R/S Configuration and Identical Compounds

• cathy
In summary, the conversation is about a problem where the speaker is trying to assign R and S configurations to two identical compounds. They are confused because they keep getting opposite answers and are unsure if they are drawing the priorities correctly. The other person points out that the priorities of the substituents may have been assigned incorrectly and explains the correct way to assign R or S configurations.
cathy

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.

Attachments

• chem.png
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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.

1. What is the difference between R and S configuration?

The R and S configuration is a way to describe the 3D arrangement of atoms in a molecule. It is based on the Cahn-Ingold-Prelog priority rules, where the highest priority group is assigned to the R position if the lowest priority group is pointing away from the viewer, and to the S position if the lowest priority group is pointing towards the viewer.

2. How is the R/S configuration determined?

The R/S configuration is determined by assigning a priority to each group attached to a chiral carbon based on the atomic number of the atoms directly attached to it. Then, if the lowest priority group is pointing away from the viewer, the remaining groups are arranged in a clockwise direction for the R configuration and counterclockwise for the S configuration.

3. Can two compounds with the same molecular formula have different R/S configurations?

Yes, two compounds with the same molecular formula can have different R/S configurations. This is because the arrangement of atoms in 3D space can result in different configurations, even if the molecular formula is the same.

4. How do enantiomers and diastereomers relate to R/S configuration?

Enantiomers are compounds that are mirror images of each other and have opposite R/S configurations. Diastereomers, on the other hand, have different R/S configurations but are not mirror images of each other. They can have different physical and chemical properties.

5. Can two identical compounds have different R/S configurations?

No, two identical compounds must have the same R/S configuration. This is because they have the same molecular formula and the same arrangement of atoms in 3D space, resulting in the same R/S configuration.

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