Additional Problems for Chapter 15 1. Draw the five resonance structures for phenantrene. Based on the examination of these structures, which carbon-carbon bond of phenantrene should be the shortest? Hornback 656 16.9 2. The conjugate acid of this ketone is quite stable. Explain. Hornback 667 16.21 3. The aminoacid histidine contains an imidazole ring. In many enzymes this ring acts as a basic catalyst. Explain which nitrogen of the imidazole ring is more basic and show the structure of the conjugate acid of imidazole. Hornback 668 16.25 4. Adenine is an important base that is found as a component of DNA. Explain whether adenine is aromatic or not. Hornback 669 16.30 5. Explain how you would distinguish each of the following compounds from the other using NMR spectroscopy. Be explicit. Loudon 736 16.356. What orbitals are used to form the bonds indicated in each molecule? Smith 581 17.2 7. Draw the structures corresponding to each name: Isobutylbenzene, o-dichlorobenzene, cis-1,2-diphenylcyclohexane, m-bromoaniline, 4-chloro-1,2-diethylbenzene. Smith 583 17.4 8. What is the structure of a compound of molecular formula C10H14O2 that shows a strong IR absorption at 3150-2850 cm-1 and gives the following 1H NMR absorptions: 1.4 (triplet, 6H), 4.0 (quartet, 4H), and 6.8 (singlet, 4H) ppm? Smith 584 17.6 9. Would [16]-, [20]- or [22]-annulene be aromatic if each ring is planar? Smith 591 17.9 10. Draw five resonance structures for the cyclopentadienyl cation. Smith 595 17.14 11. Assuming the rings are planar, which ions are aromatic? Smith 595 17.18 12. How many signals will appear in the 13C NMR spectra of the three dihydroxybenzenes (ortho, meta and para)? Jones 667 14.18 13. The compound shown below was synthesized by someone who reasoned that with six double bonds and a triple bond in the molecule, there would be 16 pi electrons and thus no aromatic character. Where was this reasoning wrong? Jones 691 14.5914. Most diazo compounds are exceedingly and sometimes disastrously, unstable. They are notorious explosives. By contrast, the brilliant, iridescent orange compound diazocyclopentadiene Is quite stable. Explain. Jones 689 14.45 15. For each of the following compounds determine which (if any) lone pairs are participating in aromaticity. Klein 835 18.15 16. Firefly luciferin is the compound that enables fireflies to glow. a) The structure exhibits three rings. Identify which of the rings is aromatic. b) Identify which lone pairs are participating in resonance. Klein 854 18.37 17. What is the value of n in Huckel’s rule when a compound has nine pairs of pi electrons? Is such a compound aromatic? Bruice 627 1 18. Draw arrows to show the movement of electrons in going from one resonance contributor to the next in the cyclopentadienyl anion. How many ring atoms share the negative charge? Bruice 629 2 19. Draw the structure of the following: Phenol, benzyl phenyl ether, benzonitrile, benzaldehyde, anisole, toluene, styrene, tert-butylbenzene, benzyl chloride. Bruice 657 3520. Purine is a heterocyclic compound with four nitrogen atoms. Which nitrogen is most apt to be protonated? Which nitrogen is least apt to be protonated? Bruice 658 44 21. Listed below are four compounds that have the molecular formula C6H6. Which of these compounds would yield only one monosubstitution product, if, for example, one hydrogen were replaced by bromine? Solomons 600 14.1 22. Explain why each compound or ion should be aromatic, antiaromatic or nonaromatic. Wade 729 16.12 23. Borazole, B3N3H6 is an unusually stable compound. Propose a structure for borazole and explain why it is aromatic. Wade 736