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.

Figure 13

The lady is holding one hand in front of the mirror and one hand behind.

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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.

Figure 14

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.