Stereochemistry

Stereochemistry



Enantiomers:
Same chemical and physical properties in achiral environments.


A mixture of these cannot be separated by normal GC or HPLC techniques
Different chemical and physical properties in chiral environments.


A mixture of these can (theoretically) be separated by chiral GC or HPLC techniques
Diastereomers:
Different physical/chemical properties in chiral/achiral environments.


Naproxen An Over-the -Counter Chiral Auxillary


Brinkman, J. A., Sowa, J. R., Jr. In Catalysis of Organic Reactions, Herkes, F. E., Ed., Marcel Dekker: New York, 1998, p. 543 – 549.
Stereogenic center (stereocenter) – a point in a molecule bearing groups such that an interchange of any two groups will produce a stereoisomer.



Number of possible stereoisomers = 2ⁿ , n = # of stereocenters.
For Crixivan determine # possible stereoisomers?




Relative vs. Absolute Configurations
Relative configuration – 3-D structure is not known but it is known that one structure is the mirror image of the other.
(+) or d – dextrorotatory – cpd that rotates light to the right.
(-) or l – levorotatory – cpd that rotates light to the left.
(�) – racemic mixture – 1:1 mixture of (+) and (-) cpds, zero rotation.
Enantiomeric excess:




Absolute configuration – 3-D structure is known.
R, S Nomenclature


(-)-alanine

1. Prioritize – assign a priority to the groups around the stereocenter

1. increasing atomic mass of the atom attached to stereocenter
2. in case of a tie move to the next atom

1. Place – place the lowest priority group in the back.
2. Connect a ® b® c

Always draw the arrow on paper as you mind can easily reverse itself!
Shortcut: if lowest priority group is unambiguously in the front then reverse assignment.
Examples: