4.7 : היטל פישר

The three-dimensional wedge–dash representation of molecules with multiple chiral centers can be very complicated and difficult to draw.

Such molecules can be more easily represented through their simple two-dimensional projections on a planar surface, known as Fischer projections.

In Fischer projections, all bonds of interest are represented as horizontal or vertical lines. While the vertical lines point away from the viewer, the horizontal lines are directed towards the viewer.

A molecule with only one chiral center, such as glyceraldehyde, can be easily rotated such that the carbon backbone points away from the viewer and the other substituents at the chiral center point towards the viewer. The projection of this orientation onto a plane creates the Fischer projection of glyceraldehyde.

In molecules with multiple chiral centers, such as ribose, there is no rotation possible such that all substituents at the chiral centers project towards the viewer while the carbon backbone points away from the viewer.

The Fischer projection of such a molecule is generated in two steps. First, the molecule is rotated such that the carbon chain is oriented top-to-bottom, conventionally with C-1 at the top.

Second, the configuration at the lowest-numbered chiral center is visualized such that the substituents point towards the viewer and the carbon backbone is slanted away.

This orientation generates the Fischer projection for that chiral center. Repeating this process for each chiral center in the molecule creates the Fischer projection of the molecule.

However, the Fischer projection is only a simplistic two-dimensional representation and does not correlate directly to the actual three-dimensional structure of the molecule.

While rotation of the Fischer projection of a molecule in plane by one hundred and eighty degrees has no effect on the overall representation, an out-of-plane rotation by one hundred and eighty degrees creates the Fischer projection of the enantiomer of the original molecule.