See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the molecule. All four structures represent alanine (2-aminopropanoic acid), which has four substituents on the chiral center: NH2, COOH, CH3, and H. Step 2 - Recall CIP priority rules for alanine's substituents: 1st priority: NH2 (nitrogen) 2nd priority: COOH (carbon bonded to two oxygens) 3rd priority: CH3 (carbon bonded to hydrogens) 4th priority: H (lowest) Step 3 - For (R) configuration, when the lowest priority group (H) points away from the viewer, the remaining three groups (NH2 > COOH > CH3) must decrease in a clockwise direction. Step 4 - Analyze option (b): The structure shows COOH on top (coming toward viewer / in plane up), H on left (in plane, horizontal), NH2 on right (in plane, horizontal), CH3 on bottom (going back / dash). In a standard wedge-dash drawing where horizontal bonds are in the plane of the paper (or coming toward viewer) and vertical bonds go behind: COOH is up (toward viewer or in plane), H is left horizontal (toward viewer), NH2 is right horizontal (toward viewer), CH3 is down (dash, going away). With CH3 going away (dash, back), we place H on the left coming toward us. To assign R/S, we need H pointing away. H is on the left coming toward us — so we need to swap perspective or use the swap method. Actually, in a standard tetrahedral wedge-dash: the two horizontal bonds (H left, NH2 right) point toward the viewer, the top bond (COOH) and bottom bond (CH3) are in the plane or going back. With COOH on top (in plane, going up-back) and CH3 on bottom (dash, going back-down): H points toward viewer (left wedge). To find configuration, rotate so H points away: the sequence NH2 (1) → COOH (2) → CH3 (3) going clockwise when viewed with H toward viewer means counterclockwise when H points away, giving S. Re-examining: H is on the left as a bold/wedge bond toward viewer. Priorities: NH2=1, COOH=2, CH3=3, H=4. With H pointing toward viewer, the observed rotation 1→2→3 (NH2 right → COOH top → CH3 bottom) appears: NH2 is right, COOH is up, CH3 is down — this sequence going right→up→down is counterclockwise as seen from front. Since H points toward us (opposite of away), we reverse: counterclockwise from front = clockwise from back = R configuration. Step 5 - Therefore option (b) represents the (R)-enantiomer of alanine. Step 6 - Why other options fail: (a) H2N on left (wedge toward viewer), COOH on right (wedge toward viewer), CH3 on top, H on bottom (dash away). With H pointing away (dash), sequence NH2(1)→COOH(2)→CH3(3) going left→right→up is counterclockwise = S configuration. (c) NH2 on top, H3C on left (wedge), H on right (wedge), COOH on bottom (dash). H is on right toward viewer; reversing gives the opposite sense = S configuration. (d) COOH on top, H3C on left (wedge), NH2 on right (wedge), H on bottom (dash). H points away (dash), sequence NH2(1)→COOH(2)→CH3(3): NH2 right→COOH top→CH3 left = counterclockwise = S configuration. Therefore, the correct answer is B.