Symmetry conditions for molecular chirality

A chiral molecule is one which exists in two forms, known as enantiomers. Each of the enantiomers is optically active, which means that they can rotate the plane of plane-polarized light. The enantiomer that rotates the plane to the right (clockwise) has been called the d (or dextro) form and the one that rotates it to the left (anticlockwise) the I (or laevo) form. Nowadays, it is more usual to refer to the d and I forms as the ( + ) and (—) forms, respectively. 

Very often, a sample of a chiral molecule exists as an equimolar mixture of (+) and (—) enantiomers. Such a mixture will not rotate the plane of plane-polarized light and is called a 

There are interesting examples of enantiomers that not only are found separately but also have different chemical properties when reacting with some reagent which is itself an enantiomer. For example (+ )-glucose is metabolized by animals and can be fermented by yeasts, but (—)-glucose has neither of these properties. The enantiomer ( + )-carvone smells of caraway whereas (—)-carvone smells of spearmint. 

A useful way of describing the difference between (+) and (—) enantiomers is that one is the mirror image of the other. In other words, neither enantiomer is superimposable on its mirror image. 

In the early days following the discovery of chirality it was thought that only molecules of the type CWXYZ, multiply substituted methanes, were important in this respect and it was said that a molecule with an asymmetric carbon atom forms enantiomers. Nowadays, this definition is totally inadequate, for two reasons. The first is that the existence of enantiomers is not confined to molecules with a central carbon atom (it is not even confined to organic molecules), and the second is that, knowing what we do about the various possible elements of symmetry, the phrase asymmetric carbon atom has no real meaning. 

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