(a) Draw the resonance forms of benzene, cyclobutadiene,

The proton \(N M R\) chemical shifts of the hydrogens in pyridine are shown. These are typical aromatic chemical shifts, except that the ortho protons (on the carbons bonded to nitrogen) are deshielded to \(\delta 8.60\). A suitable oxidizing agent (such as a peroxyacid) can add an oxygen atom to pyridine to give pyridine \(N-\text { oxide }\). The effect of this added oxygen atom is to shift the ortho protons upfield from \(\delta 8.60 \text { to } \delta 8.19\) The meta protons are shifted downfield from \(\delta 7.25\) to \(\delta 7.40\). The para protons are shifted upfield, from \(\delta 7.64 \text { to } \delta 7.32\) Explain this curious effect, shifting some protons upfield and others downfield. Equation Transcription: Text Transcription: N M R \delta 8.60 N-\text { oxide } \delta 8.60 \text { to } \delta 8.19 \delta 7.25 \delta 7.40 \delta 7.64 \text { to } \delta 7.32

Problem 1P Write Lewis structures for the Kekulé representations of benzene. Show all the valence electrons.

Problem 3P (a) Draw the resonance forms of benzene, cyclobutadiene, and cyclooctatetraene, showing all the carbon and hydrogen atoms. (b) Assuming that these molecules are all planar, show how the p orbitals on the sp2 hybrid carbon atoms form continuous rings of overlapping orbitals above and below the plane of the carbon atoms.

Using the information in Figure 16-2, calculate the values of \)\Delta H^{\circ}\) for the following reactions: Equation Transcription: Text Transcription: \Delta H^{\circ} H_2 H_2 H_2

Problem 4P Show the product of the Diels–Alder dimerization of cyclobutadiene. (This reaction is similar to the dimerization of cyclopentadiene, discussed in Section 15-11.)

Does the \(M O \) energy diagram of cyclooctatetraene (Figure 16-8) appear to be a particularly stable or unstable configuration? Explain FIGURE 16-8 The polygon rule predicts that the \(M O \) energy diagrams for these annulenes will resemble the polygonal shapes of the annulenes. Equation Transcription: Text Transcription: MO MO

Problem 6P Make a model of cyclooctatetraene in the tub conformation. Draw this conformation, and estimate the angle between the p orbitals of adjacent pi bonds.

One of the following compounds is much more stable than the other two. Classify each as aromatic, antiaromatic, or nonaromatic

(a) Use the polygon rule to draw an energy diagram (as in Figures 16-5 and 16-7) for the \(\text { MOs }\) of a planar cyclooctatetraenyl system. (b) Fill in the eight pi electrons for cyclooctatetraene. Is this electronic configuration aromatic or antiaromatic? Could the cyclooctatetraene system be aromatic if it gained or lost electrons? (c) Draw pictorial representations (like Figures 16-4 and 16-6) for the three bonding \(\text { MOs }\)and the two nonbonding \(\text { MOs }\) of cyclooctatetraene. The antibonding \(\text { MOs }\) are difficult to draw, except for the all-antibonding \(\text { MO }\). Equation Transcription: Text Transcription: MOs MOs MOs MOs MO \pi_{8}{ }^{*}

Classify the following compounds as aromatic, antiaromatic, or nonaromatic.

Explain why each compound or ion should be aromatic, antiaromatic, or nonaromatic.

(a) Draw the molecular orbitals for the cyclopropenyl case. (Since there are three p orbitals, there must be three \(\text { MOs }\): one all-bonding \(\text { MO }\) and one degenerate pair of \(\text { MOs }\).) (b) Draw an energy diagram for the cyclopropenyl \(\text { MOs }\). (The polygon rule is helpful.) Label each \(\text { MO }\) as bonding, nonbonding, or antibonding, and add the nonbonding line. Notice that it goes through the approximate average of the \(\text { MOs }\). (c) Add electrons to your energy diagram to show the configuration of the cyclopropenyl cation and the cyclopropenyl anion. Which is aromatic and which is antiaromatic? Equation Transcription: Text Transcription: MOs MO MOs MOs MO MOs

The following hydrocarbon has an unusually large dipole moment. Explain how a large dipole moment might arise.

The polarization of a carbonyl group can be represented by a pair of resonance structures:

Problem 16P (a) Explain how pyrrole is isoelectronic with the cyclopentadienyl anion. (b) Specifically, what is the difference between the cyclopentadienyl anion and pyrrole? (c) Draw resonance forms to show the charge distribution on the pyrrole structure.

Problem 17P Show which of the nitrogen atoms in purine are basic, and which one is not basic. For the nonbasic nitrogen, explain why its nonbonding electrons are not easily available to become protonated.

Problem 14P When 3-chlorocyclopropene is treated with AgBF4,AgCl precipitates. The organic product can be obtained as a crystalline material, soluble in polar solvents such as nitromethane but insoluble in hexane. When the crystalline material is dissolved in nitromethane containing KCl, the original 3-chlorocyclopropene is regenerated. Determine the structure of the crystalline material, and write equations for its formation and its reaction with chloride ion.

Explain why each compound is aromatic, antiaromatic, or nonaromatic.

The proton NMR spectrum of 2-pyridone gives the chemical shifts shown. (a) Is 2 -pyridone aromatic? (b) Use resonance forms to explain your answer to (a). Also explain why the protons at and are more deshielded than the other two and . (c) Thymine is one of the heterocyclic bases found in DNA. Do you expect thymine to be aromatic? Explain. (d) The structure of 5 -fluorouracil is shown in the box at the side of the page. Is 5 -fluorouracil aromatic? Explain.

Borazole, \(B_{3} N_{3} H_{6}\) is an unusually stable cyclic compound. Propose a structure for borazole, and explain why it is aromatic. Equation transcription: Text transcription: B{3} N{3} H{6}

Problem 22P Ciprofloxacin is a member of the fluoroquinolone class of antibiotics. (a) Which of its rings are aromatic? (b) Which nitrogen atoms are basic? (c) Which protons would you expect to appear between 6 and 8 in the proton NMR spectrum?

Problem 23P Draw and name all the chlorinated benzenes having from one to six chlorine atoms.

Name the following compounds: \(\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C}=\mathrm{CH}\) \(O H\) \(\mathrm{CH}_{3}\) \(\mathrm{NO}_{2}\) \(\mathrm{COOH}\) \(\mathrm{OCH}\left(\mathrm{CH}_{3}\right)_{2}\) \(\mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) Equation transcription: Text transcription: {CH}{2}{CH}{2}{C}={CH} O H {CH}{3} {NO}{2} {COOH} {OCH}({CH}{3}){2} {CH}{2}{OCH}{2}{CH}{3}

Problem 25P The UV spectrum of 1-phenylprop-2-en-1-ol shows an intense absorption at 220 nm and a weaker absorption at 258 nm. When this compound is treated with dilute sulfuric acid, it rearranges to an isomer with an intense absorption at 250 nm and a weaker absorption at 290 nm. Suggest a structure for the isomeric product and propose a mechanism for its formation.

Problem 26SP Draw the structure of each compound. (a) o-nitroanisole (b) 2,4-dimethoxyphenol (c) p-aminobenzoic acid (d) 4-nitroaniline (e) m-chlorotoluene (f) p-divinylbenzene (g) p-bromostyrene (h) 3,5-dimethoxybenzaldehyde (i) tropylium chloride (j) sodium cyclopentadienide (k) 2-phenylpropan-1-ol (l) benzyl methyl ether (m) p-toluenesulfonic acid (n) o-xylene (o) 3-benzylpyridine

Name the following compounds \(C l\) \(\mathrm{NO}_{2}\) \(\mathrm{OCH}_{3}\) \(B r\) \(\mathrm{COOH}\) \(\mathrm{CH}_{3} \mathrm{O}\) \(\mathrm{CH}_{3}\) \(\mathrm{CHCH}_{2} \mathrm{CH}_{3}\) \(B F_{4}^{-}\) Equation transcription: Text transcription: C l {NO}{2} {OCH}{3} B r {COOH} {CH}{3}{O} {CH}{3} {CHCH}{2}{CH}{3} B F_{4}^{-}

Problem 28SP Draw and name all the methyl, dimethyl, and trimethylbenzenes.

In Kekulé’s time, cyclohexane was unknown, and there was no proof that benzene must be a six-membered ring. Determination of the structure relied largely on the known numbers of monosubstituted and disubstituted benzenes, together with the knowledge that benzene did not react like a normal alkene. The following \(C_{6} H_{6}\) structures were the likely candidates: (a) Show where the six hydrogen atoms are in each structure. (b) For each structure, draw all the possible monobrominated derivatives (C?H?Br that would result from randomly substituting one hydrogen with a bromine. Benzene was known to have only one monobromo derivative. (c) For each of the structures that had only one monobromo derivative in part (b), draw all the possible dibromo derivatives. Benzene was known to have three dibromo derivatives, but resonance theory was unknown at the time. (d) Determine which structure was most consistent with what was known about benzene at that time: Benzene gives one monobrominated derivative and three dibrominated derivatives, and it gives negative chemical tests for an alkene. (e) The structure that was considered the most likely structure for benzene is called Ladenburg benzene, after the chemist who proposed it. What factors would make Ladenburg benzene relatively unstable, in contrast with the stability observed with real benzene? Equation transcription: Text transcription: C{6} H{6}

Four compounds are shown. These compounds react more quickly, or they react with more favorable equilibrium constants, than similar compounds with less conjugated systems. In each case, explain the enhanced reactivity.

Each of the following heterocycles includes one or more nitrogen atoms. Classify each nitrogen atom as strongly basic or weakly basic, according to the availability of its lone pair of electrons.

Some of the following compounds show aromatic properties, and others do not. 1. Predict which ones are likely to be aromatic, and explain why they are aromatic. 2. Predict which nitrogen atoms are more basic than water and which are less basic.

The following molecules and ions are grouped by similar structures. Classify each as aromatic, antiaromatic, or nonaromatic. For the aromatic and antiaromatic species, give the number of pi electrons in the ring.

The benzene ring alters the reactivity of a neighboring group in the benzylic position much like a double bond alters the reactivity of groups in the allylic position. \(H_{2} C=C H-C H_{2}-R\) \(\mathrm{CH}_{2}\) \(C H_{2}-R\) Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates. (a) Use resonance forms to show the delocalization (over four carbon atoms) of the positive charge, unpaired electron, and negative charge of the benzyl cation, radical, and anion. (b) Toluene reacts with bromine in the presence of light to give benzyl bromide. Propose a mechanism for this reaction. \(C H_{3}+B r_{2}\) \(\mathrm{CH}_{2} \mathrm{Br}+\mathrm{HBr}\) (c) Which of the following reactions will have the faster rate and give the better yield? Use a drawing of the transition state to explain your answer. \(\mathrm{CH}_{2} \mathrm{Br}\) \(\frac{\mathrm{NaOCH}_{3}}{\mathrm{CH}_{3} \mathrm{OH}}\) \(\mathrm{CH}_{2} \mathrm{OCH}_{3}\) Equation transcription: Text transcription: H{2} C=C H-C H{2}-R {CH}{2} C H{2}-R C H{3}+B r{2} {CH}{2}{Br}+{HBr} {CH}{2}{Br} frac{{NaOCH}{3}}{{CH}{3}{OH}} {CH}{2}{OCH}{3}

For each NMR spectrum, propose a structure consistent with the spectrum and the additional information provided. (a) Elemental analysis shows the molecular formula to be \(\mathrm{C}_{8} \mathrm{H}_{7} \mathrm{OCI}\). The IR spectrum shows a moderate absorption at \(1602 \mathrm{~cm}^{-1}\) and a strong absorption at \(1690 \mathrm{~cm}^{-1}\) (b) The mass spectrum shows a double molecular ion of ratio 1:1 at m/z 184 and 186. Equation transcription: Text transcription: {C}{8}{H}{7} {OCI} 1602{~cm}^{-1} 1690{~cm}^{-1}

Before spectroscopy was invented, Körner’s absolute method was used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Körner’s method involves adding a third group (often a nitro group) and determining how many isomers are formed. For example, when o-xylene is nitrated (by a method shown in Chapter 17), two isomers are formed. \(\mathrm{CH}_{3}\) \(\frac{\mathrm{HNO}_{3}}{\mathrm{H}_{2} \mathrm{SO}_{4}}\) \(\mathrm{NO}_{2}\) \(O_{2} N\) (a) How many isomers are formed by nitration of -xylene? (b) How many isomers are formed by nitration of -xylene? (c) A turn-of-the-century chemist isolated an aromatic compound of molecular formula \(C_{6} H_{4} B r_{2}\). He carefully nitrated this compound and purified three isomers of formula \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Br}_{2} \mathrm{NO}_{2}\). Propose structures for the original compound and the three nitrated derivatives. Equation transcription: Text transcription: {CH}{3} frac{{HNO}{3}}{{H}{2}{SO}{4}} {NO}{2} O{2} N C{6} H{4} B r{2} {C}{6}{H}{3}{Br}{2}{NO}{2}

Recall (Section 16-10) that two positions of anthracene sometimes react more like polyenes than like aromatic compounds. (a) Draw a Kekulé structure that shows how the reactive positions of anthracene are the ends of a diene, appropriate for a Diels–Alder reaction. (b) The Diels–Alder reaction of anthracene with maleic anhydride is a common organic lab experiment. Predict the product of this Diels–Alder reaction.

Biphenyl has the following structure. (a) Is biphenyl a (fused) polynuclear aromatic hydrocarbon? (b) How many pi electrons are there in the two aromatic rings of biphenyl? How does this number compare with that for naphthalene? (c) The heat of hydrogenation for biphenyl is about \(418 \mathrm{kj} / \mathrm{mol}\) (100 kcal/mol). Calculate the resonance energy of biphenyl. (d) Compare the resonance energy of biphenyl with that of naphthalene and with that of two benzene rings. Explain the difference in the resonance energies of naphthalene and biphenyl. Equation transcription: Text transcription: 418{kj} /{mol}

How would you convert the following compounds to aromatic compounds?

Anions of hydrocarbons are rare, and dianions of hydrocarbons are extremely rare. The following hydrocarbon reacts with two equivalents of butyllithium to form a dianion of formula \(\left[C_{8} H_{6}\right]^{2-}\) Propose a structure for this dianion, and suggest why it forms so readily. \(2 C_{4} H_{9} L i \rightarrow\left[C_{8} H_{6}\right]^{2-}\left(L i^{+}\right)_{2}+2 C_{4} H_{10} \uparrow\) Equation transcription: Text transcription: [C{8} H{6}]^{2-} 2 C{4} H{9} L i rightarrow[C_{8} H_{6}]^{2-}(L i^{+}){2}+2 C{4} H_{10} uparrow

The ribonucleosides that make up ribonucleic acid (RNA) are composed of D-ribose (a sugar) and four heterocyclic “bases.” The general structure of a ribonucleoside is The four heterocyclic bases are cytosine, uracil, guanine, and adenine. Cytosine and uracil are called pyrimidine bases because their structures resemble pyrimidine. Guanine and adenine are called purine bases because their structures resemble purine. (a) Determine which rings of these bases are aromatic. (b) Predict which nitrogen atoms are basic. (c) Do any of these bases have easily formed tautomers that are aromatic? (Consider moving a proton from nitrogen to a carbonyl group to form a phenolic derivative.)

Consider the following compound, which has been synthesized and characterized: (a) Assuming this molecule is entirely conjugated, do you expect it to be aromatic, antiaromatic, or nonaromatic? (b) Why was this molecule synthesized with three tert-butyl substituents? Why not make the unsubstituted compound and study it instead? (c) Do you expect the nitrogen atom to be basic? Explain. Why doesn’t nitrogen’s lone pair overlap with the double bonds to give a total of six electrons in the pi system? (d) At room temperature, the proton NMR spectrum shows only two singlets of ratio 1:2. The smaller signal remains unchanged at all temperatures. As the temperature is lowered to - 100 \({ }^{0} C\), the larger signal broadens and separates into two new singlets, one on either side of the original chemical shift. At - \(100^{\circ} \mathrm{C}\), the spectrum consists of three separate singlets of areas 1:1:1. Explain what these NMR data indicate about the bonding in this molecule. How does your conclusion based on the NMR data agree with your prediction in part (a)? Equation transcription: Text transcription: ^{0} C 100^{circ}{C}

An unknown compound gives the following mass, IR, and NMR spectra. Propose a structure, and show how it is consistent with the spectra. Show the fragmentations that give the prominent peaks at \(m / 127\) and 155 in the mass spectrum. Equation transcription: Text transcription: m / 127

Hexahelicene seems a poor candidate for optical activity because all its carbon atoms are hybrids and presumably flat. Nevertheless, hexahelicene has been synthesizedand separated into enantiomers. Its optical rotation is enormous: \([\alpha]_{D}=3700^{0}\) Explain why hexahelicene is optically active, and speculate as to why the rotation is so large. Equation transcription: Text transcription: [alpha]{D}=3700^{0}

Draw just the bonding \(\pi-M O^{\prime} S\) for the cycloheptatrienyl cation. Draw the energy diagram to show the relative energies of all the MO’s, and show which orbitals the electrons would occupy in the ground state. Predict whether this ion is aromatic, antiaromatic, or nonaromatic. Equation transcription: Text transcription: pi-M O^{prime} S

A student found an old bottle labeled "thymol" on the stockroom shelf. After noticing a pleasant odor, she obtained the following mass, IR, and NMR spectra. The NMR peak at \(\delta 4.8\) disappears on shaking with \(D_{2} O\). Propose a structure for thymol, and show how your structure is consistent with the spectra. Propose a fragmentation to explain the MS peak at \(m / 135\),and show why the resulting ion is relatively stable. Equation transcription: Text transcription: delta 4.8 D{2} O m / 135