Question

8. Identify the indicated faces as Re or Si (2 pts) c-CH3

Answer 1

For prochiral carbons, if when looking at a face, the priority of substituents, as dictated by the Cahn-Ingold-Prelog (CIP) rules go from the highest to the lowest group/atom in a clockwise direction, the face is labelled as the Re face. If when going from the highest to the lowest, one encounters a counter-clockwise order, that face is the Si face.

The CIP rules prioritize atoms or groups proportional to their atomic numbers. If two atoms directly bonded to a carbon of interest is the same, then the similar atoms are traced back to the point of first difference between them and thus prioritized. If there are multiple bonds between atoms, the atoms bonded are considered to be individual ones, bonded as many times as the bond order. So, C-O gets a lower priority than C=O.

In the given prochiral carbon, the carboxylate group has the highest priority. while the doubly bonded C gets the second priority, leaving H as the least priority atom. Now, when viewing this carbon from the top, as shown in the diagram, one goes in a counter-clockwise fashion, moving from the highest to the lowest priority group. Thus, the face from the top is the Si face while the face on the bottom is the Re face.