KEY CONCEPT
To determine the absolute configuration at a chiral center:FISCHER PROJECTIONS
On the MCAT, one way to represent three-dimensional molecules is by a Fischer projection. In this
system, horizontal lines indicate bonds that project out from the plane of the page (wedges),
whereas vertical lines indicate bonds going into the plane of the page (dashes). The point of
intersection of the lines represents a carbon atom.
To determine configurations using Fischer projections, we follow the same rules listed above. Once
again, we have to make sure that the lowest-priority group projects into the page. A benefit of
Fischer projections is that the lowest-priority group can be on the top or bottom of the molecule
and still project into the page.
Another advantage is that we can manipulate Fischer projections without changing the compound.
As mentioned before, switching two substituents around a chiral carbon will invert the
stereochemistry ((R) to (S), or (S) to (R)). Rotating a Fischer projection in the plane of the page by 90°
will also invert the stereochemistry of the molecule. By extension, interchanging any two pairs of
substituents will revert the compound back to its original stereochemistry, and rotating a Fischer
projection in the plane of the page by 180° will also retain the stereochemistry of the molecule.
These manipulations are shown in Figure 2.19.
Assign priority by atomic number
Arrange the molecule with the lowest-priority substituent in the back (or invert the
stereochemistry by switching two substituents)
Draw a circle around the molecule from highest to lowest priority (1 to 2 to 3)
Clockwise = (R); counterclockwise = (S)