29.4.3—Effect on plane polarised light of the two

The two enantiomers of one substance rotate plane-polarised light by equal angles in opposite directions under the same conditions. One is dextrorotatory (+), the other laevorotatory (−).

The sign of rotation is measured experimentally; it is not predicted from the R/S descriptor or from the direction in which the structure is drawn on a page.

If one enantiomer rotates light +12°, its mirror image rotates it −12° at the same concentration, path length and temperature.

R/S nomenclature and +/− optical rotation are independent labels. Never replace one with the other.