Determining R/S Nomenclature at a Glance in Three Easy Steps

Note: With this method you never have to switch groups to see what the stereochemistry is. Using the "drive-the-car" analogy, what we are doing here is defining the steering wheel in relation to the steering column (which is normally behind the steering wheel):

1. Determine the priorities of the four attached groups from highest (1) to lowest (4).

This is the same as in all methods.

2. Draw the steering wheel. Draw a curved arrow around from 1 to 2 to 3 and back to 1 and note which direction this arrow goes, clockwise (cw) or counterclockwise (ccw).

The distinct feature of this method is that you make a full circle, from 1 to 2 to 3 to 1, completely ignoring the lowest priority group.�

3a. If the lowest priority group is behind the steering wheel, then this is the standard orientation: clockwise is R and counter-clockwise is S. (Turning the steering wheel clockwise turns the car to the right--R.) �

Note that Fischer projections are best depicted as �bow ties� with horizontal groups coming out and vertical groups going back from the central carbon atom.

3b. If the lowest priority group is in front of �this curved arrow, then the assignment is reversed: clockwise is S and counterclockwise is R. (Basically, you are looking at the steering wheel from the perspective of the engine compartment!)

Ready to try for yourself? See http://www.stolaf.edu/depts/chemistry/courses/toolkits/247/js/isomers and select "R,S Quiz."