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Chapter 7: Stereochemistry



Summary | Isomer types | Enantiomers | The Stereogenic center | Optical Activity | Absolute Relative Configuration | Cahn-Ingold-Prelog R/S notation | Fischer projections | Self Assessment | Quiz |


Fischer Projections

Chapter 7 : Stereochemistry

Fischer Projections are abbreviated structural forms that allow one to convey valuable stereochemical information to a chemist without them having to draw a 3D structural representation of a molecule. These representations are only used for molecules that contain stereogenic centers, which are then represented as simple crosses.

A Fischer representation
They can be derived by considering the more accurate 3D representation using wedges and assuming the convention that horizontal lines represent bonds coming out of the plane of the paper and vertical lines represent bonds going behind the plane of the paper.
 
revealing the stereochemistry of a Fischer diagram
 
Memory Aid ? A student once told me that she remembered the relative arrangement of the bonds by the fact that the horizontal bonds were coming out to hug her !

When relating one Fischer projection to another it may only be manipulated within the 2D plane in which it is drawn (that is it may not be rotated within 3D space), and only rotated a total a 180o

 
Enantiomeric Fischer diagrams
A
B
Why can't you rotate it 90o?  A 90o rotation is equivalent to breaking bonds and exchanging two groups, which would result in the formation of the other enantiomer.
Don't rotate Fischer diagrams by 90 degrees
CAUTION Fischer projections are often confused with simpler Lewis diagrams.  Lewis diagrams, however, are not intended to give any stereochemical information!

Fischer projections a can be used to describe molecules with more than one stereogenic centre.

 
Fischer diagrams with 2 chiral centers
If a Fischer projection of this type can be divided into two-halves that are mirror images than the molecule may be identified as a meso isomer.
 
Fischer diagram of a meso compound
Assignment of the configuration at a stereogenic center, in a Fischer projection, is based on the Cahn-Ingold-Prelog rules.
1. Priorities are assigned to the substituents of a stereogenic center.
2. That of lowest priority is placed on a vertical line, this is equivalent to making sure the lowest priority group is positioned away from you as if you were looking along the C-(4) s bond (see figure far right below).
3. The configuration is assigned by moving from the substituent of highest priority to those of second and third priority.
4. If this is clockwise, then the center is R (Latin: rectus = right)
5. If this is counter clockwise, then it is S (Latin: sinister = left)
Consider the molecules A and B above. What are their configurations ?   A is R, B is S
 



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