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Stereochemistry: Fischer Projections

Stereochemistry: Fischer Projections

Dr. MJ Patterson

Fischer projections are used to represent the stereochemistry at a chiral center conveniently on paper in two dimensions. The groups on the vertical axis go away from you through the page, and the groups on the horizontal axis come out at you from the page. The following three drawings show the single enantiomer (S)-2-chlorobutane in three different representations.

Manipulating Fischer Projections:

There are only two ways that you can manipulate Fischer projections on paper without changing the configuration of the molecule.

1. Rotate the molecule 180° . When you rotate the molecule 180° , the groups that were coming out of the page are still coming out of the page, and the groups that were going into the page are still going into the page. (Rotating by 90° will invert the groups coming out and going in, and will invert the configuration.)

2. Hold one group in position, and rotate the other three groups. Use a model to convince yourself that this operation does not change the configuration. In the example, the methyl group is held still, and the other three groups are rotated .

Fischer Projections and Specifying Configuration:

To specify the configuration (R or S) of a molecule, you need to look at the molecule so that the group with lowest priority is hidden behind the chiral center. Next, you determine the direction traveled from the highest priority group to the seco nd to the third. Clockwise travel is the R configuration, and counterclockwise is the S configuration.

When using Fischer projections, the top and bottom positions are both going away from you into the paper. If the lowest priority group is placed in either the top or the bottom position, then it is effectively behind the chiral center, and you can det ermine the direction of travel among the other three groups, ignoring the lowest priority group.

If the lowest priority group is not in either the top or the bottom position, use one of the allowed manipulations to put in one of those positions.

Example: 2-chlorobutane. In this case, Cl is the highest priority, followed by ethyl, then methyl and finally hydrogen. Rotate to put the H, the lowest priority group, in the bottom position.

Next, find the direction to travel from the highest to the lowest priority group, ignoring the H.

Since the direction of travel is counterclockwise, the configuration is S.