Assigning Priorities: Rotating Groups
When determining the orientation of a stereocenter, the fourth priority must either be oriented upward or downward. It cannot be in the plane of a page. In the rare case that the fourth priority is in the plane of the page, the groups in question can be rotated such that the fourth priority is either upward or downward. It is beneficial to rotate the groups such that the fourth priority is oriented downward, such that the clockwise or counterclockwise determination does not need to be changed later.
When rotating groups, three groups change and one group remains unchanged. The group that remains unchanged should be the second group located in the plane of the page. The three groups that change can then be rotated in a clockwise or a counterclockwise orientation until the fourth priority group is in the desired position. The relative orientation of the three groups to one another should never change. For example, look at the molecule shown below. The order of the groups around the molecule, going in a clockwise direction, are bromine, chlorine, and hydrogen. Even after rotating these three groups, they will always stay in that relative order.
Consider the molecule below. The structure on the left is drawn in wedge-dash notation. The structure on the right represents the same molecule, drawn in three-space.
Notice that the fourth priority is the hydrogen (white ball), and it is located in the plane of the page. It is desirable to have the hydrogen in the dashed position, where the chlorine is currently located. In going from the hydrogen's current position to the chlorine's present position, a counterclockwise rotation is occuring.
Therefore, the fourth priority group can be placed in a desired position by rotating three groups in a clockwise direction. The other group in the plane of the page (the iodine, black ball) should not rotate. The structures below represent the molecule after it has gone through a counterclockwise rotation.
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