See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the structure. D-threose is a 4-carbon aldotetrose shown as a Fischer projection. C1 is CHO (top), C2 has HO on the left and H on the right, C3 has H on the left and OH on the right, and C4 is CH2OH (bottom). Step 2 - Assign priorities at C2. The four substituents are: OH (highest, #1), CHO group upward (#2, carbon bearing two oxygens effectively), the chain going down toward C3-C4-CH2OH (#3), and H (#4, lowest). In a Fischer projection, horizontal bonds point toward the viewer and vertical bonds point away. Step 3 - Determine configuration at C2. At C2: HO is on the left (toward viewer), H is on the right (toward viewer). The CHO is up (away) and the lower chain is down (away). Priority order: #1 = OH (left, toward viewer), #2 = CHO carbon (up, away), #3 = C3 chain (down, away), #4 = H (right, toward viewer). Since H (#4) is pointing toward the viewer (horizontal in Fischer), we first determine the rotation of 1→2→3 and then invert. The sequence OH(left) → CHO(up) → C3-chain(down) traces a clockwise arc → R, but because H is toward the viewer we invert → S. Therefore C2 is S. Step 4 - Determine configuration at C3. At C3: H is on the left (toward viewer), OH is on the right (toward viewer). The C2 chain is up (away) and CH2OH is down (away). Priority order: #1 = OH (right, toward viewer), #2 = C2 chain up (away, bears OH, CHO), #3 = CH2OH down (away), #4 = H (left, toward viewer). Rotation of 1→2→3: OH(right) → C2-chain(up) → CH2OH(down) traces counterclockwise → S, but H is toward the viewer so invert → R. Therefore C3 is R. Step 5 - Conclusion. C2 = S, C3 = R, giving configuration 2S, 3R. Step 6 - Why other options fail. (a) 2R, 3R incorrectly assigns both centers as R. (b) 2R, 3S incorrectly assigns C2 as R and C3 as S. (d) 2S, 3S incorrectly assigns C3 as S. Therefore, the correct answer is C.