The Cahn-Ingold-Prelog R/S Configuration System

The Cahn–Ingold–Prelog (CIP) system assigns the absolute configuration of a stereocenter as R (rectus, clockwise) or S (sinister, counterclockwise) by ranking the four attached groups by priority and viewing the molecule with the lowest-priority group pointing away from the viewer. In brief: in (S)-lactic acid the priorities are OH > COOH > CH₃ > H, and with the hydrogen at the back, the sequence OH → COOH → CH₃ runs counterclockwise.

Names such as “(R)-2-butanol” or “(2S,3R)-threonine” show the precise spatial arrangement at each stereocenter. The R/S system gives a name for every enantiomer, unlike the older D/L system, which is limited to sugars and amino acids. This article explains the priority rules, the procedure for assigning R and S, shows a table of group priorities, gives worked examples including a Fischer projection, and clarifies what R and S do not tell.

What are the priority rules?

1. Atomic number: compare the atoms directly attached to the stereocenter. The higher the atomic number, the higher the priority (I > Br > Cl > S > F > O > N > C > H). For isotopes, the higher mass number has priority (D > H).
2. Tie at the first atom: compare the sets of atoms attached to those atoms, working outward, one sphere at a time. Compare the atoms in order of decreasing atomic number: the first point of difference decides. For –CH₂OH (O, H, H) against –CH₂CH₃ (C, H, H): O > C, so –CH₂OH has the higher priority.
3. Multiple bonds: a double or triple bond is treated as if each bond led to a separate atom (duplicate atoms are added): –CH=CH₂ counts as (C, C, H) on the first carbon, and –CHO as (O, O, H), and –C≡N as (N, N, N).
4. Continue outward from the first point of difference, always comparing the highest-ranked branches first.

Priorities of common groups

In the table, the halogens, –OH and –NH₂ (the heteroatoms) precede the carbon groups when they are attached directly to the stereocenter. Among the halogens and heteroatoms, the order is by atomic number (I, Br, Cl, S, F, O, N). Note that a group such as –CH₂Cl, in which the first atom is carbon, ranks as a carbon group with the set (Cl, H, H), and thus above –COOH (O, O, O), because Cl is compared first with O.

How do you assign R or S?

1. Identify the stereocenter and its four groups.
2. Assign priorities 1 to 4 (1 = highest) with the rules above.
3. Orient the molecule so that the group of lowest priority (4, usually H) points away from you.
4. Look at the three remaining groups, and trace the path from 1 to 2 to 3.
5. If the path is clockwise, the configuration is R. If it is counterclockwise, it is S.
6. If the lowest-priority group points toward you, determine the direction and reverse the result.

For molecules with several stereocenters, each is assigned independently, and the configurations are given in the name with locants: (2R,3S)-2,3-dibromobutane. The descriptor is part of the name of the compound.

Worked examples

Example 1: bromochlorofluoromethane

CHFClBr. Priorities: Br (1) > Cl (2) > F (3) > H (4). With the H pointing away, if Br → Cl → F is clockwise, the compound is (R)-bromochlorofluoromethane; if it is counterclockwise, it is (S).

Example 2: lactic acid

The stereocenter in lactic acid, CH₃CH(OH)COOH, has the groups OH (O), COOH (O,O,O), CH₃ (H,H,H) and H. Priorities: OH (1) > COOH (2) > CH₃ (3) > H (4). Muscle produces (S)-(+)-lactic acid, also called L-lactic acid. For the (S) form, with H at the back, the sequence OH → COOH → CH₃ runs counterclockwise. The (R)-enantiomer is (−)-lactic acid. This shows that the sign of the rotation is not connected with R or S.

Example 3: alanine

L-Alanine has the groups NH₂ (1), COOH (2), CH₃ (3) and H (4), so the priorities are N > COOH > CH₃ > H. Natural L-alanine is (S). Almost all natural L-amino acids are (S), the exception being cysteine, which is (R), because the –CH₂SH group (S, H, H) has a higher priority than –COOH (O, O, O), since S has a higher atomic number than O.

Example 4: butan-2-ol

The groups on C2 are OH (1), CH₂CH₃ (2), CH₃ (3) and H (4). The ethyl group (C, H, H) has higher priority than methyl (H, H, H). (R)-Butan-2-ol is the enantiomer in which OH → C₂H₅ → CH₃ runs clockwise with H at the back.

Example 5: ranking a tricky case

For the stereocenter of 1-phenylethanol, the groups are OH, C₆H₅ (C, C, C), CH₃ (H, H, H) and H. Priorities: OH > phenyl > CH₃ > H.

How do you assign from a Fischer projection?

In a Fischer projection, the horizontal bonds point toward the viewer and the vertical bonds point away. If the lowest-priority group (H) is on a horizontal bond (toward the viewer), trace 1 → 2 → 3 and then reverse the result. If H is on a vertical bond, the direction read from the projection is the correct one.

Example: D-glyceraldehyde. The projection has CHO at the top, CH₂OH at the bottom, OH on the right and H on the left. Priorities: OH (1) > CHO (2) > CH₂OH (3) > H (4). Going from OH (right) to CHO (top) to CH₂OH (bottom) is counterclockwise, which would be S. But H is on a horizontal bond, so the result is reversed: the configuration is R. D-Glyceraldehyde is (R)-glyceraldehyde.

What do R and S not tell you?

• Direction of optical rotation: (R) and (S) are not related to (+) and (−). (S)-lactic acid is (+), and (S)-alanine is (+) in water, but (S)-glyceraldehyde is (−).
• D/L designation: D and L, based on the relationship to glyceraldehyde, do not directly correspond to R and S: D-glyceraldehyde is R, but L-alanine is S and L-cysteine is R.
• Similarity of two molecules: a change of the substituents can change the descriptor without changing the spatial arrangement, because the priorities change.

Who developed the system?

Robert Cahn, Christopher Ingold and Vladimir Prelog developed the sequence rules in the 1950s, and published them in 1956, with a final version in 1966. Before this, the absolute configuration of molecules was only known relative to a standard: Emil Fischer assigned in 1891 the D configuration to (+)-glyceraldehyde, arbitrarily, with a 50 % chance of being right. In 1951 Johannes Bijvoet showed by X-ray diffraction with anomalous scattering, using sodium rubidium tartrate, that Fischer’s guess was correct. Prelog shared the Nobel Prize in Chemistry in 1975 for his work on the stereochemistry of organic molecules and reactions.

Which mistakes are common?

• Looking at the first atom only: when it is a tie, compare outward.
• Forgetting to put the lowest priority group in the back: otherwise reverse the result.
• Treating a double bond as a single atom: use duplicate atoms.
• Assuming that R means (+) or D: they are unrelated systems.
• Ranking by the size of the group: only atomic numbers count.