Chemistry of Benzene: Electrophilic Aromatic Substitution

Monobromination of toluene gives a mixture of three bromotoluene products. Draw and name them.

Propose a mechanism for the electrophilic fluorination of benzene with $\mathbf{F}\mathbf{-}\mathbf{TEDA}\mathbf{-}{\mathbf{BF}}_{\mathbf{4}}$.

How many products might be formed on chlorination of o-xylene (o-dimethylbenzene), m-xylene, and p-xylene?

Question: When benzene is treated with ${\mathbf{D}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$, deuterium slowly replaces all six hydrogens in the aromatic ring. Explain.

Which of the following alkyl halides would you expect to undergo Friedel–

Crafts reaction with rearrangement and which without? Explain.

(a)  ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{Cl}$

(b) ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{CH}\left(\mathbf{Cl}\right){\mathbf{CH}}_3$

(c)  ${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2\mathbf{Cl}$

(d)  ${\left({\mathbf{CH}}_3\right)}_3{\mathbf{CCH}}_2\mathbf{Cl}$

(e) Chlorocyclohexane

What is the major monosubstitution product from the Friedel–Crafts reaction of benzene with 1-chloro-2-methylpropane in the presence of ${\mathbf{AlCl}}_3$ ?
 

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 of the following groups in order of their reactivity

to electrophilic substitution:

(a) Nitrobenzene, phenol, toluene, benzene

(b) Phenol, benzene, chlorobenzene, benzoic acid

(c) Benzene, bromobenzene, benzaldehyde, aniline

Question: Predict the major products of the following reactions:

(a) Nitration of bromobenzene

 (b) Bromination of nitrobenzene

(c) Chlorination of phenol 

(d) Bromination of aniline

Use Figure 16-11 to explain why Friedel–Crafts alkylations often give polysubstitution but Friedel–Crafts acylations do not.

An electrostatic potential map of (trifluoromethyl)benzene, ${\mathbf{C}}_6{\mathbf{H}}_5{\mathbf{CF}}_3$, is shown. Would you expect (trifluoromethyl)benzene to be more reactive or less reactive than toluene toward electrophilic substitution? Explain.

Acetanilide is less reactive than aniline toward electrophilic substitution. Explain

Draw resonance structures for the intermediates from the reaction of an electrophile at the ortho, meta, and para positions of nitrobenzene. Which intermediates are most stable?

At what position would you expect electrophilic substitution to occur in each of the following substances?

a.

b.

c.

Show the major product(s) from reaction of the following substances with

(1)  ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{Cl}$ , ${\mathbf{AlCl}}_3$  and 

(2)  ${\mathbf{HNO}}_3$ , ${\mathbf{H}}_2{\mathbf{SO}}_4$ :

a.

b.

The herbicide oxyfluorfen can be prepared by reaction between a phenol and an aryl fluoride. Propose a mechanism.

Treatment of p-bromotoluene with NaOH at $\mathbf{300}\mathbf{°}\mathbf{C}$ yields a mixture of two products, but treatment of m-bromotoluene with NaOH yields a mixture of three products. Explain.

What aromatic products would you obtain from the ${\mathbf{KMnO}}_4$ oxidation of the following substances?

a.

b.

Refer to Table 6-3 on page 170 for a quantitative idea of the stability of a benzyl radical. How much more stable (in kJ/mol) is the benzyl radical than a primary alkyl radical? How does a benzyl radical compare in stability to an allyl radical?

Styrene, the simplest alkenylbenzene, is prepared commercially for use in plastics manufacture by catalytic dehydrogenation of ethylbenzene. How might you prepare styrene from benzene using reactions you’ve studied?

How would you prepare diphenylmethane, ${\left(\mathbf{Ph}\right)}_2{\mathbf{CH}}_2$ , from benzene and an acid chloride?

Question: How might you synthesize the following substances from benzene? 

(a) m-Chloronitrobenzene 

(b) m-Chloroethylbenzene 

(c) 4-Chloro-1-nitro-2-propylbenzene 

(d) 3-Bromo-2-methylbenzenesulfonic acid

The carbocation electrophile in a Friedel–Crafts reaction can be generated by an alternate means than reaction of an alkyl chloride with. For example, reaction of benzene with 2-methylpropene in the presence of ${\mathbf{H}}_{\mathbf{3}}\mathit{P}{\mathit{O}}_{\mathbf{4}}$ yields tert-butylbenzene. Propose a mechanism for this reaction.

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 electron-withdrawing resonance effect. Explain

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.

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.

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 sulfonation of an aromatic ring with is reversible. That is, heating benzenesulfonic acid with H2SO4 yields benzene. Show the mechanism of the desulfonation reaction. What is the electrophile?

The nitroso group, - NO, is one of the few non-halogens that is an ortho- and para-directing deactivator. Explain this behaviour by drawing resonance structures of the carbocation intermediates in ortho, meta, and para electrophilic reaction on nitroso benzene,${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{N}\mathbf{=}\mathbf{O}$.

Triphenylmethane can be prepared by reaction of benzene and chloroform in the presence of ${\mathbf{AlCl}}_{\mathbf{3}}$ . 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 ${\mathbf{H}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}$ in the presence of an acidic catalyst. What is the structure of the reactive electrophile? Propose a mechanism for the reaction.

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.

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

Rank the compounds in each group according to their reactivity toward electrophilic substitution.

1. Chlorobenzene, o-dichlorobenzene, benzene
2. P-bromonitrobenzene, nitrobenzene, phenol
3. Fluorobenzene, benzaldehyde, o-xylene
4. Benzonitrile, p-methylbenzonitrile, p-methoxybenzonitrile

Question: Predict the major monoalkylation products you would expect to obtain from the reaction of the following substances with chloromethane and  .

1. Bromobenzene
2. m-bromophenol
3. p-chloroaniline
4. 2,4-Dichloronitrobenzene
5. 2,4-Dichlorophenol
6. Benzoic acid
7. p-Methylbenzenesulfonic acid
8. 2,5-Dibromotoluene

Name and draw the major product (s) of electrophilic chlorination of the following compounds:

1. m-nitrophenol
2. o-xylene
3. p-nitrobenzoic acid
4. p-Bromobenzenesulfonic acid

Predict the major product (s) you would obtain from sulfonation of the following compounds:

1. Fluorobenzene
2. m-Bromophenol
3. m-Dichlorobenzene
4. 2,4-Dibromophenol

Rank the following aromatic compounds in the expected order of their

reactivity toward Friedel–Crafts alkylation. Which compounds are

unreactive?

(a) Bromobenzene (b) Toluene (c) Phenol

(d) Aniline (e) Nitrobenzene (f) p-Bromotoluene

What product(s) would you expect to obtain from the following

reactions?

(a) 

(b) 

(c) 

(d) 

Question: Predict the major product(s) of the following reactions:

(a)

(b) 

(c) 

(d) 

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

Starting with benzene as your only source of aromatic compounds,

how would you synthesize the following substances? Assume that you

can separate ortho and para isomers if necessary.

(a) p-Chloroacetophenone (b) m-Bromonitrobenzene

(c) o-Bromobenzenesulfonic acid (d) m-Chlorobenzenesulfonic 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) 1,3,5-Trinitrobenzene

(c) 2,4,6-Tribromoaniline (d) m-Fluorobenzoic acid

As written, the following syntheses have flaws. What is wrong with

each?

At what position and on what ring do you expect nitration of 4-bromobiphenyl 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 the following substances to undergo electrophilic substitution?

(a)  

(b)  

(c)  

(d)