Enantiomorphic property

Racemic and Mesotartaric Acids.—These two acids represent two inactive types of compounds containing a< yminct7 ic carbon atoms (see above). Apart from certain well-marked differences in physical properties they also differ in one important feature racemic acid can be lesoh-ed into its optical enantiomorphs, whereas mesotartaric acid cannot. The latter belongs to what is termed the inactive indivisible type. If we examine the structuial formula of tartaric acid it will l>e seen that it possesses two asyimnetric carbon atoms, denoted in the formula by thick type. 

If a molecule is nonsuperimposable on its miixor image, the mirror image must be a different molecule, since superimposability is the same as identity. In each case of optical activity of a pure compound there are two and only two isomers, called enantiomers (sometimes enantiomorphs), which differ in structure only in the left-and right-handedness of their orientations (Fig. 4.1). Enantiomers have identical physical and chemical properties except in two important respects ... 

Pasteur thus made the important deduction that the rotation of polarized light caused by different tartaric acid salt crystals was the property of chiral molecules. The (+)- and ( )-tartaric acids were thought to be related as an object to its mirror image in three dimensions. These tartaric acid salts were dissymmetric and enantiomorphous at the molecular level. It was this dissymmetry that provided the power to rotate the polarized light. 

Many benzomorphan derivatives with morphine-like potencies or better have a low physical dependence capacity (PDC) in monkeys especially potent members, however, such as phenazocine and the /3-isomers (Table 5.2, Nos. 8 and 16) have high to intermediate PDC properties [5, 63, 68]. Unusual results have been reported. for the enantiomorphs of a-(X) (R = Me, R = = Et) the... 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

It should be recalled that whereas the enantiomers in the mixture (or racemate) (1) have identical physical properties (except for their action on the plane of polarised light), the diastereoisomers (2) and (3) have physical properties (e.g. solubility, boiling points, chromatographic behaviour, etc.) which are frequently significantly different. Resolution of the mixture (or racemate) can then be achieved provided that one of the diastereoisomers may be obtained in a pure state, and that regeneration from it of the pure enantiomorphous form is not accompanied by any degree of racemisation. 

Chiral refers to the property of chirality. As applied to a molecule, the term has been used differently by different workers. Some apply it exclusively to the whole molecule, whereas others apply it to parts of a molecule. For example, according to the latter view, a meso compound is considered to be composed of two chiral parts of opposite chirality sense this usage is to be discouraged. See enantiomorph... 

Since chirality is a geometrical property, all serious discussions on this topic require a mathematical treatment that is much out of this review. Note, however, that if you cut by the middle of a Klein bottle (an achiral object having a plane of symmetry), you obtain two Mobius strips both chiral and mutually enantiomorph (Fig. 3.5). This pure mathematical result is closely related to the situation of meso compounds described above [11]. 

As forcefully stated by Pasteur, whatever the precise arrangement of atoms in the molecule, ce qui ne peut etre l objet d un doute is that the chirality of the atomic arrangement is the necessary and sufficient condition for molecular enantiomorphism and for its manifestation in a pseudoscalar property such as optical activity. It cannot be emphasized too strongly that no recourse to structural theory was needed to arrive at this conclusion, which was based purely on a symmetry argument and which F. M. Jaeger has referred to as la loi de Pasteur ... 

The chirality of objects such as scalene triangles and oriented circles in R2 (Figure 1) and helices in R3 (Figure 3) is a property shared by both enantiomorphs the difference between them is their sense of chirality. In what follows, we shall for simplicity use the term configuration to stand for sense of chirality. Two enantiomorphs are thus said to have opposite configurations. 

Eleven acentric crystal classes are chiral, i.e., they exist in enantiomorphic forms, whereas ten are polar, i.e., they exhibit a dipole moment. Only five (1,2, 3, 4, and 6) have both chiral and polar symmetry. All acentric crystal classes except 432 possess the same symmetry requirements for materials to display piezoelectric and SHG properties. Both ferroelectricity and pyroelectricity are related to polarity a ferroelectric material crystallizes in one of ten polar crystal classes (1, 2, 3,4, 6, m, mm2, 3m, 4mm, and 6mm) and possesses a permanent dipole moment that can be reversed by an applied voltage, but the spontaneous polarization (as a function of temperature) of a pyroelectric material is not. Thus all ferroelectric materials are pyroelectric, but the converse is not true. 

Racemic and enantiomorphous 2-D and 3-D crystals display different physical and chemical properties. This difference has been utilized to enhance chirality in non-racemic systems that self-assemble in racemic and enantiomorphous crystallites. Morowetz [196] has elaborated a mathematical model that considers an evaporation/crystallization process where the racemate is less soluble than the pure enantiomorphous crystal and the enantiomer (in excess) is concentrated in the solution. A similar enrichment of chirality has... 

D-Ribose, L-ribose and D,L-ribose crystallize without water of hydration. The melting point of the optically active forms is recorded by most authors as 86-87°2,33 37 39 although Alberda van Ekenstein7 reported a value of 95° for D-ribose. D,i>Ribose, prepared by crystallizing a mixture of equal parts of the enantiomorphs, melts at 83-84°.44 The optical crystallographic properties of D-ribose have been measured by... 

Chirality is a concept well known to organic chemists and to all chemists concerned in any way with structure. The geometric property that is responsible for the nonidentity of an object with its mirror image is called chirality. A chiral object may exist in two enantiomorphic forms that are mirror images of one another. Such forms lack inverse symmetry elements, that is, a center, a plane, and an improper axis of symmetry. Objects that possess one or more of these inverse symmetry elements are superimposable on their mirror images they are achiral. All objects belong to one of these categories. 

---