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Three-Dimensional Representations

Assigning Priorities

Determining Stereochemical Relationships

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A project of the Chemical Education Digital Library
Determining Stereochemical Relationships:
Comparing Cyclic Structures Using the R and S Method

You have selected to use the R and S Method for determining the stereochemical relationships between molecules drawn in different cyclic three-dimensional representations. Using this method requires four steps:

Step 1: Determine that the molecules have the same molecular formula and connectivity.

For example, compare the molecules below.

Do these molecules have the same molecular formula and connectivity?

The above two molecules do have the same molecular formula (C6H10BrCl) and the same connectivity.

Step 2: Find the stereocenters on each molecule. Note that a cyclic molecule must have at least one stereocenter and must not have a plane of symmetry in order to be a chiral molecule. If a cyclic molecule has no stereocenters, the molecule is achiral. Two cyclic molecules with the same connectivity and no stereocenters are identical.

Again consider the above pair of molecules and find the stereocenter(s) on each.

Click Next to check your answer and continue on.

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