Optical Activity and Enantiomorphism

Enantiomers have identical physical and chemical properties except for (a) they rotate plane polarised light in opposite directions, but in equal amounts (b) they react at different rates with other enantiomers of the same substance. These rates may be almost identical or they may be widely different. Enantiomers react at the same rate with non-enantiomeric compounds. 

Mixtures of equal amounts of enantiomers are optically inactive and are called racemic mixtures or racemates. Their properties are not always the same as those of individual, enantiomers. 

The enantiomeric purity of a non-racemic mixture (i.e. not 50 50) can be expressed as the enantiomeric excess (ee) and is given by the expression  

If the molecule contains two similar asymmetric centres, the [R,S] and [S,R] forms may be 

Molecules satisfying the requirements for optical activity are also known as chiral molecules. Chirality arises in pyramidal and tetrahedral phosphorus compounds when the substituent groups are all different (Chapter 3.2). It may also arise in trigonal bipyramidal and octahedral compounds when at least three different kinds of substituent molecules are present. 

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The relation between optical activity and enantiomorphism is not quite so simple for a crystal. Of the thirty-two crystal classes eleven are enantiomorphic ... 

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