Diastereomers meso compounds

Know the meaning of diastereomer, meso compound, lactic acid, tartaric acid, resolution. 

Draw all stereoisomers for the following molecules in Fischer projection, labeling diastereomers, meso compounds, and enantiomers where appropriate ... 

Stereoisomers Compounds that have the same molecular formula, the same connectivity of their atoms, but a different orientation of their atoms in space. The term "stereoisomer"includes cis,trans isomers in cycloalkanes and alkenes as well as enantiomers, diastereomers, meso compounds, and atropisomers. Conformational isomers are also stereoisomers, whether they are isolable or not. 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

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. 

These results are quite compatible with the preferred conformations of the two-chain carbonyl diacids in aqueous media mentioned above (Porter et al, 1986b, 1988). The meso-compounds preferred collinear conformation, which places the hydrogens at the asymmetric carbons in a nearly eclipsed position relative to each other (Fig. 44), is more stable than that of the ( )-diastereomer by about 1.2kcalmol 1. In this conformation, the two carboxylic acid groups at the ends of the chain can be attached to the water surface side-by-side. The entire molecule can then behave as a good-amphiphile whose structure is similar to a pair of single-chain fatty acid molecules bound side-by-side, each chain mirroring the other about the molecules plane of symmetry. 

Since the two reactants cis and trans butenes are stereoisomers, being diastereomers, the product from cis is a meso compound and that from the trans give a pair of enantiomers, by definition both the reactions are stereospecific. 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

Remarkably, in the equilibrium mixtures, the less abundant diastereomer crystallizes (Tabic 12). A more recent example for a configurationally labile organolithium derivative has been reported49. A conceptually different approach starts with a meso-compound containing two enantiotopic functional units (A). 

In the products C retains the R configuration since none of its bonds were broken and there was no change in priority. TTie configuration at C, the newly created chiral center, can be either R or S. As a result, two diastereomers are formed, the optically active RR enantiomer and the optically inactive RS meso compound. 

B is correct. The isomer on the left is a meso compound and has no enantiomer. Both isomers have the same bond-to-bond connectivity and are therefore diastereomers. 

C is correct. A B is a racemic mixture. C I are the same meso compound. H J are diastereomers, which rotate plane-polarized light to different degrees. 

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. 

C is correct. Diastereomers - epimers, anomers, and geometric isomers - are stereoisomers that are not mirror images of each other. A meso compound is an achiral molecule, which is identical to its mirror image. 

There are no stereogenic centers. Both molecules have planes of symmetry. The cis isomer has two such planes, through opposite corners of the ring. The trans isomer has one such plane, through the opposite methyl-bearing corners. Both compounds are optically inactive and achiral. They are not meso compounds because there are no chiral centers. To summarize, the two isomers are configurational, achiral and diastereomers. 

The cw-alkene B is consequently the major product of the olefination of Figure 11.21. It can be separated chromatographically from the minor cw-alkene iso-B and also from the fraws-alkenes formed in small amounts because these species are diastereomers. Finally, the chiral auxiliary can be removed reductively from the obtained pure alkene B (cf. the analogous reaction at the bottom of Figure 11.19). The resulting allyl alcohol C is 100% enantiomerically pure. Figure 11.21 thereby gives an example of the conversion of a meso-compound into an enantiomerically pure substance a 100% yield is possible in principle in such reactions. 

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... 

Compounds that are achiral even though they have asymmetric carbon atoms are called meso compounds. The (2R,3S) isomer of 2,3-dibromobutane is a meso compound most meso compounds have this kind of symmetric structure, with two similar halves of the molecule having opposite configurations. In speaking of the two diastereomers of 2,3-dibromobutane, the symmetric one is called the meso diastereomer, and the chiral one is called the ( ) diastereomer, since one enantiomer is ( + ) and the other is (-). 

Q Identify pairs of enantiomers, diastereomers, and meso compounds, and explain how they differ in their physical and chemical properties. 

The physical properties of meso compounds, diastereomers and racemic mixtures differ from each other and from the properties of enantiomers. 

B and C are enantiomers and are optically active. Compound A is their diastereomer and is a meso compound, which is not optically active. 

Both of the diastereomers shown below are meso compounds with three chirality centers. Each is a meso compound because it has a symmetry plane, and in each structure the centra] carbon is bonded to four different groups (a group with R configuration, a group with S configuration, -OH, and -H). 

There are four stereoisomers of 2,4-dibromo-3-chloropentane. C and D are enantiomers and are optically active. A and B are optically inactive meso compounds and are diastereomers. 

Commercial 2,4-pentanediol is a mixture of three stereoisomers (R,R), (S,S), and (R,S). The meso isomer shows three signals in its 13C NMR spectrum. Its diastereomers, the R,R and S,S enantiomeric pair, also show three signals, but two of these signals occur at different 8 values from the meso isomer. This is expected, because diastereomers differ in physical and chemical properties. One resonance from the meso compound accidentally overlaps with one signal from the enantiomeric pair. 

The product of acid hydrolysis of trans-5,6-epoxydecane is a meso compound that is a diastereomer of the products formed in the previous problem. 

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