R/S Configuration and Identical Compounds

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SUMMARY

The discussion centers on the confusion surrounding the R/S configuration of two identical compounds in stereochemistry. The user consistently arrives at R for the first compound and S for the second, despite the key indicating they should share the same configuration. The issue arises from misassigning priorities, particularly between CH2CH2OH and CH2CH3, where the user incorrectly ranks them. Properly rotating the molecule to place the lowest priority substituent behind the central carbon is essential for accurate configuration assignment.

PREREQUISITES
  • Stereochemistry fundamentals
  • CIP priority rules for substituents
  • Understanding of molecular rotation
  • Familiarity with R/S nomenclature
NEXT STEPS
  • Review the Cahn-Ingold-Prelog (CIP) priority rules in detail
  • Practice drawing and rotating molecules to determine R/S configurations
  • Study examples of stereoisomers to reinforce understanding
  • Explore advanced stereochemistry concepts such as meso compounds and enantiomers
USEFUL FOR

Chemistry students, organic chemists, and anyone studying stereochemistry or working with chiral compounds.

cathy
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Homework Statement



Hello, so I think I've been looking at this problem way too long, and I'm going no where with it. It is in my attached picture, because it is easier to draw than write out on here.
The key says these two are identical compounds, meaning they should have the same configuration.
But, I keep on getting R for the first and S for the second.
I got R for the first by switching the configuration since the 4th one should be going away, but it is rather coming towards me. I got S by following the priorities. Would someone be able to tell me where I am going wrong, please? I am very confused.
 

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Am I drawing all the priorities correctly?
I keep looking at my solution manual and I'm getting all opposite answers for all the questions :(. Is there something crucial I'm missing here?
 
Last edited:
The priorities of CH2CH2OH and CH2CH3 should be 3 and 4, respectively as O has higher priority than H. You seem to have assigned them the other way round.
To assign R or S to the structures, you have to rotate the molecule so that substituent 4 lies behind the central C and then determine the direction from 1 over 2 to 3.
 

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