Organic Chemistry

Paula Yurkanis Bruice

Chapter 4

Reactions of Alkenes - all with Video Answers

Educators

Chapter Questions

Problem 1

List the carbocations in order of decreasing stability.

Shazia Naz

Shazia Naz

Numerade Educator

Problem 2

a. How many $\mathrm{C}-\mathrm{H}$ bond orbitals are available for overlap with the vacant $p$ orbital in the methyl cation?
b. Which is more stable, a methyl cation or an ethyl cation?

Temi Ajayi

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Problem 3

For each of the following reaction coordinate diagrams, tell whether the structure of the transition state will more closely resemble the structure of the reactants or the structure of the products:

Temi Ajayi

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Problem 4

What would be the major product obtained from the addition of HBr to each of the following compounds?

Temi Ajayi

Numerade Educator

Problem 5

What alkene should be used to synthesize each of the following alkyl bromides?

Dalton Hilovsky

Dalton Hilovsky

Numerade Educator

Problem 6

The addition of HBr to which of the following alkenes is more highly regioselective?

Temi Ajayi

Numerade Educator

Problem 7

The $p K_{a}$ of a protonated alcohol is about -2.5 and the $p K_{a}$ of an alcohol is about 15 . Therefore, as long as the $\mathrm{pH}$ of the solution is greater than ___________ and less than ___________ , more than $50 \%$ of 2 -propanol (the product of the previous reaction) will be in its neutral, nonprotonated form.

Shazia Naz

Numerade Educator

Problem 8

Use Figure 4.5 to answer the following questions about the acid-catalyzed hydration of an alkene:
$*$ At a pH of $4,$ for example, the concentration of $\mathrm{HO}^{-}$ is $1 \times 10^{-10} \mathrm{M}$, whereas the concentration of water in a dilute aqueous solution is $55.5 \mathrm{M}$.
a. How many transition states are there?
b. How many intermediates are there?
c. Which is more stable, the protonated alcohol or the neutral alcohol?
d. Of the six steps in the forward and reverse directions, which are the two fastest?

Temi Ajayi

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Problem 9

Give the major product obtained from the acid-catalyzed hydration of each of the following alkenes:

Shazia Naz

Numerade Educator

Problem 10

a. Give the major product of each of the following reactions:
b. What do all the reactions have in common?
c. How do all the reactions differ?

Temi Ajayi

Numerade Educator

Problem 11

How could the following compounds be prepared, using an alkene as one of the starting materials?

Dalton Hilovsky

Dalton Hilovsky

Numerade Educator

Problem 12

Propose a mechanism for the following reaction (remember to use curved arrows when showing a mechanism):

Temi Ajayi

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Problem 13

Which of the following carbocations would you expect to rearrange?

Temi Ajayi

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Problem 14

Give the major product(s) obtained from the reaction of each of the following with HBr:

Temi Ajayi

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Problem 15

What would have been the product of the preceding reaction if HBr had been used as a reagent instead of $\mathrm{Br}_{2} ?$

Shazia Naz

Numerade Educator

Problem 16

a. How does the first step in the reaction of ethene with $\mathrm{Br}_{2}$ differ from the first step in the reaction of ethene with HBr?
b. To understand why $\mathrm{Br}^{-}$ attacks a carbon atom of the bromonium ion rather than the positively charged bromine atom, draw the product that would be obtained if $\mathrm{Br}^{-}$ did attack the bromine atom.

Shazia Naz

Numerade Educator

Problem 17

There are two nucleophiles in each of the following reactions:
For each reaction, explain why there is a greater concentration of one nucleophile than the other. What will be the major product of each reaction?

Temi Ajayi

Numerade Educator

Problem 18

Why are $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ unable to form covalent bonds?

Dalton Hilovsky

Dalton Hilovsky

Numerade Educator

Problem 19

What will be the product of the addition of I-Cl to 1 -butene? [Hint: Chlorine is more electronegative than iodine (Table 1.3 ).

Dalton Hilovsky

Dalton Hilovsky

Numerade Educator

Problem 20

What would be the major product obtained from the reaction of $\mathrm{Br}_{2}$ with 1 -butene if the reaction were carried out in.
a. dichloromethane?
c. ethyl alcohol?
b. water?
d. methyl alcohol?

Shazia Naz

Numerade Educator

Problem 21

How could the following compounds be synthesized from an alkene?

Temi Ajayi

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Problem 22

How could the following compounds be synthesized from 3 -methyl- 1 -butene?

Temi Ajayi

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Problem 23

How many moles of $\mathrm{BH}_{3}$ are needed to react with 2 moles of 1 -pentene?

Shazia Naz

Numerade Educator

Problem 24

What product would be obtained from hydroboration-oxidation of the following alkenes?
a. 2 -methyl- 2 -butene
b. 1-methylcyclohexene

Shazia Naz

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Problem 25

Propose a mechanism for the following reaction:

Zubair Abdulla

Numerade Educator

Problem 26

Write out the propagation steps that occur when HBr adds to 1 -methylcyclohexene in the presence of a peroxide.

Temi Ajayi

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Problem 27

The same alkane is obtained from the catalytic hydrogenation of both alkene $A$ and alkene
B. The heat of hydrogenation of alkene $A$ is 29.8 kcal/mol $(125 \mathrm{kJ} / \mathrm{mol}),$ and the heat of hydrogenation of alkene $\mathrm{B}$ is $31.4 \mathrm{kcal} / \mathrm{mol}(131 \mathrm{kJ} / \mathrm{mol}) .$ Which alkene is more stable?

Shazia Naz

Numerade Educator

Problem 28

a. Which of the following compounds is the most stable?
b. Which is the least stable?
c. Which has the smallest heat of hydrogenation?

Lottie Adams

Numerade Educator

Problem 29

Rank the following compounds in order of decreasing stability:
trans-3-hexene, cis-3-hexene, 1-hexene, cis-2,2,5,5-tetramethyl-3-hexene

Temi Ajayi

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Problem 30

Starting with an alkene, indicate how each of the following compounds can be synthesized:

Temi Ajayi

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Problem 31

Why should 3-methylcyclohexene not be used as the starting material in Problem 30b?

Temi Ajayi

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Problem 32

Starting with an alkene, indicate how each of the following compounds can be synthesized:

Temi Ajayi

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Problem 33

Give the major product of each of the following reactions:

Shazia Naz

Numerade Educator

Problem 34

Identify the electrophile and the nucleophile in each of the following reaction steps. Then draw curved arrows to illustrate the bond-making and bond-breaking processes.

Lottie Adams

Numerade Educator

Problem 35

What will be the major product of the reaction of 2 -methyl- 2 -butene with each of the following reagents?
a. $\mathrm{HBr}$
b. HBr + peroxide
c. HI
d. HI + peroxide
e. ICl
f. $\mathrm{H}_{2} / \mathrm{Pd}$
g. $\quad \mathrm{Br}_{2}+$ excess $\mathrm{NaCl}$
h. $\mathrm{Hg}(\mathrm{OAc})_{2}, \mathrm{H}_{2} \mathrm{O}$ followed by $\mathrm{NaBH}_{4}$
i. $\quad \mathrm{H}_{2} \mathrm{O}+$ trace $\mathrm{HCl}$
j. $\quad \mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$
k. $\quad \mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}$
I. $\quad \mathrm{Br}_{2} / \mathrm{CH}_{3} \mathrm{OH}$
m. $\mathrm{BH}_{3} / \mathrm{THF}$, followed by $\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{HO}^{-}$
n. $\mathrm{Hg}\left(\mathrm{O}_{2} \mathrm{CCF}_{3}\right)_{2}+\mathrm{CH}_{3} \mathrm{OH},$ followed by $\mathrm{NaBH}_{4}$

Temi Ajayi

Numerade Educator

Problem 36

Which of the following compounds is the most stable?
3,4-dimethyl-2-hexene; 2,3-dimethyl-2-hexene; 4,5-dimethyl-2-hexene
Which would you expect to have the largest heat of hydrogenation? Which would you expect to have the lowest heat of hydrogenation?

Shazia Naz

Numerade Educator

Problem 37

When 3-methyl-1-butene reacts with HBr, two alkyl halides are formed: 2-bromo-3-methylbutane and 2-bromo-2-methylbutane. Propose a mechanism that explains the formation of these products.

Lottie Adams

Numerade Educator

Problem 38

Problem 30 in Chapter 3 asked you to give the structures of all alkenes with molecular formula $C_{6} \mathrm{H}_{12}$. Use those structures to answer the following questions:
a. Which of the compounds is the most stable?
b. Which of the compounds is the least stable?

Temi Ajayi

Numerade Educator

Problem 39

Draw curved arrows to show the flow of electrons responsible for the conversion of reactants into products.

Lottie Adams

Numerade Educator

Problem 40

Give the reagents that would be required to carry out the following syntheses:

Temi Ajayi

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Problem 41

For each pair of bonds, which has the greater strength? Briefly explain why.

Temi Ajayi

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Problem 42

Give the major product of each of the following reactions:

Temi Ajayi

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Problem 43

Using any alkene and any other reagents, how would you prepare the following compounds?

Anish Wadhwa

Numerade Educator

Problem 44

There are two alkenes that react with HBr to give 1-bromo-1-methylcyclohexane.
a. Identify the alkenes.
b. Will both alkenes give the same product when they react with HBr/peroxide?
c. With HCl?
d. With HCl/peroxide?

Temi Ajayi

Numerade Educator

Problem 45

For each of the following pairs, indicate which member is the more stable:

Temi Ajayi

Numerade Educator

Problem 46

The second-order rate constant (in units of $\mathbf{M}^{-1} \mathbf{s}^{-1}$ ) for acid-catalyzed hydration at $25^{\circ} \mathrm{C}$ is given for each of the following alkenes:
a. Calculate the relative rates of hydration of the alkenes.
b. Why does ( $Z$ )-2-butene react faster than $(E)$ - 2 -butene?
c. Why does 2 -methyl- 2 -butene react faster than $(Z)$ - 2 -butene?
d. Why does 2,3 -dimethyl- 2 -butene react faster than 2 -methyl- 2 -butene?

Temi Ajayi

Numerade Educator

Problem 47

Which compound has the greater dipole moment?

Lottie Adams

Numerade Educator

Problem 48

a. What five-carbon alkene will form the same product whether it reacts with HBr in the presence of a peroxide or with HBr in the absence of a peroxide?
b. Give three alkenes containing six carbon atoms that form the same product, whether they react with HBr in the presence of a peroxide or with HBr in the absence of a peroxide.

Shazia Naz

Numerade Educator

Problem 49

Mark Onikoff was about to turn in the products he had obtained from the reaction of HI with 3,3,3-trifluoropropene when he realized that the labels had fallen off his flasks. He didn’t know which label belonged to which flask. Another student in the next lab told him that because the product obtained by following Markovnikov’s rule was 1,1,1-trifluoro-2-iodopropane, he should put that label on the flask containing the most product and label the flask with the least product 1,1,1-trifluoro-3-iodopropane, the anti-Markovnikov product. Should Mark follow the student’s advice?

Temi Ajayi

Numerade Educator

Problem 50

a. Propose a mechanism for the following reaction (show all curved arrows):
b. Which step is the rate-determining step?
c. What is the electrophile in the first step?
d. What is the nucleophile in the first step?
e. What is the electrophile in the second step?
f. What is the nucleophile in the second step?

Lottie Adams

Numerade Educator

Problem 51

a. What product is obtained from the reaction of HCl with 1-butene? With 2-butene?
b. Which of the two reactions has the greater free energy of activation?
c. Which of the two alkenes reacts more rapidly with HCI?
d. Which compound reacts more rapidly with HCl, (Z)-2-butene or (E)-2-butene?

Temi Ajayi

Numerade Educator

Problem 52

a. How many alkenes could you treat with $\mathrm{H}_{2} / \mathrm{Pt}$ in order to prepare methylcyclopentane?
b. Which of the alkenes is the most stable?
c. Which of the alkenes has the smallest heat of hydrogenation?

Temi Ajayi

Numerade Educator

Problem 53

a. Propose a mechanism for the following reaction:
b. Is the initially formed carbocation primary, secondary, or tertiary?
c. Is the rearranged carbocation primary, secondary, or tertiary?
d. Why does the rearrangement occur?

Lottie Adams

Numerade Educator

Problem 54

When the following compound is hydrated in the presence of acid, the unreacted alkene is found to have retained the deuterium atoms:
What does the preceding statement tell you about the mechanism of hydration?

Temi Ajayi

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Problem 55

Propose a mechanism for each of the following reactions:

Lottie Adams

Numerade Educator

Problem 56

a. Dichlorocarbene can be generated by heating chloroform with $\mathrm{HO}^{-}$. Propose a mechanism for the reaction.
b. Dichlorocarbene can also be generated by heating sodium trichloroacetate. Propose a mechanism for the reaction.

Lottie Adams

Numerade Educator