Stereoisomerism optical isomers

Bis-chelate octahedral complexes such as [Co(en)2Cl2] exist in both cis- and frara-isomers depending on the arrangement of the chloro ligands. In addition, the cw-isomer (but 

The ran -isomer is non-polar Asymmetric stretch IR active Symmetric stretch IR inactive 

Box 19.1 Trans- and c/s-isomers of square planar complexes an NMR spectroscopic probe 

Similar diagnostic information can be obtained from NMR spectroscopy for square planar complexes containing metal centres with spin-active isotopes. For example, rhodium is monotopic (i.e. 100% of one isotope) with Rh 

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The steric bulk of the three iodine atoms in the 2,4,6-triiodoben2ene system and the amide nature of the 1,3,5-substituents yield rotational isomers of the 5-A/-acyl-substituted 2,4,6-triiodoisophthalamides. Rotational motion in the bonds connecting the side chains and the aromatic ring is restricted. These compounds also exhibit stereoisomerism when chiral carbon atoms are present on side chains. (R,5)-3-Amino-l,2-propanediol is incorporated in the synthesis of iohexol (11) and ioversol (12) and an (3)-2-hydroxypropanoyl group is used in the synthesis of iopamidol (10). Consequendy, the resulting products contain a mixture of stereoisomers, ie, meso-isomers, or an optical isomer. 

The second type of stereoisomerism is optical isomerism, in which two molecules that are mirror images of each other are not superimposable on each other. Consider the compound 2-butanol, CH3CH(OH)CH2CH3. It has two optical isomers, because it is not superimposable on its mirror image. 

In stereoisomerism, three dimensions must be considered. In stereoisomerism (also termed optical isomerism), there is no plane of symmetry in the molecule, so that the two forms are mirror-images, and thus cannot be turned into a position of coincidence. Thus, compounds containing a carbon atom (or other tetravalent atom) to which four different atoms or radicals are bonded are optical isomers. They receive this name from the fact that one isomer rotates the plane of polarized light to the right (d extra form) the other rotates it to the left (leva form). Lactic acid is an example. See also Lactic Acid, and formulas below ... 

The stereospecificity of living organisms is imperative to their efficiency. The reason is that it is just not possible for an organism to be so constructed as to be able to deal with all of the theoretically possible isomers of molecules with many asymmetric centers. Thus a protein molecule not uncommonly has 100 or more different asymmetric centers such a molecule would have 2100 or 1030 possible optical isomers. A vessel with a capacity on the order of 107 liters would be required to hold all of the possible stereoisomeric molecules of this structure if no two were identical. An organism so constituted as to be able to deal specifically with each one of these isomers would be very large indeed. 

The synthetic form of vitamin E exists as a mixture of eight possible optical isomers. The use of the synthetic form of the vitamin (dl-a-tocopherol) in the polymer would be expected to yield oxidative transformation products with a complex mixture of stereoisomeric structures. This was investigated by examining the oxidation of dl-a-tocopherol in solution by a mild oxidant, using HPLC and spectroscopic analysis to isolate and characterise the iso-... 

Even a fourth variant is feasible, namely the submission of meso-compounds -these contain stereogenic centers but feature internal elements of symmetry and, hence, are achiral - to a desymmetrization, which in principle generates only one stereoisomer. The classical case is the hydrolysis of a diester to form a half-acid/es-ter as a single antipode. Generation of both optical isomers of synthetically useful 1,2-diol products in good yield and stereoisomeric purity applying this methodology has recently been demonstrated (see Fig. 2.11) [65]. 

Optical isomerism is a form of isomerism (specifically stereoisomerism) where the two different isomers are the same in every way except being non-superposable mirror images of each other. Optical isomers are known as chiral molecules (pronounced ki-rall). 

According to lUPAC the term geometric isomerism" is an obsolete synonym of cis-trans isomerism and its use is strongly discouraged. Sometimes the term geometric isomerism has been used as a synonym of stereoisomerism, i.e. optical isomers being considered to be geometric isomers. This, however, is not consistent with current standard chemical nomenclature. The exact term for stereoisomers that are not optical isomers is diastereomers. 

Stereoisomerism in Place - Positional Isomers in Space - Optical Isomers... 

One problem with a special emphasis in the pharmaceutical industry involves the isolation of stereoisomers (optical isomers). An organic molecule with no asymmetric carbon atoms is denoted as achiral, but if it contains one or more asymmetric carbon atoms, it would be denoted as a stereoisomer. If a single asymmetric carbon exists, there will be two enantiomers, while with two asymmetric carbon atoms the molecule forms four diastereomers. A reader not familiar with the general concept of steriochemistry should refer to a introductory organic chemistry text. Biological systems are inherently based on enantiomeric or stereoisomeric biochemistry. Thus, there has been a trend toward selecting a single stereoisomer as a new chemical entity for pharmaceuticals (Collins et al. 1997 Stinson 1999). 

Some alkenes can also have cis and trans isomers, one of two forms of stereoisomerism. Here the homers have the same molecular formulas and the same atom-to-atom bonding sequences, but the atoms differ in their arrangement in space. The other form of stereoisomerism, optical isomerism, is discussed in Section 15.1. 

A large number of stereo- and optical isomers are possible. Iso- refers to stereo-isomerism about Cj and Neo- to stereoisomerism about C2. 

The second type of stereoisomerism listed in Figure 23.19 is optical isomerism. Optical isomers, called enantiomers, are mirror images that cannot be superimposed on each other. They bear the same resemblance to each other that your left hand bears to your right hand. If you look at your left hand in a mirror, the image is identical to your right hand ( Figure 23.22). No matter how hard you try, however, you cannot... 

The three-dimensional spatial arrangements in coordination compounds can lead to stereoisomerism. Recall from Section 4.4 that stereoisomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence, but with different three-dimensional structures. The two types of stereoisomers are geometric isomers (isomers that cannot be interconverted without breaking a chemical bond) and optical isomers (isomers that are nonsuperimposable mirror images). Coordination compounds may exhibit one or both types of stereoisomerism. Many coordination compounds, however, do not have stereoisomers. 

Enantiomers are also referred to as optical israners because different enantiomers rotate the plane of polarized light in equal but opposite directions. Optical isomerism is the second form of stereoisomerism. In these molecules the groups in the optical isomers are the same they differ in their arrangement in space, in this case around the central carbon atom. 

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