Organic Chemistry

John McMurry

Chapter 15 Benzene and Aromaticity - all with Video Answers

Educators

Chapter Questions

03:50

Problem 1

Tell whether the following compounds are ortho-, meta-, or para-disubstituted:

Mercedes Mazza

Numerade Educator

12:59

Problem 2

Give IUPAC names for the following compounds:

Mercedes Mazza

Numerade Educator

05:17

Problem 3

Draw structures corresponding to the following IUPAC names:
(a) $p$ -Bromochlorobenzene
(b) $p$ -Bromotoluene
(c) $m$ -Chloroaniline
(d) $1-$ Chloro- 3,5 -dimethylbenzene

Mercedes Mazza

Numerade Educator

04:53

Pyridine is a flat, hexagonal molecule with bond angles of $120^{\circ} .$ It undergoes substitution rather than addition and generally behaves like benzene. Draw a picture of the $\pi$ orbitals of pyridine to explain its properties. Check your answer by looking ahead to Section 15.5.

Mercedes Mazza

Numerade Educator

02:34

Problem 5

To be aromatic, a molecule must have $4 n+2 \pi$ electrons and must have cyclic conjugation. $1,3,5,7,9-$ Cyclodecapentaene fulfilis one of these criteria but not the other and has resisted all attempts at synthesis. Explain.

Mercedes Mazza

Numerade Educator

04:06

Problem 6

Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent? How many absorption lines would you expect to see in the 'H NMR and '3 C NMR spectra of the anion?

Mercedes Mazza

Numerade Educator

03:07

Problem 7

Cyclooctatetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, $\mathrm{C}_{8} \mathrm{H}_{8}^{2-} .$ Why do you suppose this reaction occurs so easily? What geometry do you expect for the cyclooctatetraene dianion?

Mercedes Mazza

Numerade Educator

03:28

Problem 8

The relative energy levels of the five $\pi$ molecular orbitals of the cyclopentadienyl system are similar to those in benzene. That is, there is a single lowest-energy MO, above which the orbitals come in degenerate pairs. Draw a diagram like that in Figure $15.5,$ and tell which of the five orbitals are occupied in the cation, radical, and anion.

Mercedes Mazza

Numerade Educator

03:47

Problem 9

Draw an orbital picture of furan to show how the molecule is aromatic.

Mercedes Mazza

Numerade Educator

05:29

Problem 10

Thiamin, or vitamin $\mathrm{B}_{1}$, contains a positively charged five-membered nitrogen-sulfur heterocycle called a thiazolium ring. Explain why the thiazolium ring is aromatic.

Mercedes Mazza

Numerade Educator

02:23

Problem 11

Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene. Is azulene aromatic? Draw a second resonance form of azulene in addition to that shown.

Mercedes Mazza

Numerade Educator

03:18

Problem 12

How many electrons does each of the four nitrogen atoms in purine contribute to the aromatic $\pi$ system?

Mercedes Mazza

Numerade Educator

04:39

Problem 13

Give IUPAC names for the following substances (red = O, blue = N):

Mercedes Mazza

Numerade Educator

02:34

Problem 14

All-cis cyclodecapentaene is a stable molecule that shows a single absorption in its $^{\text {i }}$ H NMR spectrum at 5.67 . s. Tell whether it is aromatic, and explain its NMR spectrum.

Mercedes Mazza

Numerade Educator

04:57

Problem 15

1,6 -Methanonaphthalene has an interesting $^{1} \mathrm{H}$ NMR spectrum in which the eight hydrogens around the perimeter absorb at 6.9 to $7.3 \delta,$ while the two $\mathrm{CH}_{2}$ protons absorb at -0.5 \&. Tell whether it is aromatic, and explain its NMR spectrum.

Mercedes Mazza

Numerade Educator

02:54

Problem 16

The following molecular model is that of a carbocation. Draw two resonance structures for the carbocation, indicating the positions of the double bonds.

Mercedes Mazza

Numerade Educator

04:04

Problem 17

Azulene, an isomer of naphthalene, has a remarkably large dipole moment for a hydrocarbon $(\mu=1.0 \mathrm{D}) .$ Explain, using resonance structures.

Mercedes Mazza

Numerade Educator

06:54

Problem 18

Give IUPAC names for the following compounds:

Mercedes Mazza

Numerade Educator

04:13

Problem 19

Draw structures corresponding to the following names:
(a) 3 -Methyl-1, 2 -benzenediamine
(b) 1,3,5 -Benzenetriol
(c) 3-Methyl-2-phenylhexane
(d) $o$ -Aminobenzoic acid
(e) $m$ -Bromophenol
(f) $2,4,6-$ Trinitrophenol (picric acid)

Mercedes Mazza

Numerade Educator

09:10

Problem 20

Draw and name all possible isomers of the following:
(a) Dinitrobenzene
(b) Bromodimethylbenzene
(c) Trinitrophenol

Mercedes Mazza

Numerade Educator

03:46

Problem 21

Draw and name all possible aromatic compounds with the formula $\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{Cl}$.

Mercedes Mazza

Numerade Educator

09:30

Problem 22

Draw and name all possible aromatic compounds with the formula $C_{8} \mathrm{H}_{9} \mathrm{Br}$. (There are 14.)

Mercedes Mazza

Numerade Educator

15:00

Problem 23

Propose structures for aromatic hydrocarbons that meet the following descriptions:
(a) $\mathrm{C}_{9} \mathrm{H}_{12}$; gives only one $\mathrm{C}_{9} \mathrm{H}_{11} \mathrm{Br}$ product on substitution of a hydrogen on the aromatic ring with bromine
(b) $\mathrm{C}_{10} \mathrm{H}_{14}$; gives only one $\mathrm{C}_{10} \mathrm{H}_{13} \mathrm{Cl}$ product on substitution of a hydrogen on the aromatic ring with chlorine
(c) $\mathrm{C}_{8} \mathrm{H}_{10}$; gives three $\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}$ products on substitution of a hydrogen on the aromatic ring with bromine
(d) $\mathrm{C}_{10 \mathrm{H}_{14}}$, gives two $\mathrm{C}_{10 \mathrm{H}_{13} \mathrm{Cl} \text { products on substitution of a hydrogen on }}$ the aromatic ring with chlorine

Mercedes Mazza

Numerade Educator

04:44

Problem 24

Look at the three resonance structures of naphthalene shown in Section 15.6 and account for the fact that not all carbon-carbon bonds have the same length. The C1-C2 bond is $136 \mathrm{pm}$ long, whereas the $\mathrm{C} 2-\mathrm{C} 3$ bond is $139 \mathrm{pm}$ long.

Mercedes Mazza

Numerade Educator

02:22

Problem 25

Anthracene has four resonance structures, one of which is shown. Draw the other three.

Mercedes Mazza

Numerade Educator

03:55

Problem 26

Phenanthrene has five resonance structures, one of which is shown. Draw the other four.

Mercedes Mazza

Numerade Educator

05:55

Problem 27

Look at the five resonance structures for phenanthrene (Problem 15.26 ), and predict which of its carbon-carbon bonds is shortest.

Mercedes Mazza

Numerade Educator

08:07

Problem 28

In1932, A. A. Levine and $A$. G. Cole studied the ozonolysis of $o$ -xylene and isolated three products: glyoxal, 2,3 -butanedione, and pyruvaldehyde:
In what ratio would you expect the three products to be formed if $o$ -xylene is a resonance hybrid of two structures? The actual ratio found was 3 parts glyoxal, 1 part 2,3-butanedione, and 2 parts pyruvaldehyde. What conclusions can you draw about the structure of $o$ -xylene?

Mercedes Mazza

Numerade Educator

03:37

Problem 29

3-Chlorocyclopropene, on treatment with AgBF $_{4}$, gives a precipitate of AgCl and a stable solution of a product that shows a single $^{1} \mathrm{H}$ NMR absorption at 11.04 $\delta$. What is a likely structure for the product, and what is its relation to Hückel's rule?

Mercedes Mazza

Numerade Educator

04:20

Problem 30

Draw an energy diagram for the three molecular orbitals of the cyclopropenyl system $\left(\mathrm{C}_{3} \mathrm{H}_{3}\right) .$ How are these three molecular orbitals occupied in the cyclopropenyl anion, cation, and radical? Which of the three substances is aromatic according to Hückel's rule?

Mercedes Mazza

Numerade Educator

04:06

Problem 31

Cyclopropanone is highly reactive because of its large amount of angle strain. but methylcyclopropenone, although even more strained than cyclopropanone, is nevertheless quite stable and can even be distilled. Explain, taking the polarity of the carbonyl group into account.

Mercedes Mazza

Numerade Educator

05:44

Problem 32

Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can't be isolated. Explain, taking the polarity of the carbonyl group into account.

Mercedes Mazza

Numerade Educator

03:50

Problem 33

Which would you expect to be most stable, cyclononatetraenyl radical, cation, or anion?

Mercedes Mazza

Numerade Educator

04:18

Problem 34

How might you convert 1,3,5,7 -cyclononatetraene to an aromatic substance?

Mercedes Mazza

Numerade Educator

02:56

Problem 35

Calicene, like azulene (Problem 15.17 ), has an unusually large dipole moment for a hydrocarbon. Explain, using resonance structures.

Mercedes Mazza

Numerade Educator

02:48

Problem 36

Pentalene is a most elusive molecule that has been isolated only at liquidnitrogen temperature. The pentalene dianion, however, is well known and quite stable. Explain.

Mercedes Mazza

Numerade Educator

03:45

Problem 37

Indole is an aromatic heterocycle that has a benzene ring fused to a pyrrole ring. Draw an orbital picture of indole.
(a) How many $\pi$ electrons does indole have?
(b) What is the electronic relationship of indole to naphthalene?

Mercedes Mazza

Numerade Educator

03:11

Problem 38

Ribavirin, an antiviral agent used against hepatitis $\mathrm{C}$ and viral pneumonia, contains a 1,2,4 -triazole ring. Why is the ring aromatic?

Mercedes Mazza

Numerade Educator

04:24

Problem 39

Compound $\mathrm{A}, \mathrm{C}_{8} \mathrm{H}_{10},$ yields three substitution products, $\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}$, on reaction with $\mathrm{Br}_{2}$. Propose two possible structures for A. The ${ }^{1} \mathrm{H}$ NMR spectrum of A shows a complex four-proton multiplet at $7.0 \delta$ and a six-proton singlet at $2.30 \delta .$ What is the structure of $\mathrm{A} ?$

Nicole Krahulik

Numerade Educator

05:03

Problem 40

What is the structure of a hydrocarbon that has $\mathrm{M}^{+}=120$ in its mass spectrum and has the following $1 \mathrm{H}$ NMR spectrum?
7.25 $\delta(5 \mathrm{H}, \text { broad singlet); } 2.90 \delta(1 \mathrm{H}, \text { septet, } J=7 \mathrm{Hz}) ; 1.22 \delta(6 \mathrm{H},\text { doublet, }$
$J=7 \mathrm{Hz})$

Mercedes Mazza

Numerade Educator

08:17

Problem 41

Propose structures for compounds that fit the following descriptions:
(a) $\mathrm{C}_{10} \mathrm{H}_{14}$
1 H NMR: 7.18 \delta $(4 \mathrm{H}, \text { broad singlet }) ; 2.70 \delta(4 \mathrm{H}, \text { quartet, } J=7 \mathrm{Hz})$ $1.20 \delta(6 \mathrm{H}, \text { triplet, } J=7 \mathrm{Hz})$
IR: $745 \mathrm{cm}^{-1}$
(b) $\mathrm{C}_{10} \mathrm{H}_{14}$
$^{1} \mathrm{H}$ NMR: $7.0 \delta(4 \mathrm{H}, \text { broad singlet }) ; 2.85 \delta(1 \mathrm{H}, \text { septet, } J=8 \mathrm{Hz})$
$[(6 \mathrm{H}, \text { doublet, } J=8 \mathrm{Hz})$
$2.28 \delta(3 \mathrm{H}, \text { singlet }) ; 1.20 \delta$
IR: $825 \mathrm{cm}^{-1}$

Mercedes Mazza

Numerade Educator

03:12

Problem 42

On reaction with acid, 4 -pyrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hückel $4 n+2$ rule, explain why the protonated product is so stable.

Mercedes Mazza

Numerade Educator

04:16

Problem 43

Bextra, a COX-2 inhibitor used in the treatment of arthritis, contains an isoxazole ring. Why is the ring aromatic?

Mercedes Mazza

Numerade Educator

04:26

Problem 44

N-Phenylsydnone, so-named because it was first studied at the University of Sydney, Australia, behaves like a typical aromatic molecule. Explain, using the Hückel $4 n+2$ rule.

Mercedes Mazza

Numerade Educator

04:27

Problem 45

Show the relative energy levels of the seven $\pi$ molecular orbitals of the cycloheptatrienyl system. Tell which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation.

Mercedes Mazza

Numerade Educator

02:15

Problem 46

1-Phenyl-2-butene has an ultraviolet absorption at $\lambda_{\max }=208 \mathrm{nm}(\epsilon=8000)$ On treatment with a small amount of strong acid, isomerization occurs and a new substance with $\lambda_{\max }=250 \mathrm{nm}(\epsilon=15,800)$ is formed. Propose a structure for this isomer, and suggest a mechanism for its formation.

Mercedes Mazza

Numerade Educator

09:13

Problem 47

Propose structures for aromatic compounds that have the following 'H NMR spectra:
(a) $\mathrm{C}_{8} \mathrm{H}_{9} \mathrm{Br}$
IR: $820 \mathrm{cm}^{-1}$
(b) $\mathrm{C}_{9} \mathrm{H}_{12}$
IR: $750 \mathrm{cm}^{-1}$
(c) $\mathrm{C}_{11} \mathrm{H}_{16}$
IR: $820 \mathrm{cm}^{-1}$

Mercedes Mazza

Numerade Educator

05:02

Problem 48

Propose a structure for a molecule $\mathrm{C}_{14} \mathrm{H}_{12}$ that has the following $^{1} \mathrm{H}$ NMR spectrum and has IR absorptions at $700,740,$ and $890 \mathrm{cm}^{-1}$ :

Mercedes Mazza

Numerade Educator

01:57

Problem 49

Aromatic substitution reactions occur by addition of an electrophile such as $\mathrm{Br}^{+}$ to the aromatic ring to yield an allylic carbocation intermediate, followed by loss of $\mathrm{H}^{+}$. Show the structure of the intermediate formed by reaction of benzene with $\mathrm{Br}^{+}$

Mercedes Mazza

Numerade Educator

04:22

Problem 50

The substitution reaction of toluene with $\mathrm{Br}_{2}$ can, in principle, lead to the formation of three isomeric bromotoluene products. In practice, however, only $o-$ and $p$ -bromotoluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates (Problem $15.49),$ and explain why ortho and para products predominate over meta.

Mercedes Mazza

Numerade Educator