Diastereoisomers different chemical properties

Diastereoisomers have different chemical properties and thus can often be resolved by conventional separation techniques. There are two other terms that need to be defined regarding diastereoisomers and they zx anomers and epimers. 

Most methods for the resolution of enantiomers contained in a reaction mixture consist in the conversion of the compounds into stable or transient diastereoisomers and separation of the latter on the basis of their different physico-chemical properties. 

So much for one chiral centre. The problems really begin when you come up against molecules which have two or more chiral centres With two chiral centres, we can construct four possible stereoisomers. These can be separated into two enantiomeric pairs (indistinguishable by NMR). But, (key sentence coming up) if we compare one member of each of these enantiomeric pairs, we will find that they may be distinguished from each other by NMR, because they are diastereoisomers. Diastereoisomers are stereoisomers which are not mirror images of each other - they are different compounds with distinct physical and chemical properties. See Figure 6.1 if this isn t clear. 

Diastereoisomers are stereoisomers which do NOT have a mirror image of one another. Figure 11.20 shows the diastereoisomers of 2-butene (alkenes such as this are sometimes called geometric isomers and are a consequence of the prohibition of rotation about double bonds). If a vertical mirror was placed between the two structures in Fig. 11.20 they would not reflect onto one another. If the functionality is on the same side then the isomer is the cis-form, if on the opposite side then it is the trans- form. The chemical properties are very similar because the functional groups are identical. However, as they have different shapes their physical properties are different. Interconversion requires breaking and remaking bonds so these isomers are also stable under normal conditions. 

Whereas enantiomers (e.g. 4a and 4b) have indentical chemical and physical properties (except their effect on plane polarised light), diastereoisomers (e.g. 4a, or 4b and 4c) frequently differ in their chemical properties, and have different physical properties. 

Enantiomers have identical physical and chemical properties to one another except the direction in which they rotate plane polarised light (clockwise or anticlockwise). They may be separated by interaction with a second chiral species. This gives two diastereoisomers (if the two chiral centres are the same we can describe the diastereoisomers as optically pure meso AA and the racemic or rac form which itself occurs as two pairs of enantiomers, AA and AA) which do differ in their physical properties e.g. have different NMR spectra, can be separated by achiral chromatography etc). For example, Scheme 3.1 shows the experimental resolution of [Co(en)3]3+ using tartrate. 

The physical and chemical properties of enantiomers are identical the physical and chemical properties of diastereoisomers differ. Diastereoisomer is sometimes shortened to diastereomer. ... 

Stereoisomers that are not mirror images of one another are called diastereoisomers. Both of these pairs of isomers fall into this category. Notice how the physical and chemical properties of a pair of diastereoisomers differ. 

If a compound is chiral, it can exist as two enantiomers. We ve just drawn the two enantiomers of each of the diastereoisomers of our epoxide. This set of four structures contains two diastereoisomers (stereoisomers that are not mirror images). These are the two different chemical compounds, the cis and tram epoxides, that have different properties. Each can exist as two enantiomers (stereoisomers that are mirror images) indistinguishable except for rotation. We have two pairs of diastereoisomers and two pairs of enantiomers. When you are considering the stereochemistry of a compound, always distinguish the diastereoisomers first and then split these into enantiomers if they are chiral. 

In fact, the chemists working on these compounds wanted only one enantiomer of the irons epoxide—the top left stereoisomer. They were able to separate the trans epoxide from the cis epoxide by chromatography, because they are diastereoisomers. However, because they had made both diastereoisomers in the laboratory from achiral starting materials, both diastereoisomers were racemic mixtures of the two enantiomers. Separating the top enantiomer of the trans epoxide from the bottom one was much harder because enantiomers have identical physical and chemical properties. To get just the enantiomer they wanted the chemists had to develop some completely different chemistry, using enantiomerically pure compounds derived from nature. 

The above-discussed classification of isomers is depicted schematically in the upper half of Fig. 2. Such a classification, which is considered classical and widely accepted, nevertheless fails to be fully satisfactory, as aptly demonstrated by Mislow [18]. Thus, this classification considers diastereoisomers to be more closely related to enantiomers than to constitutional isomers. In fact, diastereoisomers resemble constitutional isomers in that their energy content is different, and therefore they differ in their chemical and physical properties. In this perspective, diastereoisomers differ from enantiomers which have identical energy contents and thus display identical physical and chemical properties. 

Enantiomers possess identical physical and chemical properties but diastereoisomers can differ and these differences can be used in appropriate physical chemical procedures to separate enantiomers. The enantiomer mixture is reacted with an appropriate chiral reagent and the diastereoisomers so formed are then separated by classical means. 

The c/s and trans compounds are different diastereoisomers. Consequently, they have different chemical and physical properties and cannot interconvert simply by rotating bonds. 

Diastereoisomers when the isomers are not mirror images of each other (this includes alkene E, Z isomers). Diastereoisomers possess different physical and chemical properties. 

Diastereoisomers (unlike enantiomers) have different physical properties such as boiling point, density, etc. These differences between diastereoisomers can be exploited to resolve (or separate) mixtures of enantiomers. The principle behind this technique is to resolve the mixture of enantiomers by chemically converting them into a pair of diastereoisomers. This is achieved by reacting the racemic mixture with an optically pure reagent. These reagents are usually natural products for example, if the racemic mixture contains acidic compounds, reaction is with an optically pure alkaloid such as strychnine or brucine. 

The amine 2 is made by a chemical reaction - the reductive amination of ketone 1. The starting material 1 and the reagents are all achiral so the product 2, though chiral, must be racemic. Reaction with one enantiomer of tartaric acid 3 forms the amine salt 4, or rather the amine salts 4a and 4b. Examine these structures carefully. The stereochemistry of tartaric acid 3 is the same for both salts but the stereochemistry of the amine 2 is different so these salts 4a and 4b are diastereoisomers. They have different physical properties the useful distinction, discovered by trial and error, is that 4b crystallises preferentially from a solution in methanol leaving 4a behind in solution. Neutralisation of 4b with NaOH gives the free amine (S) -2, insoluble in water and essentially optically pure. 

Diastereoisomers can have different physical (e.g. melting point, polarity) and chemical properties. 

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