Diastereomers physical properties

Diastereomers= stereoisomers which are not rnirror images usually have different physical properties... 

Now, consider the physical properties of these stereoisomers. Enantiomers should have many of the same physical properties, such as energy and dipole moment, but diastereomers should not. Obtain the energy of each conformer and use equation (1) to calculate the composition of a large sample of each stereoisomer at 298 K. Then, obtain the dipole moment of each conformer and use equatiori (2) to calculate the dipole moment of a large sample of each stereoisomer at 298 K. Do enantiomers have the same dipole moment Do diastereomers have different dipole moments ... 

Meso compounds contain chirality centers but are achiral overall because they have a plane of symmetry. Racemic mixtures, or racemates, are 50 50 mixtures of (+) and (-) enantiomers. Racemic mixtures and individual diastereomers differ in their physical properties, such as solubility, melting point, and boiling point. 

Two compounds are diastereomers when they contain more than one chiral center. If the number of dissymmetric centers is given by N, then the number of possible diastereomers is given by 2N. Of these 2 v diastereomers, each will be characterized by its mirror image, so that the number of enantiomers is given by 2NI2. Whereas the physical properties of enantiomers in an achiral environment are necessarily identical, the physical properties (including solubility) of diastereomers are normally different. The differences arise since there is no structural requirement that the crystal lattices of different diastereomers be the same. For instance, the solubility of an (SS )-diastereomer could differ substantially from that of the (/ S)-diastereomer. However, it should be remembered that the solubility of the (SS)-diastereomer must be exactly identical to that of the (I 7 )-diastereomer, since these compounds are enantiomers of each other. At the same time, the solubilities of the (SI )-diastereomer and the (I S)-diastereomer must also be identical. 

Molecules which exhibit optical activity are molecules which have a handedness in their structure. They are chiral . Chemists often have reasons to obtain chemical pure aliquots of particular molecules. Since the chirality of molecules can influence biological effect in pharmaceuticals, the chiral purity of a drug substance can pose a challenge both in terms of obtaining the molecules and in assaying the chiral purity by instrumental methods. While diastereomers can have different physical properties including solubility, enantiomers have the same physical properties and the same chemical composition. How then to separate optically active molecules ... 

The first example of the deliberate separation of optically active molecules is appropriate as an example of physical separation in the clearest sense of the term. The molecules are referred to as optically active because polarized light interacts differently with right- and left-handed molecules. In the case of simple diastereomers the RR and SS forms are enantiomers while the RS and SR forms are not. The separation of the latter and former was first done under a microscope using crossed polarizers and the crystals which were seen were separated from those that caused little or no rotation of plane-polarized light by hand using tweezers. A truly physical separation of chemical species using a physical property of chemical origin ... 

Chiral molecules interact to form complexes that are related as enantiomers or as diastereomers. Enantiomers are perfect chemical models for each other except in their interactions with polarized light or with other chiral molecules, and this provides the basis for an absolute method for demonstrating subtle differences in physical properties that might otherwise be confused with the effects of impurities. 

The maximum number of stereoisomers is 2" where n is the number of nonidentical chiral centers. Figure 1-2 shows the four stereoisomers present in a molecule with two chiral centers. Non-superimposable mirror images are enantiomers, while the other species in the figure are diastereomers. Unlike enantiomers, diastereomers have different physical properties. 

Diastereomers are nonenantiomeric isomers that result when more than one stereocenter is present in a molecule. The distinction between diastereomers and enantiomers is not always clear but, in general, enantiomers have mirror images, whereas diastereomers are not mirror images of one another. As such diastereomers have different physical properties such as boiling and melting points, solubilities, etc. 

Geomelric (cis-trans) isomers are stereoisomers because they differ only in the spatial arrangement of the groups. They are diastereomers and have different physical properties (m.p., b.p., etc.). 

If we look at structures A and C or B and D, we have stereoisomers, but not enantiomers. These are called diastereomers. Diastereomers have different physical properties (e.g. melting point). Other pairs of diastereomers among the stereoisomers of 2,3,4-trihydroxybutanal are A and D, and B and C. 

Resolution Methods. Chiral pharmaceuticals of high enantiomeric purity may be produced by resolution methodologies, asymmetric synthesis, or the use of commercially available optically pure starting materials. Resolution refers to the separation of a racemic mixture. Classical resolutions involve the construction of a diastcrcomcr by reaction of the racemic substrate with an enantiomerically pure compound. The two diastereomers formed possess different physical properties and may be separated by crystallization, chromatography, or distillation. A disadvantage of the use of resolutions is that the best yield obtainable is. 50%, which is rarely approached. However, the yield may he improved by repeated raccmization of the undcsired enantiomer and subsequent resolution of the racemate. Resolutions are commonly used in industrial preparations of homochiral compounds. 

As expected from our previous discussions diastereomers of tartaric acid have different physical properties (Table 5-2). 

The most commonly used procedure for separating enantiomers is to convert them to a mixture of diastereomers that will have different physical properties ... 

Diastereomers are stereoisomers that are nor mirror images of each other. Diastereomers have different physical properties. And they maybe dextrorotatoiy, levorotatory or inactive. 

We have seen that individual enantiomers have identical physical properties and only can be distinguished in a chiral environment. Plane-polarized light is such a chiral environment, and one enantiomer is dextrorotatory and one is levorotatory. Another way to distinguish enantiomers is to allow them to react (or interact) with other chiral molecules. The interaction of a mixture of enantiomers with a single enantiomer of a chiral molecule produces a mixture of diastereomers as illustrated. 

Since diastereomers have different physical properties, diey can be separated on the basis of those physical properties. After separation of the diastereomers,... 

Other groups would behave similarly, with the axial isomer being higher in energy (less stable) than the equatorial isomer because of 1,3 diaxial interactions. These two isomers are conformational isomers because they are interconvertible by rotations about C-C single bonds, but they are also called conformational diastereomers since they have different physical properties and are nonsuperim-posable, non-mirror images. 

B is correct The compounds are diastereomers, so they have different physical properties (like boiling points) and could thus be separated by distillation. A and D are wrong because both compounds have the same mass and the same functional groups. Neither group rotates polarized light, since Reaction 1 produces a meso compound and Reaction 2 produces a racemic mixture. 

Comparison of physical properties of compound 41 (in Scheme 2) with the corresponding product from vertine showed that both compounds were diastereomers as predicted (24). There were significant differences in the NMR and IR spectra, and the melting point of 41 (84-85°) was depressed (77-92°) on admixture with the corresponding derivative from vertine. 

The (2S, 3R) stereoisomer is the non-superimposable mirror image of the (2R, 3S) stereoisomer and so these structures are also enantiomers having the same chemical and physical properties. Each set of enantiomers is called a diastereomer. Diastereomer are not mirror images of each other and are completely different compounds with different physical and chemical properties. Thus, threonine has two asymmetric centres, i.e. there are two possible diastereomers consisting of two enantiomers each, making a total of four stereoisomers. As the number of asymmetric centres increases, the number of possible stereoisomers and diastereomers increases. For a molecule having n asymmetric centres, the number of possible stereoisomers is 2" and the number of diastereomers is 2n J. 

Distinction should be made at this time between diastereoisomers and enantiomers. The former are characterized by the presence of at least two closely associated asymmetric centers in the molecular structure, either of which can epimerize. Altogether then there are two pairs of enantiomers for a total of four stereochemically unique individuals. Diastereoisomers have different physical properties and as a result discriminations, and even separations, can be done relatively easily. Enantiomers on the other hand differ in only one physical property, i.e. the direction of rotation of polarized light. Reaction of an enantiomeric racemic mixture with a third chiral species will produce a mixture of diastereomers therefore facilitating their identification or their separation. Early examples of this were the separations done by fractional crystallization of salts produced by a derivatization reaction with, for example, the alkaloid (-)-brucine. Fractional crystallization would never seem to be an effective analytical method yet it was used with some success in a forensic sciences context to confirm the presence of (L)-cocaine by a carefully contrived microcrystalline test. The physical properties... 

Tables IX and X also reveal some dramatic differences between NMPP and MDPP. These ligands are diastereomers more precisely, they are epimers since they differ only in configuration at C-3. It is quite reasonable that these ligands should behave differently, since diastereomers have different chemical and physical properties, although sometimes only slightly different. However, NMDPP and MDPP generate considerably disparate behavior both in terms of the activity and the chiral influence of the catalysts derived from them. Toward every substrate examined thus far the MDPP catalyst has had a very low activity, much lower activity than the NMDPP catalyst. Also, the MDPP catalyst generally gave much lower asymmetric bias than the NMDPP catalyst, and was the only chiral catalyst to give an archiral product11 (two examples).

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