Blueprint MCAT Formerly Next Step
Return to Blog Homepage

PCAT Chemical Processes – Glyceraldehyde Stereochemical Configuration

  • by Sam
  • Apr 19, 2018
  • PCAT Question of the Day

The structure of d-glyceraldehyde, an important metabolic intermediate, is shown below:

What is the absolute stereochemical configuration of the glyceraldehyde stereoisomer depicted above?


A. (R)
B. (S)
C. (+)
D. (–)

Click for Explanation

A is correct. The convention for drawing Fischer projections dictates that substituents branching horizontally from the vertical backbone of the molecule are directed out of the plane of the paper, toward the reader. Therefore, the hydrogen and oxygen atoms bonded to the chiral C-2 carbon atom are directed outward toward you, out of the page. The hydroxyl group has the highest priority, and the aldehyde carbon has the second highest priority. Moving around the molecule from highest priority downward, we travel counterclockwise, which typically denotes an S configuration. However, since our fourth-priority group (the hydrogen atom) points toward us, we must reverse this configuration. Our final answer should be R.

Submit a Comment