Organic Chemistry

John McMurry

Chapter 14

Conjugated Compounds and Ultraviolet Spectroscopy - all with Video Answers

Educators

Chapter Questions

Problem 1

Allene, $\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2},$ has a heat of hydrogenation of $-298 \mathrm{kJ} / \mathrm{mol}(-71.3 \mathrm{kca} / / \mathrm{mol})$. Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability.

Lottie Adams

Lottie Adams

Numerade Educator

Problem 2

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HCI with 1,3 -pentadiene.

Lottie Adams

Numerade Educator

Problem 3

Look at the possible carbocation intermediates produced during addition of HCI to 1,3-pentadiene (Problem 14.2 ), and predict which 1,2 adduct predominates. Which 1,4 adduct predominates?

Lottie Adams

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Problem 4

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HBr with the following compound: (FIGURE CAN'T COPY)

Lottie Adams

Numerade Educator

Problem 5

The 1,2 adduct and the 1,4 adduct formed by reaction of HBr with 1,3 -butadiene are in equilibrium at $40^{\circ} \mathrm{C}$. Propose a mechanism by which the interconversion of products takes place.

Lottie Adams

Numerade Educator

Problem 6

Why do you suppose 1,4 adducts of 1,3 -butadiene are generally more stable than 1,2 adducts?

Lottie Adams

Numerade Educator

Problem 7

Predict the product of the following Diels-Alder reaction: (EQUATION CAN'T COPY)

Lottie Adams

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Problem 8

Which of the following alkenes would you expect to be good Diels-Alder dienophiles?
(EQUATION CAN'T COPY)

Lottie Adams

Numerade Educator

Problem 9

Which of the following dienes have an $s$ -cis conformation, and which have an $s$ -trans conformation? Of the s-trans dienes, which can readily rotate to s-cis?

Lottie Adams

Numerade Educator

Problem 10

Predict the product of the following Diels-Alder reaction:
(FIGURE CANT COPY)

Lottie Adams

Numerade Educator

Problem 11

Draw a segment of the polymer that might be prepared from 2-phenyl-1,3-butadiene.

Lottie Adams

Numerade Educator

Problem 12

Show the mechanism of the acid-catalyzed polymerization of 1,3 -butadiene.

Lottie Adams

Numerade Educator

Problem 13

Calculate the energy range of electromagnetic radiation in the UV region of the spectrum from 200 to $400 \mathrm{nm}$. How does this value compare with the values calculated previously for IR and NMR spectroscopy?

Lottie Adams

Numerade Educator

Problem 14

If pure vitamin $A$ has $\lambda_{\max }=325(\epsilon=50,100),$ what is the vitamin A concentration in a sample whose absorbance at $325 \mathrm{nm}$ is $A=0.735$ in a cell with a pathlength of $1.00 \mathrm{cm} ?$

Lottie Adams

Numerade Educator

Problem 15

Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to $400 \mathrm{nm}$ range?
(FIGURE CANT COPY)

Lottie Adams

Numerade Educator

Problem 16

Show the structures of all possible adducts of the following diene with 1 equivalent of HCl:
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 17

Show the product of the Diels-Alder reaction of the following diene with 3-buten-2-one, $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCOCH}_{3}$. Make sure you show the full stereochemistry of the reaction product.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 18

The following diene does not undergo Diels-Alder reactions. Explain.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 19

The following model is that of an allylic carbocation intermediate formed by protonation of a conjugated diene with HBr. Show the structure of the diene and the structures of the final reaction products.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 20

Give IUPAC names for the following compounds:
(EQUATION CAN'T COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 21

Draw and name the six possible diene isomers of formula $\mathrm{C}_{5} \mathrm{H}_{8} .$ Which of the six are conjugated dienes?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 22

What product(s) would you expect to obtain from reaction of 1,3 -cyclohexadiene with each of the following?
(EQUATION CAN'T COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 23

Electrophilic addition of $\mathrm{Br}_{2}$ to isoprene (2-methyl-1,3-butadiene) yields the following product mixture:
(EQUATION CAN'T COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 24

Propose a structure for a conjugated diene that gives the same product from both 1,2 and 1,4 addition of HBr.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 25

Draw the possible products resulting from addition of 1 equivalent of HCl to 1-phenyl-1,3-butadiene. Which would you expect to predominate, and why?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 26

Predict the products of the following Diels-Alder reactions:
(EQUATION CAN'T COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 27

2,3-Di-tert-butyl-1,3-butadiene does not undergo Diels-Alder reactions. Explain.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 28

Show the structure, including stereochemistry, of the product from the following Diels-Alder reaction:
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 29

How can you account for the fact that cis-1,3-pentadiene is much less reactive than trans-1,3-pentadiene in the Diels-Alder reaction?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 30

Would you expect a conjugated diyne such as 1,3 -butadiyne to undergo Diels-Alder reaction with a dienophile? Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 31

Reaction of isoprene (2-methyl-1,3-butadiene) with ethyl propenoate gives a mixture of two Diels-Alder adducts. Show the structure of each, and explain why a mixture is formed.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 32

Rank the following dienophiles in order of their expected reactivity in the Diels-Alder reaction.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 33

1,3-Cyclopentadiene is very reactive in Diels-Alder cycloaddition reactions, but 1,3 -cyclohexadiene is less reactive and 1,3 -cycloheptadiene is nearly inert. Explain. (Molecular models are helpful.)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 34

1,3-Pentadiene is much more reactive in Diels-Alder reactions than 2,4 pentadienal. Why might this be?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 35

How could you use Diels-Alder reactions to prepare the following products? Show the starting diene and dienophile in each case.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 36

Diene polymers contain occasional vinyl branches along the chain. How do you think these branches might arise?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 37

Tires whose sidewalls are made of natural rubber tend to crack and weather rapidly in areas around cities where high levels of ozone and other industrial pollutants are found. Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 38

1,3 -Cyclopentadiene polymerizes slowly at room temperature to yield a polymer that has no double bonds except on the ends. On heating, the polymer breaks down to regenerate 1,3 -cyclopentadiene. Propose a structure for the product.

Mikayla Stephens

Mikayla Stephens

Numerade Educator

Problem 39

Which of the following compounds would you expect to have a $\pi \rightarrow \pi^{*}$ UV absorption in the 200 to 400 nm range?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 40

Would you expect allene, $\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2},$ to show a UV absorption in the 200 to $400 \mathrm{nm}$ range? Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 41

The following ultraviolet absorption maxima have been measured:
1,3-Butadiene 2-Methyl-1,3-butadiene
$220 \mathrm{nm}$
1,3 -Pentadiene
$223 \mathrm{nm}$
$2,3-$ Dimethyl-1,3-butadiene
$226 \mathrm{nm}$
$2,4-$ Hexadiene
$227 \mathrm{nm}$
$2,4-$ Dimethyl-1,3-pentadiene
$232 \mathrm{nm}$
$2,5-$ Dimethyl- 2,4 -hexadiene
$240 \mathrm{nm}$

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 42

$1,3,5-$ Hexatriene has $\lambda_{\max }=258 \mathrm{nm} .$ In light of your answer to Problem 14.41, approximately where would you expect 2,3 -dimethyl- 1,3,5 -hexatriene to absorb?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 43

\beta-Ocimene is a pleasant-smelling hydrocarbon found in the leaves of certain herbs. It has the molecular formula $\mathrm{C}_{10} \mathrm{H}_{16}$ and a UV absorption maximum at $232 \mathrm{nm}$. On hydrogenation with a palladium catalyst, 2,6 -dimethyloctane is obtained. Ozonolysis of $\beta$ -ocimene, followed by treatment with zinc and acetic acid, produces the following four fragments:
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 44

Treatment of 3,4 -dibromohexane with strong base leads to loss of 2 equivalents of HBr and formation of a product with formula $\mathrm{C}_{6} \mathrm{H}_{10}$. Three products are possible. Name each of the three, and tell how you would use $^{1} \mathrm{H}$ and $^{13} \mathrm{C}$ NMR spectroscopy to help identify them. How would you use UV spectroscopy?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 45

Addition of HCl to 1-methoxycyclohexene yields 1-chloro-1-methoxycyclohexane as the sole product. Use resonance structures to explain why none of the other regioisomer is formed.
(FIGURE CANT COPY)

Prashant Bana

Numerade Educator

Problem 46

Aldrin, a chlorinated insecticide now banned for use in the United States, can be made by Diels-Alder reaction of hexachloro-1,3-cyclopentadiene with norbornadiene. What is the structure of aldrin?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 47

Norbornadiene (Problem 14.46 ) can be prepared by reaction of chloroethylene with 1,3 -cyclopentadiene, followed by treatment of the product with sodium ethoxide. Write the overall scheme, and identify the two kinds of reactions.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 48

We've seen that the Diels-Alder cycloaddition reaction is a one-step, pericyclic process that occurs through a cyclic transition state. Propose a mechanism for the following reaction:
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 49

In light of your answer to Problem $14.48,$ propose a mechanism for the following reaction:
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 50

The triene shown here reacts with 2 equivalents of maleic anhydride to yield a product with the formula $\mathrm{C}_{17} \mathrm{H}_{16} \mathrm{O}_{6}$. Predict a structure for the product.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 51

Myrcene, $\mathrm{C}_{10} \mathrm{H}_{16},$ is found in oil of bay leaves and is isomeric with $\beta$ ocimene . It has an ultraviolet absorption at $226 \mathrm{nm}$ and can be cataIytically hydrogenated to yield $2,6-$ dimethyloctane. On ozonolysis followed by zinc/acetic acid treatment, myrcene yields formaldehyde, acetone, and 2- -oxopentanedial:(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 53

Adiponitrile, a starting material used in the manufacture of nylon, can be prepared in three steps from 1,3 -butadiene. How would you carry out this synthesis?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 54

Ergosterol, a precursor of vitamin $\mathrm{D},$ has $\lambda_{\max }=282 \mathrm{nm}$ and molar absorptivity $\epsilon=11,900 .$ What is the concentration of ergosterol in a solution whose absorbance $A=0.065$ with a sample pathlength $I=1.00 \mathrm{cm} ?$
(FIGURE CANT COPY)

Lottie Adams

Numerade Educator

Problem 55

Dimethyl butynedioate undergoes a Diels-Alder reaction with $(2 E, 4 E)$ hexadiene. Show the structure and stereochemistry of the product. (FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 56

Dimethyl butynedioate also undergoes a Diels-Alder reaction with $(2 E, 4 Z)$ hexadiene, but the stereochemistry of the product is different from that of the $(2 E, 4 E)$ isomer (Problem 14.55). Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 57

How would you carry out the following synthesis (more than one step is required)? What stereochemical relationship between the $-\mathrm{CO}_{2} \mathrm{CH}_{3}$ group attached to the cyclohexane ring and the - CHO groups would your synthesis produce?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 58

The double bond of an enamine (alkene $+$ amine) is much more nucleophilic than a typical alkene double bond. Assuming that the nitrogen atom in an enamine is $s p^{2}$ -hybridized, draw an orbital picture of an enamine, and explain why the double bond is electron-rich.
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 59

Benzene has an ultraviolet absorption at $\lambda_{\max }=204 \mathrm{nm},$ and para-toluidine has $\lambda_{\max }=235 \mathrm{nm} .$ How do you account for this difference?
(FIGURE CANT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator