Tartrate crystals, hemihedrism

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. 

The first experimental interpretation of the physical basis for optical activity was provided by Pasteur, who observed the hemihedrism of tartrate crystals, which was visually manifest by tetrahedral facets oriented... 

When the absolute structure has been determined, the result must be correlated with some physical property of the crystal, otherwise the result has no use to the chemist. The obvious correlation is with the direction of rotation of the plane of plane-polarized light, that is, whether the compound or crystal is dextrorotatory or levorotatory. Another correlation can be made with crystal appearance this was shown for zinc blende with its matte and shiny faces, and for silica and sodium ammonium tartrate crystals for the disposition of their hemihedral faces. If such data are not available, it may be necessary to list physical properties of diastereomers made with chiral complexing agents. Then, whenever this same compound is encountered by a chemist, its absolute structure is well known. 

Pasteur studied the crystals of both tartaric and racemic acids and found that while tartrate crystals contained nonsuperimposable hemihedral facets, racemate crystals did not. Examples of the ideal... 

Crystals composed of the R and S enantiomers of the same racemic mixture must be related by mirror symmetry in terms of both their internal structure and external shape. Enantiomorphous crystals may be sorted visually only if the crystals develop recognizable hemihedral faces. [Opposite (hid) and (hkl) crystal faces are hemihedral if their surface structures are not related to each other by symmetry other than translation, in which case the crystal structure is polar along a vector joining the two faces. Under such circumstances the hemihedral (hkl) and (hkl) faces may not be morphologically equivalent.] It is well known that Pasteur s discovery of enantiomorphism through die asymmetric shape of die crystals of racemic sodium ammonium tartrate was due in part to a confluence of favorable circumstances. In the cold climate of Paris, Pasteur obtained crystals in the form of conglomerates. These crystals were large and exhibited easily seen hemihedral faces. In contrast, at temperatures above 27°C sodium ammonium tartrate forms a racemic compound. 

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. 

Figure 35, Top Enantiomorphous crystals of sodium ammonium tartrate. Hemihedral facets are marked by an h. Bottom (+)-(2R,3R)-tartaric acid (left) and (-)-(2S,35)-tartaric acid (right).

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. 

FIGURE 9.1 Crystals of sodium ammonium tartrate, obtained under conditions yielding the hemihedral facets (darkened crystal faces) distinctive of the chiral crystalline forms. Also shown is the crystal morphology of racemic sodium ammonium tartrate. 

Figure 5 Paratarate of soda and ammonia formed by an equal mixture of hemihedral crystals of levo-tartrate (on left) and dextro-tartrate (on right). The anterior hemihedral facet "h" is on the left side of the observer in the levo-tartrate and on his or her right in the dextro-tartrate, [From Descour (17).]...

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. 

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). 

To put it simply, it is possible to say that the two enantiomers present in the solution separate and form distinct crystals. This phenomenon, which is called spontaneous resolution, is what allowed Pasteur to separate the levo- and dextrorotatory cystals of sodium ammonium tartrate tetrahydrate mechanically. In this case, the crystals of each enantiomer can be physically distinguished by their geometry as a result of their hemihedral character. Later, Werner made the same observation for the crystals of [Rh °(C204)3]K3 (2.24) where the levo- and dextrorotatory forms are mirror images of each other (Figure 2.30). 

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... 

Crystals belonging to one of the chiral space groups appear in enantiomorphic forms. The enantiomorphous crystals in some systems may be distinguished by their hemihedral faces, if these are well developed, as in the classic experiment of Pasteur on the resolution of ammonium tartrate. Other methods which can be used for distinguishing between enantiomorphous crystals include anomalous X-ray scattering, etching techniques, and gas-solid reactions with chiral gases [28]. 

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