See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: A chiral compound is one that is non-superimposable on its mirror image, meaning it lacks an internal plane of symmetry (or any improper rotation axis). A molecule is achiral if it possesses a plane of symmetry, center of symmetry, or other symmetry element that makes it superimposable on its mirror image. Step 2 - Analyze compound (A): Compound A is a norbornene-type bicyclic system bearing an exocyclic double bond (=CHCH3, i.e., a Z/E alkene with CH3 and H) at C7 and two acetyl groups at two different ring carbons. The presence of the exocyclic double bond (with CH3 and H) creates geometric asymmetry, and combined with the rigid bicyclic framework and two substituents (two acetyl groups) at distinct stereocenters on the ring, compound A has multiple stereocenters with no internal plane of symmetry. This makes compound A chiral. Step 3 - Analyze compound (B): Compound B is a norbornene framework with Me and H at the C7 bridge carbon and two NH2 groups on adjacent carbons. Although there are stereocenters, the two NH2 groups on adjacent carbons in a symmetric arrangement can create a meso-like situation or the molecule may have a plane of symmetry through the framework, rendering it achiral (meso compound). Step 4 - Analyze compound (C): Compound C appears to be cubane or a highly symmetric cage hydrocarbon. Cubane (or similar structures) possesses multiple planes of symmetry and high molecular symmetry (Oh point group for cubane), making it achiral. Step 5 - Analyze compound (D): Compound D appears to be a bicyclo[2.2.0] or similar symmetric structure. The drawing shows a symmetric polycyclic hydrocarbon with a plane of symmetry, making it achiral. Step 6 - Conclusion: Among all four compounds, only compound A lacks all symmetry elements that would make it superimposable on its mirror image. The combination of the rigid bicyclic skeleton, the exocyclic double bond bearing CH3 and H (restricting rotation), and the two acetyl-bearing stereocenters in a non-symmetric arrangement ensures chirality. Compounds B, C, and D all possess symmetry elements (planes of symmetry or meso arrangements) that render them achiral. Therefore, the correct answer is A.