The RS notation for configuration.

This notation, devised by Cahn, Ingold, and Prelog, provides an unambiguous way of specifying configuration at any chiral center. It is especially useful for classes of compounds for which no well-established DL system is available. The groups or atoms surrounding the central carbon atom, or other central atom, are ranked according to a prioritysequence. The priority of a group is determined by a number of sequence rules, the first of which is (1) Higher atomic number precedes lower. In the following illustration, the priorities of the groups in D-alanine are indicated by the letters a > b > c > d. The highest priority (a) is assigned to the NH2 groups which contain nitrogen bonded to the central atom. To establish the configuration, the observer views the molecule down the axis connecting the central atom to the group having the lowest priority, i.e., to group d. Viewed in this way, the sequence of groups a, b, and c can either be that of a right-handed turn (clockwise) as shown in the drawing or that of a left-handed turn (counterclockwise).

The view down the axis and toward the group of lowest priority (d), which lies behind the page. The right-handed turn indicates the configuration R (rectus = right); the opposite configuration is S (sinister = left).

To establish the priority sequence of groups first look at the atoms that are bonded directly to the central atom, arranging them in order of decreasing atomic number. Then if necessary, move outward to the next set of atoms, again comparing atomic numbers. In the case of alanine, groups b and c must be ordered in this way because they both contain carbon directly bonded to the central atom. When double bonds are present at one of the atoms being examined, e.g., the carboxyl group in alanine, imagine that phantom atoms that replicate the real ones are present at the ends of the bonds:


These phantom atoms fill out the valences of the atoms involved in the multiple bonds and are considered to have zero atomic number and zero mass. They are not considered in establishing priorities.
If the first rule and the expansion of multiple bonds are not sufficient to establish the priority, use these additional rules: (2) Higher atomic mass precedes lower. (3) When a double bond is present Z precedes E (see Geometrical isomers). For ring systems a cis arrangement of the highest priority substituents precedes trans. (4) When a pair of chiral centers is present R,R or S,S precedes R,S or S,R. (5) An R chiral center precedes S. For further details see Eliel et al. and Bentley. The following groups are ordered in terms of decreasing priority6: SH > OR > OH > NH–COCH3 > NH2 > COOR > COOH > CHO > CH2OH > C6H5 > CH3 > 3H > 2H > H.
Although the RS system is unambiguous, closely related compounds that belong to the same configurational family in the DL system may have opposite configurations in the RS system. Thus, L-cysteine (side chain–CH2SH) has the R configuration. This is one of the reasons that the DL system is still used for amino acids and sugars.