Right-handed crystal

Working carefully with tweezers, Pasteur was able to separate the crystal into two piles, one of "right-handed" crystals and one of "left-handed" crys tals like those shown in Figure 9.6. Although the original sample, a 50 50 mix lure of right and left, was optically inactive, solutions of the crystals from eacl of the sorted piles were optically active, and their specific rotations were equa in amount but opposite in sign. 

This is a case of optically active materials arising from inactive materials. However, it may be argued that an optically active investigator is required to use the tweezers. Perhaps a hypothetical human being constructed entirely of inactive molecules would be unable to tell the difference between left-and right-handed crystals. 

Fig. 34. Orthorhombic system. (See also Fig. 25.) a. Unit cell type.. (H.COO),Sr.2HsO. Cla8s222. Left-and right-handed crystals, c. 1-Brom, 2-hydroxy-naphthalene. Class 222. d. Picric acid, Class mm. e. Oxalic acid. Class mmm. /. C,Br. Class mmm.

In 1848, Louis Pasteur noticed that a salt of racemic ( )-tartaric acid crystallizes into mirror-image crystals. Using a microscope and a pair of tweezers, he physically separated the enantiomeric crystals. He found that solutions made from the left-handed crystals rotate polarized light in one direction and solutions made from the right-handed crystals rotate polarized light in the opposite direction. Pasteur had accomplished the first artificial resolution of enantiomers. Unfortunately, few racemic compounds crystallize as separate enantiomers, and other methods of separation are required. 

An assembly of molecules may be achiral for one of two reasons. Either all the molecules present are achiral, or the two kinds of enantiomorphs are present in equal amounts. Chemical reactions between achiral molecules lead to achiral products. Either all product molecules will be achiral or the two kinds of chiral molecules will be produced in equal amounts. Chiral crystals may sometimes be obtained from achiral solutions. When this happens, the two enantiomorphs will be obtained in (roughly) equal numbers, as was observed by Pasteur. Quartz crystals are an inorganic example of chirality (Figure 2-36b). Roughly equal numbers of left-handed and right-handed crystals are obtained from the achiral silica melt. 

Meso configurations can have enantiomeric conformers in crystals. Ribitol and xylitol have meso configurations and are optically inactive in solution. In the crystalline state, they have the bent-chain enantiomorphic conformations. Xylitol crystallizes in space group P212121, so each crystal contains either only the optically active left-handed or the optically active right-handed conformers. There are, of course, an equal number of left- and right-handed crystals in any one batch, which on dissolution give no optical activity.1... 

If the left- and right-handed crystals could be separated, by the method of Pasteur, the phenomenon of "conformational mutarotation might be observed. 

The left-handed acid likewise furnishes crystals without action on polarized light and possessing a similar hemiedry. A leftr handed crystal is superposable upon the image of a right-handed crystal in a mirror and reciprocally. 

FIGURE 14.14. (a) Hemihedral faces (shaded) of sodium ammonium tartrate compared (b) with the holohedral faces (shaded) of the racemate. The hemihedral faces in (a) were used by Peisteur to separate left-handed and right-handed crystals. 

Crystals with an asymmetric habit may be separated into groups of left-handed and right-handed crystals. These two groups rotate the plane of polarized light in opposite directions. 

The chiral crystals were spontaneously obtained from the solutions of 1 and 2,4-dichlorobenzylamine 3 (Scheme 27.1b). This kind of chiral crystallization of achiral molecules leads neccessarily to left- and right-handed crystals. The enantiomeric crystals of M- and P-1-3 were selectively prepared by seeding. Irradiation of the crystals of M-l-3 at around 350 nm through a UV filter gave a cyclopentenol (R,R)-5 as almost the sole product, achieving absolute asymmetric photocyclization [25]. In contrast, irradiation at >290 nm afforded (R,R)-5, (P)-4, and (P)-hydrol in a 6 3 1 molar ratio the enantiomeric excesses of the three products were higher than... 

Enantiomers cannot be separated by the usual separation techniques such as fractional distillation or crystallization because their identical boiling points and solubilities cause them to distill or crystallize simultaneously. Louis Pasteur was the first to separate a pair of enantiomers successfully. While working with crystals of sodium ammonium tartrate, he noted that the crystals were not identical—some of the crystals were right-handed and some were left-handed. He painstakingly separated the two kinds of crystals with a pair of tweezers. He found that a solution of the right-handed crystals rotated the plane of polarized light clockwise, whereas a solution of the left-handed crystals rotated the plane of polarized light counterclockwise. 

It was noted that the two forms of quartz were mirror images of each other, and that no amount of positioning or orientation permitted the superimposition of one form onto the other. For instance, on a right-handed crystal the s trigonal pyramid lies to the right of the m face, which is below the predominating positive rhombo-... 

Fig. 1.8 Two crystals of the radical-cation salt (di-fluoranthene) hexafluorphosphate, (Fa)2PFg. The right surface of the right-hand crystal is orientated in such a way that it reflects the light coming from the light source on the right. The reflectivity is metallic due to the high conductivity of the crystal along its long axis a axis, see Fig. 2.18). The grid corresponds to 1 mm. Cf. the coloured plates in the Appendix.

SU-32 enantiomorphs are built only from the large [4 5 8 10 ] cavity and are intersected at different heights by straight 8-ring channels, which are related by the 6i (for right-handed crystals) or 65 (for left-handed crystals) screw axes and thus run in three different directions. Under the synthesis conditions described in Tang et which lack any chiral... 

The absolute asymmetric synthesis described above is a stochastic process, as the probabihty of obtaining either a left- or a right-handed crystal, in different experiments, is identical. At the authors laboratories, attempts have been made to extend the absolute asymmetric synthesis into autocatalytic cycles, where the presence of a product formed in a given enantiomorphous crystal could induce -by ensuing fresh crystallizations cycles comprising crystallization/dissolution - the formation of crystals of the same handedness as in the first crystal formed by chance. In this way, the handedness generated in the first stochastic experiment would be both preserved and efficiently proliferated in the ensuing fresh crystallization experiments (Scheme 8.7). 

Pasteur made up a solution of the left-oriented crystals and found that it rotated polarized light in one direction. A solution of right-handed crystals gave the opposite rotation. He made up a mixture of equal amounts of both right-handed and left-handed crystals, and it had no effect on polarized light. (The term racemic mixture has come to indicate a mixture that contains equal portions of the two possible orientations of a chiral compound.) He told Biot, who by now was an elderly man, and Biot did not believe him. Pasteur went to Biot s laboratory, used Biot s chemicals, did the separation in front of him, and let Biot make up the solutions for testing. They had exactly the properties Pasteur claimed. Biot became an enthusiastic supporter of Pasteur s work. 

As discussed in the A Word About... Pasteur s Experiments and the van t Hoff-LeBel Explanation on pages 160-161, a critical experiment and observation made by Pasteur was that when he dissolved pure forms of the left- and right-handed crystals, he found that each solution was optically active, even though each set of crystals was obtained from crystallization of the racemic acid. The optical rotations of (R,R)- and (S,S)-tartaric acid are provided in Figure 5.14. Predict the specific rotation of a solution that contains 75% (R,R) and 25% (S,S) crystals of tartaric acid. 

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