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Organic Chemistry

David R. Klein

Chapter 17

Conjugated Pi Systems and Pericyclic Reactions - all with Video Answers

Educators

Chapter Questions

05:45

Problem 1

For each of the following compounds, identify whether each $\mathrm{C}=\mathrm{C} \pi$ system is cumulated, conjugated, or isolated:
Figure a-d can't copy

Bin Chen
Bin Chen
Numerade Educator
04:42

Problem 2

Identify reagents necessary to convert cyclohexane into 1,3-cylohexadiene: Notice that the starting material has no leaving groups and cannot simply be treated with a strong base. You must first introduce functionality into the starting material (for help with introducing functional groups, see Section 12.2).
Figure can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
03:09

Problem 3

Rank the three $\mathrm{C}-\mathrm{C}$ bonds of 1-butene in terms of bond length (from shortest to longest).

Ian Kaigh
Ian Kaigh
Numerade Educator
01:05

Problem 4

Compare the following three isomeric dienes:
Figure can't copy
(a) Which compound will liberate the least heat upon hydrogenation with 2 mol of hydrogen gas? Why?
(b) Which compound will liberate the most heat upon hydrogenation with 2 mol of hydrogen gas? Why? (For help, see Section 8.5).

Lottie Adams
Lottie Adams
Numerade Educator
00:44

Problem 5

Identify the most stable compound:
Figure can't copy

Aadit Sharma
Aadit Sharma
Numerade Educator
01:30

Problem 6

Draw an energy diagram showing the relative energy levels of the MOs for 1,3,5,7octatetraene and identify the HOMO and LUMO for both the ground state and the excited state.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator

Problem 7

Predict the products for each of the following reactions, and propose a mechanism that explains the formation of each product:
Figure a-f can't copy

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02:20

Problem 8

Consider the following two dienes. When treated with HBr, one of these dienes yields four products, while the other diene yields only two products. Explain.
Figure can't copy

George Bennett
George Bennett
Numerade Educator
00:46

Problem 9

Propose the structure of a conjugated diene for which 1,2-addition of HBr produces the same product as 1,4-addition of HBr.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator

Problem 10

Predict the products for each of the following reactions, and in each case, determine which product will predominate:
Figure a-c can't copy

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02:06

Problem 11

When 1,4-dimethylcyclohepta-1,3-diene is treated with HBr at elevated temperature, the 1,2-adduct predominates, rather than the 1,4-adduct. Explain this result.
Figure can't copy

Raghvendra Singh
Raghvendra Singh
Numerade Educator
02:20

Problem 12

When 1,3-cyclopentadiene is treated with HBr, the temperature does not have an influence on the identity of the major product. Explain.

George Bennett
George Bennett
Numerade Educator
07:19

Problem 13

What monomer should be used to make each of the following polymers:
Figure a-b can't copy

Nicholas Sacco
Nicholas Sacco
Numerade Educator

Problem 14

Predict the products for each of the following reactions:
Figure a-i can't copy

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02:41

Problem 15

Predict the product(s) obtained when benzoquinone is treated with excess butadiene:
Figure can't copy

Lottie Adams
Lottie Adams
Numerade Educator

Problem 16

Propose a mechanism for the following transformation:
Figure can't copy

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02:03

Problem 17

Consider the following two isomers of 2,4-hexadiene. One isomer reacts rapidly as a diene in a Diels-Alder reaction, and the other does not. Identify which isomer is more reactive, and explain your choice.
Figure can't copy

Madeline Currie
Madeline Currie
Numerade Educator
02:54

Problem 18

Rank the following dienes in terms of reactivity in DielsAlder reactions (from least reactive to most reactive):
Figure can't copy

Ian Kaigh
Ian Kaigh
Numerade Educator

Problem 19

Predict the products for each of the following reactions:
Figure a-f can't copy

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05:54

Problem 20

Consider the following $[4+4]$ cycloaddition process. Would you expect this process to occur through a thermal or photochemical pathway? Justify your answer with MO theory.
Figure can't copy

Ian Kaigh
Ian Kaigh
Numerade Educator

Problem 21

Predict the major product(s) for each of the following thermal electrocyclic reactions:
Figure a-c can't copy

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Problem 22

Predict the product for each of the following reactions:
Figure a-c can't copy

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01:16

Problem 23

Draw the structure of cis-3,4-diethylcyclobutene.
(a) Conrotatory ring opening produces only one product. Draw the product and determine whether its formation is best achieved under thermal conditions or photochemical conditions.
(b) Disrotatory ring opening can produce two possible products, but only one of these products is obtained in practice. Identify the product that is formed, and explain why the other possible product is not formed.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
02:53

Problem 24

Identify whether the product obtained from each of the following reactions is a meso compound or a pair of enantiomers:
(a) Irradiation of (2E,4Z,6Z)-4,5-dimethyl-2,4,6-octatriene with UV light
(b) Subjecting (2E,4Z,6Z)- 4,5-dimethyl-2,4,6-octatriene to elevated temperature
(c) Irradiation of (2E,4Z,6E)-4,5-dimethyl-2,4,6-octatriene with UV light

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
14:19

Problem 25

For each of the following reactions, use brackets and two numbers to identify the type of sigmatropic rearrangement taking place:
Figure a-b can't copy

Dr. Satish Ingale
Dr. Satish Ingale
Numerade Educator
06:11

Problem 26

Consider the structure of cis-1,2-divinylcyclopropane:
Figure can't copy
This compound is stable at low temperature but rearranges at room temperature to produce 1,4-cycloheptadiene.
(a) Draw a mechanism for this transformation.
(b) Using brackets and numbers, identify the type of sigmatropic rearrangement taking place in this case.
(c) This reaction proceeds to completion, which means that the concentration of the reactant is insignificant once the equilibrium has been established. Explain why this reaction proceeds to completion.

Ian Kaigh
Ian Kaigh
Numerade Educator

Problem 27

Predict the product for each of the following reactions.
Figure a-d can't copy

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Problem 28

Draw a plausible mechanism for the following transformation:
Figure can't copy

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Problem 29

Use Woodward-Fieser rules to estimate the expected $\lambda_{\max }$ for each of the following compounds:
Figure a-d can't copy

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02:07

Problem 30

Identify which of the following compounds is expected to have a larger $\lambda_{\max }$ :
Figure can't copy

Lottie Adams
Lottie Adams
Numerade Educator
01:42

Problem 31

Use the color wheel in Figure 17.37.
(a) Identify the color of a compound that absorbs orange light.
(b) Identify the color of a compound that absorbs blue-green light.
(c) Identify the color of a compound that absorbs orange-yellow light.

Susan Hallstrom
Susan Hallstrom
Numerade Educator
02:32

Problem 32

Draw the structure of each of the following compounds:
(a) 1,4-Cyclohexadiene
(b) 1,3-Cyclohexadiene
(c) (Z)-1,3-Pentadiene
(d) (2Z,4E)-Hepta-2,4-diene
(e) 2,3-Dimethyl-1,3-butadiene

Dr. Satish Ingale
Dr. Satish Ingale
Numerade Educator

Problem 33

Circle each compound that has a conjugated $\pi$ system:
Figure can't copy

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Problem 34

For each of the following pairs of compounds determine whether the two compounds would be at equilibrium at room temperature or whether they could be isolated from one another:
Figure a-c can't copy

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00:46

Problem 35

Identify the structure of the conjugated diene that will react with one equivalent of HBr to yield a racemic mixture of 3-bromocyclohexene.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
05:00

Problem 36

Draw the major product expected when 1,3-butadiene is treated with one equivalent of HBr at $0^{\circ} \mathrm{C}$, and show the mechanism of its formation.

Dr. Satish Ingale
Dr. Satish Ingale
Numerade Educator
05:00

Problem 37

Draw the major product expected when 1,3-butadiene is treated with one equivalent of HBr at $40^{\circ} \mathrm{C}$, and show the mechanism of its formation.

Dr. Satish Ingale
Dr. Satish Ingale
Numerade Educator
04:55

Problem 38

Draw all four products obtained when 2-ethyl-3-methyl-1,3-cyclohexadiene is treated with HBr at room temperature, and show the mechanism of their formation.

Ian Kaigh
Ian Kaigh
Numerade Educator
01:22

Problem 39

After performing the reaction from Problem 17.38, the reaction flask is heated to $40^{\circ} \mathrm{C}$ and two of the products become the major products. When the flask is then cooled to $0^{\circ} \mathrm{C}$, no change occurs in the product distribution. Explain why an increase in temperature causes a change in the product distribution, and explain why a subsequent reduction in temperature has no effect.

David Collins
David Collins
Numerade Educator
00:52

Problem 40

Each of the following compounds does not participate as a diene in a Diels-Alder reaction. Explain why in each case.
Figure a-d can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
01:21

Problem 41

Rank the following dienophiles (from least reactive to most reactive) in terms of reactivity in a Diels-Alder reaction:
Figure can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
02:01

Problem 42

The absorbance spectrum of 1,3-butadiene displays an absorption in the UV region ( $\lambda_{\max }=217 \mathrm{~nm}$ ), while the absorption spectrum of 1,2-butadiene does not display a similar absorption. Explain.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
02:28

Problem 43

Predict the products for each of the following DielsAlder reactions:
Figure a-f can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
02:47

Problem 44

Identify the reagents you would use to prepare each of the following compounds via a Diels-Alder reaction:
Figure a-h can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
03:12

Problem 45

Starting with 1,3-butadiene as your only source of carbon atoms, and using any other reagents of your choice, design a synthesis of the following compound:
Figure can't copy

Zubair Abdulla
Zubair Abdulla
Numerade Educator
01:14

Problem 46

The following triene reacts with excess maleic anhydride to produce a compound with molecular formula $\mathrm{C}_{14} \mathrm{H}_{12} \mathrm{O}_6$. Draw the structure of this product (ignoring stereochemistry).
Figure can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
01:58

Problem 47

Chlordane is a powerful insecticide that was used in the United States during the second half of the twentieth century. Its use was discontinued in 1988 in recognition of its persistence and accumulation in the environment. Identify the reagents you would use to prepare chlordane via a Diels-Alder reaction.
Figure can't copy

Madeline Currie
Madeline Currie
Numerade Educator
01:21

Problem 48

Cylopentadiene reacts very rapidly in Diels-Alder reactions. In contrast, 1,3-cyclohexadiene reacts more slowly and 1,3-cycloheptadiene is practically unreactive. Can you offer an explanation for this trend?

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
03:15

Problem 49

Compound $\mathbf{A}\left(\mathrm{C}_7 \mathrm{H}_{10}\right)$ exhibits a $\lambda_{\max }$ of 230 nm in its UV absorption spectrum. Upon hydrogenation with a metal catalyst, compound $\mathbf{A}$ will react with two equivalents of hydrogen gas. Ozonolysis of compound $\mathbf{A}$ yields the following two compounds. Identify two possible structures for compound $\mathbf{A}$.
Figure can't copy

Madeline Currie
Madeline Currie
Numerade Educator
02:23

Problem 50

Rank the following compounds in order of increasing $\lambda_{\max }:$
Figure can't copy

Ian Kaigh
Ian Kaigh
Numerade Educator
02:07

Problem 51

Which of the following compounds below do you expect to have a longer $\lambda_{\max }$ ?
Figure can't copy

Lottie Adams
Lottie Adams
Numerade Educator
00:58

Problem 52

Predict the expected $\lambda_{\max }$ of the following compound:
Figure can't copy

Lottie Adams
Lottie Adams
Numerade Educator
06:11

Problem 53

When 5-deutero-5-methyl-1,3-cyclopentadiene is warmed to room temperature, it rapidly rearranges, giving an equilibrium mixture containing the original compound as well as two others. Propose a plausible mechanism for the formation of these other two compounds.
Figure can't copy

Ian Kaigh
Ian Kaigh
Numerade Educator

Problem 54

Propose a plausible mechanism for the following transformation:
Figure can't copy

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02:59

Problem 55

Propose a method for achieving the following transformation:
Figure can't copy

Shazia Naz
Shazia Naz
Numerade Educator

Problem 56

Predict the product for each of the following electrocyclic reactions:
Figure a-d can't copy

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02:50

Problem 57

Predict which side of the following equilibrium is favored, and explain your choice.
Figure can't copy

Tom Rutherford
Tom Rutherford
Numerade Educator
04:45

Problem 58

When trans-3,4-dimethylcyclobutene is heated, conrotatory ring opening can produce two different products, yet only one is formed. Draw both products, identify which product is formed, and then explain why the other product is not formed.

Zubair Abdulla
Zubair Abdulla
Numerade Educator

Problem 59

Predict the product for each of the following reactions:
Figure a-c can't copy

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10:34

Problem 60

$\alpha$-Terpinene is a pleasant-smelling compound present in the essential oil of marjoram, a perennial herb. Upon hydrogenation with a metal catalyst, $\alpha$-terpinene reacts with two equivalents of hydrogen gas to produce 1-isopropyl-4-methylcyclohexane. Ozonolysis of $\alpha$-terpinene yields the following two compounds:
Figure can't copy
(a) How many double bonds are present in $\alpha$-terpinene?
(b) Identify the structure of $\alpha$-terpinene.
(c) Predict the wavelength of maximum absorption ( $\lambda_{\max }$ ) in the absorption spectrum of $\alpha$-terpinene.

Anupa Sharad Medhekar
Anupa Sharad Medhekar
Numerade Educator
02:35

Problem 61

Draw all possible conjugated dienes with molecular formula $\mathrm{C}_6 \mathrm{H}_{10}$, taking special care not to draw the same compound twice.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
02:24

Problem 62

Treatment of 1,2-dibromocycloheptane with excess potassium tert-butoxide yields a product that absorbs UV light. Identify the product.

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator

Problem 63

In each of the following pairs of compounds identify the compound that liberates the most heat upon hydrogenation.
Figure a-b can't copy

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01:21

Problem 64

Would you expect nitroethylene to be more or less reactive than ethylene in a Diels-Alder reaction? (Hint: Draw the resonance structures of nitroethylene.)
Figure can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
01:05

Problem 65

Propose a plausible mechanism for the following transformatio. (Hint: Only two sequential pericyclic reactions are required.)
Figure can't copy

Grigoriy Sereda
Grigoriy Sereda
Numerade Educator
02:13

Problem 66

When 2-methoxy-1,3-butadiene is treated with 3-buten-2-one, there are two potential regiochemical outcomes for the Diels-Alder reaction that occurs:
Figure can't copy
Nevertheless, only one of these products is obtained. Draw resonance structures for the diene and the dienophile, and use these resonance structures to make an educated guess regarding which product is formed.

Colton K
Colton K
Numerade Educator

Problem 67

Propose a mechanism for the following transformation:
Figure can't copy

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Problem 68

Based on your answer to Problem 17.67, propose a mechanism for the following transformation:
Figure can't copy

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02:07

Problem 69

Compare the structures of 1,4-pentadiene and divinyl amine:
Figure can't copy
The first compound does not absorb UV light in the region between 200 and 400 nm . The second compound does absorb light above 200 nm . Using this information, identify the hybridization state of the nitrogen atom in divinylamine, and justify your answer.

Lottie Adams
Lottie Adams
Numerade Educator