1

Organic Chemistry, 7e (Wade)

Chapter 16 Aromatic Compounds

1) Which of the following is an incorrect description of benzene?

A) The CCC bond angles are all equal to 120.

B) The molecule is planar.

C) The molecule is a 6-membered ring which contains alternating single and double carbon-

carbon bonds.

D) The molecule is aromatic.

E) The molecule can be drawn as a resonance hybrid of two Kekule structures.

Answer: C

Diff: 1

Section: 16.2

2) What is the bond order of the carbon-carbon bonds in benzene?

A) 0.5

B) 1

C) 1.5

D) 2

E) 3

Answer: C

Diff: 2

Section: 16.2

3) Cyclic hydrocarbons which can be represented as structures containing alternating single and

double bonds are called __________.

Answer: annulenes

Diff: 1

Section: 16.2

4) When cyclohexene is treated with KMnO4, H2O, the syn-1,2-diol is produced. What reaction

occurs when benzene is similarly treated?

Answer: No reaction takes place.

Diff: 2

Section: 16.2

5) Why does benzene undergo a substitution reaction with Br2 while cyclohexene undergoes an

addition reaction.

Answer: Addition of Br2 to benzene is highly unfavorable because it would result in a

nonaromatic product (aromatic stabilization would be lost).

Diff: 2

Section: 16.2

2

6) What is suggested by the fact that benzene's molar heat of hydrogenation is 36 kcal less than

three times the molar heat of hydrogenation of cyclohexene?

Answer: This suggests that the type of bonding in benzene lends special stability to the molecule.

Diff: 2

Section: 16.2

7) In the molecular orbital representation of benzene, how many molecular orbitals are present?

A) 1

B) 2

C) 4

D) 6

E) 8

Answer: D

Diff: 1

Section: 16.3

8) How many distinct nodal planes which are perpendicular to the molecular plane are present in

the 4* orbital of benzene?

A) 0

B) 1

C) 2

D) 4

E) 5

Answer: C

Diff: 2

Section: 16.3

9) How many pairs of degenerate molecular orbitals are found in benzene? A) 6

B) 5

C) 4

D) 3

E) 2

Answer: E

Diff: 2

Section: 16.3

3

10) Which of the following undergoes SN2 reaction with sodium methoxide most rapidly?

A) PhCH2Br

B) Ph3CBr

C) PhCH2CH2Br

D) PhBr

E) PhCH2CH2CH2Br

Answer: A

Diff: 2

Section: 16.3

11) Which of the following undergoes solvolysis in methanol most rapidly?

A) PhCH2Br

B) Ph3CBr

C) PhCH2CH2Br

D) PhBr

E) PhCH2CH2CH2Br

Answer: B

Diff: 2

Section: 16.3

12) Provide a diagram which depicts the relative energies of the molecular orbitals of benzene. Show which molecular orbitals are filled in benzene's ground state.

Answer:

Diff: 3

Section: 16.3

13) Show how the participating p orbitals interact to form the highest energy molecular orbital of benzene.

Answer: All adjacent interactions are antibonding.

Diff: 2

Section: 16.3

4

14) Show how the p orbitals overlap to generate the p4* of cyclobutadiene.

Answer:

Diff: 2

Section: 16.4

15) Describe the occupied p molecular orbitals in the ground state of cyclobutadiene.

Answer: Two electrons are paired in a bonding p molecular orbital, while the other two

electrons are unpaired and singly occupy two nonbonding p molecular orbitals.

Diff: 3

Section: 16.4

16) What is the major difference between an antiaromatic and aromatic compound?

A) The structure must be cyclic for aromatic but not antiaromatic compounds?

B) Antiaromatic compounds have at least one sp3 hybridized atom in the ring

C) Antiaromatic compounds can assume a chair-like structure while aromatic compounds are

nearly flat

D) Aromatic compounds cannot have a charged atom in the structure

E) Only aromatic compounds follow Huckle's rule.

Answer: E

Diff: 1

Section: 16.5

17) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: nonaromatic

Diff: 2

Section: 16.5

5

18) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: nonaromatic

Diff: 2

Section: 16.5

19) Classify the following compound as aromatic, antiaromatic, or nonaromatic.

Answer: Aromatic

Diff: 3

Section: 16.5

20) Classify the following compound as aromatic, antiaromatic, or nonaromatic.

Answer: nonaromatic (not planar nor continuous)

Diff: 3

Section: 16.5

21) Classify the following compound as being aromatic, nonaromatic or antiaromatic.

Answer: antiaromatic (8 electrons)

Diff: 3

Section: 16.5

6

22) Aromatic molecules contain __________ electrons. A) no

B) 4n + 1 (with n an integer)

C) 4n + 2 (with n an integer)

D) 4n (with n an integer)

E) unpaired

Answer: C

Diff: 1

Section: 16.6

23) Which of the following is another name for cyclobutadiene?

A) [2]annulene

B) [4]annulene

C) [6]annulene

D) Dewar benzene

E) antibenzene

Answer: B

Diff: 1

Section: 16.6

7

24) Which of the following structures, if flat, would be classified as antiaromatic?

A)

B)

C)

D)

E)

Answer: C

Diff: 2

Section: 16.6

25) Classify 1,3,5-heptatriene as aromatic, antiaromatic, or nonaromatic. Assume planarity of the

network. Answer: nonaromatic

Diff: 1

Section: 16.6

8

26) 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. Diff: 2

Section: 16.6

27) Which is more stable, cyclobutadiene or 1,3-butadiene? Explain.

Answer: 1,3-Butadiene is more stable. Cyclobutadiene is antiaromatic since it contains 4 (i.e.,

4n) electrons. Antiaromatic systems are less stable than their open-chain counterparts. Diff: 2

Section: 16.6

28) Is cyclooctatetraene a planar molecule? Explain.

Answer: No. In addition to increasing other forms of strain, a planar conformation would make

this molecule antiaromatic.

Diff: 2

Section: 16.6

29) Classify cycloheptatriene as aromatic, antiaromatic, or nonaromatic. Assume planarity of the

network. Answer: nonaromatic

Diff: 2

Section: 16.6

30) Classify cyclopentadiene as aromatic, antiaromatic, or nonaromatic. Assume planarity of the

network. Answer: nonaromatic

Diff: 2

Section: 16.6

31) Is the all-cis form of [10]annulene aromatic? Explain.

Answer: No. Planarity, which is required for aromaticity, is precluded due to excessive angle

strain.

Diff: 3

Section: 16.6

32) Is the [10]annulene shown below aromatic? Explain.

Answer: No. The ring cannot achieve a planar conformation since such a conformation would

result in severe and unfavorable steric interactions between the two H's oriented inside the ring.

Diff: 3

Section: 16.6

9

33) Classify the compound below as aromatic antiaromatic, or nonaromatic. Assume planarity of

the network.

Answer: aromatic

Diff: 3

Section: 16.6

34) Which of the following is the same as the tropylium ion?

A) cycloheptatrienyl cation

B) cycloheptatrienyl anion

C) cyclopentadienyl cation

D) cyclopentadienyl anion

E) cyclopropenyl anion

Answer: A

Diff: 2

Section: 16.8

10

35) Which of the following structures is aromatic?

A)

B)

C)

D)

E)

Answer: A

Diff: 2

Section: 16.8

11

36) Which of the labeled H atoms (1 -5) in the following molecule would be predicted to be the

most acidic?

A) 1

B) 2

C) 3

D) 4

E) 5

Answer: A

Diff: 3

Section: 16.8

37) Rank the following in order of increasing pKa (from lowest to highest pKa)

A) 1 < 2 < 3

B) 3 < 2 < 1

C) 2 < 1 < 3

D) 3 < 1 < 2

E) 2 < 3 < 1

Answer: C

Diff: 3

Section: 16.8

12

38) Which sequence ranks the indicated protons in order of increasing acidity?

A) 1 < 2 < 3

B) 2 < 3 < 1

C) 3 < 1 < 2

D) 3 < 2 < 1

Answer: D

Diff: 3

Section: 16.8

39) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: aromatic

Diff: 1

Section: 16.8

40) Provide the structure of tropylium bromide.

Answer:

Diff: 2

Section: 16.8

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41) Provide the structure of sodium cyclopentadienide.

Answer:

Diff: 2

Section: 16.8

42) What compound results when cyclooctatetraene is treated with excess potassium metal?

Answer: dipotassium cyclooctatetraene dianion

Diff: 3

Section: 16.8

43) Add six pi bonds to the structure below to produce the most stable isomer.

Answer:

Diff: 3

Section: 16.8

44) Provide the major resonance structures of the ion which results when the most acidic

hydrogen of cyclopentadiene is lost.

Answer:

Diff: 2

Section: 16.8

45) Explain the relative acidities of cyclohexene (pKa of 46) and cyclopentadiene (pKa of 16).

Answer: The conjugate base of cyclopentadiene is aromatic.

Diff: 2

Section: 16.8

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46) Why would the reaction below proceed at an extremely slow rate if at all?

Answer: The carbocation intermediate involved in this reaction is antiaromatic. Based on

Hammond's Postulate, one can surmise that the activation energy required to produce this

intermediate would be very high.

Diff: 2

Section: 16.8

47) When cycloheptatriene is deprotonated, an anion with seven resonance forms of equal energy

can be drawn. Given this fact, explain why cycloheptatriene is only slightly more acidic than

propene.

Answer: Each resonance form of the cyclopehtatrienyl anion is antiaromatic.

Diff: 2

Section: 16.8

48) 3-Chlorocyclopropene is solvolyzed in methanol at a much higher rate than is

chlorocyclopropane. Offer an explanation.

Answer: Cyclopropenyl cation, the carbocationic intermediate in the case of 3-chlorocycloprene,

is aromatic and via Hammond's Postulate we can say that the activation energy required to form

it is lower.

Diff: 2

Section: 16.8

49) Classify cyclopentadienyl cation as aromatic, antiaromatic, or nonaromatic. Assume

planarity of the network. Answer: antiaromatic

Diff: 2

Section: 16.8

50) Classify cyclopropenyl cation as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network Answer: aromatic

Diff: 2

Section: 16.8

51) Classify cycloheptatrienyl cation as aromatic, antiaromatic, or nonaromatic. Assume

planarity of the network. Answer: aromatic

Diff: 2

Section: 16.8

15

52) Nitrogen's lone pair electrons occupy what type of orbital in pyridine?

A) s

B) sp

C) sp2

D) sp3

E) p

Answer: C

Diff: 2

Section: 16.9

53) Classify pyrrole as aromatic, antiaromatic, or nonaromatic. Assume planarity of the network.

Answer: aromatic

Diff: 2

Section: 16.9

54) __________ is similar to furan, with a sulfur atom in place of the oxygen.

Answer: Thiophene

Diff: 2

Section: 16.9

55) Is the molecule below aromatic, antiaromatic, or nonaromatic?

Answer: aromatic

Diff: 2

Section: 16.9

56) Is the molecule below aromatic, antiaromatic, or nonaromatic?

Answer: aromatic

Diff: 2

Section: 16.9

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57) Is the molecule below aromatic, antiaromatic, or nonaromatic?

Answer: nonaromatic

Diff: 2

Section: 16.9

58) Is the molecule below aromatic, antiaromatic, or nonaromatic?

Answer: nonaromatic

Diff: 2

Section: 16.9

59) Is the molecule below aromatic, antiaromatic, or nonaromatic?

Answer: aromatic

Diff: 2

Section: 16.9

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60) Circle and name the aromatic heterocycles in the following structure, which has been found

to be effective against inflammatory diseases including Alzheimer's disease and rheumatoid

arthritis (J. Med. Chem. 2007, 4728).

Answer:

Diff: 2

Section: 16.9

61) Classify pyridine as aromatic, antiaromatic, or nonaromatic. Assume planarity of the network.

Answer: aromatic

Diff: 2

Section: 16.9

62) Which is more basic, pyridine or pyrrole? Explain.

Answer: Pyrrole is a much weaker base than pyridine. When pyrrole is protonated, the system's

aromaticity is destroyed.

Diff: 2

Section: 16.9

63) Provide the structure of pyrimidine.

Answer:

Diff: 2

Section: 16.9

18

64) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: aromatic

Diff: 3

Section: 16.9

65) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: aromatic

Diff: 3

Section: 16.9

66) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: antiaromatic

Diff: 2

Section: 16.10

67) Classify naphthalene as aromatic, antiaromatic, or nonaromatic. Assume planarity of the network.

Answer: aromatic

Diff: 1

Section: 16.10

68) Why are researchers interested in the properties of large polynuclear aromatic hydrocrobons?

Answer: These compounds are formed to some extent in nearly all combustion reactions of

organic compounds and are known to be potent carcinogens.

Diff: 2

Section: 16.10

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69) Provide the structure of the major organic product which results when phenanthrene is

treated with Br2 in carbon tetrachloride.

Answer:

Diff: 2

Section: 16.10

70) Provide the structure of anthracene.

Answer:

Diff: 2

Section: 16.10

71) Name two of the three common allotropes of carbon.

Answer: diamond; graphite; fullerenes (like buckminsterfullerene)

Diff: 1

Section: 16.11

72) Which of the following is not a fused-ring heterocycle?

A) purine

B) pyrimidine

C) benzofuran

D) indole

E) quinoline

Answer: B

Diff: 2

Section: 16.12

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73) Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity

of the network.

Answer: aromatic

Diff: 2

Section: 16.12

74) Provide the structure of indole.

Answer:

Diff: 3

Section: 16.12

75) How many distinct trichlorobenzene isomers are possible?

A) 2

B) 3

C) 4

D) 5

E) 6

Answer: B

Diff: 1

Section: 16.13

76) Which of the following is another accepted name for methyl phenyl ketone?

A) anisole

B) cresol

C) acetophenone

D) benzophenone

E) cresone

Answer: C

Diff: 1

Section: 16.13

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77) Which of the following is also an acceptable name for 3-nitrophenol?

A) o-nitrophenol

B) m-nitrophenol

C) p-nitrophenol

D) 3-cresol

E) hydroquinone

Answer: B

Diff: 1

Section: 16.13

78) Which of the following is also an acceptable name for 1,3,5-trimethylbenzene?

A) styrene

B) mesitylene

C) cumene

D) cymene

E) 1,3,5-xylene

Answer: B

Diff: 2

Section: 16.13

79) Provide an acceptable name for the compound below.

Answer: 1,3,5-trinitrobenzene

Diff: 1

Section: 16.13

80) Provide the structure of m-xylene.

Answer:

Diff: 2

Section: 16.13

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81) Provide the structure of o-toluic acid.

Answer:

Diff: 2

Section: 16.13

82) Provide the structure of 2,5-dichlorophenol.

Answer:

Diff: 2

Section: 16.13

83) Provide the structure of o-chlorotoluene.

Answer:

Diff: 2

Section: 16.13

84) Provide the structure of p-aminobenzoic acid.

Answer:

Diff: 2

Section: 16.13

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85) Provide the structure of 2,4,6-trichlorophenol.

Answer:

Diff: 2

Section: 16.13

86) Provide the structure of m-iodobenzoic acid.

Answer:

Diff: 2

Section: 16.13

87) Provide the name of the compound shown below.

Answer: o-chlorobenzoic zcid or 2-chlorobenzoic acid

Diff: 2

Section: 16.13

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88) Provide the name of the compound shown below.

Answer: m-iodobenzaldehyde or 3-iodobenzaldehyde

Diff: 2

Section: 16.13

89) Provide an acceptable name for the compound below.

Answer: para-dichlorobenzene or 1,4-dichlorobenzene

Diff: 1

Section: 16.13

90) Provide an acceptable name for the compound below.

Answer: ortho-xylene or 1,2-dimethylbenzene

Diff: 1

Section: 16.13

91) Provide an acceptable name for the compound below.

Answer: aniline or aminobenzene or benzeneamine

Diff: 2

Section: 16.13

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92) Provide an acceptable name for the compound below.

Answer: 2,4-dinitrotoluene

Diff: 2

Section: 16.13

93) Provide the structure of m-nitrophenol.

Answer:

Diff: 2

Section: 16.13

94) Provide the structure of anisole.

Answer:

Diff: 2

Section: 16.13

95) Provide the structure of o-bromostyrene.

Answer:

Diff: 2

Section: 16.13

96) Provide the structure of 2-bromo-4-chlorobenzoic acid.

Answer:

Diff: 2

Section: 16.13

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97) Provide the structure of 4-isopropylbenzaldehyde.

Answer:

Diff: 2

Section: 16.13

98) Provide an acceptable name for PhSO3H.

Answer: benzenesulfonic acid

Diff: 2

Section: 16.13

99) Provide an acceptable name for the compound below.

Answer: imidazole

Diff: 3

Section: 16.13

100) Which of the following compounds has the lowest boiling point?

A) 1,2,3-trichlorobenzene

B) 1,2,4-tirchlorobenzene

C) p-dichlorobenzene

D) m-dichlorobenzene

E) o-dichlorobenzene

Answer: C

Diff: 2

Section: 16.14

101) Which of the following compound has the highest melting point?

A) benzene

B) toluene

C) o-dichlorobenzene

D) m-dichlorobenzene

E) p-dichlorobenzene

Answer: E

Diff: 2

Section: 16.14

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102) In the UV-visible spectra of the following compounds, in which does lambda max appear at

the highest wavelength?

A) 3-phenylpropene

B) 1-phenylpropene

C) benzene

D) n-octylbenzene

E) isopropylbenzene

Answer: B

Diff: 1

Section: 16.15

103) In the proton NMR, in what region of the spectrum does one typically observe hydrogens

bound to the aromatic ring?

A) 1.0-1.5 ppm

B) 2.0-3.0 ppm

C) 4.5-5.5 ppm

D) 7.0-8.0 ppm

E) 9.0-10.0 ppm

Answer: D

Diff: 2

Section: 16.15

104) In the carbon NMR, in what region of the spectrum does one typically observe carbons

which are part of the aromatic ring?

A) -10-0 ppm

B) 40-60 ppm

C) 80-100 ppm

D) 120-150 ppm

E) 200-220 ppm

Answer: D

Diff: 2

Section: 16.15

105) Which of the following compounds has the most signals in the noise-decoupled 13C NMR

spectrum?

A) o-dibromobenzene

B) m-dibromobenzene

C) p-dibromobenzene

D) 1,3,5-tribromobenzene

E) 1,2,3,4-tetrabromobenzene

Answer: B

Diff: 2

Section: 16.15

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107) How many peaks are in the proton spin decoupled 13C NMR spectrum of p-

dichlorobenzene?

A) 2

B) 3

C) 4

D) 5

E) 6

Answer: A

Diff: 2

Section: 16.15

108) How many peaks are in the proton spin decoupled 13C NMR spectrum of 1,3,5-

trinitrobenzene?

A) 1

B) 2

C) 3

D) 4

E) 6

Answer: B

Diff: 2

Section: 16.15

109) How many peaks are in the proton spin decoupled 13C NMR spectrum of 1,2,3-

trichlorobenzene?

A) 1

B) 2

C) 3

D) 4

E) 6

Answer: D

Diff: 2

Section: 16.15

29

110) The IR spectrum of m-xylene contains stretches which are characteristic of most aromatic

hydrocarbons. List the peaks (in cm-1) associated with the aromatic C=C stretch and the

aromatic CH stretch.

Answer: C=C: around 1600 cm-1

CH: just above 3000 cm-1 (usually around 3030 cm-1) Diff: 2

Section: 16.15

111) How might one distinguish the isomers of trimethylbenzene by noise-decoupled 13C NMR?

Answer: 1,3,5-trimethylbenzene: 3 signals

1,2,4-trimethylbenzene: 9 signals

1,2,3-trimethylbenzene: 6 signals

Diff: 2

Section: 16.15