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14_ - P1 OXT/SRB JWDD052-14 P2 xxx JWDD052-Solomons-v2 14...

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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-14 JWDD052-Solomons-v2 May 1, 2007 14:10 14 AROMATIC COMPOUNDS SOLUTIONS TO PROBLEMS 14.1 Compounds (a) and (b) would yield only one monosubstitution product. 14.2 Resonance structures are defined as being structures that differ only in the positions of the electrons. In the two 1,3,5-cyclohexatrienes shown, the carbon atoms are in different positions; therefore, they cannot be resonance structures. 14.3 Inscribing a square in a circle with one corner at the bottom gives the following results: We see, therefore, that cyclobutadiene would be a diradical and would not be aromatic. 14.4 (a) The cyclopentadienyl anion (above) should be aromatic because it has a closed bonding shell of delocalized π electrons. (b) and (c) The cyclopentadienyl cation (below) would be a diradical. We would not expect it to be aromatic. (d) No, 4 is not a H¨uckel number. 14.5 (a) The cycloheptatrienyl cation (below) would be aromatic because it would have a closed bonding shell of delocalized π electrons. 271
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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-14 JWDD052-Solomons-v2 May 1, 2007 14:10 272 AROMATIC COMPOUNDS (b) No, the cycloheptatrienyl anion (below) would be a diradical. (c) No, 8 is not a H¨uckel number. 14.6 If the 1,3,5-cycloheptatrienyl anion were aromatic, we would expect it to be unusually stable. This would mean that 1,3,5-cycloheptatriene should be unusually acidic. The fact that 1,3,5-cycloheptatriene is not unusually acidic (it is less acidic than 1,3,5-heptatriene) confirms the prediction made in the previous problem, that the 1,3,5-cycloheptatrienyl anion should not be aromatic. 14.7 Br Br (a) Br Br Br heat HBr (b) + Tropylium bromide These results suggest that the bonding in tropylium bromide is ionic; that is, it consists of a positive tropylium ion and a negative bromide ion. 14.8 It suggests that the cyclopropenyl cation should be aromatic. 14.9 The fact that the cyclopentadienyl cation is antiaromatic means that the following hypothetical transformation would occur with an increase in π -electron energy. HC H 2 π -electron energy increases + + + 14.10 (a) The cyclopropenyl cation (below). or SbF 6 + + (b) Only one 13 C NMR signal is predicted for this ion. 14.11 (a) 3 (b) 4 (c) 7 (d) 5
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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-14 JWDD052-Solomons-v2 May 1, 2007 14:10 AROMATIC COMPOUNDS 273 14.12 The high field signal arises from the six methyl protons of trans -15,16-dimethyldihy- dropyrene, which by virtue of their location are strongly shielded by the magnetic field created by the aromatic ring current (see Figure 14.8). 14.13 Major contributors to the hybrid must be ones that involve separated charges. Contributors like the following would have separated charges, and would have aromatic five- and seven- membered rings. etc. + + + 14.14 SH OH N N N N H (a) (b) N N N N H 14.15 Because of their symmetries, p -dibromobenzene would give two 13 C signals, o -dibro- mobenzene would give three, and m -dibromobenzene would give four.
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