See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: A stereogenic (chiral) carbon is a carbon atom bearing four different substituents. Step 1: Identify all sp3 carbons with four different groups in the Allegra structure. Candidate 1 - The diphenylcarbinol carbon (bearing HO, phenyl, phenyl, and the piperidine C4 carbon): This carbon bears two phenyl groups. Two identical substituents mean this is NOT a stereogenic center. Candidate 2 - C4 of the piperidine ring (bearing H on wedge, the diphenylcarbinol carbon, and two CH2 groups of the ring): The two ring arms going up and down from C4 through the nitrogen are different (one arm goes to N directly on each side, but the nitrogen has a substituent making the two halves of the ring non-equivalent). However, looking carefully, C4 of the piperidine has: H, the diphenylcarbinol group, and two -CH2- arms of the piperidine ring leading to N. The two ring arms are: -CH2-CH2-N- on both sides, making them identical. Therefore C4 of the piperidine is NOT a stereogenic center (two identical ring arms). Candidate 3 - The CHOH carbon on the propyl chain (bearing OH, H, the aryl group, and the CH2CH2- chain to N): All four substituents are different. This IS a stereogenic center (shown with bold OH bond). Candidate 4 - The quaternary carbon on the para-substituent of the right benzene ring (bearing CH3, CH3, COOH, and aryl): This carbon bears two CH3 groups (identical substituents), so it is NOT a stereogenic center. Step 2: Count confirmed stereogenic carbons. Only the CHOH carbon on the side chain has four genuinely different substituents = 1 stereogenic carbon. Why other options fail: - (b) 2: Would require two stereogenic carbons; the diphenylcarbinol or piperidine C4 are not truly stereogenic due to identical groups. - (c) 3 and (d) 4: Even fewer carbons qualify upon rigorous analysis. Therefore, the correct answer is A.