Meso Structures

Consider the various possible structures of 2,3 dichlorobutane. as shown in figure 22. It is clear that structures (1) and (2) are neither superimposable, nor interconvertable by rotation and are, thus, enantiomers and optically active. Structures (3) and (4), on the other hand, are both superimposable and interconvertable and are, thus, neither enantiomers or optically active.

Figure 22. The Different Structures of 2,3-Dichlorobutane

Although structures (3) and (4) are not chiral and can not exist in an enantiomeric form, they, nevertheless, contain two chiral centers. These types of stereoisomers are called meso structures or meso compounds. Alternatively, meso compounds can be considered to superimposable on their mirror images even though they contain chiral centers.

Figure 23. 2,3 Dichlorobutane.

A meso compound is inactive because the rotation caused by one chiral center is exactly offset by the equal and opposite rotation produced by the other chiral center. It is often possible to recognize a meso structure by the fact that one half of the molecule is the mirror image of the other half. Consider, again, the structure of 2,3-dichlorobutane as shown in figure 23. It is seen that the groups surrounding one chiral carbon atom is the mirror image of the groups surround the other chiral atom.

As already stated, enantiomers have exactly the same properties (excepting their capacity for rotating polarized light) and, thus, can be very difficult to separate. In fact, with the exception of chromatographic and electrophoretic techniques the isolation of pure enantiomers can only be achieved by exploiting spatial differences between the respective isomers.