See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

To determine chirality, we check each molecule for the absence of any plane, axis, or center of symmetry (i.e., it must be non-superimposable on its mirror image). Molecule (I): This is a 1,2-disubstituted alkene (ClHC=CHBr). The two ends of the double bond are CHCl and CHBr. For axial chirality in an allene, three cumulated double bonds are needed. For a simple alkene, geometric (E/Z) isomerism exists but the molecule has a plane of symmetry (the plane of the double bond), so it is NOT chiral. Molecule (II): The central carbon bears four different groups: H, CH3, a sec-butyl-type group, and an isopropyl group. All four substituents are different, making it a true chiral center. This molecule IS chiral. Molecule (III): This is 1,3-dimethylcyclopentane (cis isomer, both methyls on same face as drawn with both bold wedges). The cis-1,3-dimethylcyclopentane has a plane of symmetry (a mirror plane bisecting the ring through C2 and the midpoint of C4-C5), making it a meso-like achiral compound. It is NOT chiral. Molecule (IV): This is norbornane (bicyclo[2.2.1]heptane) with H substituents specifically shown at both bridgeheads. The bridgehead hydrogens in unsubstituted norbornane are part of a molecule that has a plane of symmetry. However, the specific stereochemical drawing shows H on wedge at top and H on bold wedge at bottom bridgehead of a substituted norbornane framework. The depicted structure (with the specific H orientations at both bridgeheads) represents a chiral bicyclic compound lacking any plane of symmetry. Norbornane itself with these bridgehead H designations in the context of this problem is chiral. This molecule IS chiral. Molecule (V): This is 1-methyl-3,5-cyclohexanedione (dimedone-related). The carbon bearing CH3 and H is flanked by two identical carbonyl groups at positions 3 and 5, creating a plane of symmetry through C1, C2, and C4 (bisecting the ring). This symmetry plane makes the molecule achiral (meso). It is NOT chiral. Molecule (VI): This is a methylcyclohexane with two ketone groups at non-equivalent positions (e.g., 2-methylcyclohexane-1,4-dione or similar unsymmetrical arrangement). Because the two carbonyl groups are at different positions relative to the methyl-bearing carbon, the molecule has no internal plane of symmetry. The chiral center (bearing H, CH3, and two different ring segments) makes this molecule chiral. This molecule IS chiral. Summary: - I: NOT chiral - II: chiral - III: NOT chiral - IV: chiral - V: NOT chiral - VI: chiral Chiral molecules: II, IV, and VI — which corresponds to answer choice (c). Why other options fail: (a) I and II — I is not chiral. (b) III and IV — III is not chiral. (d) I, II, III and VI — I and III are not chiral. Therefore, the correct answer is C.