See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

To determine the number of pi electrons in each monocyclic compound, we use the degree of unsaturation (DoU) formula and account for the charge by adjusting the electron count. Concept: For a monocyclic fully conjugated system, each double bond contributes 2 pi electrons, and lone pairs on charged carbons (carbanions) or empty orbitals (carbocations) participate in the pi system. The ring itself accounts for one degree of unsaturation, so we focus on the pi electrons from double bonds and the charge. For a cyclic conjugated hydrocarbon CnHm with charge z (negative z means extra electrons, positive z means fewer electrons), the number of pi electrons can be estimated from the structure. (a) C4H4^(2-): This is the cyclobutadienyl dianion. Neutral C4H4 (cyclobutadiene) has 2 degrees of unsaturation (1 ring + 2 double bonds in the ring contributing 4 pi electrons). Adding 2 negative charges adds 2 electrons to the pi system: 4 + 2 = 6 pi electrons. So C4H4^(2-) has 6 pi electrons. Matches (q). (b) C4H4^(2+): This is the cyclobutadienyl dication. Starting from cyclobutadiene's 4 pi electrons, removing 2 electrons: 4 - 2 = 2 pi electrons. So C4H4^(2+) has 2 pi electrons. Matches (p). (c) C9H9^(+1): This is the cyclononatetraenyl cation (9-membered ring). Neutral C9H9 would be cyclononatetraene with formula suggesting 4 double bonds in a 9-membered ring contributing 8 pi electrons. With a +1 charge (removing 1 electron): 8 - 0 = 8 pi electrons (the cation removes one electron but from a sp3 CH2 that becomes a carbocation contributing an empty p orbital, so the ring goes from having one sp3 carbon to full conjugation). For C9H9^+: 9 carbons in a ring, with the formula C9H9^+ suggesting 8 pi electrons (4 double bonds worth, the + means one less electron than neutral, but the neutral sp3 CH2 had 2H; losing H+ or e- to form cation gives full conjugation with 8 pi electrons). Matches (r). (d) C9H9^(-1): This is the cyclononatetraenyl anion. For C9H9^-: similar reasoning, the anion adds 2 electrons to the sp3 carbon making it a carbanion with a lone pair entering conjugation, giving 8 + 2 = 10 pi electrons. Matches (s). Summary: - (a) C4H4^(2-) → 6 pi electrons → (q) - (b) C4H4^(2+) → 2 pi electrons → (p) - (c) C9H9^(+1) → 8 pi electrons → (r) - (d) C9H9^(-1) → 10 pi electrons → (s) Therefore, the correct answer is a-q; b-p; c-r; d-s.