16 • Additional Problems
16 • Additional Problems
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?
How would you synthesize the following compound starting from benzene? More than one step is needed.
The following compound can’t be synthesized using the methods discussed in this chapter. Why not?
Mechanisms of Electrophilic Substitutions
Triphenylmethane can be prepared by reaction of benzene and chloroform in the presence of AlCl3. Propose a mechanism for the reaction.
Using resonance structures of the intermediates, explain why bromination of biphenyl occurs at ortho and para positions rather than at meta.
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.
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.
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.
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:
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.
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 HCl in the presence of mixed CuCl/AlCl3 gives p-methylbenzaldehyde. 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.
Propose a mechanism for the Smiles rearrangement below.
Reactivity and Orientation of Electrophilic Substitutions
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.
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.
At what position, and on what ring, would you expect bromination of benzanilide to occur? Explain by drawing resonance structures of the intermediates.
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.
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.
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.
Propose a mechanism to account for the reaction of benzene with 2,2,5,5-tetramethyltetrahydrofuran.
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.
Use your knowledge of directing effects, along with the following data, to deduce the directions of the dipole moments in aniline, bromobenzene, and bromoaniline.
Identify the reagents represented by the letters a–e in the following scheme: