le Bel (1847–1930). Both followed with interest Pasteur's research in the field of optical isomerism. And then ... Here is what K. Manolov writes in his book "Great Chemists": "There was a rich library at Utrecht University. Here Henry got acquainted with an article by Professor Johannes Wislicenus on the results of a study of lactic acid. He took a piece of paper and drew the formula for lactic acid. In the center of the molecule, there is again one asymmetric carbon atom. In essence, if four different substituents are replaced by hydrogen atoms, the result is a methane molecule. Imagine that the hydrogen atoms in the methane molecule are located in the same plane as the carbon atom. Van't Hoff was struck by an unexpected thought. He left the article unread and went out into the street. The evening breeze tousled his blond hair, he did not notice anything around - before his eyes stood the formula of methane he had just drawn. But how likely is it that all four hydrogens are in the same plane? Everything in nature tends to a state of minimum energy. In this case, this happens only when the hydrogen atoms are arranged uniformly around the carbon atom in space. Van't Hoff mentally imagined what a methane molecule might look like in space. Tetrahedron! Of course, a tetrahedron! This is the best location! And if the hydrogen atoms are replaced by four different substituents? They can take two different positions in space. Is this the solution to the riddle? Van't Hoff rushed back to the library. How could such a simple thought not occur to him until now? Differences in the optical properties of substances are associated primarily with the spatial structure of their molecules. Two tetrahedra appeared on a piece of paper next to the formula for lactic acid, one being a mirror image of the other. Van't Hoff rejoiced. Molecules of organic compounds have a spatial structure! It's so simple... How has no one figured it out yet? He must immediately state his hypothesis and publish the article. A mistake is not ruled out, but if his guess turns out to be correct ... van't Hoff took out a blank sheet of paper and wrote the title of a future article: "A proposal to apply modern structural chemical formulas in space, along with a note on the relationship between the optical rotational ability and the chemical design of organic compounds ". The title turned out to be quite long, but it accurately reflected the goal and the main conclusion. "I will allow myself in this preliminary report to express some thoughts that may provoke discussion," Van't Hoff began his article. The intentions of the author were the most beautiful, the ideas original and promising, but a small article printed in Dutch remained unnoticed by European scientists. Only Bui Ballot, professor of physics at the University of Utrecht, appreciated it." Only two months have passed since Van't Hoffard's friend J. Le Bel published his work. In it, he explained the appearance of optical activity by the spatial features of the structure of molecules in much the same way as the Dutch scientist had done earlier. But the works were not quite identical. “The most significant difference was,” writes Potapov, “that Van’t Hoff spoke about the directionality of the valences of the carbon atom, using a clear geometric picture of the tetrahedron, and Le Bel represented the valences as some kind of non-oriented centripetal force. The grouping of substituents that arises around the carbon atom can be, according to Le Bel, different depending on the nature of these substituents, but not necessarily tetrahedral.In the application to the explanation of the causes of optical activity in the presence of the so-called asymmetric atom, both approaches gave the same result, but the more clearly formulated van't Hoff theory turned out to be much more fruitful in explaining the series other factors." The very idea of the spatial structure of molecules was developed by the Dutchman not only to explain the phenomena of optical isomerism. “In his article,” continues Manolov, “he gave a simple explanation of geometric isomerism. Having examined the structure of fumaric and maleic acids, he schematically showed that their two carboxyl groups can be located on one or two opposite sides relative to the plane of the double bond between carbon atoms ". Van't Hoff's new article "Chemistry in Space", where he expressed all these considerations, served as the beginning of a new stage in the development of organic chemistry. Shortly after its publication, in November 1875, van't Hoff received a letter from Professor Wieslicenus, who taught organic chemistry in Würzburg and was one of the most famous experts in this field. “I would like to get permission for the translation of your article into German by my assistant Dr. Hermann,” Wislicenus wrote. “Your theoretical development brought me great joy. I see in it not only an extremely witty attempt to explain hitherto incomprehensible facts, but that it in our science ... will acquire epoch-making significance. The translation of the article was published in 1876. By this time, van't Hoff had managed to get a job as an assistant in physics at the Veterinary Institute in Utrecht. A special "merit" in popularizing the new views of van't Hoff belonged to Professor Hermann Kolbe from Leipzig, who spoke out against the article, and, moreover, in a rather harsh tone. In his comments on van't Hoff's article, he wrote: "Some doctor J. G. van't Hoff from the Veterinary Institute in Utrecht, apparently, has no taste for exact chemical research. It is much more convenient for him to sit on Pegasus (probably taken hired at the Veterinary Institute) and proclaim in his "Chemistry in Space" that, as it seemed to him during a bold flight to chemical Parnassus, atoms are located in interplanetary space. Naturally, everyone who read this sharp rebuke was interested in Van't Hoff's theory. Thus began its rapid spread in the scientific world. Now van't Hoff could repeat the words of his idol Byron: "One morning I woke up a celebrity." A few days after the publication of the article, Kolbe van't Hoff was offered a teaching position at the University of Amsterdam, and from 1878 he became a professor of chemistry. Author: Samin D.K.
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