How do you use the stereospecific numbering system? How do you determine the proR and proS position?

Question

I believe that this different from the RS nomenclature system. From what I have researched, the C-1 is that group of the prochiral compound that occupies the proS position. I'm not sure how to determine which carbon is proS vs proR.

Answer 1

It is actually quite simple. To begin, you likely understand that a prochiral atom is one which initially is achiral, but upon reaction it becomes a new chiral center. An adaptation of the sequence rule is used to designate the pro-R and pro-S.

You will first select one of the heterotopic ligands (N,O,etc...) and just arbitrarily assign it a higher priority than the other ligand (leaving the 2 H atoms alone). You then apply the sequence rule and assign R or S to the 'equivelent' H atoms.

This rule can also be applied to assigning the faces of pro-chiral compounds. For instance, a reaction can occur from the front face or from the back face. The faces of the pro-chiral compound are assigned 're' and 'si', and are assigned in the same manner. If application of the sequence rule results in R, then that is the 're' face.

Answer 2

If I recall, think of a steering wheel around the target carbon atom. Start from the largest molecular mass to the next largest. If your wheel turns to the right, it is R. To the left, it is S.