Although none of the [10]annulenes given in Section 14.7B is aromatic, the following 10

Provide a name for each of the following compounds.

Listed below are four compounds that have the molecular formula C6H6. Which of these Practice Problem 14.2 compounds would yield only one monosubstitution product, if, for example, one hydrogen were replaced by bromine?

If benzene were 1,3,5-cyclohexatriene, the carboncarbon bonds would be alternately Practice Problem 14.3 long and short as indicated in the following structures. However, to consider the structures here as resonance contributors (or to connect them by a double-headed arrow) violates a basic principle of resonance theory. Explain.

Apply the polygon-and-circle method to cyclopentadienyl cation and explain whether it would be aromatic or not. Cyclopentadienyl cation

Apply the polygon-and-circle method to the cycloheptatrienyl anion and cation and explain whether each would be aromatic or not. (a) Cycloheptatrienyl anion (b) Cycloheptatrienyl cation

1,3,5-Cycloheptatriene is even less acidic than 1,3,5-heptatriene. Explain how this experimental observation might help to confirm your answer to part (b) of the previous problem.

When 1,3,5-cycloheptatriene reacts with one molar equivalent of bromine at 0 8C, it Practice Problem 14.7 undergoes 1,6 addition. (a) Write the structure of this product. (b) On heating, this 1,6-addition product loses HBr readily to form a compound with the molecular formula C7H7Br, called tropylium bromide. Tropylium bromide is insoluble in nonpolar solvents but is soluble in water; it has an unexpectedly high melting point (mp 203 8C), and when treated with silver nitrate, an aqueous solution of tropylium bromide gives a precipitate of AgBr. What do these experimental results suggest about the bonding in tropylium bromide?

The cyclopentadienyl cation is apparently antiaromatic. Explain what this means in terms of the p-electron energies of a cyclic and an open-chain compound.

In 1967 R. Breslow (of Columbia University) and co-workers showed that adding SbCl5 Practice Problem 14.9 to a solution of 3-chlorocyclopropene in CH2Cl2 caused the precipitation of a white solid with the composition C3H3 +SbCl6 -. NMR spectroscopy of a solution of this salt showed that all of its hydrogen atoms were equivalent. (a) What new aromatic ion had these researchers prepared? (b) How many 13C NMR signals would you predict for this ion?

In addition to a signal downfield, the Practice Problem 14.11 1 H NMR spectrum of trans-15,16-dimethyldihydropyrene has a signal far upfield at d -4.2. Account for the presence of this upfield signal.

Azulene has an appreciable dipole moment. Write resonance structures for azulene that Practice Problem 14.12 explain this dipole moment and that help explain its aromaticity.

(a) The iSH group is sometimes called the mercapto group. 6-Mercaptopurine is used in the treatment of acute leukemia. Write its structure. (b) Allopurinol, a compound used to treat gout, is 6-hydroxypurine. Write its structure.

Explain how Practice Problem 14.14 13C NMR spectroscopy could be used to distinguish the ortho-, meta-, and para-dibromobenzene isomers one from another

Four benzenoid compounds, all with the formula C7H7Br, gave the following IR peaks in the 680860-cm-1 region: A, 740 cm-1 (strong) C, 680 cm-1 (strong) and 760 cm-1 (very strong) B, 800 cm-1 (very strong) D, 693 cm-1 (very strong) and 765 cm-1 (very strong) Propose structures for A, B, C, and D.

Write structural formulas for each of the following: (a) 3-Nitrobenzoic acid (b) p-Bromotoluene (c) o-Dibromobenzene (d) m-Dinitrobenzene (e) 3,5-Dinitrophenol (f) p-Nitrobenzoic acid (g) 3-Chloro-1-ethoxybenzene (h) p-Chlorobenzenesulfonic acid (i) Methyl p-toluenesulfonate (j) Benzyl bromide (k) p-Nitroaniline (l) o-Xylene (m) tert-Butylbenzene (n) p-Methylphenol (o) p-Bromoacetophenone (p) 3-Phenylcyclohexanol (q) 2-Methyl-3-phenyl-1-butanol (r) o-Chloroanisole

Write structural formulas and give acceptable names for all representatives of the following: (a) Tribromobenzenes (c) Nitroanilines (e) Isomers of C6H5iC4H9 (b) Dichlorophenols (d) Methylbenzenesulfonic acids

Which of the following molecules would you expect to be aromatic?

Use the polygon-and-circle method to draw an orbital diagram for each of the following compounds.

Which of the hydrogen atoms shown below is more acidic? Explain your answer.

The rings below are joined by a double bond that undergoes cistrans isomerization much more readily than the bond of a typical alkene. Provide an explanation.

Although Hckels rule (Section 14.7) strictly applies only to monocyclic compounds, it does appear to have application to certain bicyclic compounds, if one assumes use of resonance structures involving only the perimeter double bonds, as shown with one resonance contributor for naphthalene below. Both naphthalene (Section 14.8A) and azulene (Section 14.8B) have 10 p electrons and are aromatic. Pentalene (below) is apparently antiaromatic and is unstable even at -100 8C. Heptalene has been made but it adds bromine, it reacts with acids, and it is not planar. Is Hckels rule applicable to these compounds? If so, explain their lack of aromaticity

In 1960 T. Katz (Columbia University) showed that cyclooctatetraene adds two electrons when treated with potassium metal and forms a stable, planar dianion, C8H8 2- (as the dipotassium salt): 2 K C8H8 2 2 equiv. K THF Use the molecular orbital diagram given in Fig. 14.7 and explain this result. (b) In 1964 Katz also showed that removing two protons from the compound below (using butyllithium as the base) leads to the formation of a stable dianion with the formula C8H6 2- (as the dilithium salt). 2 BuLi 2 Li C8H6 2 Propose a reasonable structure for the product and explain why it is stable.

Although none of the [10]annulenes given in Section 14.7B is aromatic, the following 10 p-electron system is aromatic: What factor makes this possible?

Cycloheptatrienone (I) is very stable. Cyclopentadienone (II) by contrast is quite unstable and rapidly undergoes a DielsAlder reaction with itself. (a) Propose an explanation for the different stabilities of these two compounds. (b) Write the structure of the DielsAlder adduct of cyclopentadienone.

5-Chloro-1,3-cyclopentadiene (below) undergoes SN1 solvolysis in the presence of silver ion extremely slowly even though the chlorine is doubly allylic and allylic halides normally ionize readily (Section 15.15). Provide an explanation for this behavior.

Explain the following: (a) Cyclononatetraenyl anion is planar (in spite of the angle strain involved) and appears to be aromatic. (b) Although [16]annulene is not aromatic, it adds two electrons readily to form an aromatic dianion.

Furan possesses less aromatic character than benzene as measured by their resonance energies (96 kJ mol-1 for furan; 151 kJ mol-1 for benzene). What reaction have we studied earlier that shows that furan is less aromatic than benzene and can react in a way characteristic of some dienes?

Assign structures to each of the compounds A, B, and C whose 1 H NMR spectra are shown in Fig. 14.27

The 1 H NMR spectrum of cyclooctatetraene consists of a single line located at d 5.78. What does the location of this signal suggest about electron delocalization in cyclooctatetraene?

Give a structure for compound F that is consistent with the 1 H NMR spectrum in Fig. 14.28 and IR absorptions at 3020, 2965, 2940, 2870, 1517, 1463, and 818 cm-1 .

A compound (L) with the molecular formula C9H10 reacts with bromine and gives an IR absorption spectrum that includes the following absorption peaks: 3035 cm-1 (m), 3020 cm-1 (m), 2925 cm-1 (m), 2853 cm-1 (w), 1640 cm-1 (m), 990 cm-1 (s), 915 cm-1 (s), 740 cm-1 (s), 695 cm-1 (s). The 1 H NMR spectrum of L consists of: Doublet d 3.1 (2H) Multiplet d 5.1 Multiplet d 7.1 (5H) Multiplet d 4.8 Multiplet d 5.8 The UV spectrum shows a maximum at 255 nm. Propose a structure for compound L and make assignments for each of the IR peaks.

Compound M has the molecular formula C9H12. The 1 H NMR spectrum of M is given in Fig. 14.29 and the IR spectrum in Fig. 14.30. Propose a structure for M

A compound (N) with the molecular formula C9H10O reacts with osmium tetroxide. The 1 H NMR spectrum of N is shown in Fig. 14.31 and the IR spectrum of N is shown in Fig. 14.32. Propose a structure for N.

The IR and 1 H NMR spectra for compound X (C8H10) are given in Fig. 14.33. Propose a structure for compound X

The IR and 1 H NMR spectra of compound Y (C9H12O) are given in Fig. 14.34. Propose a structure for Y.

a) How many signals would you expect to find in the 1 H NMR spectrum of caffeine? N N N N H3C O CH3 O CH3 Caffeine (b) What characteristic peaks would you expect to find in the IR spectrum of caffeine?

Consider these reactions: A B t-BuOK KCl HBr t-BuOH Cl The intermediate A is a covalently bonded compound that has typical 1 H NMR signals for aromatic ring hydrogens and only one additional signal at d 1.21, with an area ratio of 5:3, respectively. Final product B is ionic and has only aromatic hydrogen signals. What are the structures of A and B?

The final product of this sequence, D, is an orange, crystalline solid melting at 1748C and having molecular weight 186: Cyclopentadiene + Na 9: C + H2 2 C + FeCl2 9: D + 2 NaCl In its 1 H and 13C NMR spectra, product D shows only one kind of hydrogen and only one kind of carbon, respectively. Draw the structure of C and make a structural suggestion as to how the high degree of symmetry of D can be explained. (D belongs to a group of compounds named after something you might get at a deli for lunch.)

Compound E has the spectral features given below. What is its structure? MS (m/z): M # + 202 IR (cm-1 ): 30303080, 2150 (very weak), 1600, 1490, 760, and 690 1 H NMR (d): narrow multiplet centered at 7.34 UV (nm): 287 (P = 25,000), 305 (P = 36,000), and 326 (P = 33,000)

Draw all of the p molecular orbitals for (3E)-1,3,5-hexatriene, order them from lowest to highest in energy, and indicate the number of electrons that would be found in each in the ground state for the molecule. After doing so, open the computer molecular model for (3E)-1,3,5-hexatriene and display the calculated molecular orbitals. How well does the appearance and sequence of the orbitals you drew (e.g., number of nodes, overall symmetry of each, etc.) compare with the orbitals in the calculated model? Are the same orbitals populated with electrons in your analysis as in the calculated model?

Write mechanism arrows for the following step in the chemical synthesis by A. Robertson and R. Robinson (J. Chem. Soc. 1928, 14551472) of callistephin chloride, a red flower pigment from the purple-red aster. Explain why this transformation is a reasonable process.

The following reaction sequence was used by E. J. Corey (J. Am. Chem. Soc. 1969, 91, 56755677) at the beginning of a synthesis of prostaglandin F2a and prostaglandin E2. Explain what is involved in this reaction and why it is a reasonable process.

The 1 H NMR signals for the aromatic hydrogens of methyl p-hydroxybenzoate appear as two doublets at approximately 7.05 and 8.04 ppm (d). Assign these two doublets to the respective hydrogens that produce each signal. Justify your assignments using arguments of relative electron density based on contributing resonance structures.

Draw the structure of adenine, a heterocyclic aromatic compound incorporated in the structure of DNA. Identify the nonbonding electron pairs that are not part of the aromatic system in the rings of adenine. Which nitrogen atoms in the rings would you expect to be more basic and which should be less basic?

Draw structures of the nicotinamide ring in NADH and NAD+. In the transformation of NADH to NAD+, in what form must a hydrogen be transferred in order to produce the aromatic pyridinium ion in NAD+?