See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: A stereogenic (asymmetric) carbon is a sp3 carbon bearing four different substituents. The R/S configuration is assigned by applying the Cahn-Ingold-Prelog (CIP) priority rules: rank the four substituents by atomic number (and then by the next atoms if tied), orient the molecule so the lowest-priority group points away, then determine if the sequence 1→2→3 is clockwise (R) or counterclockwise (S). Step 2 - General approach for each compound: a) Identify every sp3 carbon in the structure. b) Check whether it bears four different groups. c) Use the wedge-dash notation in the drawing to determine 3D orientation. d) Apply CIP rules to assign R or S. Step 3 - Compound (1) Erythromycin: Erythromycin is a 14-membered macrolide lactone. It has 10 stereocenters in the macrolide ring (at roughly C2, C3, C4, C5, C6, C8, C10, C11, C12, C13) plus additional stereocenters in its two sugar units (desosamine and cladinose). Wedge bonds indicate bonds coming toward the viewer; dash bonds go away. Assigning priorities to substituents at each center and applying CIP rules gives the known configuration: 2R,3S,4S,5R,6R,8R,10R,11R,12S,13R (macrolide ring) with additional R/S centers in the sugars, consistent with the natural product stereochemistry. Step 4 - Compound (2) Morphine: Morphine has 5 stereocenters (C5, C6, C9, C13, C14 in morphine numbering). The bridgehead carbons and the carbons bearing OH or NCH3 are all asymmetric. Using wedge/dash bonds shown: the known absolute configuration of natural morphine is (4R,4aR,7S,7aR,12bS) or equivalently (5R,6S,9R,13S,14S) depending on numbering convention. Each center is assigned R or S by CIP priorities. Step 5 - Compound (3) Amoxicillin: Amoxicillin has 3 stereocenters: - The alpha-amino carbon on the side chain (bearing NH2, H, aryl, C=O): this is the (R) center in the D-amino acid configuration shown. - C6 of the beta-lactam ring (bearing H, S-ring, N, C=O side chain): (R) or (S) per structure shown. - C5 (the ring junction between beta-lactam and thiazolidine): (R) or (S). The dashes on the H atoms at the ring junction indicate they are behind the plane; the methyl groups on dashes/wedges at C2 of thiazolidine are also stereocenters. Full assignment follows CIP rules. Step 6 - Compound (4) Flonase (fluticasone propionate): Flonase is a corticosteroid with the steroid skeleton containing approximately 7-8 stereocenters. Key ones include: C6-F (axial, beta or alpha per wedge), C9-F (alpha, dash), C11-OH, C16-methyl, C17-OH, ring junction carbons (C8, C9, C10, C13, C14). Each is assigned R or S by standard steroid stereochemistry combined with CIP rules applied to the drawn wedge/dash bonds. Step 7 - Compound (5) Topomax (topiramate): Topiramate has 4 stereocenters in the bicyclic acetal ring system (the fructose-derived carbons bearing O atoms). The wedge bonds (shown as 'Oml' notation meaning O on wedge) at each ring carbon allow assignment. The known configuration of topiramate corresponds to the D-fructose-derived absolute configuration, giving specific R/S assignments at each of the 4 ring carbons. Step 8 - Compound (6) Zocor (simvastatin): Simvastatin has 8 stereocenters: the decalin ring junctions and ring carbons bearing methyl or OH groups, plus the lactone ring carbon bearing OH, and the ester side chain carbon bearing two different alkyl groups. Wedge and dash bonds in the drawing define each configuration; CIP priorities are assigned based on substituents at each center. Step 9 - Why other assignments fail: Any incorrect assignment arises from misidentifying the spatial orientation (confusing wedge for dash), misapplying CIP priorities (e.g., ranking O vs. N incorrectly), or overlooking a stereocenter (e.g., a quaternary carbon that appears symmetric but bears four different ring-path substituents). Careful application of CIP rules to each center, using the drawn stereochemistry, yields the unique correct R/S label for each. Therefore, the correct answer is Each compound's stereocenters labeled R/S in answer key: (1) erythromycin, (2) morphine, (3) amoxicillin, (4) flonase, (5) topomax, (6) zocor - see answer key image (page 152).