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.

  • 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 Norris Dec 22 '16 at 15:03

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:

structure of (–)-cocaine

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.

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