See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Stereoisomers include enantiomers and meso compounds. For a compound with n stereocenters, the maximum number of stereoisomers is 2^n, but this can be reduced if a meso form exists. Step 1: Identify the structure of 2,3-dichlorobutane. CH3-CHCl-CHCl-CH3 Carbon 2 and Carbon 3 are both stereocenters (each bears H, Cl, CH3, and CHClCH3 groups). Step 2: Since there are 2 stereocenters, the maximum possible stereoisomers = 2^2 = 4. Step 3: Check for meso forms. The molecule has a symmetric backbone (CH3 on both ends). When both stereocenters have opposite configurations (one R, one S), the molecule has an internal plane of symmetry and is a meso compound — it is achiral and identical to its mirror image. Step 4: List all stereoisomers: 1. (2R,3R)-2,3-dichlorobutane 2. (2S,3S)-2,3-dichlorobutane — enantiomer of (2R,3R) 3. (2R,3S)-2,3-dichlorobutane — meso compound (achiral) The (2S,3R) form is identical to (2R,3S) due to the internal symmetry, so it does NOT count as a separate stereoisomer. Step 5: Total distinct stereoisomers = 2 enantiomers + 1 meso = 3. Why other options fail: - (a) 2: Ignores the meso compound. - (c) 4: Treats (2R,3S) and (2S,3R) as separate, ignoring the meso relationship. - (d) 5: Overcounts beyond any logical framework. Therefore, the correct answer is B.