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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
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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.
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.
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pt>
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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)
Question
: Consider the molecules A and B above.
What are their configurations ? ANSWER?
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© Dr. Ian Hunt,
Department of Chemistry, University of Calgary |
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