I couldn't get the absolute configuration for the chiral center I marked. I got R, but according to my answer key it is apparently S. Would really appreciate an explanation.
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1$\begingroup$ The simplest explanation is that answer keys occasionally have mistakes. Note that the chirality where the benzoate group is attached has the $S$ configuration, as does the bridgehead chirality center to the front. It is possible that if this was an assignment or exam question, your instructor previously gave cocaine with a different chirality center marked on an older version and forgot to update the key for the different chirality center. $\endgroup$– Ben NorrisCommented Dec 22, 2016 at 15:03
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2 Answers
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You are actually right, that stereocenter does have R stereochemistry. The four substituents to the chiral center in question are:
- 1 - $\ce{CO(O)}$ = $\ce{C(O)3}$
- 2 - $\ce{C(O)(C)(H)}$
- 3 - $\ce{C(N)(C)(H)}$
- 4 - $\ce{H}$
These are ranked and numbered according to the Cahn-Ingold-Prelog priority rules. Adding the numbering to the structure:
Orienting the lowest priority group away from us and following the direction of the numbering, we confirm that it does indeed have R stereochemistry:
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ChemSpider returns the following name when searching for cocaine:
(1R,2R,3S,5S)-2-Methoxycarbonyltropan-3-yl benzoate
And the following image:
The structure is identical to the one given in your assignment and the carbon in question is position 2. The name gives carbon 2 as R-configured.
This answer was meant to supplement ringo’s answer which shows how to derive the correct CIP priorities which clearly give R as well.
