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Three-Dimensional Representations

Assigning Priorities

Determining Stereochemical Relationships

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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: Consider the fourth priority of this molecule and determine if any changes to the CW or CCW orientation need to be made.

The fourth priority is wedged (oriented upward), so the orientation needs to be changed from counterclockwise to clockwise.

Step 5: Assign the final orientation: Determine whether the stereocenter has an orientation of R or S.

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