Optical isomerism 5.
Fischer projection
This two-dimensional projection model of chiral atoms was originally developed by Emil Fischer more than 100 years ago. Apart from very simple compounds, typically amino acids and carbohydrates, its use is being phased out. It was often used in older literature, therefore its knowledge is unavoidable.
In the Fischer projections the vertical lines represent attachments pointing out the back of the paper away from us and horizontal lines represent attachments pointing out of the paper towards us. The projection cannot be arbitrary rotated, only rotation by 180 deg in the plane of paper is allowed. Interchanging two atoms (substituents) we get to the opposite optical isomer (R → S or S → R). Interchanging twice (or 4x, etc) retains the configuration.
Several possible starting positions of a molecule could be used during the projection, but conventionally glyceraldehyde is projected as shown above. In case of amino acids the carboxyl group is by convention written at the top, in analogy to the aldehyde group of carbohydrates, and the R group is written at the bottom. The amino group is then considered as analogous to the OH group of glyceraldehyde. When the amino group projects to the left, the stereoisomer is considered to be the L enantiomer.
For structures with more than one stereogenic center such as carbohydrates, the tetrahedral carbons are "stacked" on top of one another. For carbohydrates, the convention is to put the carbonyl group at the top for aldoses and closest to the top for ketoses. The carbons are numbered from top to bottom.
Important! Fischer structures cannot be rotated by 90°!
D-Erithyrose
(2R,3R)-2,3,4-Trihydroxybutanal
The next example shows how to rotate a molecule drawn with up/down bonds and its atoms to get to the Fischer representation (and vice versa).
Excercises
Which is the correct 2D representation of the rotatable molecule?
In old publications you may come across with the Fischer representation of the cyclic form of sugars (pyranoses). Notice that the ring looks angular because of technical reasons. The broken bonds do not represent atoms! Always use up/down bonds in this case.:
