9.5: Assigning Configuration to Conformationally Mobile Systems

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This is probably one of the trickiest situations to deal with, especially when the molecule is shown to us in conformations such as a cyclohexane chair, or represented by Newman projections. It is a good idea in these cases to work with models, because one cannot help but to turn the molecule around until the of the lowest priority groups are positioned where they should be, and the priorities of the groups attached to the chiral centers can be clearly seen.

In the case of cyclohexane chairs and other rings, it’s a good idea to flatten the ring and position it on the plane of the paper, with the lowest priority groups pointing towards the back when possible.

In the case of Newman projections, it helps to rotate one of the carbons around the C–C bond under consideration until as many similar groups as possible are aligned (eclipsing each other), then rotate the structure sideways to obtain a side view, rather than a projection, then assign priorities and configuration.