Chapter 22

How many structurally different monomethyl derivatives are possible for each of the following compounds? Name each. a. naphthalene b. anthracene c. phenanthrene

How many isomeric products could each of the dimethylbenzenes give on introduction of a third substituent? Name each isomer, using chlorine as the third substituent.

Name each of the following compounds by the IUPAC system: a. \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2} \mathrm{CHCl}\) b. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHCl}_{2}\) c. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CCl}_{3}\)

Why do fairly reactive arenes, such as benzene, methylbenzene, and ethylbenzene, react with excess nitric acid in nitromethane solution at a rate that is independent of the concentration of the arene (i.e., zero order in arene concentration)? Does this lack of dependencies on arene concentration mean that nitration of an equimolar mixture of benzene and methylbenzene would necessarily give an equimolar mixture of nitrobenzene and nitromethylbenzenes? Why or why not?

Reagents, besides the molecular halogens, that effect halogen substitution include hypochlorous and hypobromous acids. They are most effective when a strong acid is present and care is taken to exclude formation of halide ions. Account for the catalytic effect of acid and the anticatalytic effect of halide ions.

Arrange the following bromine-containing species in order of their expected reactivity in achieving electrophilic aromatic bromination: \(\mathrm{HOBr}, \mathrm{Br}_{2}, \mathrm{Br}^{\oplus}, \mathrm{Br}^{\ominus}, \mathrm{HBr}, \mathrm{H}_{2} \mathrm{OBr}^{\oplus}, \mathrm{BrCl}\).

Aluminum chloride is a much more powerful catalyst than ferric bromide for bromination of benzene. Would you expect the combination of aluminum chloride and bromine to give much chlorobenzene in reaction with benzene? Explain.

a. The bromination of benzene is catalyzed by small amounts of iodine. Devise a possible explanation for this catalytic effect. b. The kinetic expression for the bromination of naphthalene in ethanoic acid involves a term that is first order in naphthalene and second order in bromine. How can two molecules of bromine and one of naphthalene be involved in the rate-determining step of bromination? Explain why the kinetic expression simplifies to first order in naphthalene and first order in bromine in \(50 \%\) aqueous ethanoic acid.

Write a mechanism for the alkylation of benzene with 2 -propanol catalyzed by boron trifluoride.

Suggest possible routes for the synthesis of the following compounds: a. diphenylmethane from benzoic acid and benzene b. 1 -ethyl-4-methylbenzene from methylbenzene

a. Substitution of a chloromethyl group, \(-\mathrm{CH}_{2} \mathrm{Cl}\), on an aromatic ring is chloromethylation and is accomplished using methanal, \(\mathrm{HCl}\), and a metal-halide catalyst \(\left(\mathrm{ZnCl}_{2}\right)\). Write reasonable mechanistic steps that could be involved in this reaction: $$ \mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{CH}_{2} \mathrm{O}+\mathrm{HCl} \stackrel{\mathrm{ZnCl}_{2}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}+\mathrm{CH}_{2} \mathrm{O} $$ b. Phenylmethyl chloride can be formed from benzene and chloromethyl methyl ether, \(\mathrm{ClCH}_{2} \mathrm{OCH}_{3}\), in the presence of stannic chloride, \(\mathrm{SnCl}_{4}\). Write reasonable mechanistic steps, again supported by analogy, for this reaction. Notice that \(\mathrm{SnCl}_{4}\) is a Lewis acid.

Show explicitly how an alkyl side chain of alkylbenzenesulfonates could be formed with a quaternary carbon, if the \(\mathrm{C}_{12}\) alkane used at the start of the synthesis contained any branched-chain \(\mathrm{C}_{12}\) isomers.

Predict the favored position(s) of substitution in the nitration of the following compounds: a. 4 -nitro-1-phenylbenzene b. 4 -methylbenzenecarboxylic acid c. 3 -methylbenzenecarboxylic acid d. 1,3 -dibromobenzene e. 1 -fluoro-3-methoxybenzene f. 1,3 -dimethylbenzene

Draw the Kekulé-type valence-bond structures for napthalene, anthracene, and phenanthrene. Estimate the percentage of double-bond character for the 9,10 bond of phenanthrene, assuming that each of the valence-bond structures contributes equally to the hybrid structure.

A side reaction when reducing benzene derivatives to 1,4 -cyclohexadienes with lithium or sodium in liquid ammonia is over-reduction to give cyclohexenes. Addition of ethanol greatly reduces the importance of this side reaction. Explain what role ethanol plays in preventing over-reduction.

The rate of the Diels-Alder addition between cyclooctatetraene and tetracyanoethene is proportional to the tetracyanoethene concentration, \(\left[\mathrm{C}_{2}(\mathrm{CN})_{4}\right]\), at low concentrations of the addends but becomes independent of \(\left[\mathrm{C}_{2}(\mathrm{CN})_{4}\right]\) at high concentrations. Write a mechanism that accounts for this behavior.

Write structural formulas for all of the possible isomers of \(\mathrm{C}_{8} \mathrm{H}_{10}\) that contain one benzene ring. Show how many different mononitration products each could give if no carbon skeleton rearrangements occur but nitration is possible either in the ring or side chain. Name all of the mononitration products by an accepted system.

Write structural formulas (more than one may be possible) for aromatic substances that fit the following descriptions: a. \(\mathrm{C}_{8} \mathrm{H}_{10}\), which can give only one theoretically possible ring nitration product b. \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Br}_{3}\), which can give three theoretically possible nitration products. c. \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Br}_{2} \mathrm{Cl}\), which can give two theoretically possible nitration products. d. \(\mathrm{C}_{8} \mathrm{H}_{8}\left(\mathrm{NO}_{2}\right)_{2}\), which can give only two theoretically possible different ring monobromosubstitution products.

Predict the most favorable position for mononitration for each of the following substances. Indicate whether the rate is greater, or less, than for the nitration of benzene. Give your reasoning in each case. a. fluorobenzene b. trifluoromethylbenzene c. phenylethanone d. phenylmethyldimethylamine oxide, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \stackrel{\oplus}{\mathrm{N}}\left(\mathrm{CH}_{3}\right)_{2} \stackrel{\ominus}{\mathrm{O}}\) e. diphenylmethane f. 4 -bromo-1-methoxybenzene g. phenylsulfinylbenzene, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SOC}_{6} \mathrm{H}_{5}\) h. 1-tert-butyl-4-methylbenzene i. diphenyliodonium nitrate, \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2} \mathrm{IN}\) \ce \(\left\\{\mathrm{O}_{-} 3\right\\}()\) j. 1,3 -diphenylbenzene (meta-terphenyl) k. N-(4-phenylphenyl)ethanamide

Explain why the bromination of benzenamine (aniline) gives \(2,4,6\) -tribromobenzenamine \((2,4,6-\) tribromoaniline), whereas the nitration with mixed acids gives 3-nitrobenzenamine (meta-nitroaniline).

Starting with benzene, show how the following compounds could be prepared. Specify the required reagents and catalysts. a. 1 -bromo-4-nitrobenzene b. 4 -isopropyl-3-nitrobenzenesulfonic acid c. 4 -tert-butylbenzenecarbaldehyde d. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\) e. \(1,2,4,5\) -tetrachlorocyclohexane

Offer a suitable explanation of each of the following facts: a. Nitration of arenes in concentrated nitric acid is retarded by added nitrate ions and strongly accelerated by small amounts of sulfuric acid. b. Nitrobenzene is a suitable solvent to use in Friedel-Crafts acylation of benzene derivatives. c. Benzene and other arenes usually do not react with nucleophiles by either addition or substitution. d. Pyridine is almost inert to nitration with mixed nitric and sulfuric acids, a reaction the proceeds readily with benzene.

Trifluoroperoxyethanoic acid, \(\mathrm{CF}_{3} \mathrm{C}(\mathrm{O}) \mathrm{O}-\mathrm{OH}\) reacts with methoxybenzene to give \(2-\) and 4 methoxybenzenols: Explain the nature of this reaction. What is likely to be the substituting agent? What products would you expect from trifluoroperoxyethanoic acid and fluorobenzene? Would fluorobenzene be more, or less, reactive than methoxybenzene?

Ethanoic anhydride reacts with concentrated nitric acid to yield the rather unstable ethanoyl nitrate (acetyl nitrate), which is a useful nitrating agent. With mixtures of benzene and methylbenzene, ethanoyl nitrate products a mixture of nitrobenzene and 2 - and 4-nitromethylbenzenes. When nitrated separately, each compound reacts at the same overall rate, but when mixed together, 25 times more nitromethylbenzene is formed than nitrobenzene. a. Write equations for the formation of ethanoyl nitrate and its use in nitration of benzene derivatives. b. Consider possible mechanisms for nitrations with ethanoyl nitrate and show how the above observations with benzene and methylbenzene alone or in mixtures can be rationalized by proper choice of the rate-determining step.

Explain why the nitration and halogenation of biphenyl (phenylbenzene) goes with activation at the ortho and para positions but with deactivation at the meta position. Suggest a reason why biphenyl is more reactive than \(2,2^{\prime}\) dimethylbiphenyl in nitration.