Organic Chemistry

Section 5.6 Naming Enantiomers: The R,SSystem of Nomenclature 189

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 two-
dimensional piece of paper has the Ror the Sconfiguration. 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 asymmet-
ric 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 Rconfiguration and which has the Sconfiguration.

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 hy-
drogen 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 com-
pound has the Rconfiguration, and if it points counterclockwise, the compound has
the Sconfiguration.

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 Rconfiguration, which means the original molecule has the
Sconfiguration. In contrast, if the arrow points counterclockwise, the enantiomer
(with the switched groups) has the Sconfiguration, which means the original molecule
has the Rconfiguration.

(S)-2-bromobutane (R)-2-bromobutane

C

CH 3 CH 2

Br

H

CH 3

H C

Br

CH 2 CH 3

CH 3

11

2 2

3 3

(^44)
11
2 2
(^33)
C^44
CH 3 CH 2
Br
H
CH 3
H C
Br
CH 2 CH 3
CH 3
the enantiomers of 2-bromobutane
C
CH 3 CH 2
Br
H
CH 3
C
H
Br
CH 2 CH 3
CH 3
Clockwise specifies Rif the lowest prior-
ity substituent is on a hatched wedge.
left turn
right turn
3-D Molecules:
(R)-2-Bromobutane;
(S)-2-Bromobutane