CH 2 CH 3

H CH 3

OH

C

CH 2 CH 3

H 3 C H

OH

Hold CH 3

Rotate other

three groups

counterclockwise

CH 2 CH 3

H 3 C OH

H

Not A

Rotate

180 o

5.12C ASSIGNING R,S CONFIGURATIONS TO FISCHER PROJECTIONS:

1. Procedures for assigning R,S designations:
   
   
   1. Assign priorities to the four substituents.
   
   
   2. Perform one of the two allowed motions to place the group of lowest (fourth)
      priority at the top of the Fischer projection.
   
   
   3. Determine the direction of rotation in going from priority 1 to 2 to 3, and assign
      R or S configuration.
      COOH
      H 2 N
      CH 3
   
   
   
   

H

Serine

CH 3

H 2 N H

COOH

Hold −CH 3 steady

H

HOOC NH 2

CH 3

1 2 2 1

(^33)
4
4

Rotate counterclockwise
H
HOOC NH 2
CH 3
2 1
3
(^4) H
HOOC NH 2
CH 3
2 1
3
4

S stereochemistry
CHO
HO H
HOH
CH 2 OH
C
CHO
HO H
H C OH
CH 2 OH
1
2
3
(^4)
Threose [(2S,3R)-2,3,4-Trihydroxybutanal]