CHIRAL CONFORMERS OF MESO-CH₃CHOH-CHOHCH₃

The "RS-" and "SR-" structures from the previous page are, of course, identical.  This molecule is the meso-stereoisomer of CH₃CHOH-CHOHCH₃.  Below are shown two more structures with this constitution.  Look carefully and you will see that they are (a) different to all the structures on the previous page, and (b) different to each other.  They are in fact mirror images of each other (and, being different, are therefore chiral).  Are they yet more stereoisomers of CH₃CHOH-CHOHCH₃?   No, because, in each case, rotation about the central C-C bond will give you the RS/SR structure as depicted on the previous page, and this rotation happens very fast indeed at room temperature.  They are in fact conformers of the meso-diol.
 
 

This example highlights two important points.  Firstly, most complex molecules have a wide variety of accessible shapes (conformations), and this must be taken into account when deciding how many stereoisomers (different, isolable molecules) are possible for a given constitution.  Secondly, a molecule which does not possess an enantiomer may well have chiral conformers.  However, look for an accessible achiral conformation.  Mirror-image pairs of conformers will be able to interconvert via this structure (see also page 5), so that only one molecule, with no "net chirality", is present..
 

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