Question

The absolute configuration of

is :

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

Hint:

Assign CIP priorities at C-2 and C-3 and read the Fischer projection (horizontal groups point towards the viewer, so reverse the apparent sense).

Answer 1

The Correct Option is D

The Fischer projection (top to bottom): \(\mathrm{CO_2H}\); C-2 with H (left) and OH (right); C-3 with H (left) and Cl (right); \(\mathrm{CH_3}\).
C-2 priorities: OH > C-3 (Cl, C, H) > \(\mathrm{CO_2H}\) (O, O, O) > H, because Cl outranks O at the first point of difference. OH (right) to C-3 (bottom) to \(\mathrm{CO_2H}\) (top) is clockwise, but H is on a horizontal bond, so reverse it: C-2 is S.
C-3 priorities: Cl > C-2 (O, C, H) > \(\mathrm{CH_3}\) > H. Cl (right) to C-2 (top) to \(\mathrm{CH_3}\) (bottom) is anticlockwise; H is horizontal, so reverse: C-3 is R.
Configuration \((2S, 3R)\).
Answer: (4) \((2S, 3R)\)