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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.

Assign the priorities around the stereocenter for the molecule.


Carbons A, A', and A'' are bonded to two hydrogens and one carbon. Carbon A''' is bonded to three hydrogens. The hydrogens have a lower priority than the carbons, so the A''' chain is the fourth priority. Carbons A, A', and A'' are equivalent and cannot be used to assign priority. The next carbon in each chain must be considered.

Carbons B, B', and B'' are all bonded to two hydrogens and another carbon. They are equivalent and cannot be used to assign priority. The next carbon in each chain must be considered.

Carbon C is bonded to two carbons and one hydrogen. Carbon C' is bonded to one carbon and is double bonded to another carbon. Carbon C'' is bonded to one oxygen and two hydrogens. Of these groups, the oxygen has the highest priority, making the C'' chain the first priority. Since the C' carbon is double bonded to a carbon and is single bonded to another carbon, and the C carbon is bonded to two other carbons and a hydrogen, the C' chain has the next highest (second) priority, and the C chain is the third priority.

Click Next to see the full priority assignment.


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