Diastereomers and Meso compounds

Today we started off with some practice at identifying (R) and (S) configurations -- and saw that there is still some work to be done. Remember, the substituent of priority number 4 MUST be going back for the CIP system to work. Futhermore, if you switch two substituents to get to that scenario, the act of switching has changed the stereochemistry to give you the opposite of the stereochemistry of the original compound.

We also looked at scenarios with more than one stereocenter and saw that there are a few possibilities that can come about. If every stereocenter is changed, then you have made the enantiomer UNLESS there is a plane of symmetry in the molecule. In that case, it is a meso compound and the mirror images are identical. If at least one but not all of the stereocenters are changed, then you have made a diastereomer of the original compound; the two compounds most likely have different physical attributes.

The image shows the addition of dioxygen to a compound to make a pair of diastereomers -- one is R, S, S, R and the other is R, R, R, R. The slash through the lower arrow shows that the pathway to the R, R, R, R compound does not happen.