Pasteur, Louis optically active

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

Little was done after Biot s discovery of optical activity until 1848, when Louis Pasteur began work on a study of crystalline tartaric acid salts derived from wine. On crystallizing a concentrated solution of sodium ammonium tartrate below... 

Since drugs interact with optically active, asymmetric biological macromolecules such as proteins, polynucleotides, or glycolipids acting as receptors, many of them exhibit stereochemical specificity. This means that there is a difference in action between stereoisomers of the same compound, with one isomer showing pharmacological activity while the other is more or less inactive. In 1860, Louis Pasteur was the first to demonstrate that molds and yeasts can differentiate between (+)- and (-)-tartarates, utilizing only one of the two isomers. 

Louis Pasteur encountered the phenomenon of optical activity in 1843, during his investigation of the crystalline sediment that accumulated in wine casks (a form of tartaric acid called paratartaric acid—also called racemic acid, from Latin racemus,... 

Until recently, the phenomenon of chirality has been better known as optical isomerism, and configurational isomers that are enantiomers were referred to as optical antipodes. The reasons for this are mainly historical. It was discovered early in the nineteenth century that many compounds, whether solid, liquid, or gas, have the property of rotating the plane of polarization of polarized light and can be said to be optically active. A satisfactory explanation of the origin of optical activity came much later and developed in its modern form from the classic researches of Louis Pasteur, and from the concept of the three-dimensional carbon atom expressed independently by J. H. van t Hoff and J. A. Le Bel.2... 

The importance of being able to synthesize enantiometrically pure compounds (EPC syntheses) has continued to increase ever since Louis Pasteur about 150 years ago realized that molecular asymmetry causes optical activity not the least through the—often grievous—experience that the biological activity, of enantiomers can differ dramatically in its kind and intensity because of the chiral nature of life processes [1-3]. Current regulatory requests for an evaluation of... 

In 1848, the French scientist Louis Pasteur prepared the sodium ammonium salt of racemic tartaric acid and allowed it to crystallize in large crystals which are visually distinctive from hemihedral forms.4 By discriminating the asymmetric faces of the crystals, he picked out the two kinds of crystals mechanically with a pair of tweezers and a loupe. Finally he obtained two piles of crystals, one of (+) and one of (-)-sodium ammonium tartrate. This was the first separation of optically active compounds from their racemate. 

Chirality, in its many and varied manifestations, is ubiquitous a concept rooted in mathematics, it permeates all branches of the natural sciences.1 In 1848, Louis Pasteur announced his epochal discovery of a causal relationship between the handedness of hemihedral sodium ammonium tartrate crystals and the sense of optical rotation of the tartrates in solution.2 This discovery, which marks the beginning of modem stereochemistry, connected enantiomorphism on the macroscopic scale to enantiomorphism on the molecular scale and thus led to Pasteur s recognition that the optical activity of the tartrates is a manifestation of dissymetrie moleculaire, 3 that is, of molecular chirality. 

A chiral object and its mirror image are enantiomorphous, and they are each other s enantiomorphs. Louis Pasteur (Figure 2-37) was the first who suggested that molecules can be chiral. In his famous experiment in 1848, he recrystallized a salt of tartaric acid and obtained two kinds of small crystals which were mirror images of each other as seen by Pasteur s models in Figure 2-38 preserved at Institut Pasteur at Paris. Originally Pasteur may have been motivated to make these large-scale models because Jean Baptiste Biot, the discoverer of optical activity had very poor vision by the time of Pasteur s discovery [42], Pasteur demonstrated chirality to Biot, who was visibly affected... 

Louis Pasteur was the first scientist to study the effect of molecular chirality on the crystal structure of organic compoimds [23], finding that the resolved enantiomers of sodium ammonium tartrate could be obtained in a crystalline form that featured nonsuperimposable hemihedral facets (see Fig. 9.1). Pasteur was quite surprised to learn that when he conducted the crystallization of racemic sodium ammonium tartrate at temperatures below 28 °C, he also obtained crystals of that contained nonsuperimposable hemihedral facets. He was able to manually separate the left-handed crystals from the right-handed ones, and foimd that these separated forms were optically active upon dissolution. More surprising was the discovery that when the crystallization was conducted at temperatures exceeding 28 °C, he obtained crystals having different morphologies that did not contain the hemihedral crystal facets (also illustrated in Fig. 9.1). Later workers established that this was a case of crystal polymorphism. 

Tartaric acid, HOOCCHOHCHOHCOOH, has played a key role in the development of stereochemistry, and particularly the stereochemistry of the carbohydrates. In 1848 Louis Pasteur, using a hand lens and a pair of tweezers, laboriously separated a quantity of the sodium ammonium salt of racemic tartaric acid into two piles of mirror-image crystals and, in thus carrying out the first resolution of a racemic modification, was led to the discovery of enantiomerism. Almost exactly 100 years later, in 1949, Bijvoet, using x-ray diffraction—and also laboriously—determined the actual arrangement m space of the atoms oY the sodium rubidium salt of (-f )-tartaric acid, and thus made the first determination of the absolute configuration of an optically active substance. 

Alfred Werner was also the first person to demonstrate optical activity in an inorganic compoimd (not of biological origin). This demonstration silenced critics of his theory of coordination compoimds, and in his opinion, it was his greatest achievement. Louis Pasteur had demonstrated the phenomenon of optical activity many years earlier in organic compounds of biological origin. 

Louis Pasteur deduced in 1848 that the handedness of molecular structure is responsible for optical activity. He sorted the chiral crystals of tartaric acid salts into left-handed and right-handed forms, and discovered that the solutions showed equal and opposite optical activity. 

It was the experimental work of Louis Pasteur that first revealed a relationship between structure and optical activity. However, it was not until 1874 that the Dutch chemist van t Hoff and the French chemist LeBel independently came up with a basis for the observed optical activity tetrahedral carbon atoms bonded to four different atoms or groups of atoms. 

This evolution in QSAR was slow. As in many sciences, the evolution has been driven by discoveries of chemical behavior that could not be explained using conventional concepts and models. For example, Louis Pasteur recognized that optical activity (a phenomenon observed earlier ) was the result of the molecular dissymmetry later called chirality (from Greek cheir = hand). The concept of stereochemistry, however, was introduced by van t Hoff and Le Bel. It was V. Prelog" who pointed out that stereochemistry is not a branch of chemistry but a point of view. Part of this point of view is the description of structure that explains relevant behavior, which necessarily leads to additional levels of taxonomic analysis of chemicals. 

Optical activity was a fundamental mystery of matter during most of the nineteenth century. Jean Baptist Biot discovered that certain minerals were optically active—they rotated the plane of polarized light. In 1815 he found that certain liquids, oil of turpentine and camphor in alcohol solution for example, were also optically active. However, it was Louis Pasteur s genius that perceived the molecular connection in 1848 even though rational structural chemistry remained some fifteen years or so in the future. 

In 1815, a French physicist, Jean Biot (1774-1862), showed that certain crystals could rotate the plane of polarization of light. Later it was found that solutions of certain compounds could do the same thing (see Fig. 21.13). Louis Pasteur (1822-1895) was the first to understand this behavior. In 1848 he noted that solid sodium ammonium tartrate (NaNH4C4H404) existed as a mixture of two types of crystals, which he painstakingly separated with tweezers. Separate solutions of these two types of crystals rotated plane-polarized light in exactly opposite directions. This led to a connection between optical activity and molecular structure. 

The first glimmer of understanding of optical activity appeared in 1848, when the French chemist Louis Pasteur (1822-95) began work with crystals of sodium ammonium tartrate. 

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