See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1: Identify the compound. This is a Fischer projection of 2,3-dichloropentane (CH3-CHCl-CHCl-C2H5). C-2 has substituents CH3 (up/top), C3 chain (down/bottom), H (left), Cl (right). C-3 has C2 chain (up), C2H5 (down), Cl (left), H (right). Step 2: Recall Fischer projection convention. Horizontal bonds point toward the viewer (wedge), vertical bonds point away (dash). Step 3: Assign configuration at C-2. Substituents at C-2: Cl, CH3 (as part of chain going up = toward C1 = CH3), the chain going down (C-3 and C2H5), H. Priority order: Cl > C-3 carbon (attached to Cl, H, C2H5, C2) > CH3 > H. Actually, priorities: 1. Cl (highest) 2. C-3 (carbon bearing Cl, H, C2H5) 3. CH3 (carbon bearing H, H, H) 4. H (lowest) In the Fischer projection at C-2: Cl is on the right (toward viewer), H is on the left (toward viewer), CH3 is up (away), C-3 chain is down (away). With H (lowest priority) pointing toward the viewer (left horizontal), we must invert the rotation. Arranging 1(Cl-right), 2(C3-down), 3(CH3-up): going from Cl(right) -> C3(down) -> CH3(up) = clockwise = R. But H is toward viewer, so invert: S. C-2 configuration = S. Step 4: Assign configuration at C-3. Substituents at C-3: Cl (left, toward viewer), H (right, toward viewer), C-2 chain (up, away), C2H5 (down, away). Priorities: 1. Cl (highest) 2. C-2 carbon (bearing Cl, H, CH3) 3. C2H5 (CH2CH3, carbon bearing H, H, CH3) 4. H (lowest) H is on the right (toward viewer). Arrange remaining: Cl(left), C2(up), C2H5(down). Rotation from Cl(left) -> C2(up) -> C2H5(down): left to up to down = counterclockwise = S. Since H is toward viewer, invert: R. C-3 configuration = R. Step 5: Result is 2S, 3R. Why other options fail: - (a) 2R, 3S: incorrect assignment at both centers - (c) 2S, 3S: C-3 is R, not S - (d) 2R, 3R: C-2 is S, not R Therefore, the correct answer is B.