Lecture 2

Question 1

Assigning (R) and (S) Enantiomers

Answer 1

i.e. determining the configuration of an enantiomer

The absolute configuration defines the exact spatial
arrangement of the atoms in a chiral molecule

Rules:Cahn-Ingold-Prelog rules are used to assign configurations

Note: The R/S system is independent of the - and + assignment of optical rotation. - and + refer to the direction of optical rotation for a particular chiral molecule.

Question 2

Cahn-Ingold-Prelog Rules for Assigning (R) and (S)

Answer 2

1. Locate the chiral carbon (indicated by *)

2.Assign a highest to lowest priority 1 (highest), 2, 3, or 4 (lowest) to the four atoms bonded to the chiral carbon according to their atomic numbers

Question 3

Cahn-Ingold-Prelog Rules

Answer 3

The higher the atomic number, the higher the priority

Question 4

Cahn-Ingold-Prelog Rules

Answer 4

3. If there are 2 or more identical atoms, then priorities of the substituents are established by the sets of atoms one further bond away

Question 5

Cahn-Ingold-Prelog Rules

Answer 5

4. For multiple bonds, the order of priority is:

Question 6

Cahn-Ingold-Prelog rules

Answer 6

e.g. Double (C=O) bond takes priority over single (C-O) and then the
third atom (O versus H) distinguishes the remaining two

Question 7

Assigning a Configuration to a 3-D Chiral Molecule

Answer 7

5. Rotate molecule so that lowest priority group is at the back
6. Draw an arrow from highest to lowest priority group

Question 8

Cahn-Ingold-Prelog Rules
How to orientate the enantiomer and assignment

Answer 8

Question 9

Assigning the Absolute Configuration (R or S)

Answer 9

Question 10

Assigning the Absolute Configuration (R or S)

Answer 10

Question 11

Assigning the Absolute Configuration (R or S)

Answer 11

Question 12

Assigning the Absolute Configuration (R or S)

Answer 12

Question 13

manipulation of stereocentre for assignment of configuration - carousel method

Answer 13

Question 14

The carousel method for manipulating a stereocentre

Answer 14

Question 15

Fischer Projections: Representation of a Three-dimensional Molecule as a Flat Structure

Answer 15

Tetrahedral chiral carbon represented by two crossed lines

Question 16

3D structures and Fischer projections

Answer 16

Question 17

Manipulation of Fischer Projections

Answer 17

1. Fischer projections can be rotated by 180°without changing the configuration
   -to see that this is (R) it is best to put the group of lowest priority on a vertical line in Fischer projection (i.e. pointing away from viewer)

Question 18

Manipulation of Fischer Projections

Answer 18

2. Fischer projections when rotate by 90°will change the configuration of the structure. You
   -should not complete this type of manipulation unless you want to reverse the configuration.

Question 19

Manipulation of Fischer Projections

Answer 19

3. If one group of a Fischer projection is held steady, the other three groups can be rotated clockwise or counterclockwise.

-If you are trying to get the group of lowest priority to the vertical line don’t hold it in place. Make sure it can be rotated to one of the two vertical positions.

Question 20

Assigning R and S Configuration to Fischer Projections

Answer 20

1.Assign priorities to the four substituents according to the Cahn-Ingold-Prelog rules.
2.If necessary manipulate the structure to get the group of lowest priority on in a vertical position.
If the priority of the groups 1-2-3 are clockwise then assign the center as R, 3.if 1-2-3 are counterclockwise then assign the center as S.