Figure 2.12. PolarizerBRIDGE
While rotation of plane-polarized light can be tested in organic chemistry questions, the
polarization of light itself is fair game as a physics question. Be sure to review light
polarization, discussed in Chapter 8 of MCAT Physics and Math Review.Optical activity refers to the rotation of this plane-polarized light by a chiral molecule. At the
molecular level, one enantiomer will rotate plane-polarized light to the same magnitude but in the
opposite direction of its mirror image (assuming concentration and path lengths are equal). A
compound that rotates the plane of polarized light to the right, or clockwise, is dextrorotatory (d-)
and is labeled (+). A compound that rotates light toward the left, or counterclockwise, is
levorotatory (l-) and is labeled (–). The direction of rotation cannot be determined from the
structure of a molecule and must be determined experimentally. That is, it is not related to the
absolute configuration of the molecule.
KEY CONCEPT
The system for labeling optical activity always uses d- or (+) to refer to clockwise rotation of
plane-polarized light, while l- and (–) always go together and refer to counterclockwise
rotation of plane-polarized light. Do not confuse this with D- or L- labels on carbohydrates or
amino acids, which are based on the absolute configuration of glyceraldehyde. (R) and (S)
also refer to absolute configuration, which is determined by structure. Optical activity does
not consistently align with the other systems.