priority groups configuration

[SaintJude]

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View attachment L-tartaric_acid.jpg

So Please focus on the Fisher projection of tartaric acid, b/c that what I was given. How do I know that COOH has a higher priority (no.2) than the rest of the circled group (no.3?) Can someone spell out their thought process please!

I feel like a 3rd could understand this bs, but I haven't...

 

[MedPR]

View attachment 18948

So Please focus on the Fisher projection of tartaric acid, b/c that what I was given. How do I know that COOH has a higher priority (no.2) than the rest of the circled group (no.3?) Can someone spell out their thought process please!

I feel like a 3rd could understand this bs, but I haven't...

Because the carbon in "COOH" is bonded to two oxygens. The adjacent carbon on the circled group is bonded to one oxygen (the one in OH) and one carbon (the one in the circled COOH).

When comparing priorities, you only count the next adjacent atom.

If that doesn't make sense, read on. It's more step by step of the same thing as above.

I'm assuming you are asking about the chiral carbon on top (Carbon #2), not the chiral carbon on bottom (Carbon #3). So for carbon #2:

It is bound to an OH, an H, a COOH, and the C with the circled crap around it.

First priority is OH, since Oxygen is heavier than Carbon. Now the second priority has a tie at the first adjacent atom (both are carbons) so we move to the next adjacent atom. For the group up top, the next adjacent atom is an oxygen (COOH). For the group on the bottom the next adjacent is also an oxygen (OH), so we need to move to the third adjacent atom. The third adjacent on the top is another oxygen (COOH), and the third adjacent on the bottom is a C (the C in the bottom COOH). So the top one wins since it is bonded to two oxygens instead of only 1.

 

[SaintJude]

Oooh.

Ok, so b/c the circled carbon is attached to 3 atoms that have a combined weight of 29 g

While the other carbon is attached to 2 atoms with =32 grams. This one is higher priority?

 

[MedPR]

Oooh.

Ok, so b/c the circled carbon is attached to 3 atoms that have a combined weight of 29 g

While the other carbon is attached to 2 atoms with =32 grams. This one is higher priority?

No, priority only cares about the immediately adjacent atoms and you only consider the non-immediately adjacent atoms if there is a tie between the adjacent atoms.

The only reason the COOH "wins" is because it ties with the circled group at the first carbon (which is immediately adjacent to the chiral carbon), then it ties again because both immediately adjacent carbons are bound to one oxygen, then it wins because the bottom adjacent carbon is bound to a C (the C in the bottom COOH).

Consider this example.

It's easy to see why the groups are ordered that way. But say you stuck a CH2 between the iodine and the chiral carbon. This group (H2C-I) weighs more than Br or F, but it is now the third priority group because the chiral carbon is bound to a Carbon and not an Iodine. However, if all three halogens had a CH2 stuck between them and the chiral carbon, the priorities would be exactly the same as if they were bound directly to the chiral carbon because all 3 groups would "tie" at the C in CH2, then you would consider the next-adjacent atoms for all 3 groups (which would be the 3 halogens) and I > Br > F.