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FunmbiB
#1 Posted : Tuesday, August 23, 2011 1:55:19 AM
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When labelling atoms to determine absolute configuration, the largest atomic weight is given the highest priority. In the ExamKrackers Organic Chemistry textbook, page 21, I don't understand why Bromine got assigned as 2 while Fluorine was assigned 1. Does it have to do with a rule about electronegativity or the rule about selectivity.
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arifawan
#2 Posted : Saturday, August 27, 2011 1:27:08 PM
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Hi,

I hope that all is well!

You are correct that the atom with the largest atomic weight attached to the chiral carbon gets the highest priority when it comes to determining absolute configurations.

However, note that on page 21, the fluorine is directly attached to the chiral carbon, whereas the bromine is first attached to another carbon (labelled 2)and then the chiral carbon. As a carbon has lower molecular weight than fluorine, it gets a lower priority. If both the fluorine and the bromine were attached to the same chiral carbon directly, bromine would definitely have a higher priority.

It's a subtle thing, but you have to be careful when you examine the bonding around the chiral carbon as you can very easily make careless mistakes. Stereochemistry is also a topic that shows up quite frequently on the MCAT from my experience.

All the best!
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FunmbiB
#3 Posted : Tuesday, August 30, 2011 5:18:39 PM
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Thanks. That makes so much sense. Can't believe I didn't see that
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