Stereo and Geometrical Isomerism

In this section we shall consider three types of isomerism which are encountered in polymers. These are positional isomerism, stereo isomerism, and geometrical isomerism. We shall focus attention on synthetic polymers and shall, for the most part, be concerned with these types of isomerism occurring singly, rather than in combination. The synthetic and analytical aspects of stereo isomerism will be considered in Chap. 7. Our present concern is merely to introduce the possibilities of these isomers and some of the vocabulary associated with them. 

Stereoisomerism, on the other hand, arises primarily because of the two or more distinct ways in which adjacent repeat units containing asymmetric groups can be superimposed. The stereoregular isotactic and syndiotactic polymers are examples of this type of isomeric structures. The repeating structural unit in some polymers may exhibit both stereo and geometric types of isomerism. 

Stereo (or geometrical) isomerism is related to different orientations of the side-groups or main-chain bonds (while the sequence and the nature of all bond connections remain the same). The two main typ>es of stereoisomerism are optical isomerism (tarticity) and cis-tram isomerism. The optical isomerism is also related to macromolecular chirality (enan-tiomerism, see Figure 8 and Sertion 1.02.1.8). 

Both singlet and triplet nitrenes add to C=C double bonds, but by different mechanisms. The stereo specific addition of singlet aroyl nitrenes occurs in a single step. Triplet nitrenes add in two steps and therefore isomerization by rotation about the former C=C-bond may lead to a total loss of the geometrical information in the starting material. The standard... 

As this position is fixed in space isomeric compounds are possible in which the position of two of the elements or groups linked to the doubly bound carbon atoms are reversed, as in (A) and (B) above. Two stereo-isomeric compounds should therefore be possible according to such a space arrangement and the two isomeric crotonic acids may thus be explained. This kind of stereo-isomerism is termed geometric isomerism. Without taking up in detail the proofs as to which of the two stereo-chemical formulas applies to each of the two crotonic acids, we may simply state the fact, that the properties of the solid or ordinary crotonic acid prove that it must be represented by formula (A), above, in which the methyl and carboxyl groups are 12... 

J. Wislicenus, The space arrangement of the atoms in organic molecules and the resulting geometric isomerism in unsaturated compounds. The Foundations of Stereo Oiemisfry Memoirs by Pasteur. Van t Hoff, Le M, and Wislkenus (G. 

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

Isomerism. Distinguish between the following types of isomerism (a) stereo and structural (b) geometrical and optical. 

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. 

Isomerism of Maleic and Fumaric Acids.—The isomerism of maleic and fumaric acids is stereo-isomerism of the geometric type. It is exactly like that of the two crotonic acids (p. 177). 

Explanation of Isomeric Glucoses and Glucosides.— As the lactone constitution fixes the position of the hydrogen and hydroxyl which are linked to the end carbon atom of the lactone ring stereo-isomerism of the geometric type is possible. The two isomeric glucoses and the two isomeric methyl glucosides are thus represented as follows ... 

Hantzsch Stereo Formula.—A development of the Kekule formula to agree with the existence of isomeric diazotates was suggested by Hantzsch. He assumed geometric stereo-isomerism due to the double linking of two nitrogen atoms, and this assumption has been supported not only by the isomerism of the diazotates, but also of other similar... 

Isomerism.—Cinnamic acid occurs as geometric stereo-isomers, as cis and trans forms like maleic and fumaric acids (p. 291) and crotonic and iso-crotonic acids (p. 177). This is apparent as it is... 

Coumaric and Coumarinic Acids.—Like cinnamic acid it exists in geometric stereo-isomeric forms, the trans form being known as coumaric acid and the cis form as coumarinic acid. 

There are five principal forms of isomerism related to the spatial arrangement of atoms in organic molecules structural, positional, geometrical, conformational and stereo. 

Isomers are molecules which have the same empirical formulae, i.e. they have the same total composition in terms of constituent atoms, but are different in the way the atoms are bonded together or arranged in space. In this chapter we will ignore functional isomers (e.g. allylic alcohol versus aldehyde) and tautomers (e.g. ketone versus enol) and will concentrate on those forms of isomerism relating to the spatial arrangement of atoms. There are five principal forms of such isomerism in organic molecules structural, positional, geometrical, conformational and stereo. In fact, most of the discussion will concern stereoisomerism. 

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