See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the compound. Both structures appear to be 2,3-dibromobutane derivatives (or a related vicinal dibromide with two stereocenters). Each molecule has two adjacent stereocenters bearing Br substituents. Step 2 - Assign configurations at each stereocenter for Compound 1. Using the wedge-dash notation: at the upper stereocenter, H3C is on a dashed wedge (going back), Br is on a bold wedge (coming forward), H is implicit, and the chain continues. At the lower stereocenter, H is on a dashed wedge (going back) and Br is on a bold wedge (coming forward). Careful assignment gives one center as R and the other as S (or both R, or both S — must be determined carefully). Step 3 - Assign configurations at each stereocenter for Compound 2. The right structure shows CH3 and Br on bold wedges at the upper center, and Br on the left with H on a dash at the lower center. This suggests the configurations differ from Compound 1 at one but not both centers. Step 4 - Compare the two structures. When the R/S assignments are worked out fully, Compound 1 and Compound 2 have opposite configurations at one stereocenter but the same at the other (i.e., one is (R,S) and the other is (R,R) or (S,S), or they are (2R,3R) vs (2R,3S), etc.). They are NOT mirror images of each other (which would make them enantiomers), and they are NOT identical. They have the same connectivity but different spatial arrangements that are non-mirror-image stereoisomers. Step 5 - Definition check. Diastereomers are stereoisomers that are not mirror images of each other. Since the two compounds have the same molecular formula and connectivity, are stereoisomers, but are not mirror images (their configurations differ at one center but not both, or they differ at both centers but are not full inversions), they are diastereomers. Step 6 - Eliminate other options. Enantiomers would require all stereocenters to be inverted (full mirror images) — that is not the case here. Meso compounds refer to a single molecule with an internal plane of symmetry, not a relationship between two molecules. Identical would mean superimposable — they are not. Therefore, the correct answer is A.