Stereoisomers of tartaric acid
Consider the following set of stereoisomers of tartaric acid (Please note the axial bonds are deliberately, and unnaturally, elongated to make the diagrams clearer) :
In each case, there are two stereocentres. 1 and 2 are clearly enantiomers, however, what may not be immediately apparent is that 3 and 4 are in fact the same molecule. Molecules that have stereocentres, yet are themselves achiral (like 3/4) are known as mesocompounds.
Conformation describes the relative location of atoms in a molecule. This is best illustrated by example: The following molecule is shown in two different conformations (the one on the left is of low energy, and the one on the right is of high energy), they are known as staggered (left), and eclipsed (right). A conformational change can be (and is) achieved without breaking any bonds.
Of course, the two conformations shown above are merely convenient to represent; there are an infinite number of possible conformations of the molecule above, and indeed of any molecule with a small rotational energy barrier round a single bond.
Configuration describes the exact three dimensional location of each atom in space, and their connectivities.
To change a configuration, bonds must be broken.
There are special rules to describe the absolute configuration of a molecule. These are known as Cahn-Ingold-Prelog rules, and will be dealt with on the next page. Note that in the meso form of tartaric acid, the configurations of the two stereocentres are opposite. The molecule is descibed as being internally compensated, and thus has no optical activity.
Epimerisation is the process, during which, a molecule with more than one stereocentre undergoes a change in configuration at one of its stereocentres. 1 and 2 above are both epimers of 3/4.