Mirror-image stereoisomerism

Enantiomers are characterized as nonsuperimposable mirror images. Enantiomers are said to be chiral (note that some diastereomers may be chiral as well). In the context of the same bonding pattern or connectivity, which atoms are bonded to which, enantiomers have handedness and are related to each other as the right hand is related to the left hand. In the specific example we saw earlier, the carbon atom is linked to four different atoms. Such molecules have non-superimposable mirror images. Stereoisomerism occurs in some molecules that do not have such a carbon atom but these cases are more exotic than we need to worry about here. Stereoisomers frequently have different, and sometimes strikingly different, biological properties, exemplified by the thalidomide case. 

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

It has been mentioned (p. 9) that the two carbon atoms of a C—C double bond and the four atoms directly attached to them are all in the same plane and that rotation around the double bond is prevented. This means that in the case of a molecule WXC=CYZ, stereoisomerism exists when W X and Y Z. There are two and only two isomers (E and F), each superimposable on its mirror image unless... 

The second type of stereoisomerism is optical isomerism, in which two molecules that are mirror images of each other are not superimposable on each other. Consider the compound 2-butanol, CH3CH(OH)CH2CH3. It has two optical isomers, because it is not superimposable on its mirror image. 

If a molecule contains more than one chiral center, there are other forms of stereoisomerism. As mentioned in Section 1.1, nonsuperimposable mirror images are called enantiomers. However, substances with the same chemical constitution may not be mirror images and may instead differ from one another... 

Figure 11.21 Stereoisomerism in 2-iodobutane (CH3CH2CHICH3). The dotted line represents a mirror plane between the two structures, showing that one is the mirror image of the other.

Since all the molecules are asymmetric and have no plane of symmetry, all are optically active. Further structures I and II are enantiomorphs and so are structures III and IV. But structures III and I or IV and I are although stereoisomers but are not enantiomorphs. Such pairs of steroisomers which possess chirality but are not the mirror images of each other are called diastereomers. Thus III and IV are diastereomers of 1. So diastereomers will always be formed when the compound contains two dissimilar asymmetric carbon atoms and will exist in four stereoisomeric forms. 

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 ... 

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). 

Figure 1.11 Stereoisomers a carbon atom bonded to fonr different substituents in a chiral carbon or a stereogenic center. Such molecules cannot be superimposed upon their mirror image. A receptor will recognize one stereoisomer but not another. Such stereoisomers are designated as either R or S. Stereoisomerism may also occur around double bonds, producing cis or trans orientations of the substituents on either face of the double bond.

Now we do know. X-ray crystallographic studies in 1951 confirmed that the levorotatory and dextrorotatory forms of tartaric acid are mirror images of each other at the molecular level and established the absolute configuration of each (Fig. 1). The same approach has been used to demonstrate that although the amino acid alanine has two stereoisomeric forms (designated d and l), alanine in proteins exists exclusively in one form (the l isomer see Chapter 3). 

FIGURE 3-3 Stereoisomerism in n-amino acids, (a) The two stereoisomers of alanine, l- and o-alanine, are nonsuperimposable mirror images of each other (enantiomers), (b, c) Two different conventions for showing the configurations in space of stereoisomers. In perspective formulas (b) the solid wedge-shaped bonds project out of the plane of the paper, the dashed bonds behind it. In projection formulas (c) the horizontal bonds are assumed to project out of the plane of the paper, the vertical bonds behind. However, projection formulas are often used casually and are not always intended to portray a specific stereochemical configuration. 

Only three, not four, stereoisomeric 2,3-butanediols are possible. These three are shown in Figure 7.10. The (2R,3R) and (25,35) forms are enantiomers of each other and have equal and opposite optical rotations. A third combination of chirality centers, (2/ ,35), however, gives an achiral structure that is superimposable on its (25,37 ) mirror image. Because it is achiral, this third stereoisomer is optically inactive. We call achiral molecules that have chirality centers meso forms. The meso form in Figure 7.10 is known as /Mcso-2,3-butanediol. 

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... 

In stereoisomerism, three dimensions must be considered. In stereoisomerism (also termed optical isomerism), there is no plane of symmetry in the molecule, so that the two forms are mirror-images, and thus cannot be turned into a position of coincidence. Thus, compounds containing a carbon atom (or other tetravalent atom) to which four different atoms or radicals are bonded are optical isomers. They receive this name from the fact that one isomer rotates the plane of polarized light to the right (d extra form) the other rotates it to the left (leva form). Lactic acid is an example. See also Lactic Acid, and formulas below ... 

The most important type of stereoisomerism is that which arises when molecules possess two structures that are not identical and also are mirror images of one another. This is not a difficult or unfamiliar concept. Many things around us, such as our hands, and pairs of shoes, are not identical and also are mirror images of one another. In the same way, nonidentical molecules exist in which the only distinction between them is that one is the mirror image of the other. A common statement is that such isomers are mirror images of one another, but these images are not superimposable. A simple example of this type of... 

Allenes of the type RR C=C=CRR are chiral molecules and can exist in two stereoisomeric forms, one being the mirror image of the other and neither being superimposable on the other (i.e., enantiomers, Figure 13-5). 

Diasteromers stereoisomeric chemical structures that are not enantiomers (mirror images) of one another. 

Molecules such as 2-chlorobutane are termed chiral. Chiral molecules exist as either of two stereoisomeric structures. These stereoisomers, a pair of nonsuperimposable mirror images, are called enantiomers. 

The simplest type of stereoisomerism for hexacovalent systems occurs in cases where four of the six substituents on a complex are the same. The remaining two, which may or may not be different from each other, may occupy either cis or irans positions (Fig. 22-14). Both of these complexes are superimposable on their mirror images and cannot be separated into enantiomorphic forms. This can be checked by imagining the complexes to be reflected in a mirror (as was done in Fig. 22-7), or it may be noted that both complexes have at least one plane of symmetry (that is, that there is at least one way in which a plane can bisect the complex into two mirror-image halves). The presence of a plane of symmetry is a convenient indication of the nonresolvability of a complex. The apparent absence of such a plane, however, should be regarded with reserve, for without considerable experience or a good sense of spatial... 

Any molecule with n asymmetric carbon atoms can exist in the form of 2 stereoisomers including 2W enantiomeric (mirror image) pairs. When the number of asymmetric carbon atoms exceeds two, so-called dia-stereoisomeric forms become possible. They possess different physical properties and are not mirror images. Enantiomers, however, are identical in physical properties with the exception of their behavior toward polarized light. The aldohexoses comprise 16 stereoisomers (8 enantiomeric pairs) belonging to the respective series. Within the d or l series the individual aldohexoses are diastereoisomers. The aldoses in D series are shown in Fig. 2-3. 

The 2R,3S and 2S,3R structures are identical because the molecule has a plane of symmetry and is therefore achiral. The symmetry plane cuts through the C2-C3 bond, making one half of the molecule a mirror image of the other half (Figure 9.11). Because of the [ lane of symmetry, the molecule is achiral, despite the fact that it has two chirality centers. Compounds that are achiral, yet contain chirality centers, are called meso (me-zo) compounds. Thus, tartaric acid exists in three stereoisomeric forms two enantiomers and one meso form. 

The molecular basis for the left- and right-handedness of distinct crystals of the same chemical substance and the associated differences in optical rotation was developed from the hypothesis of Paterno (1869) and Kekule that the geometry about a carbon atom bound to four ligands is tetrahedral. Based on the concept of tetrahedral geometry, Van t Hoff and LeBel concluded that when four different groups or atoms are bound to a carbon atom, two distinct tetrahedral molecular forms are possible, and these bear a non-superimposable mirror-image relationship to one another (Fig. 3). This hypothesis provided the link between three-dimensional molecular structure and optical activity, and as such represents the foundation of stereoisomerism and stereochemistry. 

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