[noahshragowitz]

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!

[henrytudorvii]

The double bond in C1 gets priority over the single OH bond in C3.

You are misreading the CHO part. It's not C-OH (that would make only 3 bonds) its H-C=O instead

[noahshragowitz]

The double bond in C1 gets priority over the single OH bond in C3.

You are misreading the CHO part. It's not C-OH (that would make only 3 bonds) its H-C=O instead

Thanks for a quick response. When looking at c3- that is also attached to an Oxygen an H and another carbon. Surely then that trumps c1 with it's greater atomic weight? Do they not both tie with the O which prompts you to look at the next atoms?

Thanks again. Apologies- I'm just starting out with this stuff!

[Dan]

When looking at c3- that is also attached to an Oxygen an H and another carbon. Surely then that trumps c1 with it's greater atomic weight? Do they not both tie with the O which prompts you to look at the next atoms?

Multiple bonds are treated as bonds to multiple atoms (usually called ghost atoms) for the purposes of CIP priority - i.e. C=O is treated as a C bonded to two O atoms.

See page 3 of this pdf for an example.

[noahshragowitz]

Brilliant- thanks Dan! You've just cured a morning of pain for me.

Much appreciated.