Organic Chemistry: Stereoisomers


Optical Activity

Summary Optical Activity

Optical Rotation of Enantiomers

A chiral compound has a specific rotation that is equal in magnitude but opposite in direction from its enantiomer. This type of symmetry is expected given that enantiomers are mirror images. For instance, one enantiomer of 2- butanol has a specific rotation of +13.5 degrees while the other enantiomer of 2-butanol has a specific rotation of -13.5 degrees. The pair can be designated as (+)-2-butanol and (-)-2-butanol or (d)-2-butanol and (l)-2-butanol, where (d) and (l) correspond to dextrorotary and levorotary, accordingly. Note, however, that designations of optical activity do not correlate with absolute (R)/(S) configurations. A (+) compound might have an (R) or (S) configuration. It is impossible to tell the absolute configuration of a substance from its specific rotation. This holds for the reverse process as well: it is difficult to predict the specific rotation of a compound given its absolute configuration.