See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1: Identify the molecule. This is a Fischer projection of a compound with two stereocenters: C2 bears COOH (up), H (left), Br (right), and C3 (down); C3 bears C2 (up), H (left), CN (right), and OH (down). Step 2: Assign priorities at C2. Substituents at C2: Br (atomic number 35) > COOH (C with two O, O, O phantom) > C3 chain (C with O, N, H...) > H. Priority order: Br (1) > COOH (2) > C3-chain bearing OH and CN (3) > H (4). In Fischer projection, horizontal bonds point toward viewer, vertical bonds away. At C2: Br is on the right (toward viewer), H is on the left (toward viewer), COOH is up (away), C3 is down (away). Arranging 1→2→3 (Br→COOH→C3-chain): In the Fischer projection, looking from the front plane, Br is right, COOH is top-back, C3 is bottom-back. The lowest priority H is on the left (toward viewer). To determine R/S, we look at 1→2→3 with H toward us (not away). When the lowest priority is pointing toward the viewer, we reverse the observed rotation. In Fischer projection at C2: going from Br (right) → COOH (up) → C3 (down): right → up → down is clockwise (R), but since H is horizontal (toward viewer), we reverse: giving S... Let me redo carefully. In a Fischer projection, horizontal substituents point toward viewer, vertical substituents point away. At C2: H (left, toward viewer), Br (right, toward viewer), COOH (up, away from viewer), C3 (down, away from viewer). Lowest priority is H. H is toward the viewer. We read 1→2→3 and then reverse. Priorities: 1=Br, 2=COOH, 3=C3-chain (with OH,CN), 4=H. Reading Br→COOH→C3: Br is at right, COOH is at top, C3 is at bottom. Right→Top→Bottom: this is counterclockwise (CCW) = S direction. Since H is toward viewer, we reverse: S reversed = R. So C2 = R. Step 3: Assign priorities at C3. Substituents at C3: OH (bottom, away from viewer in Fischer), CN (right, toward viewer), H (left, toward viewer), C2-chain (up, away from viewer). Priority order: OH (O directly attached, highest among O,N,C,H) = 1; CN (N attached) = 2; C2-chain (C with Br, COOH, H) = 3; H = 4. Lowest priority H is toward viewer (left, horizontal). Reading OH→CN→C2: OH is at bottom (away), CN is at right (toward), C2 is at top (away). Bottom→Right→Top: this is clockwise (CW) = R direction. Since H is toward viewer, reverse: R reversed = S... Wait, let me reconsider priority at C3. C2-chain has Br(35), which is very high. Compare C2-chain vs CN: C2 carbon attached to Br(35), COOH carbon, H. CN carbon attached to N(7),N(7)(phantom). So C2-chain > CN because Br > N at first point of difference. Revised priorities at C3: 1=OH, 2=C2-chain (has Br), 3=CN, 4=H. Reading OH→C2-chain→CN: OH is bottom, C2-chain is top, CN is right. Bottom→Top→Right: counterclockwise = S. H is toward viewer, reverse: S reversed = R. So C3 = R. Step 4: Result is 2(R), 3(R), which corresponds to answer (b). Therefore, the correct answer is B.