Properties of Diastereomers

Properties of Diastereomers In contrast to enantiomeric pairs, the correpond-ing spatial distances in diastereomeric pairs are not all identical. For example, errand trans-1,2-difluoroethene (Figure 1.2.4), differ in their F-F and H-H distances. This results into different energy contents and different properties between diastereomeric molecules. The difference in properties of diastereomers is illustrated with cis- and trans-1-phenyl-1,3-butadiene, which show markedly different physicochemical properties [12] (Figure 1.2.5). Further investigation of stereochemical isomers is beyond the scope of this book, and discussion in subsequent chapters is limited to constitutional isomers. 

Summary Fischer Projections andTheir Use 201 Diastereomers 201 Summary Types of Isomers 203 5-12 Stereochemistry of Molecules withTwo or More Asymmetric Carbons 204 5-13 Meso Compounds 205 5-14 Absolute and Relative Configuration 207 5-15 Physical Properties of Diastereomers 208 5-16 Resolution of Enantiomers 209 EssentialTerms 213 Study Problems 215... 

Although the basic principle and procedure of diastereomeric resolution are not difficult to understand, the chiral discrimination mechanism involved in the selective crystallization of one diastereomer from the mixture is very complicated. The chiral discrimination mechanism for diastereomeric resolution changes in accord with the resolving system, since not only the properties of diastereomeric crystals but also the conditions for crystallization strongly influence the chiral discrimination mechanism. In particular, the polymorphism of crystal, the severe solvent effect on solubility, and the kinetic factor for crystal growth are still not perfectly understood regarding this chiral discrimination phenomenon. The study is therefore limited in its investigation of the chiral discrimination mechanism for the diastereomeric resolution, as the mechanism involves both the crystal and solution properties of diastereomers.7... 

Diastereomers are optical isomers that are not related as an object and its mirror image. Unlike enantiomers, the physical and chemical properties of diastereomers can differ and it is not unusual for them to have different melting and boiling points, refractive indices, solubilities, etc. Their optical rotations can differ in both sign and magnitude. 

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. 

A compound with two dissimilar chiral carbon atoms has two possible pairs of enantiomers. The mirror image structures of one enantiomeric pair are diastereomers of those of the other enantiomeric pairs. Diastereomers are stereoisomers that are not mirror images. All physical properties of diastereomers are different including, usually, their rotation of plane polarized light. 

A compound with two similar chiral carbon atoms has one pair of enantiomers and one meso compound. A meso compound has more than one chiral center and is superimposable on its mirror image meso compounds are optically inactive. A meso compound is a diastereomer of each of the enantiomers. Diastereomers are stereoisomers that are not mirror images all physical properties of diastereomers are usually different. 

The thermal properties of diastereomers of mixed cobalt(III) complexes with aromatic amino acids and diamine were studied by Miodragovi and coworkers [140] to obtain information about stereochemical effects on their thermal stability. The thermal decompositions of these complexes were shown to be multi-step degradation processes, some of which can be satisfactorily separated into individual steps, depending on the molecular symmetry. For diastereomers which crystallize with water molecules, preliminary dehydration occurs. 

Physical Properties of DIastereomers Resolution, a Method of Separating Enantiomers from Each Other... 

While compounds which are enantiomers have identical chemical and physical properties and equal and opposite optical rotation, compounds which are diastereomeric with each other can have completely different chemical and physical properties and optical rotations. This feature provides the basis for the resolution of chiral compounds. In this procedure a racemic mixture is derivatised by reaction with an enantiomerically pure compound which leads to a mixture of two diastereomers. These can be separated by crystallisation or chromatography as a result of their different properties and the pure enantiomers of the starting compound obtained by cleavage of each diastereomer separately. In the synthesis of the chiral phosphines (section 1.7) the phosphine oxide (37) is obtained by resolution of the starting material (34) via the diastereomeric esters (35) and (36). A further important application of the differing properties of diastereomers is in the determination of e.e. by derivatisation with an enantiomerically pure compound followed by chromatographic or NMR analysis (see section 3.4.1). 

The physical properties of the three stereoisomers of tartaric acid are listed in Table 4.2. The meso compound and either of the enantiomers are diastereomers. Notice that the physical properties of enantiomers are the same, whereas the physical properties of diastereomers are different. 

The differing physical properties of diastereomers is also the basis for a particularly sensitive method of assessing the optical purity of compounds. Although, in principle, this can be done by measuring the optical rotation, this value is reliable only if the rotation of the pure enantiomer is accurately known. This will never be the case for a newly prepared material and is also often in question for previously prepared compounds. If a derivative is prepared in which a new chiral center is introduced, the two enantiomers will give different diastereomers. Since these will have different physical properties in general, they have different NMR spectra. These spectra are distinguished by somewhat different chemical shifts. A pure... 

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