4. Assigning R and S

8/9/2019 4. Assigning R and S

1/13

Assigning R,S configurations to Fischer

projections:

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 or

bottom of the Fischer projection.

3) Determine the direction of rotation in going from

priority 1 to 2 to 3, and assign RorSconfiguration.


2/13

Serine


3/13

How can you assign the configuration of a

molecule in R & S system ? Or

What is the sequence rule for R & Sconfiguration?

Let us consider an asymmetric carbon Cabde and

consider the priority of a,b,d,e is 1,2,3,4 gradually, wherea>b>d>e according to priority.

Now by following priority, if eye moves clockwise

direction, then the configuration is R-configuration (Latin

Rectus,-right) and if eye moves anticlockwise direction, thenthe configuration will be S-configuration (Latin Sinistus,-left).


4/13


5/13

The Cahn Ingold Prelog sequence rules

Cahn, Ingold and Prelog introduced this systematic

notation during the period 1951-1956. The sequence rules are-

y Priority is assigned by considering the decreasing order

of the atomic number of the atom by which the group is directly

attached to asymmetric carbon. The atom of highest atomic

number gets the highest priority (1). e.g.

Group Atomic no Priority

Br 35 1Cl 17 2

F 9 3H 1 4

Memorize order of priority: I > Br > Cl > F > O > N > C > H


6/13

y If two atoms on a stereogenic center are the same,

assign priority based on the atomic number of the atoms

bonded to these atoms. One atom of higher priority determines

the higher priority.


7/13

y If two isotopes are bonded to the stereogenic center,

assign priorities in order of decreasing mass number. Thus, in

comparing the three isotopes of hydrogen, the order of priorities

is:

y If any substitution is done on groups, then higher

substituents containing group will get more priority.

-CHCl2> -CH2Cl> -CH3

CH

CH3

CHCl2 1

3

CH2Cl

4

2


8/13

y To assign a priority to an atom that is part of a multiple

bond, treat a multiply bonded atom as an equivalent number of

singly bonded atoms. For example, the C of a C=O is considered

to be bonded to two O atoms.

Other common multiple bonds are drawn below:


9/13


10/13

Examples Assigning Priorities


11/13

R,S System for isomers with more than one chiralitycenter

(2R,3S)-3-bromo-2-butanol

CH3

CH3

OHHBrH

2

3

When a compound has more than one stereogenic

center, the R and S configuration must be assigned to each of

them.

j Enantiomers have exactly opposite R,S designations.

j Diastereoisomers have the same R,S designation for at least one chiral center and

the opposite for at least one of the other chiral center.

j Meso compounds have the same R,S designations at every chiral center.


12/13

Molecules with more than one chirality center have mirror

image stereoisomers that are enantiomers.

In addition they can have stereoisomeric forms that are

not mirror images, called diastereomers.

2R,3S 2S,3R2R,3R 2S,3S

2

3


13/13

4. Assigning R and S

Documents