Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.
Chapter 16: Chemistry of Benzene: Electrophilic Aromatic Substitution
23 problems · 18 figures16-1 §16.1 source ↗
Monobromination of toluene gives a mixture of three bromotoluene products. Draw and name them.
Answer
o-, m-, and p-Bromotoluene
16-2 §16.2 source ↗
Propose a mechanism for the electrophilic fluorination of benzene with SelectFluor.
Answer
16-3 §16.2 source ↗
How many products might be formed on chlorination of o-xylene (o-dimethylbenzene), m-xylene, and p-xylene?
Answer
o-xylene: 2; p-xylene: 1; m-xylene: 3
16-4 §16.2 source ↗
When benzene is treated with D2SO4, deuterium slowly replaces all six hydrogens in the aromatic ring. Explain.
Answer
D+ does electrophilic substitutions on the ring.
16-5 §16.3 source ↗
Which of the following alkyl halides would you expect to undergo Friedel–Crafts reaction with rearrangement and which without? Explain.
(a) CH3CH2Cl (b) CH3CH2CH(Cl)CH3 (c) CH3CH2CH2Cl (d) (CH3)3CCH2Cl
(e) Chlorocyclohexane
Answer
No rearrangement: (a), (b), (e)
16-6 §16.3 source ↗
What is the major monosubstitution product from the Friedel–Crafts reaction of benzene with 1-chloro-2-methylpropane in the presence of AlCl3?
Answer
tert-Butylbenzene
16-7 §16.3 source ↗
Identify the carboxylic acid chloride that might be used in a Friedel–Crafts acylation reaction to prepare each of the following acylbenzenes:
(a)
(b)
Answer (2 parts)
(a)
(CH3)2CHCOCl
(b)
PhCOCl
16-8 §16.4 source ↗
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
Answer (3 parts)
(a)
Phenol > Toluene > Benzene > Nitrobenzene
(b)
Phenol > Benzene > Chlorobenzene > Benzoic acid
(c)
Aniline > Benzene > Bromobenzene > Benzaldehyde
16-9 §16.4 source ↗
Predict the major products of the following reactions:
(a)
Nitration of bromobenzene
(b)
Bromination of nitrobenzene
(c)
Chlorination of phenol
(d)
Bromination of aniline
Answer (4 parts)
(a)
o- and p-Bromonitrobenzene
(b)
m-Bromonitrobenzene
(c)
o- and p-Chlorophenol
(d)
o- and p-Bromoaniline
16-10 §16.4 source ↗
Use Figure 16.12 to explain why Friedel–Crafts alkylations often give polysubstitution but Friedel–Crafts acylations do not.
Classification of substituent effects in electrophilic aromatic substitution. All activating groups are ortho- and para-directing, and all deactivating groups other than halogen are meta-directing. Halogens are unique in being deactivating but ortho- and para-directing.
Answer
Alkylbenzenes are more reactive than benzene itself, but acylbenzenes are less reactive.
16-11 §16.4 source ↗
An electrostatic potential map of (trifluoromethyl)benzene, C6H5CF3, is shown. Would you expect (trifluoromethyl)benzene to be more reactive or less reactive than toluene toward electrophilic substitution? Explain.
Answer
Toluene is more reactive; the trifluoromethyl group is electron-withdrawing.
16-12 §16.4 source ↗
Acetanilide is less reactive than aniline toward electrophilic substitution. Explain.
Answer
The nitrogen electrons are donated to the nearby carbonyl group by resonance and are less available to the ring.
16-13 §16.4 source ↗
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?
Answer
The meta intermediate is most favored.
16-14 §16.5 source ↗
At what position would you expect electrophilic substitution to occur in each of the following substances:
(a)
(b)
(c)
Answer (3 parts)
(a)
Ortho and para to − OCH3
(b)
Ortho and para to − NH2
(c)
Ortho and para to − Cl
16-15 §16.5 source ↗
Show the major product(s) from reaction of the following substances with (1) CH3CH2Cl, AlCl3 and (2) HNO3, H2SO4:
(a)
(b)
Answer (2 parts)
(a)
Reaction occurs ortho and para to the − CH3 group.
(b)
Reaction occurs ortho and para to the − OCH3 group.
16-16 §16.6 source ↗
The herbicide oxyfluorfen can be prepared by reaction between a phenol and an aryl fluoride. Propose a mechanism.
Answer
The phenol is deprotonated by KOH to give an anion that carries out a nucleophilic acyl substitution reaction on the fluoronitrobenzene.
16-17 §16.7 source ↗
Treatment of p-bromotoluene with NaOH at 300 °C yields a mixture of two products, but treatment of m-bromotoluene with NaOH yields a mixture of three products. Explain.
Answer
Only one benzyne intermediate can form from p-bromotoluene; two different benzyne intermediates can form from m-bromotoluene.
16-18 §16.8 source ↗
What aromatic products would you obtain from the KMnO4 oxidation of the following substances?
(a)
(b)
Answer (2 parts)
(a)
m-Nitrobenzoic acid
(b)
p-tert-Butylbenzoic acid
16-19 §16.8 source ↗
Refer to Table 6.3 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?
| Bond | D (kJ/mol) | Bond | D (kJ/mol) | Bond | D (kJ/mol) |
|---|---|---|---|---|---|
| H―H | 436 | (CH3)2CH―H | 410 | C2H5―CH3 | 370 |
| H―F | 570 | (CH3)2CH―Cl | 354 | (CH3)2CH―CH3 | 369 |
| H―Cl | 431 | (CH3)2CH―Br | 299 | (CH3)3C―CH3 | 363 |
| H―Br | 366 | (CH3)3C―H | 400 | H2C═CH―CH3 | 426 |
| H―I | 298 | (CH3)3C―Cl | 352 | H2C═CHCH2―CH3 | 318 |
| Cl―Cl | 242 | (CH3)3C―Br | 293 | H2C═CH2 | 728 |
| Br―Br | 194 | (CH3)3C―I | 227 |  | 427 |
| I―I | 152 | H2C═CH―H | 464 |  | 325 |
| CH3―H | 439 | H2C═CH―Cl | 396 |  | 374 |
| CH3―Cl | 350 | H2C═CHCH2―H | 369 | HO―H | 497 |
| CH3―Br | 294 | H2C═CHCH2―Cl | 298 | HO―OH | 211 |
| CH3―I | 239 |  | 472 | CH3O―H | 440 |
| CH3―OH | 385 |  | 400 | CH3S―H | 366 |
| CH3―NH2 | 386 |  | 375 | C2H5O―H | 441 |
| C2H5―H | 421 |  | 300 |  | 352 |
| C2H5―Cl | 352 |  | 336 | CH3CH2O―CH3 | 355 |
| C2H5―Br | 293 |  | 464 | NH2―H | 450 |
| C2H5―I | 233 | HC≡C―H | 558 | H―CN | 528 |
| C2H5―OH | 391 | CH3―CH3 | 377 |
Answer
A benzyl radical is more stable than a primary alkyl radical by 52 kJ/mol and is similar in stability to an allyl radical.
16-20 §16.8 source ↗
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?
Answer
1. CH3CH2Cl, AlCl3; 2. NBS; 3. KOH, ethanol
16-21 §16.9 source ↗
How would you prepare diphenylmethane, (Ph)2CH2, from benzene and an acid chloride?
Answer
1. PhCOCl, AlCl3; 2. H2/Pd
16-22 §16.10 source ↗
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
Answer (4 parts)
(a)
1. HNO3, H2SO4; 2. Cl2, FeCl3
(b)
1. CH3COCl, AlCl3; 2. Cl2, FeCl3; 3. H2/Pd
(c)
1. CH3CH2COCl, AlCl3; 2. Cl2, FeCl3; 3. H2/Pd; 4. HNO3, H2SO4
(d)
1. CH3Cl, AlCl3; 2. Br2, FeBr3; 3. SO3, H2SO4
16-23 §16.10 source ↗
In planning a synthesis, it’s as important to know what not to do as to know what to do. As written, the following reaction schemes have flaws in them. What is wrong with each?
(a)
(b)
Answer (2 parts)
(a)
Friedel–Crafts acylation does not occur on a deactivated ring.
(b)
Rearrangement occurs during Friedel–Crafts alkylation with primary halides; chlorination occurs ortho to the alkyl group.
Loading chapter 17…
Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.