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[Solved] Cahn-Ingold-Prelog ACS Question on YouTube


I'm watching an ACS Ochem 2 Exam Review:

At timestamp 4:30, we are assigning R and S configuration for the epoxide's carbon #3. We are comparing the carbon with O, C, H, and the carbon connected to Br, H, H. The carbon connected to OCH is given 2nd priority, and the carbon with BHH is given 3rd priority. Why is that? I thought since Br has the higher mass, it would have 2nd priority?

I also found a similar question on R/MCAT:

I would appreciate the clarification. Thank you for your time and patience!

The video looks incorrect to me.

It should be the cis - 4-bromo-2R,3R epoxy- butan-1-ol.

I agree. Also, it doesn't make sense for it to be 2R, 3S and it's cis. It would be 2R, 3R, if I'm not mistaken.
edit: exactly, @Hunter2. Thank you!


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