Fischer Projection Formulas
The problem of drawing three-dimensional configurations on a two-dimensional surface,
such as a piece of paper, has been a long-standing concern of chemists. The wedge and
hatched line notations we have been using are effective, but can be troublesome when
applied to compounds having many chiral centers. As part of his Nobel Prize-winning
research on carbohydrates, the great German chemist Emil Fischer, devised a simple
notation that is still widely used. In a Fischer projection drawing, the four bonds to a chiral
carbon make a cross with the carbon atom at the intersection of the horizontal and vertical
lines. The two horizontal bonds are directed toward the viewer (forward of the stereogenic
carbon). The two vertical bonds are directed behind the central carbon (away from the
viewer). Since this is not the usual way in which we have viewed such structures, the
following diagram shows how a stereogenic carbon positioned in the common two-bonds-in-
a-plane orientation ( x–C–y define the reference plane ) is rotated into the Fischer projection
orientation (the far right formula). When writing Fischer projection formulas it is important to
remember these conventions. Since the vertical bonds extend away from the viewer and the
horizontal bonds toward the viewer, a Fischer structure may only be turned by 180º within
the plane, thus maintaining this relationship. The structure must not be flipped over or
rotated by 90º.
A model of the preceding diagram may be examined by .
In the above diagram, if x = CO2H, y = CH3, a = H & b = OH, the resulting formula describes
(R)-(–)-lactic acid. The mirror-image formula, where x = CO2H, y = CH3, a = OH & b = H,
would, of course, represent (S)-(+)-lactic acid.
