See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the compound: L-threonine is (2S, 3R)-2-amino-3-hydroxybutanoic acid. It has two stereocenters: C2 (S configuration, bearing NH2) and C3 (R configuration, bearing OH). Step 2 - Fischer projection conventions: In a Fischer projection, horizontal bonds point toward the viewer and vertical bonds point away. The most oxidized carbon (CO2H) is placed at the top, and CH3 at the bottom. Step 3 - Determine C2 (2S) configuration in Fischer projection: For the (S) configuration at C2 with CO2H above and CH3 below (both going away), NH2 and H on horizontal bonds. Priority order: NH2 (1) > CO2H (2) > CH3 (3) > H (4). With H on the right (pointing toward viewer), if the 1→2→3 sequence is counterclockwise in the drawing, that corresponds to S. Placing NH2 on the left and H on the right at C2 gives S configuration. This matches options (b) and (d). Step 4 - Determine C3 (3R) configuration in Fischer projection: At C3, groups are OH, H (horizontal, toward viewer), C2 chain above, and CH3 below. Priority: OH (1) > C2-chain (2) > CH3 (3) > H (4). With H on the right (toward viewer), if 1→2→3 is clockwise as drawn it appears R but must account for H pointing toward viewer (invert). In option (b): at C3, H is on the left and OH is on the right. With OH on right and H on left: H points toward viewer (left side), priorities OH(1) is right, C2-chain(2) is up, CH3(3) is down — 1→2→3 goes counterclockwise as seen, but since H is toward viewer, we invert to get clockwise = R. This gives 3R configuration. Step 5 - Verify option (b): C2 has H2N on left, H on right → 2S. C3 has H on left, OH on right → 3R. This matches (2S, 3R)-2-amino-3-hydroxybutanoic acid = L-threonine. Step 6 - Eliminate other options: (a) has NH2 on right at C2 → 2R, incorrect. (c) has NH2 on right at C2 → 2R and OH on left at C3 → 3S, incorrect. (d) has NH2 on left at C2 → 2S but OH on left at C3 → 3S, incorrect. Therefore, the correct answer is B.