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:
We use the Cahn-Ingold-Prelog (CIP) priority rules to assign R/S configurations. In a Fischer projection, if the lowest priority group (H) is on a horizontal line, the observed configuration (CW for R, CCW for S) must be reversed.

Step 2: Detailed Explanation:
At Carbon 2 (C2): 1. Priority: \(-\text{OH}>-\text{CO}_2\text{H}>\text{C3}>\text{H}\). 2. Path from 1 \(\to\) 2 \(\to\) 3 is Clockwise (R). 3. Since H is horizontal, reverse R \(\to\) S. (Wait, let's re-verify priority: \(-\text{OH}\) (1), \(-\text{CO}_2\text{H}\) (2), \(-\text{CHClCH}_3\) (3). 1\(\to\)2\(\to\)3 is CCW. Reverse \(\to\) R). At Carbon 3 (C3): 1. Priority: \(-\text{Cl}>\text{C2}>-\text{CH}_3>\text{H}\). 2. Path from 1 \(\to\) 2 \(\to\) 3 is Clockwise (R). 3. Since H is horizontal, reverse R \(\to\) S.

Step 3: Final Answer
The configuration is (2R, 3S).