Hemihedral facets

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).]...

Examples of twinned qnartz crystals, illnstrating (a) the lack of hemihedral facets in a completely interpenetrating crystal, and (b) the existence of oppositely handed qnartz crystals in a single twiimed stractme. (The fignre was adapted from Ref 14.)... 

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... 

In 1855 Pasteur, remarking that Biot in 1849 had found that amyl alcohol is optically active, found that fermentation amyl alcohol is a mixture of an active and an inactive form. He prepared active and inactive barium sulphamylates, having very different solubilities but isomorphous, and, what was remarkable, the crystals of the active salt had no hemihedral facets, although the cinchonine salt had. 

Heavy water, ao, 49 Hebron, 23 Heidelberg, 173, 174 Helicon, 66 van Helmont, toa, 123 Hemihedral facets, 183 Kennel), t66 Hephaestus, 36, 62 Herbalist, 99 Hercules, 6a Hermes, 23, 6a Hermetic androgyne, aa, 164 seal, 24, 31 stream, 20, 25,49 Henchel, 190 Hesperides, 63 tlexagon, 181 Hippomenes, 65... 

In recent years, stereochemistry, dealing with the three-dimensional behavior of chiral molecules, has become a significant area of research in modern organic chemistry. The development of stereochemistry can, however, be traced as far back as the nineteenth century. In 1801, the French mineralogist Haiiy noticed that quartz crystals exhibited hemihedral phenomena, which implied that certain facets of the crystals were disposed as nonsuperimposable species showing a typical relationship between an object and its mirror image. In 1809, the French physicist Malus, who also studied quartz crystals, observed that they could induce the polarization of light. 

Ihsteur begins his first lecture by discussing the precedents that led up to his research and then defines hemihedral crystals. These are cubical crystals with four little facets inclined at the same angle to the adjacent surfaces and arranged alternately so the same edge of the cube does not contain two facets (Fig. 4). Under these conditions, no point or plane of symmetry exists in the cube. 

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... 

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