Question

Why is it not possible to determine in which direction a compound rotates light based upon the R or S prefix?

Answer 1

Knowing whether a chiral molecule has the R configuration or the S configuration does not tell us the direction in which the compound rotates the plane of polarization because some compound with the R configuration rotate the plane to the right (+) and some rotate the plane to the left(-). We can tell by looking at the structure of a compound whether it has the R configuration of the S configuration, but the only way we can tell whether a compound is dextrorotatory (+) or levorotatory (-) is to put the compound in a polarimeter.