Tb

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CHAPTER 21—BENZENE AND THE CONCEPT OF AROMATICITY

MULTIPLE CHOICE

1. What is the IUPAC name of the following compound?

a. 2,4-dibromotoluene b. 2,4-dibromophenol

c. 2,4-dibromohydroxybenzene d. 4,6-dibromophenol

ANS: B

a. 2-nitro-6-carboxybromobenzene b. 2-bromo-1-nitro-4-benzoic acid c. 2-bromo-4-carboxynitrobenzene d. 3-bromo-4-nitrobenzoic acid ANS: D

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a. 1-bromo-2-fluoro-4-toluene b. 4-bromo-2-fluorotoluene c. 2-bromo-4-fluorophenol d. 4-bromo-2-fluoroxylene ANS: B

4. What is the correct assignment of the names of the following substituted benzenes?

a. 1 = phenol; 2 = aniline; 3 = anisole b. 1 = benzaldehyde; 2 = anisole; 3 = toluene c. 1 = anisole; 2 = xylene; 3 = toluene d. 1 = anisole; 2 = aniline; 3 = toluene ANS: D

5. What is the correct assignment of the names of the following substituted benzenes?

a. 1 = styrene; 2 = benzaldehyde; 3 = cumene b. 1 = anisole; 2 = benzaldehyde; 3 = toluene c. 1 = styrene; 2 = xylene; 3 = toluene d. 1 = aniline; 2 = phenol; 3 = cumene ANS: A

6. Which is the correct assignment of the names of the following substituted benzenes?

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a. 1 = phenol; 2 = m-cresol; 3 = toluene b. 1 = m-cresol; 2 = resorcinol; 3 = m-xylene c. 1 = anisole; 2 = catechol; 3 = m-xylene d. 1 = m-cresol; 2 = anisole; 3 = cumene ANS: B

7. Which of the following represents the energy levels of the molecular orbitals of benzene?

a. 1 b. 2 c. 3 d. 4 ANS: C

8. Which of the following is not one of Hückel's criteria for aromaticity? a. The compound must be cyclic

b. The compound must have one p-orbital on each atom of a ring

c. The compound must be planar or nearly planar so that there is a continuous overlap of p orbitals

d. The compound must have a closed loop of six pi electrons ANS: D

9. What is the correct assignment of the names of the following heterocycles?

a. 1 = pyrrole; 2 = thiophene; 3 = pyridine b. 1 = thiophene; 2 = furan; 3 = pyrrole c. 1 = pyridine; 2 = thiophene; 3 = furan d. 1 = pyridine; 2 = thiophene; 3 = pyrrole ANS: C

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10. What is the correct assignment of the names of the following heterocycles?

a. 1 = pyrrole; 2 = thiophene; 3 = pyridine b. 1 = thiophene; 2 = furan; 3 = pyrrole c. 1 = furan; 2 = pyrrole; 3 = thiophene d. 1 = furan; 2 = thiophene; 3 = pyrrole ANS: C

11. What is the correct assignment of the names of the following fused arenes?

a. 1 = naphthalene; 2 = anthracene; 3 = pyrene b. 1 = naphthalene; 2 = phenanthrene; 3 = coronene c. 1 = naphthalene; 2 = anthracene; 3 = benzo[a]pyrene d. 1 = anthracene; 2 = naphthalene; 3 = coronene ANS: A

12. What is the approximate length of the carbon-carbon bonds in benzene? a. 110 pm

b. 121 pm c. 139 pm d. 156 pm ANS: C

13. Which of the following statements is not true about the structure of benzene? a. the carbon-carbon bonds are all the same length

b. the structure rapidly transforms between two resonance contributors c. the structure is an average of two resonance contributors

d. the ring of six carbon atoms is planar ANS: B

14. What is the hybridization of the oxygen atom of furan? a. s

b. sp c. sp2 d. sp3

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ANS: C

15. What is the hybridization of the nitrogen atom of pyridine? a. s

b. sp c. sp2 d. sp3 ANS: C

16. Which orbital contains the lone pair on the nitrogen atom of pyridine? a. s

b. p c. sp d. sp2 ANS: D

17. Which orbital contains the lone pair on the nitrogen atom of pyrrole? a. s

b. p c. sp d. sp2 ANS: B

18. Which orbitals contain the lone pairs on the nitrogen atoms labeled i and ii in the imidazole ring?

a. i = sp2;_{; ii = sp}3 b. i = p;_{; ii = sp}3 c. i = p;_{; ii = sp}3 d. i = p;_{; ii = sp}2 ANS: D

19. Which of the following statements is not true about the structure of benzene? a. six atomic 2p-orbitals overlap to form six -molecular orbitals

b. there are three bonding -molecular orbitals and three -antibonding molecular orbitals c. the ground state electronic configuration of benzene has six electrons in three -bonding

molecular orbitals

d. the bonds alternate in length around the ring ANS: D

20. Which of the following compounds is aromatic? a. ethane

b. cyclobutadiene c. benzene

d. cyclooctatetraene

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ANS: C

21. Which of the following heterocycles is not aromatic?

a. 1 b. 2 c. 3 d. 4 ANS: C

22. Which of the following ions is aromatic?

a. 1 b. 2 c. 3 d. 4 ANS: B

23. Which of the following compounds is antiaromatic? a. ethane

b. cyclobutadiene c. benzene

d. cyclooctatetraene ANS: B

24. Which of the following molecules is the most acidic?

a. 1 b. 2 c. 3 d. 4

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ANS: B

25. Which of the following compounds undergoes heterolytic carbon-halogen bond cleavage to form a stable organic cation?

a. 1 b. 2 c. 3 d. 4 ANS: D

26. Which of the following has the compounds shown in the correct order of decreasing acidity (i.e., more acidic > less acidic)?

a. 1 > 2 > 3 b. 2 > 1 > 3 c. 3 > 2 > 1 d. 1 > 3 > 2 ANS: A

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a. 1 > 2 > 3 b. 2 > 3 > 1 c. 3 > 2 > 1 d. 1 > 3 > 2 ANS: A

a. 1 > 2 > 3 b. 2 > 1 > 3 c. 3 > 2 > 1 d. 1 > 3 > 2 ANS: A

a. 1 > 2 > 3 b. 2 > 3 > 1 c. 3 > 2 > 1 d. 1 > 3 > 2 ANS: B

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a. 1 > 2 > 3 b. 2 > 3 > 1 c. 3 > 2 > 1 d. 2 > 1 > 3 ANS: D

31. What is the major organic product obtained from the following reaction?

a. 1 b. 2 c. 3 d. 4 ANS: A

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a. 1 b. 2 c. 3 d. 4 ANS: A

a. 1 b. 2 c. 3 d. 4 ANS: C

a. 1 b. 2 c. 3 d. 4 ANS: B

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35. What is the major organic product obtained from the following reaction? a. 1 b. 2 c. 3 d. 4 ANS: A

a. 1 b. 2 c. 3 d. 4 ANS: A

37. What is the best choice of reagent to achieve the following reaction?

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a. H2SO4

b. NaOH, H2O

c. K2Cr2O7, H2SO4 d. LiAlH4

ANS: C

a. H2SO4

b. NaOH, H2O

c. K2Cr2O7, H2SO4 d. H2/Pd

ANS: D

a. Br2, CCl4 b. HBr, H2O c. Br2, light d. NaBr ANS: C

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40. Which of the following represents the energy levels of the molecular orbitals of cyclopentadienyl anion? a. 1 b. 2 c. 3 d. 4 ANS: A

41. Which of the following represents the energy levels of the molecular orbitals of cyclobutadiene?

a. 1 b. 2 c. 3 d. 4 ANS: A

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42. Which of the following is not true about cyclooctatetraene? a. The planar conformation is antiaromatic

b. In the planar conformation there are two electrons in degenerate nonbonding pi orbitals. c. In the tub conformation the pi system is not conjugated

d. The tub conformation is aromatic ANS: D

43. What are the relative positions of the substituents in the following structure?

a. anti b. meta c. ortho d. para ANS: C

a. anti b. meta c. ortho d. para ANS: B

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a. anti b. meta c. ortho d. para ANS: D

46. Which of the following is not true about [18]annulene?

a. [18]annulene is planar b. [18]annulene is aromatic

c. [18]annulene gives one peak in the 1_{H NMR spectrum} d. [18]annulene has 18 pi bonds

ANS: D

47. Which of the following is true about [10]annulene?

a. [10]annulene is planar b. [10]annulene is nonaromatic

c. [10]annulene undergoes addition reactions similar to simple alkenes d. [10]annulene has 10 pi electrons

ANS: A

48. Which of the following is not true about [the bridged [10]annulene shown below?

a. all of the carbon atoms of bridged [10]annulene are all in the same plane b. bridged [10]annulene is aromatic

c. bridged [10]annulene undergoes substitution reactions similar to benzene d. bridged [10]annulene has 10 pi electrons

ANS: A

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49. Which of the following does not undergo oxidation in the presence of H2CrO4? a. 1 b. 2 c. 3 d. 4 ANS: D

50. What is the intermediate in the reaction of ethylbenzene with NBS in the presence of benzoyl peroxide to give 1-bromo-1-phenylethane? a. Benzylic anion b. Benzylic cation c. Benzylic radical d. Benzylic carbene ANS: C

51. How many p orbital electrons are present in furan? a. 4

b. 6 c. 7 d. 8 ANS: B

52. How many p orbital electrons are present in cyclopentadienyl anion? a. 4 b. 6 c. 7 d. 8 ANS: B TRUE/FALSE

1. The following compound is named (Z)-2,3-diphenyl-2-hexene.

ANS: F

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2. Furan has the structure shown below.

The  orbitals in furan are shown below.

ANS: T

3. Consider the following structures,

A B

Structure A is more acidic than B. ANS: F

4.

Phenols are stronger acids than alcohols because of the resonance stabilization of alkoxide

ions.

ANS: F

5. The product of the following reaction would be a dicarboxylic acid.

ANS: F

6. The product of the following reaction would be 1-bromo-1-phenylethane.

ANS: T

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7. The following compound should be name as 1-((1S, 3R)-3-methylcyclohexyl)benzene.

ANS: T

8. Consider the following structure.

The 1_{H NMR spectrum of this compound}_₆₀__{C shows a peak at 7.6 ppm, this would indicate} aromaticity.

ANS: T

9. The following compound could be produced by reaction of phenol with bromobenzene.

ANS: F

10. Consider the following general structure.

Positions 1,4 and 2,6 and 3,5 are termed para, ortho, and meta, respectively. ANS: T

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COMPLETION

Answer the following questions concerning sulfathiazole below by filling ieach blank with the appropriate response.

1. The hybridization of the nitrogen atom in sulfathiazole is ______. A. sp

B. sp2 C. sp3 ANS: B

2. Assuming that the sulfur atom is sp2_{-hybridized, there are _____}__{-electrons in the sulfathiazole ring.}

ANS: Six 6

3. According to Hückel criteria, sulfathiazole is predicted to be ___________. ANS: aromatic

Consider the cycloheptatrienyl anion. Complete the following questions by filling in the blank with the appropriate word.

4. According the Hückel criteria this anion would be classified as ______________. ANS: antiaromatic

5. This anion has______  electrons.

ANS: 8 eight

6. Consider the following two compounds.

A B

An ether solution containing these two substances was mixed with an aqueous 1 M NaOH solution. Substance _____ will be found in the basic layer.

ANS: B

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7. Consider the following compound.

When treated with acidic K2Cr2O7 the product would be classified as a ___________. ANS: quinone

Consider the following structure.

8. If named as an annulene, the name would be [ __ ] annulene. ANS: 16

9. This structure contains ____  electrons. ANS: 16

10. Using the Hückel criteria, the compound would be ______________. ANS: antiaromatic

Consider the following structure.

The 1_{H NMR spectrum of [14]annulene at}_₆₀__{C shows two peaks, one at 0 ppm and one at 7.6 ppm.} 11. The NMR data would seem to indicate that this compound is ____________.

ANS: aromatic

12. The ratio of peak areas, respectively for the 7.6 ppm and 0 ppm, peaks would be_______.

ANS: 5:2 5 to 2 5/2

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PROBLEM

ANS: 3-bromo-4-nitrophenol

ANS: 4-chloro-3-nitrobenzoic acid

ANS: 3,4-dichloroaniline

ANS:

4-bromo-2-propylbenzonitrile

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ANS:

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ANS:

8. What are the two major organic products obtained from the following reaction?

ANS:

9. List all of the requirements of a molecule for it to be considered aromatic.

ANS: An aromatic molecule has an uninterrupted cycle of overlapping p orbitals (i.e., the cyclic structure should be planar) that contains 2n+2 (i.e., 2, 6, 8, ...) electrons

10. Provide a brief explanation of why furan (below) is aromatic. Your answer should identify the hybridization of the oxygen atom and account for which electrons are in the pi system.

ANS: The oxygen is sp2_{hybridized. One lone pair is in a p orbital and is part of the pi system, along} with one electron in the p orbital on each of the four carbon atoms. The other lone pair of the oxygen is in an sp2_{orbital, which is in the plane of the ring and not involved in the pi system.}

11. Provide a brief explanation of why the cyclopentadienyl anion (below) is aromatic. Your answer should identify the hybridization of each carbon atom and account for which electrons are in the pi system.

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ANS: The all five carbons are sp2_{hybridized. The lone pair is in a p orbital and is part of the pi} system, along with one electron in the p orbital on each of the other four carbon atoms for a total of six pi electrons.