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16.16: Additional Problems

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    16 • Additional Problems 16 • Additional Problems

    Visualizing Chemistry

    Draw the product from reaction of each of the following substances with (1) Br2, FeBr3 and (2) CH3COCl, AlCl3. (a)

    the= (b)

    The ball-and-stick model has a benzene ring. C 1 is bonded to an aldehyde group. C 4 is bonded to a methyl group.

    The following molecular model of a dimethyl-substituted biphenyl represents the lowest-energy conformation of the molecule. Why are the two benzene rings tilted at a 63° angle to each other rather than being in the same plane so that their p orbitals overlap? Why doesn’t complete rotation around the single bond joining the two rings occur? the=

    How would you synthesize the following compound starting from benzene? More than one step is needed.

    the=

    The following compound can’t be synthesized using the methods discussed in this chapter. Why not?

    the=

    Mechanism Problems

    Mechanisms of Electrophilic Substitutions

    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.
    The sulfonation of an aromatic ring with SO3 and H2SO4 is reversible. That is, heating benzenesulfonic acid with H2SO4 yields benzene. Show the mechanism of the desulfonation reaction. What is the electrophile?
    The carbocation electrophile in a Friedel–Crafts reaction can be generated by an alternate means than reaction of an alkyl chloride with AlCl3. For example, reaction of benzene with 2-methylpropene in the presence of H3PO4 yields tert-butylbenzene. Propose a mechanism for this reaction.
    The N,N,N-trimethylammonium group,  –   N + (CH3)3, is one of the few groups that is a meta-directing deactivator yet has no electron- withdrawing resonance effect. Explain.
    The nitroso group,  – N = O, is one of the few nonhalogens that is an ortho- and para-directing deactivator. Explain this behavior by drawing resonance structures of the carbocation intermediates in ortho, meta, and para electrophilic reaction on nitrosobenzene, C6H5N  = O.

    Triphenylmethane can be prepared by reaction of benzene and chloroform in the presence of AlCl3. Propose a mechanism for the reaction.

    benzene=

    Using resonance structures of the intermediates, explain why bromination of biphenyl occurs at ortho and para positions rather than at meta.

    biphenyl=

    Benzene and alkyl-substituted benzenes can be hydroxylated by reaction with H2O2 in the presence of an acidic catalyst. What is the structure of the reactive electrophile? Propose a mechanism for the reaction.

    benzene=

    Additional Mechanism Practice

    Addition of HBr to 1-phenylpropene yields only (1-bromopropyl)benzene. Propose a mechanism for the reaction, and explain why none of the other regioisomer is produced.

    benzene=

    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.

    benzene=

    Benzenediazonium carboxylate decomposes when heated to yield N2, CO2, and a reactive substance that can’t be isolated. When benzene-diazonium carboxylate is heated in the presence of furan, the following reaction is observed:

    when=

    What intermediate is involved in this reaction? Propose a mechanism for its formation.

    4-Chloropyridine undergoes reaction with dimethylamine to yield 4-dimethylaminopyridine. Propose a mechanism for the reaction.

    4-chloropyridine=

    Propose a mechanism to account for the following reaction:

    benzene=

    In the Gatterman–Koch reaction, a formyl group ( – CHO) is introduced directly onto a benzene ring. For example, reaction of toluene with CO and HCl in the presence of mixed CuCl/AlCl3 gives p-methylbenzaldehyde. Propose a mechanism.

    toluene=
    Treatment of p-tert-butylphenol with a strong acid such as H2SO4 yields phenol and 2-methylpropene. Propose a mechanism.

    Benzyl bromide is converted into benzaldehyde by heating in dimethyl sulfoxide. Propose a structure for the intermediate, and show the mechanisms of the two steps in the reaction.

    a=

    Propose a mechanism for the Smiles rearrangement below.

    when=
    benzene= (b)

    A disubstituted napthalene reacts with a diazo compound to form Allura Red. (c)

    A disubstituted napthalene reacts with a diazo compound to form Lithol Rubine BK..

    Reactivity and Orientation of Electrophilic Substitutions

    Problem 16-46 Identify each of the following groups as an activator or deactivator and as an o,p-director or m-director: (a)

    The structure of a group where N H (C H 3) 2 is bonded to an open single bond that has a wavy line across it. (b)

    The structure of a group where cyclopentane ring is bonded to an open single bond that has a wavy line across it. (c)

    The structure of a group where O C H 2 C H 3 is bonded to an open single bond that has a wavy line across it. (d)

    A cyclohexane ring bonded to a carbonyl group. The carbonyl carbon is bonded to an open single bond that has a wavy line across it.

    Problem 16-47 Predict the major product(s) of nitration 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) ethoxybenzene
    Chlorobenzene, o-dichlorobenzene, benzene
    (b) p-Bromonitrobenzene, nitrobenzene, phenol (c) Fluorobenzene, benzaldehyde, o-xylene (d) Benzonitrile, p-methylbenzonitrile, p-methoxybenzonitrile
    3: (a)
    Bromobenzene
    (b) m-Bromophenol (c) p-Chloroaniline (d) 2,4-Dichloronitrobenzene (e) 2,4-Dichlorophenol (f) Benzoic acid (g) p-Methylbenzenesulfonic acid (h) 2,5-Dibromotoluene
    m-Nitrophenol
    (b) o-Xylene (c) p-Nitrobenzoic acid (d) p-Bromobenzenesulfonic acid
    Fluorobenzene
    (b) m-Bromophenol (c) m-Dichlorobenzene (d) 2,4-Dibromophenol
    Bromobenzene
    (b) Toluene (c) Phenol (d) Aniline (e) Nitrobenzene (f) p-Bromotoluene
    benzene= (b)

    ortho-Dibromobenzene reacts with nitric acid in the presence of sulfuric acid followed by iron and actd forming unknown product(s), depicted by a question mark (c)

    In water, benzene fused to a cyclohexane ring reacts with potassium permanganate to form unknown product(s), depicted by question mark. (d)

    Benzene bonded to a methoxy group and a chlorine atom para to one another reacts with propyl chloride in the presence of aluminum trichloride to form unknown product(s), depicted by question mark.

    chlorobenzene= (b)

    A compound containg an oxygen atom bonded to two benzene rings reacts with propanoyl chloride in the presence of aluminum trichloride to form unknown product(s), depicted by question mark. (c)

    Benzoic acid reacts with nitric acid in the presence of sulfuric acid to form unknown product(s), depicted by question mark. (d)

    Benzene bonded to N (C H 2 C H 3) 2 reacts with sulfur trioxide in presence of sulfuric acid to form unknown product(s), depicted by question mark.

    Organic Synthesis

    Problem 16-55 How would you synthesize the following substances starting from benzene or phenol? Assume that ortho- and para-substitution products can be separated. (a)

    o-Bromobenzoic acid
    (b)

    p-Methoxytoluene
    (c) 2,4,6-Trinitrobenzoic acid (d) m-Bromoaniline
    p-Chloroacetophenone
    (b) m-Bromonitrobenzene (c) o-Bromobenzenesulfonic acid (d) m-Chlorobenzenesulfonic acid
    2-Bromo-4-nitrotoluene
    (b) 1,3,5-Trinitrobenzene (c) 2,4,6-Tribromoaniline (d) m-Fluorobenzoic acid
    toluene= (b)

    Chlorobenzene reacts with nitric acid in sulfuric acid, followed by methyl chloride and aluminum trichloride, then iron and acid, and finally sodium hydroxide in water to form a substituted benzene. (c)

    Toluene reacts with acetyl chloride in the presence of aluminum trichloride, then with nitric acid in the presence of sulfuric acid, then hydrogen and a palladium catalyst to form a substituted benzene.

    General Problems

    Problem 16-59

    At what position and on what ring do you expect nitration of 4-bromo-biphenyl to occur? Explain, using resonance structures of the potential intermediates.

    4-Bromobiphenyl has a benzene ring bonded to a second benzene ring bearing a bromine atom in the para position.

    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.

    the=
    a= (b)

    A molecule in which a central N H group is bonded to a benzene ring and a second benzene ring bearing a bromine atom in the para position (c)

    A molecule in which a benzene ring is bonded to a second benzene ring bearing a methyl group in the meta position. (d)

    A molecule in which a central carbonyl group is bonded to a benzene ring and to a meta-substituted chlorobenzene ring.

    Problem 16-62

    At what position, and on what ring, would you expect bromination of benzanilide to occur? Explain by drawing resonance structures of the intermediates.

    Benzanilide has a benzene ring bonded to a carbonyl group, which is bonded to an N H group. The nitrogen of N H group is bonded to another benzene ring.
    Would you expect the Friedel–Crafts reaction of benzene with (R)-2- chlorobutane to yield optically active or racemic product? Explain.
    a= (b)

    A benzene ring with two hydroxymethyl groups para to one another (c)

    A benzene ring, with a 2-carbon chain attached to it, the othermost carbon of which is itself attached to a hydroxyl group.

    Problem 16-65

    The compound MON-0585 is a nontoxic, biodegradable larvicide that is highly selective against mosquito larvae. Synthesize MON-0585 using either benzene or phenol as a source of the aromatic rings.

    MON-0585 has a central carbon bonded to a benzene ring, two methyl groups, and a phenol group, in which the two carbons next to the O H are each bonded to a tertiaru butyl group.
    Phenylboronic acid, C6H5B(OH)2, is nitrated to give 15% ortho- substitution product and 85% meta. Explain the meta-directing effect of the  –B(OH)2 group.

    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.

    naphthalene=

    Propose a mechanism for the reaction of 1-chloroanthraquinone with methoxide ion to give the substitution product 1-methoxyanthraquinone. Use curved arrows to show the electron flow in each step.

    1-chloroanthraquinone=
    p-Bromotoluene reacts with potassium amide to give a mixture of m- and p-methylaniline. Explain.

    Propose a mechanism to account for the reaction of benzene with 2,2,5,5-tetramethyltetrahydrofuran.

    benzene=
    a= (b)

    Benzene with C 1, C 2, and C 4 bonded to a sulfonic acid group, a 3-carbon chain with a methyl group at C 2, and a chlorine atom, respectively.

    Problem 16-72

    You know the mechanism of HBr addition to alkenes, and you know the effects of various substituent groups on aromatic substitution. Use this knowledge to predict which of the following two alkenes reacts faster with HBr. Explain your answer by drawing resonance structures of the carbocation intermediates.

    Methoxy benzene with ethene group at the para position and nitrobenzene with an ethene at the para position.

    Use your knowledge of directing effects, along with the following data, to deduce the directions of the dipole moments in aniline, bromobenzene, and bromoaniline.

    structures=

    Identify the reagents represented by the letters a–e in the following scheme:

    a=
    Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The pKa 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.
    Would you expect p-methylphenol to be more acidic or less acidic than unsubstituted phenol? Explain. (See Problem 16-75.)

    Problem 16-77 Predict the product(s) for each of the following reactions. In each case, draw the resonance forms of the intermediate to explain the observed regiochemistry. (a)

    benzene= (b)

    Toluene reacts with 2-chloropropane in the presence of aluminum trichloride to form unknown product(s), depicted by a question mark. (c)

    Benzene bonded to a cyano group reacts with C I 2 in the presence of iron trichloride to form unknown products, depicted by a question mark. (d)

    Benzene bonded to a methoxy group reacts with iodine in the presence of copper (2) chloride to form unknown product(s), depicted by a question mark.

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