Mirror images, nonsuperimposable

Structures A and A are nonsuperimposable mirror images of each other Thus although as 1 2 dichloro cyclohexane is chiral it is optically inactive when chair-chair interconversion occurs Such interconver Sion IS rapid at room temperature and converts opti cally active A to a racemic mixture of A and A Because A and A are enantiomers interconvertible by a conformational change they are sometimes re ferred to as conformational enantiomers... 

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. 

Enantiomers (Section 7.1) Stereoisomers that are related as an object and its nonsuperimposable mirror image. 

Enantiotopic (Section 7.9) Describing two atoms or groups in a molecule whose environments are nonsuperimposable mirror images of each other. The two protons shown in bold in CH3CH2CI, for example, are enantiotopic. Replacement of first one, then the other, by some arbitrary test group yields compounds that are enantiomers of each other. 

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

Same atoms, same partners nonsuperimposable mirror images... 

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. 

We mentioned before that enantiomers are two compounds that are nonsuperim-posable mirror images. Let s first clear up the term enantiomers, since students will often use this word incorrectly in a sentence. Let s compare it to people again. If two boys are born to the same parents, those boys are called brothers. Each one is the brother of the other. If you had to describe both of them, you say that they are brothers. Similarly, when you have two compounds that are non-superimposable mirror images, they are called enantiomers. Each one is the enantiomer of the other. Together, they are a pair of enantiomers. But what do we mean by nonsuperimposable mirror images Let s go back to the brother analogy to explain it. 

The first thing you need to realize is that enantiomers always come in pairs. Remember that they are mirror images of each other. There are only two sides to a mirror, so there can be only two different compounds that have this relationship (nonsuperimposable mirror images). This is very much like the twin brothers. Each brother only has one twin brother, not more. 

One of the fundamental concepts of structural chemistry is that of molecular asymmetry or chirality. The most typical example is that of a tetrahedral carbon atom with four different substituents, C(abcd), which can produce two different arrangements, which are nonsuperimposable mirror images of one another. Such a carbon atom is usually called asymmetric or chiral. In contrast, when two of the substituents are alike, as in C(abc2), the system is usually termed symmetrical or achiral, except for a special class of compounds... 

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... 

Enantiomer Two stereoisomers that are nonsuperimposable mirror images of each other. 

An example is l-methyl-2-chlorocyclopropane which exists as a pair of geometric isomers and since each isomer has a nonsuperimposable mirror image, it has a pair of enantiomers. Thus its forms are ... 

The monosaccharides have a couple of characteristics that prove to be important in terms of their structure and function. They all have at least one carbon that is bonded to four different groups (a chiral carbon), and most form five-and six-membered rings easily. The presence of chiral carbons allows these compounds to exist as two different optical isomers that are nonsuperimposable mirror images of each other. (Your feet are nonsuperimposable mirror images. Try putting a left shoe on a right foot )... 

If two structures are nonsuperimposable mirror images, what can you say about the direction that each wih rotate the plane of polarized hght Would a mixture of the two be optically active Explain. 

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. 

Enantiomers, nonsuperimposable mirror-image isomers, could also be designated as dd or // instead of it dldl instead of st and, for example, dllddl-dlldld instead of at. The isotacticity of commercially produced PP may be increased by the hexane extraction of solvent extraction step. 

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. 

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