Chapter Questions
Tell whether the following compounds are ortho-, meta-, or para-disubstituted:a.b.c.
Give IUPAC names for the following compounds:a.b.c.d.e.f.
Draw structures corresponding to the following IUPAC names:(a) $p$ -Bromochlorobenzene(b) $p$ -Bromotoluene(c) $m$ -Chloroaniline(d) 1 -Chloro-3,5-dimethylbenzene
Pyridine is a flat, hexagonal molecule with bond angles of $120^{\circ} .$ It undergoes substitution rather than addition and generally behaves like benzene. Draw a picture of the $\pi$ orbitals of pyridine to explain its properties. Check your answer by looking ahead to Section $9-4$.
To be aromatic, a molecule must have $4 n+2 \pi$ electrons and must be planar for cyclic conjugation. Cyclodecapentaene fulfills one of these criteria but not the other and has resisted all attempts at synthesis. Explain.
Cycloocta-1,3,5,7-tetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, $\mathrm{C}_{8} \mathrm{H}_{8}{ }^{2-}$. Why do you suppose this reaction occurs so easily? What geometry do you think the cyclooctatetraene dianion might have?
Draw an orbital picture of furan to show how the molecule is aromatic.
Thiamin, or vitamin $\mathrm{B}_{1}$, contains a positively charged five-membered nitrogensulfur heterocycle called a thiazolium ring. Explain why the thiazolium ring is aromatic.
Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene. Is azulene aromatic? Draw a second resonance form of azulene in addition to the one shown.
How many electrons does each of the four nitrogen atoms in purine contribute to the aromatic $\pi$ system?
Propose a mechanism for the electrophilic fluorination of benzene with $\mathrm{F}-\mathrm{TEDA}-\mathrm{BF}_{4}$.
Monobromination of toluene gives a mixture of three bromotoluene products. Draw and name them.
How many products might be formed on chlorination of $o$ -xylene (o-dimethylbenzene), $m$ -xylene, and $p$ -xylene?
Which of the following alkyl halides would you expect to undergo FriedelCrafts reaction without rearrangement? Explain.(a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}$(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{3}$(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$(d) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{Cl}$(e) Chlorocyclohexane
What is the major product from the Friedel-Crafts reaction of benzene with 1-chloro-2-methylpropane in the presence of $\mathrm{AlCl}_{3}$ ?
Identify the carboxylic acid chloride that might be used in a Friedel-Crafts acylation reaction to prepare each of the following acylbenzenes:a.b.
Rank the compounds in each group in order of their reactivity to electrophilic substitution:(a) Nitrobenzene, phenol, toluene, benzene(b) Phenol, benzene, chlorobenzene, benzoic acid(c) Benzene, bromobenzene, benzaldehyde, aniline
Use Figure 9.18 to explain why Friedel-Crafts alkylations often give polysubstitution products but Friedel-Crafts acylations do not.
An electrostatic potential map of (trifluoromethyl)benzene, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CF}_{3},$ is shown. Would you expect (trifluoromethyl)benzene to be more reactive or less reactive than toluene toward electrophilic substitution? Explain.
Predict the major products of the following reactions:(a) Nitration of methyl benzoate, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}$(b) Bromination of nitrobenzene(c) Chlorination of phenol(d) Bromination of aniline
Write resonance structures for $o-, m-,$ and $p$ - intermediates in the nitration of chlorobenzene to show the electron-donating resonance effect of the chloro group.
Predict the major product you would expect from reaction of each of the following substances with $\mathrm{Cl}_{2}$ and $\mathrm{FeCl}_{3}$ (blue $=\mathrm{N}$, reddish brown $=\mathrm{Br}$ ):a.b.
The herbicide oxyfluorfen can be prepared by reaction between a phenol and an aryl fluoride. Propose a mechanism.
What aromatic products would you obtain from the $\mathrm{KMnO}_{4}$ oxidation of the following substances?a.b.
How might you prepare diphenylmethane, $(\mathrm{Ph})_{2} \mathrm{CH}_{2},$ from benzene and an appropriate acid chloride? More than one step is needed.
How might you synthesize the following substances from benzene?(a) $m$ -Chloronitrobenzene(b) $m$ -Chloroethylbenzene(c) $p$ -Chloropropylbenzene(d) 3 -Bromo-2-methylbenzenesulfonic acid
In planning a synthesis, it's as important to know what not to do as to know what to do. As written, the following reaction schemes have flaws in them. What is wrong with each?a.b.
Give IUPAC names for the following substances (red $=\mathrm{O}$, blue $=\mathrm{N}$ ):a.b.
The following molecular model is that of a carbocation. Draw two resonance structures for the carbocation, indicating the positions of the double bonds.
Draw the product from reaction of each of the following substances with (a) $\mathrm{Br}_{2}, \mathrm{FeBr}_{3}$ and $(\mathrm{b}) \mathrm{CH}_{3} \mathrm{COCl}, \mathrm{AlCl}_{3}(\mathrm{red}=\mathrm{O})$.a.b.
How would you synthesize the following compound starting from benzene? More than one step is needed (red $=\mathrm{O}$, blue $=\mathrm{N}$ ).
Azulene, an isomer of naphthalene, has a remarkably large dipole moment for a hydrocarbon $(\mu=1.0 \mathrm{D})$. Explain using resonance structures.
Draw structures corresponding to the following names:(a) 3-Methyl-2-nitrobenzoic acid(b) Benzene- 1,3,5 -triol(c) 3-Methyl-2-phenylhexane(d) $o$ -Aminobenzoic acid(e) $m$ -Bromophenol(f) 2,4,6 -Trinitrophenol (picric acid)
Draw and name all possible isomers of the following:(a) Dinitrobenzene(b) Bromodimethylbenzene(c) Trinitrophenol
Draw and name all possible aromatic compounds with the formula $\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{Cl}$.
Draw and name all possible aromatic compounds with the formula $\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}$. (There are 14.)
Look at the three resonance structures of naphthalene shown in Section $9-5,$ and account for the fact that not all carbon-carbon bonds have the same length. The C1-C2 bond is 136 pm long, whereas the C2-C3 bond is $139 \mathrm{pm}$ long.
Anthracene has four resonance structures, one of which is shown. Draw the other three
Phenanthrene has five resonance structures, one of which is shown. Draw the other four.
Look at the five resonance structures for phenanthrene (Problem 9.40 ) and predict which of its carbon-carbon bonds is shortest.
Calicene, like azulene (Problem 9.32), has an unusually large dipole moment for a hydrocarbon. Explain using resonance structures.
Draw resonance structures of the intermediate carbocations in the bromination of naphthalene, and account for the fact that naphthalene undergoes electrophilic substitution at C1 rather than C2.
Which would you expect to be most stable, cyclononatetraenyl radical, cation, or anion?
How might you convert cyclonona-1,3,5,7-tetraene to an aromatic substance?
Pentalene is a most elusive molecule that has been isolated only at liquid-nitrogen temperature. The pentalene dianion, however, is well known and quite stable. Explain.
3-Chlorocyclopropene, on treatment with AgBF $_{4}$, gives a precipitate of AgCl and a stable solution of a carbocation product. What is a likely structure for the product, and what is its relation to Hückel's rule?
Cyclopropanone is highly reactive because of its large amount of angle strain. But methylcyclopropenone, although even more strained than cyclopropanone, is nevertheless quite stable and can even be distilled. Explain, taking the polarity of the carbonyl group into account.
Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can't be isolated. Explain, taking the polarity of the carbonyl group into account.
Indole is an aromatic heterocycle that has a benzene ring fused to a pyrrole ring. Draw an orbital picture of indole.(a) How many $\pi$ electrons does indole have?(b) What is the electronic relationship of indole to naphthalene?
Identify each of the following groups as an activator or deactivator and as an $o, p$ -director or $m$ -director:a.b.c.d.
Identify each of the following groups as an activator or deactivator and as an $o, p$ -director or $m$ -director:Predict the major product(s) of mononitration of the following substances. Which react faster than benzene, and which slower?(a) Bromobenzene(b) Benzonitrile(c) Benzoic acid(d) Nitrobenzene(e) Benzenesulfonic acid(f) Methoxybenzene
Rank the following aromatic compounds in their expected order of reactivity toward Friedel-Crafts alkylation. Which compounds are unreactive?(a) Bromobenzene(b) Toluene(c) Phenol(d) Benzoic acid(e) Nitrobenzene(f) $p$ -Bromotoluene
Rank the compounds in each group according to their reactivity toward electrophilic substitution.(a) Chlorobenzene, $o$ -dichlorobenzene, benzene(b) $p$ -Bromonitrobenzene, nitrobenzene, phenol(c) Fluorobenzene, benzaldehyde, $o$ -xylene(d) Benzonitrile, $p$ -methylbenzonitrile, $p$ -methoxybenzonitrile
Propose structures for aromatic hydrocarbons that meet the following descriptions:(a) $\mathrm{C}_{9} \mathrm{H}_{12}$; gives only one $\mathrm{C}_{9} \mathrm{H}_{11} \mathrm{Br}$ product on substitution with bromine(b) $\mathrm{C}_{10} \mathrm{H}_{14}$; gives only one $\mathrm{C}_{10} \mathrm{H}_{13} \mathrm{Cl}$ product on substitution with chlorine(c) $\mathrm{C}_{8} \mathrm{H}_{10}$; gives three $\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}$ products on substitution with bromine(d) $\mathrm{C}_{10} \mathrm{H}_{14}$; gives two $\mathrm{C}_{10} \mathrm{H}_{13} \mathrm{Cl}$ products on substitution with chlorine
Predict the major product(s) of the following reactions:a.b.c.
Predict the major monoalkylation products you would expect to obtain from reaction of the following substances with chloromethane and $\mathrm{AlCl}_{3}:$(a) $p$ -Chloroaniline(b) $m$ -Bromophenol(c) 2,4-Dichlorophenol(d) 2,4 -Dichloronitrobenzene(e) $p$ -Methylbenzenesulfonic acid(f) 2,5 -Dibromotoluene
Name and draw the major product(s) of electrophilic chlorination of the following compounds:(a) $m$ -Nitrophenol(b) $o$ -Xylene (dimethylbenzene)(c) $p$ -Nitrobenzoic acid(d) $p$ -Bromobenzenesulfonic acid
Aromatic iodination can be carried out with a number of reagents, including iodine monochloride, ICl. What is the direction of polarization of ICl? Propose a mechanism for the iodination of an aromatic ring with ICl.
When benzene is treated with $\mathrm{D}_{2} \mathrm{SO}_{4}$, deuterium slowly replaces all six hydrogens in the aromatic ring. Explain.
The carbocation electrophile in a Friedel-Crafts reaction can be generated in ways other than by reaction of an alkyl chloride with $\mathrm{AlCl}_{3} .$ For example, reaction of benzene with 2 -methylpropene in the presence of $\mathrm{H}_{3} \mathrm{PO}_{4}$ yields tert-butylbenzene. Propose a mechanism for this reaction.
The nitroso group, $-\mathrm{N}=\mathrm{O}$, is one of the few nonhalogens that is an ortho- and para-directing deactivator. Explain by drawing resonance structures of the carbocation intermediates in ortho, meta, and para electrophilic reaction on nitrosobenzene, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}=\mathrm{O}$.
The $N, N, N$ -trimethylammonium group, $-\mathrm{N}^{+}\left(\mathrm{CH}_{3}\right)_{3},$ is one of the few groups that is a meta-directing deactivator yet has no electronwithdrawing resonance effect. Explain.
Using resonance structures of the intermediates, explain why bromination of biphenyl occurs at ortho and para positions rather than at meta.
How would you synthesize the following substances starting from benzene? Assume that ortho- and para-substitution products can be separated.(a) $p$ -Bromoaniline(b) $m$ -Bromoaniline(c) 2,4,6 -Trinitrobenzoic acid(d) 3,5-Dinitrobenzoic acid
Starting with either benzene or toluene, how would you synthesize the following substances? Assume that ortho and para isomers can be separated.(a) 2 -Bromo-4-nitrotoluene(b) 2,4,6 -Tribromoaniline(c) 3-Bromo-4-tert-butylbenzoic acid(d) 1,3 -Dichloro-5-ethylbenzene
As written, the following syntheses have flaws. What is wrong with each?a.b.
Ribavirin, an antiviral agent used against hepatitis C and viral pneumonia, contains a 1,2,4 -triazole ring. Why is the ring aromatic?
Valdecoxib (Bextra), a COX-2 inhibitor once used in the treatment of osteoarthritis, contains an isoxazole ring. Why is the ring aromatic?
On reaction with acid, 4 -pyrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hückel $4 n+2$ rule, explain why the protonated product is so stable.
$N$ -Phenylsydnone, so named because it was first studied at the University of Sydney, Australia, behaves like a typical aromatic molecule. Explain using the Hückel $4 n+2$ rule.
Draw resonance structures of the potential ortho, meta, and para intermediates, and explain at what position and on what ring you would expect bromination of benzanilide to occur.
Electrophilic substitution on 3-phenylpropanenitrile occurs at the ortho and para positions, but reaction with 3 -phenylpropenenitrile occurs at the meta position. Explain, using resonance structures of the intermediates.
Addition of HBr to 1 -phenylpropene yields only (1-bromopropyl)benzene. Propose a mechanism for the reaction, and explain using resonance structures why none of the other regioisomer is produced.
Phenylboronic acid, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~B}(\mathrm{OH})_{2}$, is nitrated to give $15 \%$ orthosubstitution product and $85 \%$ meta. Explain the meta-directing effect of the $-\mathrm{B}(\mathrm{OH})_{2}$ group.
Benzene and alkyl-substituted benzenes can be hydroxylated by reaction with $\mathrm{H}_{2} \mathrm{O}_{2}$ in the presence of a strong acid catalyst. What is the likely structure of the reactive electrophile? Review Figure 9.13 on page 288 , and then propose a mechanism for the reaction.
Propose a mechanism to account for the following reaction:
In the Gatterman-Koch reaction, a formyl group (-CHO) is introduced directly onto a benzene ring. For example, reaction of toluene with CO and $\mathrm{HCl}$ in the presence of $\mathrm{AlCl}_{3}$ gives $p$ -methylbenzaldehyde. Propose a mechanism.
Hexachlorophene, a substance used in the manufacture of germicidal soaps, is prepared by reaction of 2,4,5 -trichlorophenol with formaldehyde in the presence of concentrated sulfuric acid. Propose a mechanism for the reaction.
Use your knowledge of directing effects, along with the following data, to deduce the directions of the dipole moments in aniline and bromobenzene.
Identify the reagents represented by the letters a-c in the following scheme:
Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The $\mathrm{pK}_{\mathrm{a}}$ of unsubstituted phenol, for example, is 9.89 , while that of $p$ -nitrophenol is $7.15 .$ Draw resonance structures of the corresponding phenoxide anions, and explain the data.
In light of your answer to Problem $9.82,$ would you expect $p$ -methylphenol to be more acidic or less acidic than unsubstituted phenol? What about $p$ -bromophenol? Explain.