# 26.E: More on Aromatic Compounds (Exercises)


Exercise 26-1 Predict which one of the following pairs of compounds will be the stronger acid. Give your reasoning.

a. 3-nitrobenzenol or 4-nitrobenzenol
b. 3,5-dimethyl-4-nitrobezenol or 2,6-dimethyl-4-nitrobenzenol
c. 4-methoxybenzenol or 3-methoxybenzenol
d. azabenzen-4-ol or azabenzen-3-ol

Exercise 26-2* Use bond and stabilization energies to calculate $$\Delta H^0 \left( g \right)$$ for the reaction of Equation 26-1 on the assumption that the extra stabilization energy of 2-naphthalenol relative to naphthalene is $$5 \: \text{kcal mol}^{-1}$$ (see Tables 4-3 and 21-1). Compare your answer to $$\Delta H^0$$ calculated for the corresponding reactions of benzenol (Section 26-1A).

Exercise 26-3* Treatment of 1,3-benzenediol (resorcinol) with an ammonia-ammonium chloride solution under pressure at $$200^\text{o}$$ (no sulfite) gives 3-aminobenzenol. Write a reasonable mechanism for this transformation. Would you expect benzenol itself to react similarly? Why?

Exercise 26-4* Explain why benzenol with bromine gives tribromobenzenol readily in water solution but 2- and 4-monobromobenzenol in nonpolar solvents. Notice that 2,4,6-tribromobenzenol is at least a 300-fold stronger acid than phenol in water solution.

Exercise 26-5 When 2-naphthalenol is treated with bromine in ethanoic acid it first gives 1-bromo-2-naphthalenol then 1,6-dibromo-2-naphthalenol. Explain the order of substitution, giving attention to why disubstitution does not lead to 1,3-dibromo-2-naphthalenol.

Exercise 26-6 The herbicide "2,4-D" is (2,4-dichlorophenoxy)ethanoic. Show how this substance could be synthesized starting from benzenol and ethanoic acid.

Exercise 26-7 1,3-Benzenediol (resorcinol) can be converted to a carboxylic acid with carbon dioxide and alkali. Would you expect 1,3-benzenediol to react more, or less, readily than benzenol? Why? Which is the most likely point of monosubstitution? Explain.

Exercise 26-8 Explain how you would use proton NMR spectra to show that the product of oxidation of 2,4,6-tri-tert-butylbenzenol in the presence of butadiene links the aromatic rings at the 4-position, not at the 2-position.

Exercise 26-9* Benzenol samples that have been allowed to stand in air always are pink or red because of oxidation. Write a mechanism for the oxidation of benzenol by oxygen that could lead to one or more products that may be expected to be colored.

Exercise 26-10* Explain why gallic acid decarboxylates on heating more readily than benzoic acid. Would you expect 2,5-dihydroxybenzoic acid to decarboxylate as readily as its 2,4 isomer? Explain.

Exercise 26-11* Work out the course of hydrolysis and decarboxylation of 2,4,6-triaminobenzoic acid to 1,3,5-benzenetriol.

Exercise 26-12* Devise a reasonable sequence of synthetic steps for conversion of eugenol to the flavoring material vanillin, which is 3-methoxy-4-hydroxybenzenecarbaldehyde (Section 26-5).

Exercise 26-13* Reduction of the carbonyl group of quercetin gives a substance that loses water readily to give a brilliant-violet compound. This compound, when treated with hydrochloric acid, is converted reversibly to a red salt. Consider possible ways that the reduction product could dehydrate to give a violet substance and show how addition of a proton to it could occur in such a way as to give a substantial color change.

Exercise 26-14* Natural usnic acid is optically active but racemizes on heating without need for acid or base catalysts. Write a mechanism involving a reversible electrocyclic reaction for this racemization that also accounts for the fact that when usnic acid is heated in ethanol, an optically inactive ethyl ester of a carboxylic acid is formed. (Review Sections 21-10D and 17-6B.)

Exercise 26-15 Arrange the following quinones in the order of expected increasing half-cell potential for reduction (the larger the potential the greater the tendency for reduction): 1,4-benzenedione, 4,4'-biphenyldione, cis-2,2'-dimethyl-4,4'-biphenyldione, 9,10-anthracenedione, and 1,4-naphthalenedione. Your reasoning should be based on differences expected in the stabilization of the arenediones and arenediols, including steric factors, if any.

Exercise 26-16 Reduction of 9,10-anthracenedione with tin and hydrochloric acid in ethanoic acid produces a solid, pale-yellow ketone (mp $$156^\text{o}$$), which has the formula $$\ce{C_{14}H_{10}O}$$. This ketone is not soluble in cold alkali but does dissolve when heated with alkali. Acidification of cooled alkaline solutions of the ketone precipitates a brown-yellow isomer of the ketone (mp $$120^\text{o}$$), which gives a color with ferric chloride, couples with diazonium salts (Section 23-10C), reacts with bromine, and slowly reverts tot he isomeric ketone.

What are the likely structures of the ketone and its isomer? Write equations for the reactions described and calculate $$\Delta H^0$$ for interconversion of the isomers in the vapor phase. (Review Sections 26-1 and 21-7.)

Exercise 26-17* Write resonance structures that account for the stability of the cation of Wurster's salts, such as Wurster's Blue, $$2$$. Explain why $$\ce{N}$$,$$\ce{N}$$,$$\ce{N'}$$,$$\ce{N'}$$-2,3,5,6-octamethyl-1,4-benzenediamine does not form a similarly stable cation radical.

Exercise 26-18* Acidification of a solution containing semiquinone radicals such as $$1$$ tends to cause the radicals to disproportionate to the arenediol and arenedione. Why should acid cause changes in the relative stabilities of the semiquinones and the corresponding diol-dione pairs?

Exercise 26-19* The biologically important quinone called plastoquinone is similar to CoQ, except that the $$\ce{CH_3O}-$$ groups of CoQ are replaced by $$\ce{CH_3}-$$ groups. What differences in properties would you expect between plastoquinone and CoQ and their respective reduction products? Consider half-cell potentials (see Exercise 26-15), solubility in polar and nonpolar solvents, and relative acidity.

Exercise 26-20* You will see that the natural quinones, vitamin K$$_1$$, plastoquinone, and CoQ, all have three or four groups on the quinone ring. What kind of possible destructive side reactions would ring substituents tend to prevent? Give your reasoning.

Exercise 26-21* Devise a synthesis of dimethoxy- and dihydroxy-1,2-cyclobutenedione based on the expected dimerization product of trifluorochloroethene (Section 13-3D).

Exercise 26-22 Tropone (2,4,6-cycloheptatrienone) is an exceptionally strong base for a ketone. Explain.

Exercise 26-23 At which position would you expect tropolone to substitute most readily with nitric acid? Explain.

Exercise 26-24 Would you expect benzotropylium ion to form the corresponding $$\ce{OH}$$ derivative more readily or less readily than tropylium ion itself in water solution? At which position would you expect the $$\ce{C-O}$$ bond to be formed? Explain.

Exercise 26-25 What principal product would you expect from each of the following reactions? Show the steps involved.

a.

b.

c.

d.

Exercise 26-26 Devise syntheses of the following compounds from the specified starting materials, giving reagents and approximate reactions conditions. (If necessary, review the reactions of Chapter 22 as well as reactions discussed in the sections of this chapter.)

a. 4-aminobenzenecarbaldehyde from methylbenzene
b. 2,2'-biphenyldicarboxylic acid $$\left[ \ce{C_6H_4} \left( 2-\ce{CO_2H} \right) \ce{C_6H_4} \left( 2'-\ce{CO_2H} \right) \right]$$ from phenanthrene
c. 4-nitrotrifluoromethylbenzene from methylbenzene
d. 9,9,10,10-tetrafluoro-9,10-dihydroanthracene from 1,2-dimethylbenzene and benzene
e. 4-methylphenylethanenitrile from methylbenzene
f. 4-chlorobenzenecarbaldehyde from methylbenzene
g. 2-hydroxy-3-methylbenzenecarbaldehyde from methylbenzene
h. 4-ethylphenylmethanol from benzene, methylbenzene, or ethylbenzene
i. 2-chloro-4-ethoxybenzenecarbaldehyde from benzene or methylbenzene

Exercise 26-27

a. Explain why the energy of ionic dissociation of triarylmethyl chlorides in liquid sulfur dioxide decreases in the order $$7$$ $$>$$ $$8$$ $$>$$ $$9$$. (Review Section 22-8A and also consider possible effects of steric hindrance in the starting material and the cations formed.)

b. Which alcohol would you expect to form a carbocation more readily in sulfuric acid, $$10$$ or $$11$$? Explain.

c. When triphenylmethanol is dissolved in $$100\%$$ sulfuric acid, it gives a freezing-point depression that corresponds to formation of four moles of particles per mole of alcohol dissolved. Explain.

Exercise 26-28 Explain why 9-phenylfluorene is a stronger acid than triphenylmethane.

Exercise 26-29

a. Why should 3-phenyl-1-propene be appreciably more reactive than methylbenzene in hydrogen-abstraction reactions?

b. Would you expect 1-phenyl-1-propene $$\left( \ce{C_6H_5CH=CHCH_3} \right)$$ to be more, or less, reactive than 3-phenyl-1-propene $$\left( \ce{C_6H_5CH_2CH=CH_2} \right)$$ if account is taken of the stabilization of the ground state as well as the stabilization of the radicals?

Exercise 26-30 Which of the following pairs of compounds would you expect to be the more reactive under the specified conditions? Give your reasons and write equations for the reactions involved.

a. 4-$$\ce{NO_2C_6H_4CH_2Br}$$ or 4-$$\ce{CH_3OC_6H_4CH_2Br}$$ on hydrolysis in 2-propanone-water solution
b. $$\ce{(C_6H_5)_3CH}$$ or $$\ce{C_6H_5CH_3}$$ in the presence of phenyllithium
c. $$\ce{(C_6H_5)_3C-C(C_6H_5)_3}$$ or $$\ce{(C_6H_5)_2CH-CH(C_6H_5)_2}$$ on heating
d. $$\ce{(C_6H_5)_2N-N(C_6H_5)_2}$$ or $$\ce{(C_6H_5)_2CH-CH(C_6H_5)_2}$$ on heating
e. $$\ce{(C_6H_5CH_2CO_2)_2}$$ or $$\ce{(C_6H_5CO_2)_2}$$ on heating
f. $$\ce{C_6H_5COC_6H_5}$$ or $$\ce{C_6H_5CH_2C_6H_5}$$ on reduction with sodium borohydride

Exercise 26-31 The following equilibrium is established readily in the presence of bases:

The mechanism of the reaction could be either a base-induced enolization reaction (Section 17-1) or ionization of the $$\ce{OH}$$ proton followed by a Cannizzaro-type reaction (Section 16-4E). Write each mechanism in detail and devise experiments that could be used to distinguish between them.

Exercise 26-32 Devise methods of synthesis of the following compounds based on the given starting materials:

a. 1,2-di-(4-methoxyphenyl)ethane from 4-methoxybenzenecarbaldehyde
b. 4-(2-nitrophenyl)-3-buten-2-one from benzene or methylbenzene
c. 2-methyl-1-azanaphthalene (quinalidine) from 4-(2-nitrophenyl)-3-buten-2-one
d. diphenylmethanone (benzophenone) from benzenecarbaldehyde

Exercise 26-33 Write a mechanism based on analogy for the formation of quinoxalines from benzils and 1,2-benzenediamines. (Review Section 16-4C.)

Exercise 26-34 The ionization constants of 3- and 4-cyanobenzoic acids at $$30^\text{o}$$ are $$2.51 \times 10^{-4}$$ and $$2.82 \times 10^{-4}$$, respectively. Benzoic acid has $$K_a$$ of $$6.76 \times 10^{-5}$$ at $$30^\text{o}$$. Calculate $$\sigma_\text{meta}$$ and $$\sigma_\text{para}$$ for the cyano substituent.

Exercise 26-35 The magnitudes and signs of the $$\sigma$$ constants associated with meta and para substituents can be rationalized in terms of inductive and electron-delocalization influences. Show how it is possible, within this framework, to account for the following facts:

a. Fluorine has a sizable positive $$\sigma$$ constant when meta but almost zero when para.

b. The $$\sigma$$ constant of the methoxy group $$\left( \ce{-OCH_3} \right)$$ is positive in the meta position an negative in the para position.

c. The $$-\overset{\oplus}{\ce{N}} \ce{(CH_3)_3}$$ group has a slightly larger positive $$\sigma$$ constant in the meta position than in the para position, but the reverse is true for the $$\ce{-N_2^+}$$ group.

d.* The $$\sigma$$ constant of the $$\ce{-CF_3}$$ group is more positive when para than when meta.

Exercise 26-36 Predict whether the meta and para $$\sigma$$ constants for the following groups would be positive or negative, and large or small. Give your reasoning.

a. $$\ce{-C \equiv N}$$
b. $$\ce{-CH_2} \overset{\oplus}{\ce{N}} \ce{(CH_3)_3}$$
c. $$\ce{-OCF_2H}$$
d. $$\ce{-CO_2^-}$$

Exercise 26-37 Would you expect a Hammett type of relationship to correlate data for the dissociation of acids of the following type with rate data for hydrolysis of the corresponding esters? Explain.

Exercise 26-38 Account for the large difference in the $$\rho$$ values of Reactions 10 and 11 of Table 26-7.

Exercise 26-39 The $$\rho$$ constant for the ionization of benzoic acid is 1.000 for water solutions at $$25^\text{o}$$. Would you expect $$\rho$$ for acid ionization to increase, or decrease, in going to a less polar solvent such as methanol? Explain.

Exercise 26-40 Explain why $$\rho$$ for ionization of benzoic acids is larger than $$\rho$$ for phenylethanoic acids. Estimate a value of $$\rho$$ for the ionization of substituted 4-phenylbutanoic acids. Why should we expect the value of $$\rho$$ for alkaline hydrolysis of ethyl benzoates to be larger than for acid ionization and to have the same sign?

Exercise 26-41 From the data of Tables 26-6 and 26-7 and given that $$K_a$$ for benzenol at $$25^\text{o}$$ is $$1.3 \times 10^{-10}$$, calculate $$K_a$$ for 3-nitrobenzenol and 4-nitrobenzenol. The experimental values are $$1.0 \times 10^{-8}$$ for 3-nitrobenzenol and $$6.5 \times 10^{-8}$$ for 4-nitrobenzenol. Do the calculated and experimental values agree satisfactorily (within a factor of 2 to 3) and, if not, why?

Exercise 26-42 From appropriate $$\rho$$ values (Table 26-7) and $$\sigma$$ values (Table 26-6), calculate the rates of hydrolysis of 4-$$\ce{CH_3}-$$, 4-$$\ce{CH_3O}-$$, 4-$$\ce{NO_2}-$$phenylmethyl chlorides relative to phenylmethyl chloride (a) in water at $$30^\text{o}$$ in the presence of base, and (b) in $$48\%$$ ethanol at $$30^\text{o}$$. Explain why there is a greater spread in the relative rates in (b) than in (a).

Exercise 26-43 For each of the following pairs of compounds give a chemical test, preferably a test-tube reaction, that will distinguish between the two compounds. Write a structural formula for each compound and equations for the reactions involved.

a. benzenol and cyclohexanol
b. methyl 4-hydroxybenzoate and 4-methoxybenzoic acid
c. 1,4- and 1,3-benzenediol
d. 1,4-benzenediol and tropolone
e. 9,10-anthracenedione and 1,4-anthracenedione

Exercise 26-44 Show by means of equations how each of the following substances may be synthesized, starting with the indicated materials. Specify reagents and approximate reaction conditions.

a. methyl 2-methoxybenzoate from benzenol
b. 1,3-dibromo-5-tert-butyl-2-methoxybenzene from benzenol
c. (4-cyanophenoxy)ethanoic acid from benzenol
d. 2-hydroxy-5-nitrobenzoic acid from benzenol
e. 2-naphthaleneamine from naphthalene
f. tetramethyl-1,4-benzenedione from 1,2,4,5-tetramethylbenzene
g. 2-cyano-1,4-benzenedione from 1,4-benzenediol
h. from 1,4-benzenedione

Exercise 26-45 Rearrangement of 2-propenyloxybenzene labeled with radioactive carbon $$\left( \ce{^{14}C} \right)$$ at $$\ce{C_3}$$ of the 2-propenyl group forms 2-propenylbenzenol labeled at $$\ce{C_1}$$ of the 2-propenyl group. Can the rearrangement mechanism involve dissociation into $$\ce{C_6H_5O} \cdot$$ and $$\ce{^{14}CH_2=CH-CH_2} \cdot$$ followed by recombination? Can the rearrangement be a concerted pericyclic reaction (Section 21-10)? Where would you expect the $$\ce{^{14}C}$$ label to be found in the rearrangement of 2,6-dimethyl(2-propenyloxy)benzene to 2,6-dimethyl-4-(2-propenyl)benzenol?

Exercise 26-46 Write structural formulas for substances (one for each part) that fit the following descriptions:

a. an arenol that would be a stronger acid than benzenol itself
b. the dichlorobenzenol isomer that is the strongest acid
c. the Claisen rearrangement product from 1,3-dimethyl-2-(1-methyl-2-propenyloxy)benzene
d. a quinone that would not undergo Diels-Alder addition
e. a quinone that would be a better charge-transfer agent than 1,4-benzenedione
f. the expected product from addition of hydrogen cyanide to 2-cyano-1,4-benzenedione

Exercise 26-47 Predict the positions to which coupling would occur (or whether coupling would occur at all) with benzenediazonium chloride in slightly alkaline solution for the following compounds. Give your reasoning.

a. 2,4,6-trimethylbenzenol
b. 2-naphthalenol
c. 1-methyl-2-naphthalenol
d. 9-phenanthrenol

Exercise 26-48 When 2-hydroxybenzoic acid (salicylic acid) is treated with excess bromine in aqueous solution, it forms 2,4,6-tribromobenzenol. Write a reasonable mechanism for this reaction. Would you expect the same type of reaction to occur with 3-hydroxybenzoic acid?

Exercise 26-49 Account for the formation of the byproduct, $$15$$, in the reaction of 4-methylbenzenol with trichloromethane in alkali:

Exercise 26-50 The important polymer intermediate "bis-phenol A" [2,2-bis-(4-hydroxyphenyl)propane] used, among other things, in epoxy resins, is made by an acid-induced condensation of 2-propanone and benzenol. Write a stepwise mechanism for this reaction that is consistent with the nature of the reactants and the products. (Review Section 15-4E on electrophilic reactions of carbonyl compounds, Section 22-4E, and Section 26-1E.)

Exercise 26-51* Devise syntheses from benzene of each of the photographic developers whose structure is shown in Section 26-2C. Some reactions you will need are discussed in Chapters 22 and 23.

Exercise 26-52 Addition of hydrogen chloride to 1,4-benzenedione yields, among other products, 2,3,5,6-tetrachloro-1,4-benzenedione. Explain how this substance might be formed, with the knowledge that equilibria such as the following are established rapidly:

Exercise 26-53 Nitrous acid can substitute the more reactive aromatic derivatives by attack of $$\ce{NO}^\oplus$$ on the ring and form $$\ce{Ar-N=O}$$ compounds. A product obtained from benzenol by this kind of reaction has the formula $$\ce{C_6H_5O_2N}$$. Exactly the same substance is formed from treatment of one mole of 1,4-benzenedione with one mole of azanol (hydroxylamine; Section 16-4C). On the basis of the reactions by which it is formed, write two likely structures for this substance and explain how you would decide which one was correct on the basis of chemical and spectroscopic tests.

Exercise 26-54 Consider possible benzil-benzilic acid-type rearrangements occurring with 9,10-phenanthrenedione and 9,10-anthracenedione. Give your reasoning as to how easily these rearrangements might occur, relative to rearrangement of benzil itself (Section 26-4E).

Exercise 26-55 The [2 + 2] cycloadduct of tetrafluoroethene and 1,3-cyclopentadiene, when pyrolyzed at $$700^\text{o}$$ to $$750^\text{o}$$ and $$5$$-$$\text{mm}$$ pressure, produces (as the result of a sigmatropic rearrangement; Section 21-10) a mixture of two new substances, each having two double bonds. The pyrolysis mixture, when heated in aqueous ethanoic acid containing potassium ethanoate, forms tropolone in $$70\%$$ yield. Write equations for the reactions involved, with particular attention to possible structures for the pyrolysis products.

Exercise 26-56 How would you expect the properties of 3- and 4-hydroxy-2,4,6-cycloheptatrienone to compare with those of tropolone? Explain.

Exercise 26-57 Make an atomic-orbital model of benzenol, showing in detail the orbitals and electrons at the oxygen atom. From your model, would you expect one, or both, pairs of unshared electrons on oxygen to be delocalized over the ring? What would be the most favorable orientation of the hydrogen of the hydroxyl group for maximum delocalization of an unshared electron pair?

Exercise 26-58 It has been reported that compound $$16$$ with alkali rearranges to phenyl-1,2-cyclobutenedione, $$3$$ (Section 26-2E). This reaction appears to be the first reported reverse benzil-benzilic acid rearrangement (Section 26-4E). Explain how and why this process occurs.

## Contributors

John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

26.E: More on Aromatic Compounds (Exercises) is shared under a not declared license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.