Dextrorotation and levorotation
Dextrorotation and
levorotation refer to the properties of rotating plane polarized light. If the light rotates clockwise as it approaches an observer, this is known as dextrorotation, light with a rotation to the right. If the light rotates counterclockwise as it approaches the observer, then the light exhibits levorotation, rotation to the left. A compound with dextrorotation is called
dextrorotatory or
dextrorotary, while a compound with levorotation is called
levorotatory or
levorotary. Compounds with these properties are said to have optical activity and consist of chiral molecules. If a chiral molecule is dextrorotary, its enantiomer will be levorotary, and vice-versa. In fact, the enantiomers will rotate plane polarized light the same number of degrees, but in opposite directions. Apart from direct measurement of the optical rotation of an actual sample, it is only possible to determine whether a given chiral molecule will be levorotatory or dextrorotatory, directly from its absolute configuration, via detailed computer modeling. That is to say, both "R" and "S" stereocenters have the ability to be dextrorotatory or levorotatory.