Hemihedrism

Bornyl Acetate.—The acetic acid ester is the most important of the series. It is a constituent of pine-needle and rosemary oils, and has a most fragrant and refreshing odour. It is prepared artificially by the action of acetic anhydride on borneol, in the presence of sodium acetate, or by the condensation of borneol with glacial acetic acid in the presence of a small amount of a mineral acid. It is absolutely necessary in the reproduction of any pine odour. It is a crystalline body, crystallising from peDroleum ether in rhombic hemihedric crystals melting at 29°. The optical activity depends on that of the borneol from which it has been prepared. It has the following characters —... 

Halb-erzeugnis, -fabrikat, n. intermediate product, semimanufacture. -fertigwaren, /.pi. semifinished goods, halb-fest, a. semisolid semifixed semipermanent. -fett, a. Coal) semibituminous (of oil varnish) medium, -flachig, a. Cryst.) hemihedraL... 

Hemi-eder, n. hemihedron, hemihedral form or cr> stal. -edrie,/, hemihedrism. hemiedrisch, a. hemihedral. Hemimeilit(h)saure,/. hemimellitic acid, hemitrop, a. (Cryat.) hemitrope, twinned, Hemme, /. hindrance, restraint, impediment, obstruction arrest brake, hemmen, v.t. stop, check, arrest, inhibit, restrain retard brake clog,... 

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. 

In 1822, the British astronomer Sir John Herschel observed that there was a correlation between hemihedralism and optical rotation. He found that all quartz crystals having the odd faces inclined in one direction rotated the plane of polarized light in one direction, while the enantiomorphous crystals rotate the polarized light in the opposite direction. 

The formation of hemihedral crystal faces in a chiral crystal as induced by solvent. 

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. 

Some of the difficulties encountered in establishing the effect of solvent on crystal growth may be circumvented by focusing on polar crystals. This is because the difference in the rates of growth of opposite faces (hid) and (hkl) along a polar direction must arise primarily from differences in their solvent-surface interactions. Thus, one generally does not have to be concerned with faces other than the hemihedral ones in question. We illustrate below an approach to understanding solvent-surface interactions in the polar crystals of resorcinol (102). 

Til, Til, and lTT) is reduced, while that of Til (andT 11, llT, and lTl) is not, and the resulting crystal is entirely bounded by the first-mentioned set of planes and thus has a hemihedral form. To produce an effect of this sort, molecules of the dissolved impurity need not be entirely without symmetry, but they must lack planes of symmetry, inversion axes, and a centre of symmetry. 

Optically Active compounds crystallize as recognizable, hemihedral crystals of the two 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. 

An equimolar mixture of two enantiomers is called a racemate. The separation of two enantiomers that constitute a racemate is called optical resolution or resolution. Their crystalline forms best characterize types of racemates. A racemic mixture is a crystal where two enantiomers are present in equal amounts. A conglomerate is a case where each enantiomer has its own crystalline form. Sometimes their crystals have so-called hemihedral faces, which differentiate left and right crystals. For over a hundred years, crystallization processes have been used for the separation and purification of isomers and optical resolution, both in the laboratory and on an industrial scale. 

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. 

In general, however, the occurence of spontaneous resolution of racemate is rare. Moreover cases in which the crystals of the enantiomers have visually distinct hemihedral face are extremely rare. 

Figure 5 Hemihedral crystals of the salt of a-methylbenzylamine with cinnamic acid, which were crystallized from water, and spontaneously resolved.

G. B. Kauffman and I. Bernal, Alfred Werner s awareness of spontaneous resolutions and the meaning of hemihedral faces in optically active crystals , Structural Chem., 1993, 4 (2), 131-138. 

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. 

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).

PROP White crystalline powder or rhombic hemihedral crystals sidy sweet taste. Mp 234°. Sol in water insol in ale, ether. 

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. 

The first method of enantiomeric separation by direct crystallization is the mechanical technique use by Pasteur, where he separated the enan-tiomorphic crystals that were simultaneously formed while the residual mother liquor remained racemic. Enantiomer separation by this particular method can be extremely time consuming, and not possible to perform unless the crystals form with recognizable chiral features (such as well-defined hemihedral faces). Nevertheless, this procedure can be a useful means to obtain the first seed crystals required for a scale-up of a direct crystallization resolution process. When a particular system has been shown to be a conglomerate, and the crystals are not sufficiently distinct so as to be separated, polarimetry or circular dichroism spectroscopy can often be used to establish the chirality of the enantiomeric solids. 

The symmetry of the model of a molecule or of a molecular ensemble depends on the conditions of the relevant physical (or chemical) measurement, and may vary for the same system according to time scale of observation and instrumental sensitivity. Whether the model of a chemical system is chiral or achiral may therefore depend on the conditions of observation. There is no ambiguity when chirality properties are observed the hemihedrality of quartz crystals, the optical rotation of hexahelicene, and the enantiospecificity of hog-kidney acylase, for example, are all unmistakable manifestations of an underlying structural chirality. On the other hand, achirality is not so simply implied by the absence of such observations. 

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. 

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