Geometrical and Optical Isomerization

Optical and geometrical isomerization represent the two classes of reaction where change of relative position of ligands is involved. 

Mechanisms for optical and geometrical isomerization reactions similar to those employed for substitution reactions can be envisaged. Additionally possible is a twist mechanism, involving distortion of the polyhedral framework in the activated state but in which no ligands depart or join the coordination sphere. 

Fortman, J. J., andR. E. Sievers, Coordination Chem. Rev., 1971, 6, 331 (optical and geometric isomerism). 

So far, we have considered the stereochemistry of macromolecules in a general sense. Next, we shall examine the conditions for stereoregularity in polymers and the types of stereoregular polymers that can result from optical and geometrical isomerism. 

A 1-substituted butadiene, such as piperylene, offers many possibilities for stereoregular polymers because the polymers can possess optical and geometrical isomerism. If one assumes that crystalline polymers can only occur if both centers of stereoisomerism are regular, then eleven crystalline polymers are possible. 

After van t Hoff published his theories on the tetrahedral carbon atom and on optical and geometric isomerism in 1874, much work was done to confirm his ideas by Johannes Adolf Wislicenus (1835-1902). He showed how the chemistry of maleic and fumaric acids was best explained if they were geometric isomers. The isomerism of Pasteur s tartaric acid was explained by proposing that it possessed two asymmetric carbon atoms, and that the meso form was inactive by internal compensation. Paratartaric acid was also known as racemic acid since it sometimes crystallises out from wine (the Latin racemus means a bunch of grapes). Inactive mixtures of optical antipodes are now known as racemic mixtures. 

A compound that has two or more alternate structures but the same molecular formula. Isomers differ in physical properties from each other. There are many different types of isomerism, e.g. structural, optical, functional group and geometrical isomerism. The term is most widely used in organic chemistry. 

MABCD One asymmetric pair Three geometric (1,2,3,4 1,2,4,3 1,3,2,4) Three geometric (1,2,3,4 1,2,4,3 1,3,2,4) each consists of an asymmetric pair Optical isomerism without geometric isomerism would prove I geometric isomerism without optical isomerism would prove II both optical isomerism and geometric isomerism would prove III... 

Structural-, positional-, stereo-isomerism (optical and geometric) in aliphatic hydrocarbon systems. 

Chemical compounds that have the same molecular formula but different structural formulas are said to be isomers of each other. These constitutional isomers (or structural isomers) differ in their bonding sequence, i.e. their atoms are connected to each other in different ways. Stereoisomers have the same bonding sequence, but they differ in the orientation of their atoms in space. Stereoisomerism can be further divided into optical isomerism (enantiomerism) and geometrical isomerism (cis—trans isomerism). The relationships between the different types of isomerism are shown in Figure 4.1. 

There are a large number of unsymmetrical chelates which present the feature of having both geometrical and optical isomers, as shown by the examples in Figure 4.7. The fac and mer isomers each have optical isomers thus, both racemization and geometrical isomerization can be observed in one system. This can serve to eliminate certain rearrangement processes. 

TayW43 Taylor, W. J. Applications of Polya s Theorem to optical, geometrical and structural isomerism. J. Chem. Phys. 11 (1943) 532. 

The structural regularity in such polymers gives rise to optical (D-L isomerism) and tacticity of polymers and geometrical cross-trans isomerism. 

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

The two optical forms designated A and A are shown in Fig. 7.4. Since the bidentate ligand is symmetrical, geometrical isomerism cannot arise. 

The two shapes associated with four-coordination are tetrahedral and square-planar. In the former only optical activity and in the latter only geometrical isomerism is generally encountered in the appropriate molecules. 

You have already covered structural isomerism in earlier work and we will now explore stereoisomerism. There are two types of stereoisomerism geometric isomerism and optical isomerism. 

Kinetic work on the isomeric 1,2-diphenylcyclopropanes (Scheme 2) made evident a substantial reduction in Ed and thus implied a stabilization of trimethylene diradical transition structure(s) by phenyl substituents142. In further work with 0.2 M (-)-l,2-diphenylcyclopropane in 1 -butanol, Crawford and Lynch143 uncovered a direct route from one trans antipode to the other at 220.7 °C the measured ratio of rate constants /trac(for loss of optical activity) to kK (for trans to cis geometrical isomerization) was found to be 1.49 0.05 and since krdC is (2k12 + 2/c,). and klc is 2/c,(Scheme 2), the implication is that one-center epimerizations (2kt) are favored over the two-center epimerization process (ka) by... 

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