R/S problems

[equityrange]

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B,C,D are all (R) right?

is A (R) or (S)? It should be (R) because sulfur takes priority right? thanks

 

[Mr.Patel]

Yes you are right about all your assumptions. I also double checked in my Organic MCAT EK book!

"If two atoms are the same element, then their substituents are sequentially compared in order of DECREASING (thus you do S vs O first: in which S wins and the battle is over 🙂 ) priority until one of the substiutents is found to have a greater priority.

 

[Pharmaniac]

Don't u mean b,c,d are all 'S'- or it may just be me.... (started learning about these last night)

 

[Eye Cue]

If I had to take a shot at it, I'd say the carboxyl group has priority over the methylthiol group, just because of the carbonyl double bond, making the molecule 'S'. Anyone agree/disagree?

(I am talking about structure A, btw. I believe you are correct in saying B,C, and D are 'R')

 

[Akarat]

All of the structures are of the R configuration.

In regards to structure A, the amino acid cysteine, the R group (CH2SH) gets a higher priority than the carboxyl group because S has a higher atomic number than O. Even though there are two O atoms, the higher priority still goes to the R group.

It's not the total atomic number from all of the atoms that counts, but the group with the single atom that has the highest atomic number that does.

 

[Eye Cue]

All of the structures are of the R configuration.

In regards to structure A, the amino acid cysteine, the R group (CH2SH) gets a higher priority than the carboxyl group because S has a higher atomic number than O. Even though there are two O atoms, the higher priority still goes to the R group.

It's not the total atomic number from all of the atoms that counts, but the group with the single atom that has the highest atomic number that does.

Thank you for clearing that up 👍