Organic Chemistry

190 CHAPTER 5 Stereochemistry

4.In drawing the arrow from group 1 to group 2, you can draw past the group with the

lowest priority (4), but never draw past the group 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 enan-

tiomer has the Rconfiguration; 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 horizontalbond, 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

Rconfiguration, it actually has the Sconfiguration; if the arrow points counterclock-

wise, suggesting that the asymmetric carbon has the Sconfiguration, it actually has the

Rconfiguration. In the following example, the group with the lowest priority is on a

horizontal bond, so clockwise signifies the Sconfiguration, not the Rconfiguration.

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).

H CH 3

COH

OH

O

3

1

(S)-lactic acid

CH 3 H

COH

OH

O

4 3 4

1

2

(R)-lactic acid

2

H OH

CH 3

CH 2 CH 3

33

2

1

(S)-2-butanol

HO H

CH 3

CH 2 CH 3

2

4 4

1

(R)-2-butanol

CH 3 CH 2 CH 2 CH 2 CH 3

Cl

H

1

2

4

(R)-3-chlorohexane

CH 3 CH 2 CH 2 CH 2 CH 3

Cl

H

1

2 3

4

(S)-3-chlorohexane

3

(R)-1-bromo-3-pentanol

C

CH 3 CH 2

OH

H

CH 2 CH 2 Br

1

(^32)
4 C
CH 3 CH 2
OH
H
CH 2 CH 2 Br
1
(^32)
4
22
4 1
3
3
4
1
CH 3 and H
what is its configuration?
switch
this molecule has the R configuration;
therefore, it had the S configuration
before the groups were switched
HO
CH 2 CH 3
CH 3
H
C C
HO
CH 2 CH 3
H
CH 3
Clockwise specifies Rif the lowest prior-
ity substituent is on a vertical bond.
Clockwise specifies Sif the lowest priori-
ty substituent is on a horizontal bond.