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Fischer Projections

Article ID: 142
Last updated: 25 Jan, 2013
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Check out this interactive stereochemistry tutorial on the ChemEdDL

Fischer Projections are abbreviated 2D 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 (chiral centers), each of which are represented as simple crosses.  Typically the longest carbon chain is drawn up and down in a line.  Non-carbon substituents are usually drawn on the horizontal.


They can be explained by considering the more accurate and descriptive 3D representation using wedges and dashes 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 said that she remembered the relative arrangement of the bonds by the fact that the horizontal bonds were coming out to hug her! You can also visualize the horizontal bonds coming out like a bow tie.

Many times in considering stereochemistry, you are asked to compare two compounds to find their relationship (same compound, enantiomers, diastereomers, etc.).  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 can only rotated in increments of 180 degrees.  Notice molecules A and B have one pair of groups (H and OH) switched on the horizontal bonds, therefore making them enantioners.  You can see that A and B are non-superimposable mirror images below.  Neither B or its rotated form (both the same compound) can be superimposed on A.






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.  It may seem limiting, but this is how Fischer projections are able to represent 3D with 2D drawings. Let's consider another possible switch below.  Notice switching CHO and CH3 on A still leads to the enantiomer B.



CAUTION Fischer projections are often confused with simpler Lewis diagrams.  Lewis diagrams without wedges and dashes, however, are not intended to give any stereochemical information!

Fischer projections can also be used to describe molecules with more than one stereogenic center.

 


If a Fischer projection with multiple stereogenic centers can be divided into two-halves that are mirror images then 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-dash diagram (as seen in the first drawing above)

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.
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)

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
    • 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)

Assign the R or S label to Compound A and Compound B above.

Blank Quiz Question 2.

Blank Quiz Question 3.

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Blank Quiz Question 5. 

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Ways to Draw 3D Structures in 2D     Newman Projections

Department of Chemistry & Chemical Biology