Hi,
I'm having difficulty with an MCAT sterochemistry question- any help would be enormously appreciated.
In the question (please see attached) it asks what the absolute configuration of the C2 and C5 carbons are. I understand assigning R/S stereochemistry, it is the prioritisation part of the question which is posing difficulties.
My thinking was as follows:
I gave OH priority 1.
C3 I gave priority 2 as it's attached to OH, H and C
C1 I gave priority 3 as it's attached to OH (and presumably two other Hs). Therefore it's collective atomic number is less than C3 giving it a lower priority.
However, in the answer (attached) it says that C1 is priority 2 and not 3. How can this be? The carbons tie with the OH but then surely c3 trumps C1 with the additional carbon?
Is the question wrong or have I missed something?
Thanks!