See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: A molecule is achiral if it is superimposable on its mirror image, which is typically the case when it possesses an internal plane of symmetry (or other improper symmetry element such as a center of inversion or an Sn axis). Step 1 – Analyze option (a): This is a monosubstituted [2.2]paracyclophane with one CO2H group. Monosubstituted [2.2]paracyclophane derivatives lack an internal mirror plane because the two CH2–CH2 bridges are identical but the substituted ring has broken symmetry relative to the unsubstituted ring. The molecule is chiral (pseudo-asymmetric environment of the cyclophane cage gives planar chirality). So (a) is chiral. Step 2 – Analyze option (b): This is also a monosubstituted [2.2]paracyclophane with one CO2H group, drawn with different perspective but still having only one substituent. Monosubstituted [2.2]paracyclophanes exhibit planar chirality and are chiral. So (b) is chiral. Step 3 – Analyze option (c): This is a disubstituted [2.2]paracyclophane with two CO2H groups, one on each ring, both shown on wedge bonds (same face / pseudo-syn relationship). When two identical substituents are placed in a pseudo-geminal or pseudo-ortho arrangement on [2.2]paracyclophane with the same stereochemical relationship, the molecule can possess an internal plane of symmetry (C2v or containing a mirror plane that bisects both bridges and both CO2H-bearing positions symmetrically). With both CO2H groups on the same face (syn) and located at equivalent positions on each ring, the molecule has a plane of symmetry passing through the midpoints of both CH2–CH2 bridges and bisecting both rings, making the two halves mirror images of each other that are identical — hence the molecule is a meso-like achiral compound. So (c) is achiral. Step 4 – Analyze option (d): The structure shown is a large helicene-type or twisted polycyclic aromatic hydrocarbon (resembling a [7]helicene or a fused triphenylene dimer). Such helical PAHs are inherently chiral due to their helical twist (P and M enantiomers) and lack any plane or center of symmetry. So (d) is chiral. Conclusion: Only option (c), the syn-disubstituted [2.2]paracyclophane dicarboxylic acid, possesses an internal plane of symmetry and is therefore achiral. Therefore, the correct answer is C.