See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

A meso compound is one that has chiral centers but is achiral overall due to an internal plane of symmetry (or other symmetry element), making it superimposable on its mirror image. Step 1 - Concept: For a cyclic compound with two stereocenters to be meso, the two chiral centers must have opposite configurations (one R, one S) such that an internal mirror plane exists, OR the substituents must be arranged so the molecule is internally symmetric. Step 2 - Analyze Structure (1): 2,5-dimethyl-1,3-dioxane. The ring has methyl groups at C2 and C5. Both methyls are shown on bold wedge bonds (same face of the ring). In the chair-like or flat representation, having both substituents on the same face means cis relationship. For a 1,3-dioxane with methyls at C2 and C5, the cis isomer (both same face) possesses an internal plane of symmetry bisecting the ring - making it meso. Structure (1) is meso. Step 3 - Analyze Structure (2): Same ring but one methyl is on a dashed wedge and one on a bold wedge (opposite faces = trans). The trans isomer of 2,5-dimethyl-1,3-dioxane has a C2 axis but the two stereocenters have the same configuration (both R or both S), making it chiral (a pair of enantiomers), not meso. Structure (2) is NOT meso. Step 4 - Analyze Structure (3): Glycolide or 1,4-dioxane-2,5-dione derivative (cyclic diester) with methyls at the two alpha carbons, both on bold wedge (same face/cis). This 6-membered cyclic diester with two stereocenters having cis methyl groups has an internal mirror plane through the two carbonyl carbons and oxygens, making it meso. Structure (3) is meso. Step 5 - Analyze Structure (4): Same cyclic diester ring as (3) but the methyl groups appear to be on the same face (both bold wedge). However, examining the drawing carefully, structure (4) shows methyls in a trans arrangement relative to the ring plane (one up, one down based on the wedge/dash pattern shown). The trans isomer of this cyclic diester has both stereocenters with the same configuration - it is chiral, not meso. Structure (4) is NOT meso. Step 6 - Analyze Structure (5): 4,6-dimethyl-1,3-dioxan-2-one (cyclic carbonate). One methyl is on dashed wedge (going back) and one on bold wedge (coming forward) = trans arrangement. For this cyclic carbonate, the trans isomer places the two methyl-bearing stereocenters in opposite configurations; however, due to the asymmetry introduced by the single carbonyl (carbonate) group, there is no internal mirror plane in the trans isomer. The molecule is chiral. Structure (5) is NOT meso. Step 7 - Summary: Only structures 1 and 3 are meso compounds, which corresponds to answer choice (a). Why other options fail: - (b) includes 5, which is not meso - (c) includes 4, which is not meso - (d) neither 2 nor 5 are meso Therefore, the correct answer is A.