Superimposable mirror images

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

Molecules that are not superimposable on their mirror images are chiral. If two compounds are related as non-superimposable mirror images, they are called enantiomers. 

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. 

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

Compounds that have a single chiral centre and form non-superimposable mirror images of each other. 

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. 

The stereoisomers related to eaeh other as non-superimposable mirror images are ealled enantiomers (Fig. 10.5). 

The two arrangements (non-superimposable mirror images) are called enantiomers... 

A carbon atom with four different groups attached is chiral. A chiral carbon rotates plane-polarized light, light whose waves are all in the same plane, and has an enantiomer (non-superimposable mirror image). Rotation, which may be either to the right (dextrorotatory) or to the left (levorotatory), leads to one optical isomer being d and the other being 1. Specific rotation (represented... 

The maximum number of stereoisomers is 2" where n is the number of nonidentical chiral centers. Figure 1-2 shows the four stereoisomers present in a molecule with two chiral centers. Non-superimposable mirror images are enantiomers, while the other species in the figure are diastereomers. Unlike enantiomers, diastereomers have different physical properties. 

Molecules with a plane or center of symmetry have superimposable mirror images they are achiral. 

Enantiomers are non-superimposable mirror images whereas diastereoisomers are stereoisomers which are not mirror images. 

To show the validity of this rule, we first prove that if an Sn axis does exist the molecule cannot be dissymmetric that is, it must have a superimposable mirror image. 

In describing a stereoisomer, it is perhaps most important initially to define whether or not it is chiral. The origins of chirality (optical activity) in coordination compounds and important experimental results have been recently reviewed.112,113,121,122 The classical example of chirality or enantiomerism in coordination chemistry is that of octahedral complexes of the type [M-(bidentate)3]. These exist in the propeller-like,123 non-superimposable, mirror-image forms (13a) and (13b). Synthesis of this type of complex from M and the bidentate ligand in an achiral environment such as water results in an equimolar mixture of the two stereoisomers. The product... 

Optical isomers are special kinds of stereoisomers they are non-superimposable mirror images of each other (Fig. 16.27). Both geometrical and optical isomerism can occur in an octahedral complex, as in [CoCI2(en)2]+ the trans isomer is green (14a) and the two alternative cis isomers (14b) and (14c), which are optical isomers of one another, are violet. 

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. EnrirTm mers have identical physical and chemical (except towards optically aQtJvp reagents) properties except for the direction in which plane-polarized light is rotated. Enantiomers account for a compound s optical activity. 

The upper half of(the )nolecule is a non-superimposable mirror image of the lower half, making the top half an enantiomer of the lower half However, since the two halfs are in the same molecule the rotation of plane-polarized light by the upper half is cancelled by the lower half and the compound is optically inactive. 

One simple practical method of assessing the possibility of the existence of non-superimposable mirror images, particularly with complex structures, is to construct models of the two molecules. The property of chirality may alternatively be described in terms of the symmetry elements of the molecule. If there is a lack of all elements of symmetry (i.e. a simple axis, a centre, a plane, or an n-fold alternating axis) the chiral molecule is asymmetric, and will possess two non-superimposable mirror image structures (e.g. 2a and 2b). If, however, the molecule possesses a simple axis of symmetry (usually a C2 axis) but no other symmetry elements, the chiral molecule is dissymmetric. Thus 4a and 4b are dissymmetric and the simple C2 axis of symmetry, of for example 4a, is shown below. If the molecule possesses a centre of symmetry (C.) or a plane of symmetry (alternating axis of symmetry (S ), the mirror images of the molecule are superimposable and the molecule is optically inactive. These latter three symmetry elements are illustrated in the case of the molecule 4c. 

---