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Chapter 7 : Stereochemistry |
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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.
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
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.
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....
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Identify the chirality centers (most commonly an sp3 C with
4 different groups attached).
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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
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Now assess the direction of high to low priority (1 to 3)
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If this is clockwise, then the center is R (Latin: rectus = right)
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If this is counter clockwise, then it is S (Latin: sinister =
left)
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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)
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If this is clockwise, then the center is R (Latin: rectus = right)
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If this is counter clockwise, then it is S (Latin: sinister =
left)
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BUT NOW SWITCH THE ASSIGNMENT (it's like looking at a glass clock
face from opposite sides)
Consider the molecules A and B above. What
are their configurations ? 
