Definitions in Stereochemistry
Having discussed some of the basic concepts of stereochemistry, it is necessary to understand some of the language used to describe stereochemistry.
We have already met the term enantiomer. It is defined as: “A species with a non-superimposable mirror image.” An example of a pair of enantiomers is lactic acid:
Optical isomerism is used to describe the phenomenon of molecules that rotate plane-polarized light.
A racemic mixture, or racemate is a mixture of exactly equal quantities of both enantiomers, so that it does not rotate plane-polarized light in either direction (as the effects of both enantiomers cancel out.
Compounds that exist in two enantiomeric forms are said to be chiral (from the Greek for “hand”). Chiral molecules do not have a plane of symmetry (and equally, molecules without a plane of symmetry are chiral). The examples above used a carbon with four different groups (an asymmetric carbon, or stereocentre) to achieve the condition of not having a mirror-image. However, that is by no means the only way; consider the following molecule, an allene:
It does not have an asymmetric carbon, and yet is chiral.
A compound with more than one stereocentre is more complicated to deal with in terms of stereochmistry. For example:
The above two molecules are enantiomers, however, they are not the only stereoisomers of this molecule, there is another, distinct species:
This molecule, however, is not an enantiomer of the above two, as it does have a plane of symmetry. It is known as a diastereomer (or diastereoisomer).