See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 – Assign CIP priorities to the four substituents of the chiral center in cysteine (H2N-CH(CH2SH)-COOH): - Priority 1: NH2 (nitrogen directly attached; atomic number 7) - Priority 2: COOH (carbon attached to two oxygens; O,O,O phantom atoms give high priority) - Priority 3: CH2SH (carbon attached to S; sulfur has atomic number 16, but first atom is C bonded to S,H,H vs COOH first atom C bonded to O,O,O — COOH ranks higher than CH2SH) Wait — sulfur (Z=16) vs oxygen (Z=8). For CH2SH, the first atom on the chain is C (bonded to S, H, H). For COOH, the first atom is C (bonded to O, O, O phantom). Comparing: O,O,O > S,H,H? No — sulfur (16) > oxygen (8), so at the second shell, CH2SH's carbon has S attached, which outranks COOH's carbon with O attached. Therefore: Corrected priorities: - Priority 1: NH2 (N, Z=7) - Priority 2: CH2SH (first atom C; attached atoms S,H,H — S has Z=16) - Priority 3: COOH (first atom C; attached atoms O,O,O — O has Z=8; but three oxygens... duplicated atoms rule: O,O,O vs S,H,H — compare highest first: S(16) > O(8), so CH2SH > COOH) - Priority 4: H (lowest) So: 1=NH2, 2=CH2SH, 3=COOH, 4=H. The image labels: (1)=NH2, (2)=CH2SH, (3)=COOH, (4)=H — consistent with above. Step 2 – Determine configuration (R or S): The lowest priority group (4, H) is on a bold wedge (coming toward the viewer). This means H points toward us, which is opposite to the standard requirement (lowest priority pointing away). Looking at the structure: H is on a wedge bond (toward viewer). The curved arrow from 1(NH2) → 2(CH2SH) → 3(COOH) as drawn in the image goes counterclockwise (the arrow sweeps from left, down through bottom, to top — CCW direction). Since the lowest priority (H) points toward the viewer instead of away, we must reverse the observed rotation. Observed rotation: CCW (S). Reversed: CW = R. Therefore, the configuration is R. Step 3 – Assign D or L: The D/L system is based on the Fischer projection convention relative to glyceraldehyde. In a Fischer projection of an amino acid, the amino group on the left corresponds to L-configuration, and on the right corresponds to D-configuration. To determine D/L: orient the molecule in a Fischer projection with the COOH at top and CH2SH at bottom (vertical chain away from viewer). In the given structure, NH2 is on the left side. In the standard Fischer projection of L-amino acids, the NH2 group is on the left. Therefore, this structure is L. Note: Although cysteine is R in the CIP system (unlike most L-amino acids which are S), it is still classified as L in the D/L system because the priority order of substituents in cysteine is unusual due to the sulfur-containing side chain elevating CH2SH above COOH in CIP ranking, while the D/L system does not use CIP rules. Therefore, the correct answer is (a) R (b) L.