Organic Chemistry

T.W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder

Chapter 14 Aromatic Compounds - all with Video Answers

Educators

Chapter Questions

06:09

Problem 1

Provide a name for each of the following compounds.
(Check your book to see figure)

Mercedes Mazza

Numerade Educator

07:37

Listed below are four compounds that have the molecular formula $\mathrm{C}_{6} \mathrm{H}_{6} .$ Which of these compounds would yield only one monosubstitution product, if, for example, one hydrogen were replaced by bromine?
(Check your book to see figure)

Pahk Thepchatri

Numerade Educator

05:08

Problem 3

If benzene were 1,3,5-cyclohexatriene, the carbon-carbon bonds would be alternately long and short as indicated in the following structures. However, to consider the structures here as resonance contributors (or to connect them by a double-headed arrow) violates a basic principle of resonance theory. Explain.
(Check your book to see figure)

Mercedes Mazza

Numerade Educator

04:13

Problem 4

Apply the polygon-and-circle method to the cyclopentadienyl cation and explain whether it would be aromatic or not.
(Check your book to see figure)

Mercedes Mazza

Numerade Educator

05:39

Problem 5

Apply the polygon-and-circle method to the cycloheptatrienyl anion and cation and explain whether each would be aromatic or not.
(Check your book to see figure)

Mercedes Mazza

Numerade Educator

02:11

Problem 6

1,3,5-Cycloheptatriene is even less acidic than 1,3,5-heptatriene. Explain how this experimental observation might help to confirm your answers to the previous problem.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

03:27

Problem 7

When 1,3,5 -cycloheptatriene reacts with one molar equivalent of bromine at $0^{\circ} \mathrm{C},$ it undergoes 1,6 addition. (a) Write the structure of this product. (b) On heating, this 1,6-addition product loses HBr readily to form a compound with the molecular formula C_ H_H tropylium bromide. Tropylium bromide is insoluble in nonpolar solvents but is soluble in water; it has an unexpectedly high melting point (mp $203^{\circ} \mathrm{C}$ ), and when treated with silver nitrate, an aqueous solution of tropylium bromide gives a precipitate of AgBr. What do these experimental results suggest about the bonding in tropylium bromide?

Pahk Thepchatri

Numerade Educator

02:34

Problem 8

The cyclopentadienyl cation is apparently antiaromatic. Explain what this means in terms of the $\pi$ -electron energies of a cyclic and an open-chain compound.

Zubair Abdulla

Numerade Educator

02:04

Problem 9

In 1967 R. Breslow (of Columbia University) and co-workers showed that adding $\mathrm{SbCl}_{5}$ to a solution of 3-chlorocyclopropene in $\mathrm{CH}_{2} \mathrm{Cl}_{2}$ caused the precipitation of a white solid with the composition $\mathrm{C}_{3} \mathrm{H}_{3}^{+} \mathrm{SbCl}_{6}^{-}$. NMR spectroscopy of a solution of this salt showed that all of its hydrogen atoms were equivalent. (a) What new aromatic ion had these researchers prepared? (b) How many $^{13} \mathrm{C}$ NMR signals would you predict for this ion?

Zubair Abdulla

Numerade Educator

07:15

Problem 10

How many $^{13} \mathrm{C}$ NMR signals would you predict for (a) naphthalene, (b) anthracene, (c) phenanthrene, and (d) pyrene?

Ronald Prasad

Numerade Educator

05:24

Problem 11

In addition to a signal downfield, the $^{1}$ H NMR spectrum of trans-15,16-dimethyldihydropyrene has a signal far upfield at $\delta-4.2 .$ Account for the presence of this upfield signal.

Zubair Abdulla

Numerade Educator

04:05

Problem 12

Azulene has an appreciable dipole moment. Write resonance structures for azulene that explain this dipole moment and that help explain its aromaticity.

Ronald Prasad

Numerade Educator

02:50

Problem 13

(a) The - SH group is sometimes called the mercapto group. 6-Mercaptopurine is used in the treatment of acute leukemia. Write its structure. (b) Allopurinol, a compound used to treat gout, is 6-hydroxypurine. Write its structure.

Zubair Abdulla

Numerade Educator

04:04

Problem 14

Explain how $^{13} \mathrm{C}$ NMR spectroscopy could be used to distinguish the ortbo-, meta-, and para-dibromobenzene isomers one from another.

Zubair Abdulla

Numerade Educator

03:09

Problem 15

Four benzenoid compounds, all with the formula $\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{Br}$, gave the following IR peaks in the $680-860 \mathrm{cm}^{-1}$ region:
A, $740 \mathrm{cm}^{-1}(\text {strong })$
B, $800 \mathrm{cm}^{-1}$ (very strong)
C, $680 \mathrm{cm}^{-1}(\text {strong })$ and $760 \mathrm{cm}^{-1}$ (very strong)
D, $693 \mathrm{cm}^{-1}$ (very strong) and $765 \mathrm{cm}^{-1}$ (very strong)
Propose structures for $\mathbf{A}, \mathbf{B}, \mathbf{C},$ and $\mathbf{D}.$

Zubair Abdulla

Numerade Educator

05:34

Problem 16

Write structural formulas for each of the following:
(a) 3-Nitrobenzoic acid
(b) $p$-Bromotoluene
(c) $o$-Dibromobenzene
(d) $m$-Dinitrobenzene
(e) 3,5-Dinitrophenol
(f) $p$-Nitrobenzoic acid
(g) 3-Chloro-1-ethoxybenzene
(h) $p$-Chlorobenzenesulfonic acid
(i) Methyl $p$-toluenesulfonate
(i) Benzyl bromide
(k) $p$-Nitroaniline
(1) $o$-Xylene
(m) tert-Butylbenzene
(n) $p$-Methylphenol
(o) $p$-Bromoacetophenone
(p) 3-Phenylcyclohexanol
(q) 2-Methyl-3-phenyl-1-butanol
(r) $\varrho$-Chloroanisole

Ronald Prasad

Numerade Educator

12:49

Problem 17

Write structural formulas and give acceptable names for all representatives of the following:
(a) Tribromobenzenes
(c) Nitroanilines
(e) Isomers of $\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{C}_{4} \mathrm{H}_{9}$
(b) Dichlorophenols
(d) Methylbenzenesulfonic acids

Zubair Abdulla

Numerade Educator

07:01

Problem 18

Which of the following molecules would you expect to be aromatic?
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

03:49

Problem 19

Use the polygon-and-circle method to draw an orbital diagram for each of the following compounds.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

01:48

Problem 20

Write the structure of the product formed when each of the following compounds reacts with one molar equivalent of HCl.
(Check your book to see figure)

Ronald Prasad

Numerade Educator

03:49

Problem 21

Which of the hydrogen atoms shown below is more acidic? Explain your answer.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

04:48

Problem 22

The rings below are joined by a double bond that undergoes cis-trans isomerization much more readily than the bond of a typical alkene. Provide an explanation.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

03:12

Problem 23

Although Hückel's rule (Section 14.7) strictly applies only to monocydic compounds, it does appear to have application to certain bicyclic compounds, if one assumes use of resonance structures involving only the perimeter double bonds, as shown with one resonance contributor for naphthalene below.
(Check your book to see figure)
Both naphthalene (Section $14.8 \mathrm{A}$ ) and azulene (Section $14.8 \mathrm{B}$ ) have $10 \pi$ clectrons and are aromatic. Pentalene (below) is apparently antiaromatic and is unstable even at $-100^{\circ} \mathrm{C}$. Heptalene has been made but it adds bromine, it reacts with acids, and it is not planar. Is Hückel's rule applicable to these compounds? If so, explain their lack of aromaticity.

Zubair Abdulla

Numerade Educator

06:43

Problem 24

(a) In 1960 T. Katz (Columbia University) showed that cyclooctatetraene adds two electrons when treated with potassium metal and forms a stable, planar dianion, $\mathrm{C}_{8} \mathrm{H}_{8}^{2-}$ (as the dipotassium salt):
(Check your book to see figure)
Use the molecular orbital diagram given in Fig. 14.7 and explain this result.
(b) In 1964 Katz also showed that removing two protons from the compound below (using butyllithium as the base) leads to the formation of a stable dianion with the formula $\mathrm{C}_{8} \mathrm{H}_{6}^{2-}$ (as the dilithium salt).
(Check your book to see figure)
Propose a reasonable structure for the product and explain why it is stable.

Zubair Abdulla

Numerade Educator

02:27

Problem 25

Although none of the [10]annulenes given in Section 14.7 B is aromatic, the following 10 $\pi$ -electron system is aromatic:
(Check your book to see figure)
What factor makes this possible?

Zubair Abdulla

Numerade Educator

04:44

Problem 26

Cycloheptatrienone (I) is very stable. Cyclopentadienone (II) by contrast is quite unstable and rapidly undergoes a Diels-Alder reaction with itself.
(a) Propose an explanation for the different stabilities of these two compounds.
(b) Write the structure of the Diels-Alder adduct of cyclopentadienone.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

01:38

Problem 27

5-Chloro-1,3-cyclopentadiene (below) undergoes S $_{N} 1$ solvolysis in the presence of silver ion extremely slowly even though the chlorine is doubly allylic and allylic halides normally ionize readily. Provide an explanation for this behavior.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

04:19

Problem 28

Explain the following: (a) Cyclononatetraenyl anion is planar (in spite of the angle strain involved) and appears to be aromatic. (b) Although [16]annulene is not aromatic, it adds two electrons readily to form an aromatic dianion.

Zubair Abdulla

Numerade Educator

02:10

Problem 29

Furan possesses less aromatic character than benzene as measured by their resonance energies $(96 \mathrm{kJ}) \mathrm{mol}^{-1}$ for furan; 151 kJ mol $^{-1}$ for benzene). What reaction have we studied earlier that shows that furan is less aromatic than benzene and can react in a way characteristic of some dienes?
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

09:17

Problem 30

For each of the pairs below, predict specific aspects in their 'H NMR spectra that would allow you to distinguish one compound from the other.
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

06:38

Problem 31

Assign structures to each of the compounds $\mathbf{A}, \mathbf{B},$ and $\mathbf{C}$ whose $^{1}$ H NMR spectra are shown in Fig. 14.27 .

Zubair Abdulla

Numerade Educator

04:52

Problem 32

The ${ }^1 \mathrm{H}$ NMR spectrum of cyclooctatetraene consists of a single line located at $\delta 5.78$. What does the location of this signal suggest about electron delocalization in cyclooctatetraene?

Ian Kaigh

Numerade Educator

02:27

Problem 32

The $^{1} \mathrm{H} \mathrm{NMR}$ spectrum of cyclooctatetraene consists of a single line located at $\delta 5.78 .$ What does the location of this signal suggest about electron delocalization in cyclooctatetraene?

Adriano Chikande

Numerade Educator

06:06

Problem 33

Give a structure for compound $\mathbf{F}$ that is consistent with the 'H NMR spectrum in Fig. 14.28 and IR absorptions at 3020,2965 $2940,2870,1517,1463,$ and $818 \mathrm{cm}^{-}.$

Zubair Abdulla

Numerade Educator

06:52

Problem 34

A compound (L) with the molecular formula C $_{9} \mathrm{H}_{10}$ reacts with bromine and gives an IR absorption spectrum that includes the following absorption peaks: $3035 \mathrm{cm}^{-1}(\mathrm{m}), 3020 \mathrm{cm}^{-1}(\mathrm{m}), 2925 \mathrm{cm}^{-1}(\mathrm{m}), 2853 \mathrm{cm}^{-1}(\mathrm{w}), 1640 \mathrm{cm}^{-1}(\mathrm{m}), 990 \mathrm{cm}^{-1}(\mathrm{s}), 915 \mathrm{cm}^{-1}(\mathrm{s})$ $740 \mathrm{cm}^{-1}(\mathrm{s}), 695 \mathrm{cm}^{-1}(\mathrm{s}) .$ The 'H NMR spectrum of L consists of:
Doublet $\delta 3.1(2 \mathrm{H})$
Multiplet $\delta 4.8$
Multiplet $\delta 5.1$
Multiplet $\delta 5.8$
Multiplet $\delta 7.1(5 \mathrm{H})$
The UV spectrum shows a maximum at 255 nm. Propose a structure for compound $L$ and make assignments for each of the IR peaks.

Zubair Abdulla

Numerade Educator

06:06

Problem 35

Compound $\mathbf{M}$ has the molecular formula $\mathrm{C}_{9} \mathrm{H}_{12}$. The $^{1} \mathrm{H} \mathrm{NMR}$ spectrum of $\mathbf{M}$ is given in Fig. 14.29 and the IR spectrum in Fig. $14.30 .$ Propose a structure for $\mathbf{M}$.

Zubair Abdulla

Numerade Educator

03:47

Problem 36

The IR and $^{1} \mathrm{H} \mathrm{NMR}$ spectra for compound $\mathbf{X}\left(\mathrm{C}_{8} \mathrm{H}_{10}\right)$ are given in Fig. $14.31 .$ Propose a structure for compound $\mathbf{X}$.

Zubair Abdulla

Numerade Educator

06:30

Problem 37

The IR and $^{1} \mathrm{H} \mathrm{NMR}$ spectra of compound $\mathbf{Y}\left(\mathrm{C}_{9} \mathrm{H}_{12} \mathrm{O}\right)$ are given in Fig. $14.32 .$ Propose a structure for $\mathbf{Y} .$

Zubair Abdulla

Numerade Educator

04:06

Problem 38

(a) How many signals would you expect to find in the $^{1} \mathrm{H} \mathrm{NMR}$ spectrum of caffeine? (b) What characteristic peaks would you expect to find in the IR spectrum of caffeine?
(Check your book to see figure)

Zubair Abdulla

Numerade Educator

08:31

Problem 39

Given the following information, predict the appearance of the $^{1} \mathrm{H} \mathrm{NMR}$ spectrum arising from the vinyl hydrogen atoms of $p$ -chlorostyrene. Deshiclding by the induced magnetic field of the ring is greatest at proton $c(\delta 6.7)$ and is least at proton b $(\delta 5.3) .$ The chemical shift of a is about $\delta 5.7 .$ The coupling constants have the following approximate magnitudes: $J_{\mathrm{cc}} \cong 18 \mathrm{Hz}, J_{\mathrm{bc}} \cong 11 \mathrm{Hz},$ and $J_{\mathrm{ab}} \cong 2 \mathrm{Hz}$. (These coupling constants are typical of those given by vinylic systems: coupling constants for trans hydrogen atoms are larger than those for cis hydrogen atoms, and coupling constants for geminal vinylic hydrogen atoms are very small.)

Zubair Abdulla

Numerade Educator

11:44

Problem 40

Consider these reactions:
(Check your book to see figure)
The intermediate $\mathbf{A}$ is a covalently bonded compound that has typical 'H NMR signals for aromatic ring hydrogens and only one additional signal at $\delta 1.21,$ with an area ratio of $5: 3,$ respectively. Final product $B$ is ionic and has only aromatic hydrogen signals. What are the structures of $\mathbf{A}$ and $\mathbf{B} ?$

Zubair Abdulla

Numerade Educator

05:58

Problem 41

The final product of this sequence, $\mathbf{D}$, is an orange, crystalline solid melting at $174^{\circ} \mathrm{C}$ and having molecular weight 186:
$$\begin{array}{c}\text { Cyclopentadiene }+\mathrm{Na} \rightarrow \mathrm{C}+\mathrm{H}_{2} \\2 \mathrm{c}+\mathrm{FeCl}_{2} \rightarrow \mathrm{D}+2 \mathrm{NaCl}\end{array}$$
In its $^{1} \mathrm{H}$ and $^{13} \mathrm{C}$ NMR spectra, product $\mathbf{D}$ shows only one kind of hydrogen and only one kind of carbon, respectively. Draw the structure of $\mathbf{C}$ and make a structural suggestion as to how the high degree of symmetry of $\mathbf{D}$ can be explained. (D belongs to a group of compounds named after something you might get at a deli for lunch.)

Zubair Abdulla

Numerade Educator

06:24

Problem 42

Compound E has the spectral features given below. What is its structure?
MS $(m / z): \quad \mathrm{M}^{\pm}, 202$
IR $\left(\mathrm{cm}^{-1}\right): 3030-3080,2150$ (very weak), $1600,1490,760,$ and 690
$^{1} \mathrm{H} \mathrm{NMR}(\delta):$ narrow multiplet centered at 7.34 UV $(\mathrm{nm}): 287(\epsilon=25,000), 305(\epsilon=36,000),$ and $326(\epsilon=33,000)$

Zubair Abdulla

Numerade Educator

07:18

Problem 43

Draw all of the $\pi$ molecular orbitals for $(3 E)-1,3,5$ -hexatriene, order them from lowest to highest in energy, and indicate the number of electrons that would be found in each in the ground state for the molecule. After doing so, open the computer molecular model for ( $3 E$ )-1.3.5-hexatriene and display the calculated molecular orbitals. How well does the appearance and sequence of the orbitals you drew (e.g., number of nodes, overall symmetry of each, etc.) compare with the orbitals in the calculated model? Are the same orbitals populated with electrons in your analysis as in the calculated model?

Zubair Abdulla

Numerade Educator