Organic Chemistry II

Fischer Projections
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 chirality centers, which are then represented as simple crosses.

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.









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 180^o



Why can't you rotate it 90^o?  A 90^o rotation is equivalent to breaking bonds and exchanging two groups, which would result in the formation of the other enantiomer.

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 chirality center.
 


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.   



Assignment of the configuration at a chirality center, in a Fischer projection, is based on the same Cahn-Ingold-Prelog rules.
The safest method for assigning the configuration is probably to convert it to a wedge-hash diagram (as shown above)
Alternatively....

• Identify the chirality centers (most commonly an sp³ C with 4 different groups attached).
• If the group of lowest priority is placed on a vertical line, this means the lowest priority group is already positioned away from you as if you were looking along the C-(4) s bond
• Now assess the direction of high to low priority (1 to 3)
• If this is clockwise, then the center is R (Latin: rectus = right)
• If this is counter clockwise, then it is S (Latin: sinister = left)
• If the group of lowest priority is placed on a horizontal line, this means the lowest priority group is actually positioned towards you (so we have to be very careful)
• Now assess the direction of high to low priority (1 to 3)
• If this is clockwise, then the center is R (Latin: rectus = right)
• If this is counter clockwise, then it is S (Latin: sinister = left)
• BUT NOW SWITCH THE ASSIGNMENT (it's like looking at a glass clock face from opposite sides