Lecture 2 Flashcards
Assigning (R) and (S) Enantiomers
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.
Cahn-Ingold-Prelog Rules for Assigning (R) and (S)
- 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
Cahn-Ingold-Prelog Rules
The higher the atomic number, the higher the priority
Cahn-Ingold-Prelog Rules
- If there are 2 or more identical atoms, then priorities of the substituents are established by the sets of atoms one further bond away
Cahn-Ingold-Prelog Rules
- For multiple bonds, the order of priority is:
Cahn-Ingold-Prelog rules
e.g. Double (C=O) bond takes priority over single (C-O) and then the
third atom (O versus H) distinguishes the remaining two
Assigning a Configuration to a 3-D Chiral Molecule
- Rotate molecule so that lowest priority group is at the back
- Draw an arrow from highest to lowest priority group
Cahn-Ingold-Prelog Rules
How to orientate the enantiomer and assignment
Assigning the Absolute Configuration (R or S)
Assigning the Absolute Configuration (R or S)
Assigning the Absolute Configuration (R or S)
Assigning the Absolute Configuration (R or S)
manipulation of stereocentre for assignment of configuration - carousel method
The carousel method for manipulating a stereocentre
Fischer Projections: Representation of a Three-dimensional Molecule as a Flat Structure
Tetrahedral chiral carbon represented by two crossed lines
3D structures and Fischer projections
Manipulation of Fischer Projections
- 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)
Manipulation of Fischer Projections
- 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.
Manipulation of Fischer Projections
- 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.
Assigning R and S Configuration to Fischer Projections
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.
