See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: Resonance structures of polycyclic aromatic hydrocarbons (PAHs) are drawn by distributing the pi electrons (double bonds) in all valid Kekulé-type arrangements where every carbon has a valid valence and the structure retains alternating single and double bonds. Anthracene consists of three linearly fused six-membered rings sharing 10 pi electrons (5 double bonds) across 14 carbon atoms (with 10 carbons in the ring framework contributing to pi system, actually 14 carbons total, 7 double bonds in the full structure... let us count properly). Anthracene has the molecular formula C14H10, with 7 degrees of unsaturation from the ring system, meaning 7 double bonds must be placed in any Kekulé structure. To enumerate valid Kekulé (resonance) structures of anthracene: - Structure 1: Double bonds at positions 1-2, 3-4, 4a-9a, 5-6, 7-8, 9-10, and one bridging bond — systematically, one places double bonds such that every carbon is satisfied. - By systematic enumeration, anthracene has exactly 4 valid Kekulé resonance structures. These can be derived by considering the possible placements of the 5 double bonds (the C9-C10 bond can be a double bond or not, and the remaining double bonds fill in accordingly in the two outer rings). - When C9=C10 is a double bond: the two outer rings each independently take one of two arrangements, giving 2 × 1 = 2 structures (the outer rings are forced once C9=C10 is fixed, but each outer ring has 2 options — actually yielding 2 structures here: both outer rings in same orientation or alternating, but due to symmetry constraints only 2 are valid). - When C9-C10 is a single bond: each outer ring has 2 independent Kekulé arrangements, giving 2 × 1 = 2 structures (the middle ring must have its double bonds at C4a-C8a and C9a-C10a positions). - Total: 2 + 2 = 4 resonance structures. Why other options fail: - (a) 6: Overcounts; naphthalene has 3 resonance structures, and anthracene does not simply triple that. - (b) 5: Incorrect enumeration; no valid fifth Kekulé structure exists for anthracene. - (d) 2: Undercounts; ignores the valid structures where C9=C10 is a double bond and the two outer ring configurations. Therefore, the correct answer is C.