Stereoisomerism of Molecules

Identical Molecules Not distinguishable under any conditions, chiral or achiral. 

Enantiomers The same in all scalar properties and distinguishable only under chiral conditions. Only molecules of which the point groups are Cn (n 1), Dn (n 1), T, O, or / are chiral and can exist in enantiomeric forms. 

Constitutional Isomers and Diastereomers Differ in all scalar properties and are distinguishable in principle under any conditions, chiral or achiral. Geometric isomers, which are related by the orientation of groups around a double bond, are a special case of diastereomers. 

The concepts of chirality and isomerism may readily be extended to pairs or larger assemblages of molecules, hence the reference to chiral and achiral environments above. 

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Once we grasp the idea of stereoisomerism m molecules with two or more chirality cen ters we can explore further details of addition reactions of alkenes... 

Let us examine the properties of molecules where a central atom M is surrounded by five ligands A, B, C, D, E. We assume that the ligands are at the vertices of a trigonal bipyramid. This assumption is adequate for most pentacoordinate complexes but we ougth to mention that the description of stereoisomerization we propose could be applied if another polytopal form—the tetragonal pyramid, for example—was the stable one. The same type of description has already been undertaken for hexacoordinate octaedral complexes . ... 

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

The stereochemistry deals with the study of spatial structure of molecules and its effect on the physical and chemical properties of the compound. Until recently stereochemistry was thought to be purely a theoretical area of study but since it not only affects the properties but also controls the rate of reaction, it has assumed great practical importance. Now stereochemistry is applied to study physiological properties, biochemistry, molecular biology, pharmacy and even in medicine. So the scope of the subject has become enormous. Stereoisomerism is classified into two types. 

In general, organic molecules having a central carbon atom to which are attached four altogether different moieties, as C (WXYZ) thereby rendering the molecule asymmetric, are all optically active. Such types of molecules usually exist in two stereoisomeric forms as mirror images of each other. For example ... 

Since the three-dimensional structure of molecules is so tightly linked to their chemical and biological properties, we are going to encounter mnltiple examples of stereoisomerism as we move forward. So it seems reasonable and nsefnl to make a little snmmary of what we have said abont them here. 

Many drugs are racemates, including 13-blockers, nonsteroidal anti-inflammatory agents, and anticholinergics (e.g benzetimide A). A racemate consists of a molecule and its corresponding mirror image which, like the left and right hand, cannot be superimposed. Such chiral ( handed ) pairs of molecules are referred to as enantiomers. Usually, chirality is due to a carbon atom (C) linked to four different substituents ( asymmetric center ). Enantiomerism is a special case of stereoisomerism. Non-chiral stereoisomers are called diaster-eomers (e.g., quinidine/quinine). 

Note 1 A stereoselective polymerization is defined as a polymerization in which a polymer molecule is formed from a mixture of stereoisomeric monomer molecules by the incorporation of only one stereoisomeric species [1]. Thus, an asymmetric enantiomer-differentiating polymerization is a stereoselective polymerization, in which all the polymer molecules are formed by the incorporation of only one type of stereoisomeric species. 

Stereoisomers Structural isomers having an identical chemical constitution but exhibiting differences in the spatial arrangement of their atoms are called stereoisomers [7], One case of stereoisomerism, denoted asymmetric chirality, comprises molecules that are mirror images of each other. Such pairs of molecules are called enantiomers. Figure 1.2.3 illustrates the two chiral molecules of 1-bromo-1-chloroethane. The line in the middle represents a symmetry plane. Note that it is... 

Stereoisomerism of Organic Molecules the longest continuous chain is trans as it passes through the double bond ... 

The stereospecificity of living organisms is imperative to their efficiency. The reason is that it is just not possible for an organism to be so constructed as to be able to deal with all of the theoretically possible isomers of molecules with many asymmetric centers. Thus a protein molecule not uncommonly has 100 or more different asymmetric centers such a molecule would have 2100 or 1030 possible optical isomers. A vessel with a capacity on the order of 107 liters would be required to hold all of the possible stereoisomeric molecules of this structure if no two were identical. An organism so constituted as to be able to deal specifically with each one of these isomers would be very large indeed. 

A three-bladed propeller whose skeleton is of D3 symmetry may alternatively be viewed as the composite of three distinct two-bladed propellers of the same helicity, opposite to that of the three-bladed propeller. For example, triphenylcarbenium ion contains, in addition to the primary three-bladed propeller axis (C3), three secondary two-bladed propeller axes coincident with the local C2 axes of the phenyl rings. However, stereoisomerism resulting from secondary propellers is subsumed by the primary propeller, and therefore isomerism in molecules of this type may always be completely analyzed in terms of the primary propeller. In the case of molecules (propellers) of D2 symmetry, the choice of any of the three C2 axes as the primary propeller axis suffices for a complete description of the system. 

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