E-StoreStereo isomerism
In stereoisomerism, the molecules have the same molecular formula and the same structural formula (the atoms are connected in the same order in each molecule). However, in each molecule, the atoms have a different three dimensional arrangement in space which makes them non-superimposable. This means that no matter how you twist and turn the molecules, one isomer cannot fit exactly on top of the other.
There are two main types of stereoisomerism as shown in Figure 1.
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Geometric isomerism can arise when there is lack of free rotation around a bond, often a C
C bond.Consider 1,2-dichloroethene. There are two geometric isomers (Table 3).
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They exist because rotation about the C
C bond is very difficult.
Rotation is restricted because this would involve breaking the 
bond. The two geometric isomers are not superimposable.
Geometric isomers caused by restricted rotation around a bond are distinguished by
using the terms - 'cis' and 'trans':
- cis - both groups are on the same side of the double bond
- trans - the groups are on opposite sides of the double bond ('trans' means across)
Molecule(a) in Table 3 is trans-1,2-dichloroethene, while molecule(b) in Table 3 is cis-1,2-dichloroethene. A similar situation occurs with alkenes containing 4 or more carbon atoms, e.g.
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Geometric isomerism can also arise in disubstituted cycloalkanes, e.g.
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Optical isomerism. |    |
In the same way as your left hand and your right-hand are mirror images of each other, many chemical compounds can exist in two mirror image forms. Right and left-hands cannot be superimposed on top of each other so that all the fingers coincide and are therefore not identical. A right hand glove does not fit a left hand and is said to be chiral . The hands and the compounds have no centre of symmetry, plane of symmetry or axis of symmetry. Chirality arises from a lack of symmetry. Lack of symmetry is called asymmetry. Look at this picture (Figure 13) and answer the questions.
The lady is holding one hand in front of the mirror and one hand behind. |
Carbon atoms and symmetry.
Optical isomerism occurs in other compounds such as transition metal complexes. In this Topic however, we are concentrating on carbon based compounds.
The four single bonds around a carbon atom are arranged tetrahedrally. If there are four different atoms or groups attached to the carbon, it is asymmetric. It can exist in two isomeric forms. If you have access to molecular models or a kit from which to build models, it is recommended that you build models of the two isomeric forms shown in the following animation. This will help in understanding why they are not identical.
Play the animation (Figure 14) through a number of times and use your models to see why the two isomers are non-superimposable.
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The tetrahedral carbon atom with four different groups is asymmetric. Molecules containing one or more asymmetric carbon atoms are usually (but not always) also asymmetric. Asymmetric atoms or molecules are described as chiral with the carbon atom being called the chiral centre. They differ from each other only in that they are mirror images of each other and exhibit optical activity. Such isomers are called enantiomers or optical isomers .The crystals of enantiomers are mirror images of each other. A mixture containing equal amounts of each enantiomer is known as a racemic mixture .
| Learning Points Optical isomerism can occur in compounds with four different groups arranged around a carbon atom. This leads to chiral molecules that are non-superimposable mirror images of each other. |
Optical isomers are stereoisomers since the atoms are connected in the same order but the arrangement of the atoms in space is different.
Optical isomerism is immensely important in biological systems. In most biological systems only one optical isomer of each organic compound is usually present. For example, when the amino acid alanine is synthesised in the laboratory, a mixture of the two possible isomers (a racemic mixture) is produced (Figure 15). When alanine is isolated from living cells, only one of the two forms is seen.
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Optical isomers of alanineThe proteins in our bodies are built up using only one of the enantiomeric forms of amino acids. Only the isomer of alanine with structure 2 (Figure 16) occurs naturally in organisms such as humans. Since amino acids are the monomers in proteins such as enzymes, enzymes themselves will be chiral. The active site of an enzyme will only be able to operate on one type of optical isomer. An interesting example of this is the action of penicillin, which functions by preventing the formation of peptide links involving the enantiomer of alanine present in the cell walls of bacteria. This enantiomer is not found in humans. Penicillin can therefore attack and kill bacteria but not harm the human host, as this enantiomer is not present in human cells.
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