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Cis ,trans isomerism. Optical isomerism

In organic chemistry, cis/trans isomerism (also known as geometric isomerism) is a form of stereoisomerism describing the relative orientation of functional groups within a molecule. It is not to be confused with E/Z isomerism, which is the related absolute stereochemical description, only to be used with alkenes.

A second type of stereoisomerism is known as optical isomerism. Optical isomers are mirror images that cannot be superimposed on each other. Such isomers are called enantiomers. They bear the same resemblance to each other that our left hand bears to our right hand. If you look at your left hand in a mirror, the image is identical to your right hand. Furthermore, your two hands are not superimposable on one another. A good example of a complex that exhibits this type of isomerism is the [Co(en)3]3+ ion. Figure 24.20 shows the two enantiomers of [Co(en)3]3+ and their mirror-image relationship to each other. Just as there is no way that we can twist or turn our right hand to make it look identical to our left, so also there is no way to rotate one of these enantiomers to make it identical to the other.


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