Step 3: Draw a Circle
Now, imagine drawing a circle connecting the substituents from number 1 to 2 to 3. Pay no
attention to the lowest-priority group; it can be skipped because it projects directly into the page. If
the circle is drawn counterclockwise, the asymmetric atom is called (S) (Latin: sinister, “left”). If it is
clockwise, it is called (R) (rectus, “right”), as shown in Figure 2.18. Remember to correct the
stereochemistry if the modified version of Step 2 was used.
Figure 2.18. Drawing a Circle to Determine Absolute Configuration
Counterclockwise = (S); clockwise = (R)MNEMONIC
A clockwise arrangement is like turning a steering wheel clockwise, which makes a car turn
Right—so the chirality at that center is (R).Step 4: Write the Name
Once the (R)/(S) designation has been determined, the name can be written out. (R) and (S) are put
in parentheses and separated from the rest of the name by a hyphen. If we have a compound with
more than one chiral center, location is specified by a number preceding the R or S within the
parentheses and without a hyphen.