Stereoisomerism geometric isomerism

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. 

The isomers that differ with respect to the relative orientation of bonds are known as stereo isomers or space isomers. There are two forms of stereoisomerism geometrical isomerism, also known as diastereoisomerism, and Optical isomerism, also known as mirror image isomerism. 

In Chapter 14 (pages 195-6) we learnt about two types of stereoisomerism geometric isomerism and optical isomerism. The presence of a double bond in 1,2-dibromoethane means that two geometrical isomers (cis-trans isomers) are possible. Geometric isomerism is also possible in transition metal complexes, where... 

Geometrical isomerism is a type of stereoisomerism the compounds have the same molecular and structural formulae but the arrangement of their atoms in space is different. 

So for a molecule with two different asymmetric carbon atoms we will have not more than four stereoisomeric forms. Similarly other cyclic molecules show a similar stereoisomerism. However, with even-numbered rings certain molecules show only geometrical isomerism as in 1-methyl-3 -chlorocyclobutane. 

The construction of models emphasises all these stereoisomeric features also note that in a homologous sequence of cumulative polyenes, the presence of a chiral axis, or of geometric isomerism, alternates [i.e. diene (R/S), triene (E/Z), tetraene (R/S), etc.]... 

Pyrethroids have two kinds of stereoisomerism, i.e., geometric isomerism and enantiom-erism. The cyclopropene ring shown in the general structure acts like a double bond, resulting in geometric isomers, cis/trans or Z/E forms, depending on whether the two substitutes (C-l and C-3) are on the same side (cis or Z) or on opposite sides (trans or E), as shown in the following text ... 

Dienes. The dienes present a particularly challenging case since both stereoisomerism and geometric isomerism can be observed in the products (the dienes are not only polyfunctional but also permit different tacticities). For butadiene, the following possibilities exist trans-1,4 cis-1,4 atactic 1,2 isotactic 1,2 syndiotactic 1,2 cyclic oligomers and cyclic high polymers (Reaction 16). 

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. 

Three kinds of structural isomerism are observed in polymers regioisomerism, stereoisomerism, and geometrical isomerism, depending on the polymerization mechanism. For example, with modern catalysts, structures contained in polyolefins can be controlled to... 

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. 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

In stereoisomerism all atoms in the compounds are attached to the same partners, but the partners are arranged differently in space. Geometric isomerism, the isomerism shown by some alkenes because there is no free rotation about the double bond, is one type of isomerism in this class. The geometric isomers of 1,2-dichloroethene are shown below ... 

---