Organic and Inorganic Chemistry Lesson of the Day – The Difference Between (+)/(-) and (R)/(S) in Stereochemical Notation
September 11, 2014 Leave a comment
In a previous Chemistry Lesson of the Day, I introduced the concept of optical rotation (a.k.a. optical activity). You may also be familiar with the Cahn-Ingold-Prelog priority rules for designating stereogenic centres as either (R) or (S). There is no direct association between the (+)/(-) designation and the (R)/(S) designation. In other words, an (R)-enantiomer can be dextrorotary or levorotary – it must be determined on a case-by-case basis. The same holds true for an (S)-enantiomer.
(R)/(S) can be used to distinguish between enantiomers in one exception: If the stereoisomer has only 1 stereogenic centre, then this designation can also serve as a way to distinguish between 2 enantiomers.
Furthermore, note that the designation of optical rotation applies to a molecule, whereas the R/S designation applies to a particular stereogenic centre within a molecule. Thus, a molecule with 2 stereogenic centres may have one (R) stereogenic centre and one (S) stereogenic centre. However, a chiral compound consisting purely of one enantiomer can rotate linearly polarized light in only one direction, and that direction must be determined on a case-by-case basis by a polarimeter.