Geometrical isomerism also

Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

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. 

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 also occurs in octahedral complex ions. For example, the compound [Co(NH3)4Cl2]Cl has cis and trans isomers (Fig. 20.12). 

Remember that conformers are different conformations of the same molecule, whereas configurational isomers are different molecules. Geometric isomers (cis-trans isomers) are one type of configurational isomer. As we will see in Chapter 3, geometric isomerism also occurs in alkenes. Also, we will see other types of configurational isomers in Chapter 5. 

The instances of geometric isomerism also multiplied with the realisation that cis and trans isomerism was to be found in double-bonded nitrogen compounds, and also in saturated cyclic compounds. Werner s work on inorganic coordination compounds provided further instances of both geometrical and optical isomerism (Chapter 12). 

Coordinated metal centers may also provide the added nuance of stereoisomerism the component center(s) may possess chirality (i.e., be asymmetric or dissymmetric so that they are nonsuperimposable on their mirror image), or in cases where the coordination geometry of a center involves more than one relative orientation of Ug-ands—for example, square planar (where two coordination positions can be disposed at 90° or 180°), trigonal bipyra-midal (90°, 120°, or 180°), or octahedral (90° or 180°)— geometric isomerism/diastereoisomerism can give rise to alternate forms. Geometrical isomerism also arises in bis(bidentate) or tris(bidentate) centers where the bidentate ligands are unsymmetrical. ... 

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. 

Geometrical isomerism also occurs in azoxy compounds ... 

When two different substituents are attached to each carbon atom of the double bond, cis-trans isomers can exist. In the case of c T-2-butene (Fig. 1.11a), both methyl groups are on the same side of the double bond. The other isomer has the methyl groups on opposite sides and is designated as rran5--2-butene (Fig. l.llb). Their physical properties are quite different. Geometric isomerism can also exist in ring systems examples were cited in the previous discussion on conformational isomers. 

In spite of the assortment of things discussed in this chapter, there are also a variety of topics that could be included but which are not owing to space limitations. We do not discuss copolymers formed by the step-growth mechanism, for example, or the use of Ziegler-Natta catalysts to regulate geometrical isomerism in, say, butadiene polymerization. Some other important omissions are noted in passing in the body of the chapter. 

It is not the purpose of this book to discuss in detail the contributions of NMR spectroscopy to the determination of molecular structure. This is a specialized field in itself and a great deal has been written on the subject. In this section we shall consider only the application of NMR to the elucidation of stereoregularity in polymers. Numerous other applications of this powerful technique have also been made in polymer chemistry, including the study of positional and geometrical isomerism (Sec. 1.6), copolymers (Sec. 7.7), and helix-coil transitions (Sec. 1.11). We shall also make no attempt to compare the NMR spectra of various different polymers instead, we shall examine only the NMR spectra of different poly (methyl methacrylate) preparations to illustrate the capabilities of the method, using the first system that was investigated by this technique as the example. 

Geometric isomerism can also occur in chelated octahedral complexes (Figure 15.7, p. 416). Notice that an ethylenediamine molecule, here and indeed in all complexes, can only bridge cis positions. It is not long enough to connect to trans positions. 

The trans compound melts at approximately 90 °C, and continued heating leads to isomerization to the cis structure. Geometrical isomerizations can also lead to a change in structure of the complex. For example, a change from square planar to tetrahedral structure has been observed for the complex [Ni(P(C2H5)(C6H5)2)2Br2]. 

In the above arrangement the two hydrogen atoms and the two carboxyl groups are all in one plane, or the molecule is flat. The cis-trans isomerism is also called geometrical isomerism and we know that this isomerism is shown not only by olefines but by a large variety of compounds which may be broadly classified as follows ... 

The Arrhenius equations for the various reactions are shown in Table 1. The geometrical isomerizations of 1,2,3-trimethylcyclopropane and l-ethyl-2-methylcyclopropane have also been studied. In both cases geometrical isomerization is faster than the structural isomerization reactions to yield olefins. The Arrhenius equations obtained were ... 

Although geometric isomerism is most common in compounds containing a C=C double bond, it can also arise in saturated rings where rotation about the C-C single bonds is restricted. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

With a double bond, rotation would destroy the tt bond that arises from overlap of p orbitals consequently, there is a very large barrier to rotation. It is of the order of 263 kJmol , which is very much higher than any of the barriers to rotation about single bonds that we have seen for conformational isomerism. Accordingly, cis and trans isomers do not interconvert under normal conditions. Ring systems can also lead to geometric isomerism, and cis and trans isomers... 

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