Question

I know that if the atom is the same, I have to look at the next atom. In the first choice, Oxygen is not connected to any other atom (besides the chiral carbon). It is an anion. Whereas in the second choice, OH has Hydrogen attached to Oxygen. So how do I decide which one gets higher priority?

Answer 1

To determine the priority of substituents at a chiral center, the atomic number of atoms directly attached to the center are compared. The OH group has higher priority over an oxygen anion because hydrogen has a higher atomic number than the absence of a second atom.

Step-by-step explanation:

When assigning priority to substituents attached to a chiral center in a molecule, the Cahn-Ingold-Prelog priority rules are used. These rules state that priority is assigned based on the atomic number of the atoms directly attached to the chiral center.

In the case of an oxygen atom bonded to a hydrogen (OH) versus an oxygen atom that is not bonded to another atom (anion O-), the first atom in both groups is oxygen, which has the same atomic number.

However, when comparing the second atoms, hydrogen (atomic number 1) versus the absence of a second atom in the case of the anion, the OH group has a higher priority because it has a higher atomic number than the absence of an atom.

Thus, the OH group will have a higher priority than the oxygen anion in determining the configuration of the chiral center.