Essential Organic Chemistry (Bruice)

Essential Organic Chemistry (Bruice)

Chapter 7

Essential Organic Chemistry (Bruice)

Chapter 7Aromaticity: Reactions of Benzene and Substituted Benzenes

1)

The following molecule is not aromatic. Why?

Answer:

It does not have an odd number of pairs of pi electrons (i.e., it does not obey Huckel's rule) and it is not planar.

Section: 7.1

2)

Is the following molecule aromatic? Explain.

Answer:

Yes, it is aromatic since the carbon framework for the system of the molecule is planar, monocyclic with 5 pairs of electrons.

Section: 7.1

3)

Is the following molecule aromatic? Explain.

Answer:

No, it is not aromatic since the molecule is not a conjugated system.

Section: 7.1

4)

List the criteria which compounds must meet in order to be considered aromatic.

Answer:

1)The structure must be cyclic.

2) Each atom in the ring must have an unhybridized p orbital.

3)The structure must be planar or nearly planar so that overlap of these p orbitals is effective.

4)The network must contain 4n + 2 electrons (where n is a whole number), so that delocalization of the electrons results in a lowering of the molecule's electronic energy.

Section: 7.1

5)

Aromatic molecules contain __________ electrons.

A)

no

B)

4n + 2 (with n being an integer)

C)

4n + 2 (with n being 0.5)

D)

4n (with n an integer)

E)

unpaired

Answer:

B

Section: 7.1

6)

Which of the following is an aromatic hydrocarbon?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.2

7)

Which of the following is aromatic?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.2

8)

Which of the following is aromatic?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

B

Section: 7.2

9)

Cyclic hydrocarbons which can be represented as structures containing alternating single and double bonds are called __________.

Answer:

annulenes

Section: 7.2

10)

Which of the structures below would be aromatic?

A)

I and IV

B)

I, III, and IV

C)

III and IV

D)

II

Answer:

C

Section: 7.2

11)

Which of the following is aromatic?

A)

cyclopentadienyl cation

B)

1,3-cyclohexadiene

C)

cyclobutenyl anion

D)

1,3,5-hexatriene

E)

cycloheptatrienyl cation

Answer:

E

Section: 7.2

12)

Which of the following compounds is aromatic?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

D

Section: 7.3

13)

Which of the following is aromatic?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

A

Section: 7.3

14)

Provide the structure of the biologically significant heterocycle purine.

Answer:

Section: 7.3

15)

What is the name of the following structure?

Answer:

furan

Section: 7.3

16)

What is the name of the following structure?

Answer:

pyridine

Section: 7.3

17)

What is the name of the following structure?

Answer:

pyrrole

Section: 7.3

18)

Provide the structure of the biologically significant heterocycle pyrimidine.

Answer:

Section: 7.3

19)

Provide the structure of indole.

Answer:

Section: 7.3

20)

Provide the structure of anisole.

Answer:

Section: 7.4

21)

Provide the structure of benzenesulfonic acid.

Answer:

Section: 7.4

22)

Provide the structure of dibenzyl ether.

Answer:

Section: 7.4

23)

What is the structure of phenol?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

D

Section: 7.4

24)

Provide the structure of benzoic acid.

Answer:

Section: 7.4

25)

Why does benzene undergo electrophilic substitution rather than electrophilic addition?

Answer:

When benzene undergoes electrophilic substitution, the product retains the stability associated with an aromatic system. This stability is lost in electrophilic addition because the product is not aromatic.

Section: 7.5

26)

Discuss the role of benzene in the first step of an electrophilic aromatic substitution reaction.

Answer:

Benzene is a nucleophile, i.e. the electrons of the benzene ring attack the electrophile and a carbocation results.

Section: 7.5

27)

Describe how the aromatic character of the benzene ring is re-created from the nonaromatic carbocation intermediate during an electrophilic aromatic substitution reaction.

Answer:

An electron pair donating (Lewis base) species attacks the hydrogen adjacent to the carbocation then the electrons between this hydrogen and its carbon are donated to the ring to re-create the aromatic benzene ring.

Section: 7.5

28)

Which of the following is not a correct statement about the electrophilic substitution mechanism of benzene?

A)

Benzene functions as a nucleophile.

B)

Formation of a carbocation intermediate is the rate-determining step.

C)

The carbocation intermediate contains an sp3 hybridized carbon in the ring.

D)

The addition product is a frequently observed minor product.

E)

Aromaticity is regained by loss of H+.

Answer:

D

Section: 7.5

29)

Which of the following is most likely to be the first step in the general mechanism for electrophilic substitution reactions?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

B

Section: 7.6

30)

Provide the major resonance structures of the intermediate sigma complex in the reaction of benzene with the generic electrophile E+.

Answer:

Section: 7.6

31)

Which step in the general mechanism for electrophilic aromatic substitution is rate-determining? Explain.

Answer:

The first step involves addition of the electrophile to benzene to yield a nonaromatic cationic intermediate. The second step involves the loss of H+ from this cationic intermediate to yield an aromatic product. The rate-determining or slow step is the one in which the aromatic reagent is converted into a much less stable nonaromatic cation.

Section: 7.6

32)

In the bromination of benzene using Br2 and FeBr3, is the intermediate carbocation aromatic? Explain.

Answer:

No, the carbocation is not aromatic since the ring contains an sp3 center.

Section: 7.6 and 7.7

33)

Which of the following is one of the resonance contributors of the intermediate produced in a Friedel-Crafts alkylation of benzene?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

A

Section: 7.6 and 7.11

34)

Which of the following is the first step in the mechanism of bromination?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.7

35)

Provide a detailed, stepwise mechanism for the reaction of benzene with Br2 and FeBr3. Make sure to include the activating reaction between Br2 and FeBr3 in your mechanism.

Answer:

Section: 7.7

36)

Bromination of benzene requires a Lewis acid catalyst while bromination of cyclohexene does not. Explain this difference in reactivity.

Answer:

An alkene is a better nucleophile than benzene, and thus the Br-Br bond does not have to be weakened to form a better electrophile before reaction.

Section: 7.7

37)

What purpose does FeCl3 serve in the electrophilic aromatic substitution reaction between chlorine and benzene?

A)

It serves as a radical initiator to produce the chlorine radical needed to propagate the chain reaction.

B)

It functions by destabilizing the carbocationic intermediate and thereby increases the rate of H+ loss.

C)

It serves as a Lewis base catalyst by reacting with Cl2 to generate chloride ions.

D)

It functions by destabilizing the benzene through formation of a -complex.

E)

It serves as a Lewis acid catalyst by reacting with the Cl2 and thereby activates it toward attack by benzene's electrons.

Answer:

E

Section: 7.7

38)

Explain the role of nitric acid and sulfuric acid in the first step in the nitration of benzene.

Answer:

A hydrogen from sulfuric acid protonates the hydroxy group of the nitric acid forming a molecule of water which is then a good leaving group, thereby forming the nitronium ion (NO2+)(the electrophile).

Section: 7.8

39)

Which of the following is the electrophile that attacks the aromatic ring during nitration?

A)

NO2

B)

HNO3

C)

D)

E)

Answer:

D

Section: 7.8

40)

Which of the following is the electrophile that attacks the aromatic ring during sulfonation?

A)

B)

C)

D)

H2SO4

E)

Answer:

A

Section: 7.9

41)

Which of the following is the electrophile that attacks the aromatic ring during Friedel-Crafts acylation?

A)

B)

C)

D)

E)

AlCl3

Answer:

B

Section: 7.10

42)

What is the role of AlCl3 in Friedel-Crafts acylation of benzene and Friedel-Crafts akylation of benzene?

Answer:

AlCl3 reacts with the acyl chloride in the acylation to give an acylium ion and -AlCl4. AlCl3 reacts with the alkyl halide to give a carbocation and -AlCl4.

Section: 7.10 and 7.11

43)

Which of the following is not a correct statement concerning the Friedel-Crafts acylation of benzene?

A)

An alkyl group substitutes for a hydrogen.

B)

The benzene ring attacks an acylium ion.

C)

The acylium ion is resonance stabilized.

D)

The acylium ion is often produced from an acyl chloride.

E)

More than one equivalent of Lewis acid must be used.

Answer:

A

Section: 7.10

44)

What is the major product of the following Friedel-Crafts alkylation?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.11

45)

What is the major organic product of the reaction between benzene and isobutyl chloride in the presence of AlCl3?

A)

tert-butylbenzene

B)

isobutylbenzene

C)

n-butylbenzene

D)

chlorobenzene

E)

sec-butylbenzene

Answer:

A

Section: 7.11

46)

What is the name of the following compound?

A)

m-Bromomethylbenzene

B)

m-Bromotoluene

C)

3-Bromotoluene

D)

A and B

E)

B and C

Answer:

E

Section: 7.12

47)

What is the structure of 3-phenylpentane?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

E

Section: 7.12

48)

What is the name of the following compound?

A)

p-Dichlorobenzene

B)

1,4-Dichlorobenzene

C)

Phenyldichloride

D)

A and B

E)

B and C

Answer:

D

Section: 7.12

49)

What is the structure of p-toluidine?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.12

50)

What is the name of the following compound?

A)

o-nitro-m-bromotoluene

B)

3-bromo-6-nitrotoluene

C)

m-bromo-o-nitrotoluene

D)

5-bromo-2-nitrotoluene

E)

2-nitro-5-bromotoluene

Answer:

D

Section: 7.12

51)

The name 2,4,6-tribromobenzene is incorrect. Which of the following is the correct name?

A)

tribromobenzene

B)

m,m-dibromobromobenzene

C)

3,5-dibromobromobenzene

D)

1,3,5-tribromobenzene

E)

m,m,m-tribromobenzene

Answer:

D

Section: 7.12

52)

What is the proper name of the compound below?

A)

1-phenoxyethane

B)

ethyl phenyl ether

C)

m-sec-butylhydroxybenzene

D)

o-butylhydroxybenzene

E)

m-butylphenol

Answer:

E

Section: 7.12

53)

Provide the best name for the organic compound below.

Answer:

o-propylphenol or 2-propylphenol

Section: 7.12

54)

Provide the best name for the organic compound below.

Answer:

m-toluidine

Section: 7.12

55)

Which of the following compounds reacts most slowly during nitration?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.13

56)

Which of the following compounds reacts most rapidly during nitration?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

D

Section: 7.13

57)

Which of the following substrates is an electron donating group overall?

A)

Br

B)

C)

OCH3

D)

E)

CCl3

Answer:

C

Section: 7.13

58)

Which of the following substrates is an electron withdrawing group overall?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

E

Section: 7.13

59)

Rank the following groups in order of increasing activating power in electrophilic aromatic substitution reactions:

-OCH3, -OCOCH2CH3, -CH2CH3, -Br.

Answer:

-Br < -CH2CH3 < -OCOCH2CH3 < -OCH3

Section: 7.13

60)

Which of the following compounds reacts most rapidly with HNO3/H2SO4?

A)

toluene

B)

anisole

C)

nitrobenzene

D)

benzonitrile

E)

fluorobenzene

Answer:

B

Section: 7.13

61)

List the following compounds in order of decreasing reactivity toward electrophilic aromatic substitution: toluene, benzene, fluorobenzene, nitrobenzene, phenol.

Answer:

phenol > toluene > benzene > fluorobenzene > nitrobenzene

Section: 7.13

62)

Which of the following substituents acts as a moderate activator and o/p director in electrophilic aromatic substitution reactions?

A)

-Br

B)

-SO3H

C)

-CO2H

D)

-NHC(=O)R

E)

-CHO

Answer:

D

Section: 7.13

63)

Which of the following structures is the most important contributor to the resonance hybrid formed when toluene undergoes para nitration?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

A

Section: 7.14

64)

Which of the following structures is the most important contributor to the resonance hybrid formed when anisole undergoes o-bromination?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

A

Section: 7.14

65)

What is the major product of the following reaction?

A)

I

B)

II

C)

III

D)

IV

E)

V

Answer:

C

Section: 7.14

66)

In electrophilic aromatic substitution reactions a chlorine substituent:

A)

is a deactivator and a m-director.

B)

is a deactivator and an o,p-director.

C)

is an activator and a m-director.

D)

is an activator and an o,p-director.

E)

none of the above

Answer:

B

Section: 7.14

67)

Draw the four major resonance structures of the carbocation intermediate in the reaction of anisole with HNO3/H2SO4 to yield p-nitroanisole.

Answer:

Section: 7.14

68)

Draw the three major resonance structures of the carbocation intermediate in the reaction of acetophenone with HNO3/H2SO4 to yield o-nitroacetophenone. Circle the resonance form which is less stable than the other two.

Answer:

Section: 7.14

69)

Compare the effect of electron withdrawing group on an alcohol relative to that of an electron donating group on an alcohol.

Answer:

An electron withdrawing group stabilizes a base and increases the strength of its conjugate acid while an electron donating group destabilizes a base and decreases the strength of its conjugate acid. Thus electron withdrawing groups on an alcohol will increase the acidity of the alcohol.

Section: 7.15

70)

List the following compounds in order of decreasing basicity.

Answer:

Section: 7.15

71)

List the following compounds in order of decreasing acidity.

Answer:

Section: 7.15