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Three-Dimensional Representations

Assigning Priorities

Determining Stereochemical Relationships

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Assigning Priorities: Priority Rules

Finally, if a molecule has a branched chain, all of the branches must be taken into consideration when assigning priority. This can be thought of in a way similar to multiple bonds.

For example, consider the following molecule and assign the priorities to the groups attached to the carbon with the asterisk (*).

The missing hydrogen should be drawn in first, and it can be assigned as the fourth priority. The oxygen can be assigned as the first priority. The carbons can be distinguished by going down the branches, as done before. It can be seen that the carbon on the right is attached to a longer branch than is the carbon on the left. Therefore, the carbon on the right has the higher priority and is the second priority overall. The carbon on the left is the third priority.

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