pro-R, pro-S
A stereoheterotopic group c (as in tetrahedral Xabc2) is described as pro-R if, when it is arbitrarily assigned CIP priority over the other stereoheterotopic group c, the configuration of the thus generated chiral centre is assigned the stereodescriptor R. The other group c is then described as pro-S. This method for distinguishing between stereoheterotopic groups can be applied to other kinds of prochiral molecular entities or prochiral parts of molecular entities considered on their own.
See: prochirality centre
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2214 (