February 21, 2024, 02:15:05 PM
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Topic: Assigning R/S descriptor to Enantiomer  (Read 273 times)

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Offline Freakylabs4

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Assigning R/S descriptor to Enantiomer
« on: February 06, 2024, 01:34:42 AM »
I have seen this molecule with the R-stereodescriptor on the C-Atom, that binds to the hydroxy-group, but every time I do this myself I get the S-descriptor. I've used ChemDraw and got the R-descriptor, but when I use the CIP rules then I assign the highest priority to the oxygen, the second highest to the C-Br and third highest to the ethyl-group.CC[CH@@](O)[CH@@](Br)C
« Last Edit: February 06, 2024, 02:06:59 AM by Freakylabs4 »

Offline Babcock_Hall

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Re: Assigning R/S descriptor to Enantiomer
« Reply #1 on: February 06, 2024, 10:49:54 AM »
In the drawing that you provided, does the lowest priority group exist in front of the plane of the screen or in back of plane?

Offline Freakylabs4

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Re: Assigning R/S descriptor to Enantiomer
« Reply #2 on: February 07, 2024, 01:49:04 AM »
As it's drawn (or not drawn), it's in the front. I was told, that if it's not drawn into the structure, but another atom/group (in this case the hydroxy-group) is for example in the back (dash), it must by default then be in the front.
The structure though is as given in the picture I provided.

Offline Babcock_Hall

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Re: Assigning R/S descriptor to Enantiomer
« Reply #3 on: February 07, 2024, 10:43:27 AM »
To assign a configuration as R or S, the lowest priority group, which is "H" in this situation, must be in the back.  My preferred ways to do this is to rotate the molecule on paper or make a model and rotate that, but there are other ways.  What method are you using?

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