See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: A plane of symmetry (mirror plane) in a molecule passes through certain atoms, bisecting the molecule into two mirror-image halves. Atoms that lie directly in this plane are said to be 'bisected' by it. Step 1: Identify the compound. The structure is 1,3-dichlorocyclobutane with one Cl on a wedge (above plane) at C1 and one Cl on a dash (below plane) at C3. This is the trans isomer (trans-1,3-dichlorocyclobutane), which is a meso-like compound — actually it is the compound that has a plane of symmetry making it achiral (the cis isomer is the meso compound; let us re-examine). Step 2: Re-examine stereochemistry. The wedge at the upper carbon and dash at the lower-left carbon — in a cyclobutane drawn as a square, C1 (top-right) has Cl on wedge (coming toward viewer) and C3 (bottom-left) has Cl on dash (going away from viewer). This is the trans-1,3-dichlorocyclobutane. Step 3: Identify the plane of symmetry. For trans-1,3-dichlorocyclobutane, the plane of symmetry passes through C2 and C4 (the two carbons that do NOT bear chlorine), bisecting the C1–C3 diagonal. This plane is perpendicular to the ring and passes through C2, C4, and the midpoints of C1–C3 bonds — actually the plane passes through C2 and C4 atoms themselves. Step 4: Count atoms in the plane of symmetry. The mirror plane that makes this molecule achiral passes through: C2, the two H atoms on C2 (2 H's), C4, and the two H atoms on C4 (2 H's). That accounts for: C2 + 2H + C4 + 2H = 6 atoms lying in the plane and thus bisected by it. Step 5: Verify. C1 and C3 (bearing Cl) are reflected into each other by this plane — they are NOT in the plane. Each Cl and its attached H at C1 and C3 are reflected counterparts, not in the plane. The atoms IN the plane are: C2, C4, and the 2 hydrogens on each of C2 and C4 = 2 + 4 = 6 atoms. Why other options fail: (a) 2 — too few, ignores H atoms; (b) 4 — only counts carbons and misses H atoms; (d) 8 — overcounts by including atoms not actually in the plane. Therefore, the correct answer is C.