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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You are actually right, that stereocenter does have R stereochemistry. The four substituents to the chiral center in question are:
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:
ChemSpider returns the following name when searching for cocaine:
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.