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Naming Enantiomers: The R,S System of Nomenclature

We need a way to name the individual stereoisomers of a compound such as 2-bromobutane so that we know which stereoisomer we are talking about. In other words, we need a system of nomenclature that indicates the configuration (arrangement) of the atoms or groups about the asymmetric carbon. Chemists use the letters R and S to indicate the configuration about an asymmetric carbon. For any pair of enantiomers with one asymmetric carbon, one will have the R configuration and the other will have the S configuration. The R,S system was devised by Cahn, Ingold, and Prelog.
Let’s first look at how we can determine the configuration of a compound if we have a three-dimensional model of the compound.
three-dimensional model of the compound Vladimir Prelog

1. Rank the groups (or atoms) bonded to the asymmetric carbon in order of priority. The atomic numbers of the atoms directly attached to the asymmetric carbon determine the relative priorities. The higher the atomic number, the higher the priority. (This should remind you of the way relative priorities are determined for the E, Z system of nomenclature because the system of priorities was originally devised for the R, S system of nomenclature and was later borrowed for the E,Z system. You may want to revisit Section 3.5 to review how relative priorities are determined before you proceed with the R,S system.
Rank the groups bonded to the asymmetric carbon in order of priority
2. Orient the molecule so that the group (or atom) with the lowest priority (4) is directed away from you. Then draw an imaginary arrow from the group (or atom) with the highest priority (1) to the group (or atom) with the next highest priority (2). If the arrow points clockwise, the asymmetric carbon has the R configuration (R is for rectus, which is Latin for “right”). If the arrow points counter clockwise, the asymmetric carbon has the S configuration (S is for sinister, which is Latin for “left”).
clockwise means R configuration
If you forget which is which, imagine driving a car and turning the steering wheel clockwise to make a right turn or counterclockwise to make a left turn.
If you are able to easily visualize spatial relationships, the above two rules are all you need to determine whether the asymmetric carbon of a molecule written on a twodimensional piece of paper has the R or the S configuration. Just mentally rotate the molecule so that the group (or atom) with the lowest priority (4) is directed away from you, then draw an imaginary arrow from the group (or atom) with the highest priority to the group (or atom) with the next highest priority.
If you have trouble visualizing spatial relationships and you don’t have access to a model, the following will allow you to determine the configuration about an asymmetric carbon without having to mentally rotate the molecule.
First, let’s look at how you can determine the configuration of a compound drawn as a perspective formula. As an example, we will determine which of the enantiomers of 2-bromobutane has the R configuration and which has the S configuration

enantiomers of 2-bromobutane
1. Rank the groups (or atoms) that are bonded to the asymmetric carbon in order of priority. In the following pair of enantiomers, bromine has the highest priority (1), the ethyl group has the second highest priority (2), the methyl group is next (3), and hydrogen has the lowest priority (4). (Revisit Section 3.5 if you don’t understand how these priorities are assigned.)

2. If the group (or atom) with the lowest priority is bonded by a hatched wedge, draw an arrow from the group (or atom) with the highest priority (1) to the group (or atom) with the second highest priority (2). If the arrow points clockwise, the compound has the R configuration, and if it points counterclockwise, the compound has the S configuration.

3. If the group with the lowest priority (4) is NOT bonded by a hatched wedge, then switch two groups so group 4 is bonded by a hatched wedge. Then proceed as in step #2 (above): Draw an arrow from the group (or atom) with the highest priority (1) to the group (or atom) with the second highest priority (2). Because you have switched two groups, you are now determining the configuration of the enantiomer of the original molecule. So if the arrow points clockwise, the enantiomer (with the switched groups) has the R configuration, which means the original molecule has the S configuration. In contrast, if the arrow points counterclockwise, the enantiomer (with the switched groups) has the S configuration, which means the original molecule has the R configuration.

4. In drawing the arrow from group 1 to group 2, you can draw past the group (or atom) with the lowest priority (4), but never draw past the group (or atom) with the next lowest priority (3).

Now let’s see how to determine the configuration of a compound drawn as a Fischer projection.
1. Rank the groups (or atoms) that are bonded to the asymmetric carbon in order of priority.
2. Draw an arrow from the group (or atom) with the highest priority (1) to the group (or atom) with the next highest priority (2). If the arrow points clockwise, the enantiomer has the R configuration; if it points counterclockwise, the enantiomer has the S configuration,provided that the group with the lowest priority (4) is on a vertical bond.

3. If the group (or atom) with the lowest priority is on a horizontal bond, the answer you get from the direction of the arrow will be the opposite of the correct answer. For example, if the arrow points clockwise, suggesting that the asymmetric carbon has the R configuration, it actually has the S configuration; if the arrow points counterclockwise, suggesting that the asymmetric carbon has the S configuration, it actually has the R configuration. In the following example, the group with the lowest priority is on a horizontal bond, so clockwise signifies the S configuration, not the R configuration.

4. In drawing the arrow from group 1 to group 2, you can draw past the group (or atom) with the lowest priority (4), but never draw past the group (or atom) with the next lowest priority (3).

It is easy to tell whether two molecules are enantiomers (nonsuperimposable) or identical molecules (superimposable) if you have molecular models of the molecules— just see whether the models superimpose. If, however, you are working with structures on a two-dimensional piece of paper, the easiest way to determine whether two molecules are enantiomers or identical molecules is by determining their configurations. If one has the R configuration and the other has the S configuration, they are enantiomers. If they both have the R configuration or both have the S configuration, they are identical molecules.
When comparing two Fischer projections to see if they are the same or different, never rotate one 90° or turn one over, because this is a quick way to get a wrong answer. A Fischer projection can be rotated 180° in the plane of the paper, but this is the only way to move it without risking an incorrect answer.

R or S configuration
R or S configuration
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