Deck 16: Aromatic Compounds

Question 1

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

Answer 1

annulenes

Question 2

Even assuming the molecule below is planar, it would still be classified as non-aromatic. Why?

Answer 2

The center carbon of the allene is sp hybridized and the p orbitals are 90° to each other. There is no overlap of the pi system.

Question 3

When toluene is treated with bromine (shown below), the bromine doesn't react with the double bond to form a viscinal dihalide, but instead a substitution of one of the benzylic hydrogens takes place. Why?

Answer 3

Addition of Br₂ to the double bonds of benzene is highly unfavorable because it would result in a nonaromatic product (aromatic stabilization would be lost). Instead, a radical reaction to form a highly stabilized radical would occur.

Question 4

How many pairs of degenerate π molecular orbitals are found in benzene?

A) 6
B) 5
C) 4
D) 3
E) 2

Question 5

The bond lengths in a substituted cyclobutadiene compounds were determined by computational methods to display very unequal bond lengths (1.354 and 1.608

A). What does this suggest about resonance structures in this compound? (J. Phys. Chem. A, 2000, 104, 1246-1255)

Question 6

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.

Question 7

How many distinct nodal planes which are perpendicular to the molecular plane are present in the π₄^* orbital of benzene?

A) 0
B) 1
C) 2
D) 4
E) 5

Question 8

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

Question 9

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.

Question 10

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

Question 11

Which would have a smaller heat of hydrogenation? Circle your answer.

Question 12

Use the polygon rule to draw the MO energy diagram of the cyclononatetraenyl anion. Assuming planarity, would this ion be aromatic or antiaromatic?

Question 13

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

A) 0.5
B) 1
C) 1.5
D) 2
E) 3

Question 14

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

A) 1
B) 2
C) 4
D) 6
E) 8

Question 15

When cyclohexene is treated with KMnO₄, H₂O, the syn-1,2-diol is produced. What reaction occurs when benzene is similarly treated?

Question 16

The MO energy diagram for cyclobutadiene is shown below. Fill in the π electrons for the neutral compound. Does cyclobutadiene react like a cation, anion or radical in reactions?

Question 17

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?

Question 18

Show how the p orbitals overlap to generate the π₄* of cyclobutadiene.

Question 19

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

Question 20

Describe the occupied π molecular orbitals in the ground state of cyclobutadiene.

Question 21

Which of the following is another name for cyclobutadiene?

A) [2]annulene
B) [4]annulene
C) [6]annulene
D) Dewar benzene
E) antibenzene

Question 22

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

Question 23

Classify cyclopentadiene as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 24

Provide the structure of sodium cyclopentadienide.

Question 25

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

Question 26

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

Question 27

Is cyclooctatetraene a planar molecule? Explain.

Question 28

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

Question 29

Classify cycloheptatriene as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 30

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

Question 31

Aromatic molecules contain ________ π 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

Question 32

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 33

Which of the following structures is aromatic?

A)
B)
C)
D)
E)

Question 34

Classify the compound below as aromatic antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 35

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

Question 36

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

Question 37

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

Question 38

Provide the structure of tropylium bromide.

Question 39

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

A)
B)
C)
D)
E)

Question 40

Provide the major resonance structures of the ion which results when the most acidic hydrogen of cyclopentadiene is lost.

Question 41

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 42

________ is similar to furan, with a sulfur atom in place of the oxygen.

Question 43

Classify cyclopentadienyl cation as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 44

Classify cyclopropenyl cation as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network

Question 45

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

Question 46

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.

Question 47

Classify cycloheptatrienyl cation as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 48

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

Question 49

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 50

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

A) s
B) sp
C) sp²
D) sp³
E) p

Question 51

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

Question 52

Use resonance structures to explain why the compound shown below is considered to be antiaromatic.

Question 53

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

Question 54

Is the molecule below aromatic, antiaromatic, or nonaromatic?

Question 55

Which sequence correctly ranks the following substrates in order of increasing reactivity in an S_N1 reaction?

A) 1 < 2 < 3
B) 2 < 3 < 1
C) 3 < 1 < 2
D) 1 < 3 < 2

Question 56

Styrene (shown below) has 8 π electrons but is aromatic. Why?

Question 57

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 58

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

Question 59

3-Chlorocyclopropene is solvolyzed in methanol at a much higher rate than is chlorocyclopropane. Offer an explanation.

Question 60

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

Question 61

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 62

Which is more basic, pyridine or pyrrole? Explain.

Question 63

Which of the following compounds may correctly be classified as being aromatic?
(More than one answer is possible.)

A)
B)
C)
D)

Question 64

Is the molecule below aromatic, antiaromatic, or nonaromatic?

Question 65

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

A) purine
B) pyrimidine
C) benzofuran
D) indole
E) quinoline

Question 66

Is the molecule below aromatic, antiaromatic, or nonaromatic?

Question 67

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).

Question 68

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

Question 69

Provide the structure of anthracene.

Question 70

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 71

Pyrene has been tentatively identified in the interstellar medium. Use Huckel Rule to determine is pyrene is aromatic or antiaromatic (assuming planarity of the π system).

Question 72

Which orbital is the lone pair of each N in, sp² or p?

Question 73

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.

Question 74

Name two of the three common allotropes of carbon.

Question 75

Provide the structure of pyrimidine.

Question 76

Is the molecule below aromatic, antiaromatic, or nonaromatic?

Question 77

Is the molecule below aromatic, antiaromatic, or nonaromatic?

Question 78

Provide the structure of the major organic product which results when phenanthrene is treated with Br₂ in carbon tetrachloride.

Question 79

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

Question 80

Classify the compound below as aromatic, antiaromatic, or nonaromatic. Assume planarity of the π network.