See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: A meso compound is an achiral molecule that contains two or more stereocenters but has an internal plane of symmetry, making it optically inactive despite having chiral centers. Step 1: Identify which compounds have two or more stereocenters. - (a) 2-chlorobutane: Only one stereocenter (C2). Cannot be meso. Eliminated. - (b) 2,3-dichlorobutane: Two stereocenters at C2 and C3. Structure: CH3-CHCl-CHCl-CH3. The two end groups (CH3) on either side are identical. - (c) 2,3-dichloropentane: Two stereocenters at C2 and C3. Structure: CH3-CHCl-CHCl-CH2CH3. The groups on C2 (CH3) and C3 (CH2CH3) differ, so no internal plane of symmetry is possible. Cannot be meso. - (d) 2-hydroxypropanoic acid (lactic acid): Only one stereocenter (C2). Cannot be meso. Eliminated. Step 2: Check 2,3-dichlorobutane for meso form. The molecule is CH3-CHCl-CHCl-CH3. C2 has substituents: Cl, H, CH3, and CHClCH3. C3 has substituents: Cl, H, CH3, and CHClCH3. Because the molecule is symmetric (same groups on both ends: CH3), there exists a stereoisomer where C2 is R and C3 is S (or vice versa), giving an internal mirror plane through the C2-C3 bond. This (R,S) stereoisomer is the meso form — it is superimposable on its mirror image and is achiral. Step 3: Why other options fail. - (a) and (d) have only one chiral center, so meso is impossible. - (c) 2,3-dichloropentane has two different substituents on either side (CH3 vs CH2CH3), so no internal plane of symmetry can exist; no meso form is possible. Therefore, the correct answer is B.