Geometrical isomerizations

The alkenes make up a homologous series of hydrocarbons with the general formula C H2 . Alkenes show two types of structural isomerism, position isomerism and chain isomerism. Geometrical isomerism also exists because of the lack of free rotation about the C=C double bond. 

Effects of Isomerism Geometrical isomers of coordination compounds can exhibit different values of redox potential, as accounted for, in various cases, by simple djr orbital level splitting diagrams or by MO calculations. The dependence of the relative stability and redox behavior of the geometrical isomers on the electronic configuration of the metal is also... 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

We have developed an understanding of structural isomerism, i.e., straight chain and branched chain. We have seen structural isomerization connected with the location of the double bond. We must recognize another type of isomerism—geometric isomerism, also called diastereo-merism or simply cis-trans isomerism. 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

FIGURE 1 The different types of stereoisomerism I, central chirality II, axial chirality III, atropisomerism IV i /Z-isomerism (geometrical). 

Isomerism (geometric) Configuration around double bonds is specified by the characters / and indicating relative directionality between the connected (by double bond) atoms—for example,... 

For alkenes, in addition to structural isomerism, geometrical isomerism can also occur. Geometrical isomerism differs only in the arrangement of the atoms in space. 

Rule 11. Designation of structural isomerism. Geometrical isomerism is designated alternatively by numbers or by the words cis- and tram-. 

The next three Sections, 12.1 to 12.3, will discuss some of the biological consequences of optical isomerism, geometrical isomerism, and conformational behaviour. 

The stereoisomeric relation of the nonplanar D2mirror image. As enantiomers have identical scalar properties (energies, nmr shifts, etc.), but different pseudoscalar properties an experimental differentiation of these chiral molecules is bound to rely on chiral properties, such as optical rotations. The planar E-dimethylbutatriene (5) and Z-dimeth-ylbutatriene (6) exhibit cis-trans isomerism (geometrical isomerism). 5 and 6 differ in their scalar molecular properties. 

Because rotation at carbon-carbon double bonds is restricted, cis-trans isomerism (geometric isomerism) is possible in appropriately substituted alkenes. For example, 1,2-dichloroethene exists in two different forms ... 

Figure 1.2 The different type of stereoisomerisms (a) central chirality, (b) axial chirality, (c) atropisomerism and (d) -/Z-isomerism (geometric).

There are numerous complexes of Cr(III) their stability is related to the (t2g) configuration. The chelate effect confers extra stability on the oxalato-(ethanedioato-) complexes. When water in the hexaaqua ion is replaced by three oxalate ligands, a trisoxalato-complex is formed in solution and can be readily prepared as a solid potassium salt. This exhibits optical isomerism. Geometrical isomerism is encountered in the cis- and tram- bisoxalato-complexes. The former exhibits optical isomerism. Although no solid containing the monooxalato-cation has been obtained, there is spectral evidence of its formation in solution. 

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