Assigning Priorities: Stereocenters
Priority rules can also be used to determine the spatial orientation of groups around a stereocenter. In order to determine this spatial orientation, a series of steps should be followed.
Step 1: Find all of the stereocenters in a molecule. Recall that a stereocenter is an atom bonded to four different groups.
Step 2: Assign the priority of the groups around each stereocenter, with 1 being the highest priority and 4 being the lowest priority. Recall the priority rules discussed earlier in this section of the tutorial.
Step 3: Consider the first three priorities with respect to one another. Ignore the fourth priority for now. Determine whether the first three priorities are oriented such that they increase in a clockwise (CW) fashion or in a counterclockwise (CCW) fashion.
Step 4: Now consider the fourth priority:
- If the fourth priority is oriented downward (away from you, or dashed), continue to Step 5.
- If the fourth priority is oriented upward (toward you, or wedged), reverse the direction that you determined in Step 3. In other words, if you determined that the original orientation was clockwise, change it to counterclockwise. If you determined that the original orientation was counterclockwise, change it to clockwise.
- If the fourth priority is oriented in the plane of the page, rotate the groups such that the fourth priority is either oriented downward or oriented upward. (Note: If a rotation needs to be done, it is preferable to rotate the groups such that the fourth priority is oriented downward. This way the CW or CCW orientation should not have to be altered.) If you do not know how to rotate groups around a bond, click here. After rotating the groups, go back to Step 3 to check your clockwise or counterclockwise determination.
Step 5: Assign the final orientation:
- If the final orientation is clockwise, the orientation of the stereocenter is R. R is an abbreviation for the Latin word rectus, which means "to the right" (or clockwise, if considering rotations).
- If the final orientation is counterclockwise, the orientation of the stereocenter is S. S is an abbreviation for the Latin word sinister, which means "to the left" (or counterclockwise, if considering rotations).
Determine whether the stereocenter has an orientation of R or S.
Since the final orientation of the groups is clockwise, the orientation of the stereocenter is
R.
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