Question

The absolute configuration of: 

 

• A. (2S,3S)
• B. (2R,3R)
• C. (2R,3S)
• D. (2S,3R)

Hint:

"H on the Horizon, flip the sign!" If Hydrogen is on the horizontal arm of the Fischer projection, your final R/S result must be flipped.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept: 
The absolute configuration (R/S) of chiral centers in a Fischer projection is determined using the Cahn-Ingold-Prelog (CIP) priority rules. If the lowest priority group is on a horizontal bond, the actual configuration is opposite to the apparent direction (clockwise vs. counter-clockwise). Step 2: Key Formula or Approach: 
1. Assign priority numbers (1 to 4) to the four groups attached to the chiral center based on atomic number. 
2. Trace the path from 1 $\rightarrow$ 2 $\rightarrow$ 3. 
3. If the path is clockwise, the apparent configuration is R. If counter-clockwise, it is S. 
4. Since the lowest priority group (-H) is on the horizontal bond in both centers, reverse the result: R $\rightarrow$ S and S $\rightarrow$ R. 
Step 3: Detailed Explanation: 
For C2 (Top chiral center): 
Groups attached to C2: -OH, -CH(Cl)CH$_3$ (C3 group), -CO$_2$H, -H. 
Priorities at C2: 1. -OH (Oxygen has the highest atomic number, 8) 2. -CH(Cl)CH$_3$ (Carbon is attached to Cl, C, H. Max atomic number is 17) 3. -CO$_2$H (Carbon is attached to O, O, O. Max atomic number is 8) 4. -H (Lowest priority) Note: Group 2 wins over Group 3 because the first point of difference yields Cl (17) vs O (8). Positions on Fischer projection: Top(3), Bottom(2), Right(1), Left(4). Path 1 $\rightarrow$ 2 $\rightarrow$ 3 goes from Right to Bottom to Top, which forms a clockwise circle. Apparent = R. 
Since -H is horizontal (Left), we reverse the result. Therefore, actual configuration = 2S. For C3 (Bottom chiral center): 
Groups attached to C3: -Cl, -CH(OH)CO$_2$H (C2 group), -CH$_3$, -H. 
Priorities at C3: 1. -Cl (Highest atomic number, 17) 2. -CH(OH)CO$_2$H (Carbon attached to O, C, H) 3. -CH$_3$ (Carbon attached to H, H, H) 4. -H Positions on Fischer projection: Right(1), Top(2), Bottom(3), Left(4). Path 1 $\rightarrow$ 2 $\rightarrow$ 3 goes from Right to Top to Bottom, which forms a counter-clockwise circle. Apparent = S. 
Since -H is horizontal (Left), we reverse the result. Therefore, actual configuration = 3R. 
Step 4: Final Answer: 
The absolute configuration is (2S, 3R).