Sodium ammonium tartrate, optical activity

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. 

L. Pasteur (aged 26) began work on optically active sodium ammonium tartrate. 

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... 

Many substances can rotate the plane of polarization of a ray of plane polarized light. These substances are said to be optically active. The first detailed analysis of this phenomenon was made by Biot, who found not only the rotation of the plane of polarization by various materials (rotatory polarization) but also the variation of the rotation with wavelength (rotatory dispersion). This work was followed up by Pasteur, Biot s student, who separated an optically inactive crystalline material (sodium ammonium tartrate) into two species which were of different crystalline form and were separately optically active. These two species rotated the plane of polarized light equally but in opposite directions and Pasteur recognized that the only difference between them was that the crystal form of one was the mirror image of the other. We know to-day, in molecular terms, that the one necessary and sufficient condition for a substance to exhibit optical activity is that its molecular structure be such that it cannot be superimposed on its image obtained by reflection in a mirror. When this condition is satisfied the molecule exists in two forms, showing equal but opposite optical properties and the two forms are called enantiomers. 

It was the optical resolution of [Co(en)2(NH3)Cl]2+ that firmly established Werner s theory and which initiated the study of the optical activity of complex ions. The realization that some octahedral complexes are chiral evidently did not occur to Werner until several years after he published his theory of coordination. He then realized that the demonstration of this property would furnish an almost irrefutable argument in favor of his theory, and he and his students devoted several years to attempts to effect such resolution. Had he but known it, the problem could have been easily solved, for cis-[Co(en)2(N02)2]X (X = Cl, Br) crystallizes in hemihedral crystals which can be separated mechanically, just as Pasteur separated the optical isomers of sodium ammonium tartrate. 

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. 

It was Pasteur, in the middle of the 19th century, who first recognized the breaking of chiral symmetry in life. By crystallizing optically inactive sodium anmonium racemates, he separated two enantiomers of sodium ammonium tartrates, with opposite optical activities, by means of their asymmetric crystalline shapes [2], Since the activity was observed even in solution, it was concluded that optical activity is due to the molecular asymmetry or chirality, not due to the crystalline symmetry. Because two enantiomers with different chiralities are identical in every chemical and physical property except for optical activity, in 1860 Pasteur stated that artificial products have no molecular asymmetry and continued that the molecular asymmetry of natural organic products establishes the only well-marked line of demarcation that can at present be drawn between the chemistry of dead matter and the chemistry... 

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. 

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. 

The separation of the enantiomorphous crystals of racemic sodium ammonium tartrate by Pasteur in 1848, and his observation that the two forms were optically active in solution, linked the concept of molecular chirality to optical activity [1]. When Emil Fischer began the first serious attempts at asymmetric synthesis in the latter 19th century, the polarimeter was the most reliable tool available to evaluate the results of an enantioselective reaction (by determination of optical purity), and it remained the primary tool for nearly 100 years. Only recently has analytical chemistry brought us to the point where we can say that polarimetry has been superceded as the primary method of analysis in asymmetric synthesis. 

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 sodium ammonium salt crystallized from racemic tartaric acid has been found to crystallize in the orthorhombic P2j2j2j space group and contains four molecules in the unit cell [28]. This particular crystal class is noncentrosymmetric, and as a result individual crystals will be optically active. In fact, efficient growth of this tartrate salt only takes place if all the (i ,i )-tartrate molecules crystallize in one ensemble of crystals, and if all the (, /S)-tartrate molecules crystallize in another ensemble. When formed below a temperature of 26°C, the preferred molecular packing does not permit the intermingling of the enantiomers to yield a true racemic crystal. The crystallization of sodium ammonium tartrate below 26°C results in a spontaneous resolution of the substance into physically separable enantiomers. Interestingly, a different polymorph forms above 26°C that requires a completely different packing pattern that allows for the formation of a racemic modification of sodium ammonium tartrate. 

It was the observation of the hemihedral crystals of sodium ammonium tartrate tetrahydrate that enabled Pasteur (1822-1895) to make a decisive step forward in stereochemistry. The problem he encountered was the contamination of the potassium salt of tartaric acid with that of another acid (which Gay-Lussac (1778-1850) called the racemic acid) that made it unsuitable for commercial use. The two acids had the same chemical composition, and Biot showed that whereas tartaric acid and its salts could rotate the plane of polarized light, the racemic acid itself was inactive. In 1848, Pastern-found the solution to this problem.He noticed that crystals of tartaric acid, like its salts, have hemihedral faces, but that the racemic sodium ammonium tartrate exists as two distinct crystals in which the hemihedral faces are mirror images of each other. One of these crystalline forms is identical to the optically active tartrate. In solution, it rotates the plane of polarized light in a dextrorotatory manner, while the other form (a mirror image of the first) is levorotatory, that is in solution it rotates the plane of polarization towards the left (Figure 2.5). 

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. 

It has already been necessary to refer to the phenomenon of optical isomerism. A brief discussion is provided here, along with a few examples of optically-active metal complexes. Optical isomerism has been recognized for many years. The classical experiments in 1848 of Louis Pasteur, one of the most illustrious and humane of all men of science, showed that sodium ammonium tartrate exists in two different forms. Crystals of the two forms differ, and Pasteur was able to separate them by the laborious task of hand picking. 

The first enantiomeric separation was credited to Louis Pasteur, who in 1848 manually sorted crystals of sodium ammonium tartrate based on their different crystalline appearances. Advances in technology and separation science have been paralleled by similar advances in enantiomeric separation methods. These have included preferential crystallization, crystallization from optically active solvents, fractional crystallization of diaslereo-... 

Pasteur was convinced that there must be some molecular difference between the two salts, and he made the problem the subject of his first major piece of research. He prepared several salts of tartaric acid and found that in all cases the crystals were asymmetric (Pasteur used the term dissymmetric), and displayed hemihedral faces. Pasteur was tempted to speculate that such asymmetric crystals were typical of optically active materials, and were the manifestation of asymmetry of the molecules. He then found that crystals of the optically inactive sodium ammonium paratartrate also displayed hemihedral faces, but on careful examination he saw that two types of crystal were present, one the mirror image of the other (Figure 10.13). He carefully sorted some of the crystals by hand. Those with right-handed hemihedry gave a solution which was dextrorotatory and identical with a solution of sodium ammonium tartrate. A solution of equal concentration of the crystals with left-handed hemihedry rotated polarised light to an equal extent in the opposite direction. A solution of equal concentrations of each crystalline form was optically inactive. Pasteur thereby demonstrated that paratartaric acid was... 

The answer to this puzzle was found in 1844 by the great French chemist Louis Pasteur (1822-1895). He added sodium hydroxide and ammonium hydroxide to the solution of the optically inactive tartaric acid and allowed the solution to evaporate, so that crystals of sodium ammonium tartrate, NaNH4C4H406, were formed. On examining the crystals he first noticed that they appeared to be identical with the crystals similarly made from the optically active tartaric acid. Then, as he continued to scrutinize them carefully, he suddenly recognized that only half of them were truly identical the others were their mirror images... 

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 28 °C, Pasteur made the surprising observation that two distinct kinds of crystals precipitated. Furthermore, the two kinds of crystals were non-superimposable mirror images and were related in the same way that a right hand is related to a left hand. 

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