Chemistry Forum

Posted: **Mon Jul 07, 2025 7:09 pm**

I was doing practice questions and I answered this incorrectly, and I can't wrap my head around it. It'll be hard to explain w/o a picture, but I'll try my best.

The structure is a synthetic macrocyclic ion consisting of a 18-crown-6 backbone (has a bunch of ether groups). It contains two substituents "X" and "Y". Both of them are located in b/w the two ether groups, like this. O-CH(Y)-CH(X)-O.

Y is wedged and X is dashed.

X = CO₂H
Y = C(=O)N(n-Pr)CH₂CH₂OCH₂CH₂N(n-Pr)CO₂CH₂C₆H₅

My initial answer is that the chiral carbon w/ X is (S) and the chiral carbon w/ Y is also (S). But that is wrong, it's saying the answer is R,R but how?

In the case of Y, isn't the O group connected to the chiral carbon the 1st priority? (due to higher atomic #). If that's the case, the priority would be counterclockwise making it (S). Same with X, it would be R, flip it b/c hydrogen is wedged and it would also be S. But it's saying R,R.

Please help. I apologize for the long amount of text.

Posted: **Mon Jul 07, 2025 11:24 pm**

Without a photo it's difficult to say.One thing is that you have to rearrange the substituents in the way that hydrogen is on bottom, means get dashed. The other 3 then like a Mercrdes Star. Priorität is O, then CO then CHX or CHY in the other case.

https://www.masterorganicchemistry.com/ ... log-rules/

Posted: **Fri Jul 11, 2025 2:07 am**

the dajjal wrote: ↑Mon Jul 07, 2025 7:09 pm I was doing practice questions and I answered this incorrectly, and I can't wrap my head around it. It'll be hard to explain w/o a picture, but I'll try my best.

The structure is a synthetic macrocyclic ion consisting of a 18-crown-6 backbone (has a bunch of ether groups). It contains two substituents "X" and "Y". Both of them are located in b/w the two ether groups, like this. O-CH(Y)-CH(X)-O.

Y is wedged and X is dashed.

X = CO₂H
Y = C(=O)N(n-Pr)CH₂CH₂OCH₂CH₂N(n-Pr)CO₂CH₂C₆H₅

My initial answer is that the chiral carbon w/ X is (S) and the chiral carbon w/ Y is also (S). But that is wrong, it's saying the answer is R,R but how?

In the case of Y, isn't the O group connected to the chiral carbon the 1st priority? (due to higher atomic #). If that's the case, the priority would be counterclockwise making it (S). Same with X, it would be R, flip it b/c hydrogen is wedged and it would also be S. But it's saying R,R.

Please help. I apologize for the long amount of text.

The key is that you're assigning priorities correctly, but remember to trace the path from the highest to lowest priority while placing the lowest priority group (usually hydrogen) in the back, and in both cases here, the wedge/dash orientation means the configurations appear inverted—so your (S) assignments flip to R once you properly account for the 3D geometry.

Posted: **Mon Aug 04, 2025 1:48 am**

the dajjal wrote: ↑Mon Jul 07, 2025 7:09 pm I was doing practice questions and I answered this incorrectly, and I can't wrap my head around it. It'll be hard to explain w/o a picture, but I'll try my best.

The structure is a synthetic macrocyclic ion consisting of a 18-crown-6 backbone (has a bunch of ether groups). It contains two substituents "X" and "Y". Both of them are located in b/w the two ether groups, like this. O-CH(Y)-CH(X)-O.

Y is wedged and X is dashed.

X = CO₂H
Y = C(=O)N(n-Pr)CH₂CH₂OCH₂CH₂N(n-Pr)CO₂CH₂C₆H₅

My initial answer is that the chiral carbon w/ X is (S) and the chiral carbon w/ Y is also (S). But that is wrong, it's saying the answer is R,R but how?

In the case of Y, isn't the O group connected to the chiral carbon the 1st priority? (due to higher atomic #). If that's the case, the priority would be counterclockwise making it (S). Same with X, it would be R, flip it b/c hydrogen is wedged and it would also be S. But it's saying R,R.

Please help. I apologize for the long amount of text.

You're assigning priorities correctly in theory, but for both chiral centers, you're likely misinterpreting the 3D orientation, remember to first assign priorities then orient the molecule so the lowest priority group (usually H) is pointing away before determining R/S.

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