Organic Chemistry

Paula Yurkanis Bruice

Chapter 15

Aromaticity • Reactions of Benzene - all with Video Answers

Educators

Chapter Questions

Problem 1

a. What is the value of $n$ in Huckel's rule when a compound has nine pairs of $\pi$ electrons?
b. Is such a compound aromatic?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 2

Which of the following compounds are aromatic?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 3

a. How many monobromonaphthalenes are there?
b. How many monobromophenanthrenes are there?
SOLUTION TO 3a There are two monobromonaphthalenes. Substitution cannot occur at either of the carbons shared by both rings, because those carbons are not bonded to a hydrogen. Naphthalene is a flat molecule, so substitution for a hydrogen at any other carbon will result in one of the compounds shown.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 4

The $[10]-$ and $[12]-$ annulenes have been synthesized, and neither has been found to be aromatic. Explain.

Mercedes Mazza

Numerade Educator

Problem 5

In what orbitals are the electrons represented as lone pairs when drawing the structures of quinoline, indole, imidazole, purine, and pyrimidine?

Madeline Currie

Madeline Currie

Numerade Educator

Problem 6

Answer the following questions by examining the electrostatic potential maps on p. 598 :
a. Why is the bottom part of the electrostatic potential map of pyrrole blue?
b. Why is the bottom part of the electrostatic potential map of pyridine red?
c. Why is the center of the electrostatic potential map of benzene more red than the center of the electrostatic potential map of pyridine?

Madeline Currie

Madeline Currie

Numerade Educator

Problem 7

Predict the relative $\mathrm{p} K_{\mathrm{a}}$ values of cyclopentadiene and cycloheptatriene.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 8

a. Draw arrows to show the movement of electrons in going from one resonance contributor to the next in
1. the cyclopentadienyl anion
2. pyrrole
b. How many ring atoms share the negative charge in
1. the cyclopentadienyl anion?
2. pyrrole?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 9

Draw the resonance contributors of the cyclooctatrienyl dianion.
a. Which of the resonance contributors is the least stable?
b. Which of the resonance contributors makes the smallest contribution to the hybrid?

Prashant Bana

Numerade Educator

Problem 10

a. Predict the relative $p K_{a}$ values of cyclopropene and cyclopropane.
b. Which is more soluble in water, 3 -bromocyclopropene or bromocyclopropane?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 11

Which of the compounds in Problem 2 are antiaromatic?

Nicholas Sacco

Numerade Educator

Problem 12

How many bonding, nonbonding, and antibonding $\pi$ molecular orbitals does cyclobutadiene have? In which molecular orbitals are the $\pi$ electrons?

Crystal Wang

Numerade Educator

Problem 13

Can a radical be aromatic?

Nicholas Sacco

Numerade Educator

Problem 14

Following the instructions for drawing the $\pi$ molecular orbital energy levels of the compounds shown in Figure $15.2,$ draw the $\pi$ molecular orbital energy levels for the cycloheptatricnyl cation, the cycloheptatricnyl anion, and the cyclopropenyl cation. For each compound, show the distribution of the $\pi$ electrons. Which of the compounds are aromatic? Which are antiaromatic?

Lottie Adams

Numerade Educator

Problem 15

Draw the structure of each of the following compounds:
a. 2-phenylhexane
c. 3-benzylpentane
b. benzyl alcohol
d. bromomethylbenzene

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 16

If electrophilic addition to benzene is an endergonic reaction overall, how can electrophilic addition to an alkene be an exergonic reaction overall?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 17

Which compound will undergo an electrophilic aromatic substitution reaction more rapidly, benzene or hexadeuteriobenzene?

Ramesh Singh

Numerade Educator

Problem 18

Why does hydration inactivate $\mathrm{FeBr}_{3} ?$

Anish Wadhwa

Numerade Educator

Problem 19

The reaction coordinate diagram in Figure 15.5 shows that the rate-determining step for sulfonation is the slower of the two steps, whereas the rate-determining step for desulfonation is the faster of the two steps. Explain how the faster step can be the rate-determining step.

Stephen Ho

Numerade Educator

Problem 20

Show the mechanism for the generation of the acylium ion if an acid anhydride is used instead of an acyl chloride in a Friedel-Crafts acylation reaction.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 21

Propose a mechanism for the following reaction:

Zubair Abdulla

Numerade Educator

Problem 22

Show the mechanism for alkylation of benzene by an alkene.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 23

What would be the major product of a Fricdel-Crafts alkylation reaction using the following alkyl halides?
a. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}$
b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$
c. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{3}$
d. $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{Cl}$
e. $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{Cl}$
f. $\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{Cl}$

Nicholas Sacco

Numerade Educator

Problem 24

Describe how the following compounds could be prepared from benzene:

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 25

Which of the following compounds are aromatic? Are any antiaromatic? (Hint: If possible, a ring will be nonplanar to avoid being ntiaromatic.)

Nicholas Sacco

Numerade Educator

Problem 26

Give the product of the reaction of excess benzene with each of the following reagents:
a. isobutyl chloride $+\mathrm{AlCl}_{3}$
b. propene + HF
c. neopentyl chloride $+\mathrm{AlCl}_{3}$
d. dichloromethane $+\mathrm{AlCl}_{3}$

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 27

Which ion in each of the following pairs is more stable?

Madeline Currie

Madeline Currie

Numerade Educator

Problem 28

Which can lose a proton more readily, a methyl group bonded to cyclohexane or a methyl group bonded to benzene?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 29

How could you prepare the following compounds with benzene as one of the starting materials?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 30

Benzene underwent a Friedel-Crafts acylation reaction followed by a Clemmensen reduction. The product gave the following $^{1} \mathrm{H} \mathrm{NMR}$ spectrum. What acyl chloride was used in the Friedel-Crafts acylation reaction?

Narayan Hari

Numerade Educator

Problem 31

Give the products of the following reactions:

Ricajoy Montero

Ricajoy Montero

Numerade Educator

Problem 32

Which compound in each of the following pairs is a stronger base? Why?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 33

a. In what direction is the dipole moment in fulvene? Explain.
b. In what direction is the dipole moment in calicene? Explain.

Madeline Currie

Madeline Currie

Numerade Educator

Problem 34

Purine is a heterocyclic compound with four nitrogen atoms.
a. Which nitrogen is most apt to be protonated?
b. Which nitrogen is least apt to be protonated?

Madeline Currie

Madeline Currie

Numerade Educator

Problem 35

Give the product of each of the following reactions:

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 36

Propose a mechanism for each of the following reactions:

Lottie Adams

Numerade Educator

Problem 37

Show two ways that the following compound could be synthesized:

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 38

In a reaction called the Birch reduction, benzene can be partially reduced t cyclohexadiene by an alkali mctal $(Na, Li, or K)$ in liquid ammonia and a low-molecular-weight alcohol. Propose a mechanism for this reaction. (Hint: See Section 6.8.)

George Bennett

Numerade Educator

Problem 39

The principle of least motion, which states that the reaction that involves the least change in atomic positions or electronic configuration (all else being equal) is favored, has been suggested to explain why the Birch reduction forms only 1,4 -hexadiene. How does this account for the observation that no 1,3 -cyclohexadienc is obtained from a Birch reduction?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 40

Investigation has shown that cyclobutadiene is actually a rectangular molecule rather than a square molecule. In addition, it has been established that there are two different 1,2-dideuterio-1,3-cyclobutadienes. Explain the reason for these unexpected observations.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator