See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the convention used in each structure. In all three structures, the dashed wedge bonds point behind the plane (away from viewer) and the bold wedge bonds point in front of the plane (toward viewer), while the standard dashed lines represent bonds going away from the viewer in a Fischer-like horizontal/vertical sense. More precisely: vertical bonds (up/down with dashed lines) go into the page; horizontal bonds (bold wedges) come out of the page. Step 2 - Assign the four substituents and their spatial positions for Structure 1: - Into page (up): CH2CH3 - Into page (down): CH3 - Toward viewer (left): Br - Toward viewer (right): Cl Step 3 - Assign substituents for Structure 2: - Into page (up): CH3 - Into page (down): Br - Toward viewer (left): CH3CH2 (= CH2CH3) - Toward viewer (right): Cl Step 4 - Assign substituents for Structure 3: - Into page (up): CH2CH3 - Into page (down): Cl - Toward viewer (left): H3C (= CH3) - Toward viewer (right): Br Step 5 - Determine R/S configuration or check if structures are identical by rotating. For Structure 1, the four groups are: CH2CH3 (back-up), CH3 (back-down), Br (front-left), Cl (front-right). For Structure 3, the four groups are: CH2CH3 (back-up), Cl (back-down), CH3 (front-left), Br (front-right). Step 6 - Check if Structure 3 is a rotation of Structure 1. In Structure 1: Br is front-left, Cl is front-right, CH2CH3 is back-up, CH3 is back-down. In Structure 3: CH3 is front-left, Br is front-right, CH2CH3 is back-up, Cl is back-down. Rotate Structure 1 by 180° around the vertical axis (the CH2CH3–CH3 axis): Br (front-left) moves to front-right, Cl (front-right) moves to front-left, CH2CH3 stays back-up, CH3 stays back-down. This gives: CH2CH3 back-up, CH3 back-down, Cl front-left, Br front-right — which matches Structure 3 exactly. A 180° rotation about the vertical axis is a valid physical rotation (same molecule), so Structures 1 and 3 represent the same compound. Step 7 - Check Structure 2 vs Structure 1. Structure 2 has CH3 back-up and Br back-down, whereas Structure 1 has CH2CH3 back-up and CH3 back-down. These are different spatial arrangements and represent a different stereoisomer (or the same connectivity but different configuration), so Structure 2 does not depict the same compound as Structure 1. Step 8 - Conclusion: Structures 1 and 3 depict the same compound; Structure 2 is different. Therefore, the correct answer is B.