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16.23 a) Although this compound is not completely planar, the nitrogen bridge holds it nearly planar. There are 10 TT electrons along the periphery of the ring, so the compound is aromatic. b) This compound has 14 TT electrons along the periphery of the ring, so it is aromatic. 16.24 This compound has 14 TT electrons (only one pair of the triple bond is part of the TT system), so it is aromatic. The hydrogens on the periphery of the ring should appear downfield due to the diamagnetic ring current. The hydrogens on the inside of the ring should appear further upfield. 16.25 The nitrogen with no hydrogen attached is more basic because the lone pair of electrons is in an H + sp² atomic orbital and is not part of the aromatic N cycle of p orbitals. If the other nitrogen were to be protonated, the conjugate acid produced is N no longer aromatic because it does not have a NH₂ H cycle of p orbitals completely around the ring. 16.26 This compound is a very strong acid because its CF₃ CF₃ conjugate base is aromatic and is also stabilized by the electron withdrawing inductive effects of the five CF₃ groups. CF₃ CF₃ CF₃ 16.27 Using cyclopentene as a model, hydrogenation of the two double bonds of pyrrole should produce 2 X 26.6 kcal/mol (111 kJ/mol) = 53.2 kcal/mol (222 kJ/mol). But hydrogenation of pyrrole only produces 31.6 kcal/mol (132 kJ/mol). The difference, 53.2 - 31.6 = 21.6 kcal/mol (90 kJ/mol) is the resonance stabilization of pyrrole. 16.28 Butyllithium is a very strong base. Removal of two protons occurs readily because an aromatic dianion : with 10 TT electrons is formed. 243