Nonsuperimposable mirror images Chiral molecules

Section 7 1 A molecule is chiral if it cannot be superimposed on its mirror image Nonsuperimposable mirror images are enantiomers of one another Mol ecules m which mirror images are superimposable are achiral... 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

FIGURE 4.12 Enantiomeric molecules based on a chiral carbon atom. Enantiomers are nonsuperimposable mirror images of each other. 

This molecule has no chiral carbons, nor does it have a rigid shape, but it too has neither a plane nor an alternating axis of symmetry. Compound 32 has been synthesized and has, in fact, been shown to be chiral. Rings containing 50 or more members should be able to exist as knots (33, and see 37 on p. 114 in Chapter 3). Such a knot would be nonsuperimposable on its mirror image. Calixarenes, ° crown ethers, catenanes, and rotaxanes (see p. 113) can also be chiral if suitably substituted. For example, A and B are nonsuperimposable mirror images. 

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... 

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. 

A chiral stereoisomer is not superimposable on its mirror image. It does not possess a plane or center of symmetry. The nonsuperimposable mirror images are called enantiomers. A mixture of equal numbers of molecules of each enantiomer is a racemic form (racemate). The conversion of an enantiomer into a racemic form is called racemization. Resolution is the separation of a racemic form into individual enantiomers. Stereomers which are not mirror images are called diastereomers. 

The mirror images of bromochlorofluoromethane have the same constitution. That is, the atoms are connected in the same order. But they differ in the arrangement of their atoms in space they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable mirror image are classified as enantiomers. The word enantiomer describes a particular relationship between two objects. One cannot look at a single molecule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, Is that person a cousin Furthermore, just as an object has one, and only one, mirror image, a chiral molecule can have one, and only one, enantiomer. 

Chiral separations are concerned with separating molecules that can exist as nonsuperimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers, are illustrated in Figure 1. Although chirality is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphorus or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahelicene, tetrasubstituted adamantanes, and substituted allenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

When a molecule is chiral, then it will have two isomeric forms called enantiomers, each of which is the nonsuperimposable mirror image of the other. Enantiomers are distinct stereoisomers because they are compounds that have die same molecular formula and sequence of bonded elements but which differ in tile spatial arrangement of groups in the molecule. If a molecule is chiral, and thus has two enantiomers, it usually (but not always) contains at least one chiral center. In organic compounds a chiral center usually corresponds to an asymmetric tetrahedral carbon atom. 

Stereoisomers have the same atom connectivities but different arrangements of the atoms in space. They may be chiral or achiral. A stereoisomer is chiral if its mirror image is not identical or superimposable on the original molecule. It is achiral if the molecule and its mirror image are identical or superimposable. Enantiomers are a pair of molecules related as nonsuperimposable mirror images. 

Optical isomerism is when two molecules are nonsuperimposable mirror images of each other (Think of a pair of gloves. They can only stack palm to palm, not one on top of the other.) Such molecules are termed asymmetric or chiral. (Note Chiral carbons have 4 different substituents.) Two optical isomers can be distinguished in their names by the prefixes dextro- or levo- according to whether a solution of the compound rotates a beam of polarized light to the right or the left. The abbreviations d- and l- are commonly used for dextro- and levo-. A new system of naming uses R- (rectus) and S- (sinister). 

Construct the model of the image projected in the mirror. You now have two models. If one is the object, what is the other (3a) Do either have a plane of symmetry (3b) Are both chiral (3c) Now try to superimpose one model onto the other, that is, to place one model on top of the other in such a way that all five elements (i.e., the colored atoms) fall exactly one on top of the other. Can you superimpose one model onto the other (3d) Enantiomers are two molecules that are related to each other such that they are nonsuperimposable mirror images of each other. Are the two models you have a pair of enantiomers (3e) ... 

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 mirror image of trans- 1,2-dichlorocyclopentane is different from (nonsuper-imposable with) the original molecule. These are two different compounds, and we should expect to discover two mirror-image isomers of trans- 1,2-dichlorocyclopentane. Make models of these isomers to convince yourself that they are different no matter how you twist and turn them. Nonsuperimposable mirror-image molecules are called enantiomers. A chiral compound always has an enantiomer (a nonsuperimposable mirror image). An achiral compound always has a mirror image that is the same as the original molecule. Let s review the definitions of these words. 

A fundamental subclassification is that of stereoisomers, which can be divided into enantiomers and diastereoisomers. Either two stereoisomers are related to each other as object and nonsuperimposable mirror image, or they are not. In the former case, they share an enantiomeric relationship. This implies that the molecules are dissymmetric (chiral), and chirality is the necessary and sufficient condition for the existence of enantiomers. An example of an enantiomeric relationship is illustrated in diagram III which shows the (R)- and (S)-enantiomers (see Section 4.b) of... 

In Section 4.3 we mentioned two important stereochemical terms, enantiomers and enantiotopic nuclei. Enantiomers are structures related as the left hand is related to the right nonsuperimposable mirror images. Any chiral (dissymmetric) molecule can exist in two (and only two) enantiomeric forms. For example, chiral alcohol 10-11 has two enantiomeric configurations, labeled R and S.10 Enantiotopic nuclei are those related by a plane of symmetry. The methylene hydrogens of benzyl alcohol (10-12) are enantiotopic and are labeled pro-R and pro-S10 ... 

Many molecules are not superimposable on their mirror image. Such molecules, labeled chiral or dissymmetric, may have important chemical properties as a consequence of this nonsuperimposability. An example of a chiral organic molecule is CBrClFI, and many examples of chiral objects can also be found on the macroscopic scale, as in Figure 4-18. 

When molecules composed of the same constituents have the same structural formulas but differ only with respect to the spatial arrangement of certain atoms or groups of atoms, they are defined as stereoisomers. Stereoisomers can be optical isomers or geometrical isomers. Optical isomers are members of a set of stereoisomers, at least two of which are optically active or chiral geometrical isomers are members of a set of Stereoisomers that contains no optically active members. If the relationship between optical isomers is one of nonsuperimposable mirror images, the isomers are defined as enantiomers. Molecules having at least one pair of enantiomers are considered chiral. Optical isomers not related to each other as enantiomers are diastereomers. 

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

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