See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1: Identify the compound. The structure is 1,4,5,8-tetrachloronaphthalene — a naphthalene ring system with Cl atoms at positions 1, 4, 5, and 8 (the peri positions on both ends of the naphthalene). Step 2: Understand the molecular symmetry. Naphthalene itself belongs to the D2h point group, with three mutually perpendicular planes of symmetry. The introduction of substituents can reduce or maintain this symmetry. Step 3: Analyze the substitution pattern. In 1,4,5,8-tetrachloronaphthalene, the four Cl atoms are placed symmetrically: positions 1 and 8 are on one end (peri to each other on the left), and positions 4 and 5 are on the other end (peri to each other on the right). This pattern preserves the D2h symmetry of the parent naphthalene. Step 4: Identify the planes of symmetry in D2h: - Plane 1: The molecular plane itself (the plane of the naphthalene ring system). All atoms lie in this plane, so it is a plane of symmetry. - Plane 2: A vertical plane perpendicular to the molecular plane and passing through the long axis of naphthalene (the C2 axis through C4a and C8a, bisecting the molecule left-right). The left ring (with Cl at 1 and 4) maps onto the right ring (with Cl at 5 and 8), making this a valid mirror plane. - Plane 3: A vertical plane perpendicular to the molecular plane and passing through the short axis (perpendicular to the long axis, bisecting the central C-C bond). This plane maps the top Cl atoms (at 1 and 8) onto the bottom Cl atoms (at 4 and 5), and left maps to right symmetrically. Step 5: Verify no additional planes exist. There are no other mirror planes because the Cl substitution removes any additional diagonal symmetry elements that might otherwise exist in a more symmetric molecule. Step 6: Count: Plane of the ring (molecular plane) + longitudinal vertical plane + transverse vertical plane = 3 planes of symmetry. Therefore, the correct answer is 3.