Mirror-image isomers

An interesting and important fact is that almost all amino acids isolated from proteins L-configuration have the L-configuration at the a-carbon, although some amino acids isolated from microbiological sources are the mirror image isomers, ie in the D-configuration. We shall consider amino add stereochemistry in more detail in section 8.3. 

Enantiomers, nonsuperimposable mirror-image isomers, could also be designated as dd or // instead of it dldl instead of st and, for example, dllddl-dlldld instead of at. The isotacticity of commercially produced PP may be increased by the hexane extraction of solvent extraction step. 

Biopolymers in Chiral Chromatography. Biopolymers have had a tremendous impact on the separation of nonsupernnposable. mirror-image isomers known as enantiomers. Enantiomers have identical physical and chemical properties in an achiral environment except that they rotate the plane of polarized light in opposite directions. Thus separation of enantiomers by chromatographic techniques presents special problems. Direct chiral resolution by liquid chromatography (lc) involves diastereomenc interactions between the chiral solute and the chiral stationary phase. Because biopolymers are chiral molecules and can form diastereomeric... 

Exercise 12-22 If the twist-chair conformation 8 were rigid rather than flexible, how many different monochlorocycloheptanes would you expect (a) excluding mirror-image isomers and (b) including mirror image isomers ... 

Optical resolution did not provide pure enantiomers of 28 in any cases (Table 8) but repeated resolution of the complex forming enantiomeric mixture yielded (IR,2S,5R)-2H in 92-94 % ee. Treatment of the (1S,2R,5>S)-28 containing enantiomeric mixture with the same resolving agent (//.//-isomer of DBTA) did not affect dramatic change of the enantomeric ratio. Pure (lS,2R,5S)-28 could be prepared by the use of the mirror image isomer of DBTA during a repeated resolution (Table 9). 

When a molecule can be assembled such that all the atom connections are the same, but with arrangements in space such that one isomer is the nonsuperimposable mirror image of the other, the isomers are called optical isomers. Most of the chemical and physical properties of optical isomers are identical, but the two isomers interact with polarized light differently, hence the name. Many molecules of biochemical relevance are optical isomers, with one isomer biologically active, the other inactive. The amino acid alanine (amino acids are the building blocks of proteins), shown in Figure 8-9, has two mirror image isomers. 

The two mirror-image isomers (R) and (5) of a chiral molecule are called enantiomers. Racemic mixtures contain the enantiomers (R) and... 

The permutation of five different ligands L -L5 around the positions of a square pyramid gives rise to a total of 30 different isomers, which may be subdivided into 15 pairs of enantiomers. However, it has been shown how the application of three constraints reduces this large number of isomers to only two, a pair of mirror-image isomers (73). 

Constraints (a) and (b) decrease the number of possible isomers for C5H5M(CO)2L2L3 derivatives to only three, namely, two enantiomeric isomers having L2 and L3 cis to each other, and one achiral isomer having L2 and L3 trans to each other, shown in the first line of Scheme 8. Because of constraint (c), the achiral trans isomer is excluded, and the two isomers shown in Scheme 8 (second line), are left as the only possible isomers. Thus, by introducing three constraints, the stereochemistry of a square pyramid can provide a pair of mirror-image isomers (51, 73). 

The mirror image of trans- 1,2-dichlorocyclopentane is different from (nonsuper-imposable with) the original molecule. These are two different compounds, and we should expect to discover two mirror-image isomers of trans- 1,2-dichlorocyclopentane. Make models of these isomers to convince yourself that they are different no matter how you twist and turn them. Nonsuperimposable mirror-image molecules are called enantiomers. A chiral compound always has an enantiomer (a nonsuperimposable mirror image). An achiral compound always has a mirror image that is the same as the original molecule. Let s review the definitions of these words. 

We can draw two nonsuperimposable mirror images of the most stable chair conformation of trans-1,2-dibromocyclohexane with both bromines equatorial. These structures cannot interconvert by ring-flips or other rotations about bonds, however. They are mirror-image isomers enantiomers. 

We have defined stereoisomers as isomers whose atoms are bonded together in the same order but differ in how the atoms are directed in space. We have also considered enan- Diastereomers tiomers (mirror-image isomers) in detail. All other stereoisomers are classified as diastereomers, which are defined as stereoisomers that are not mirror images. Most diastereomers are either geometric isomers or compounds containing two or more chirality centers. 

This mirror image isomer attracts the femaies... 

The two ketones are all flavour compounds too. The first, (-)-carvone, is the chief component (70%) of spearmintoil. Carvone is an interesting compound in Chapter 16 you will meet mirror-image isomers known as enantiomers, and (-)-carvone s mirror image (+)-carvone, is the chief component (35%) of dill oil. Our taste can tell the difference, though an NMR machine can t and both carvones have identical NMR spectra. See Chapter 16 for more detailf The second ketone is raspberry ketone and is largely responsible for the flavour of... 

The majority of syntheses of organic compounds is performed under macroscopic symmetrical conditions, thus optical activity is not resulted. The product is the racemate containing two mirror image isomers that can be separated by molecular chiral recognition. 

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