Fischer Projections: Assign R/S Study guide
A Fischer projection flattens a molecule onto a cross: horizontal bonds point toward you and vertical bonds point away. That fixed convention is what makes the drawing readable, and it is also the source of the classic trap: when the lowest CIP priority sits on a horizontal bond, the rotation you see is the reverse of the true assignment.
Assign the configuration of the highlighted stereocenter below.
Common Questions
How do you assign R and S from a Fischer projection?
Rank the four groups by CIP priority. If the lowest priority sits on a vertical bond, it already points away from you: read the ①→②→③ rotation directly, clockwise = R. If the lowest priority sits on a horizontal bond, it points toward you, so reverse whatever rotation you see.
Why do horizontal bonds reverse the R/S assignment?
In a Fischer projection every horizontal bond points toward the viewer. The CIP rules assume you view a stereocenter with the lowest priority pointing away; when the lowest priority faces you instead, you are viewing the center from the wrong side, and the apparent rotation is the mirror of the true one.
What do the lines in a Fischer projection mean?
The carbon backbone runs vertically, and each cross is a carbon. Every vertical bond points away from the viewer (dashes in a line-angle drawing) and every horizontal bond points toward the viewer (wedges). This fixed convention is what makes the projection readable without any wedges drawn.
Is there a shortcut for assigning R and S in sugars?
Yes. In a standard sugar drawn with the most oxidized carbon on top, the priorities at each CHOH center run OH > chain up > chain down > H, and the H is horizontal, so the reversal is already accounted for: OH on the right reads R, and OH on the left reads S. The explanation on each problem points out when this shortcut applies.
Why is L-cysteine R when the other L-amino acids are S?
The D/L label depends only on the drawing (NH2 on the left = L), while R/S depends on CIP priorities. In cysteine the side chain is CH2SH, and sulfur outranks the carboxyl oxygen-bearing carbon, so the priority order changes and the same spatial arrangement reads R. This bank includes cysteine for exactly that trap.