Priority ranking is the critical control point because the same planar structure can appear different when viewed from opposite sides. After the three substituents receive an ordered ranking, the observer follows their 1–2–3 sequence from the selected face. This procedure separates the two sides consistently rather than relying on the molecule’s orientation in a drawing.
Re or Si analysis identifies which side of a planar functional group reacts, but the product’s R or S assignment must be determined after the new stereocenter forms. The resulting configuration depends on the attacked face and the priority order of substituents in the product. Thus, face selection provides a route to predicting stereochemical outcome, not a universal one-to-one label conversion.
Chiral catalysts and enzymes can distinguish the two faces by creating an asymmetric reaction environment around the planar substrate. Preferential interaction with one face makes one reaction pathway more favorable than the other, leading to selective formation of a stereochemical outcome. Re/Si analysis provides a framework for describing which face is favored and for interpreting the resulting selectivity.
First, identify the trigonal-planar or other planar prochiral center and the three attached substituents. Rank those substituents by priority, then view the center from one face and trace the 1–2–3 order. A clockwise sequence identifies one designation and a counterclockwise sequence the other; examining the opposite side provides the complementary face assignment.
A chemist can analyze both faces of a planar carbonyl or related functional group before considering a reaction pathway. Determining which face an incoming reagent or catalyst approaches helps connect the proposed mechanism with the stereochemistry of the new center. This supports reaction planning when selective formation of a defined R or S product is important.
Re and Si designations describe the distinguishable faces of a planar prochiral starting structure, whereas R and S describe the configuration of a stereocenter after reaction has created it. The two systems are therefore related but not interchangeable. Face-selective attack can produce a defined R or S center, yet the final assignment requires evaluating the product’s own substituent priorities.