11. Radical Reactions

(••••) Because of the angle strain present in cyclopropanes, they tend to open up in the presence of nearby radicals. Show a mechanism for the following reaction that demonstrates this principle.

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Assessments 11.57–11.61 should be answered in order. Each question should be used to help you answer the next. At the end of these assessments, you should have learned a new skill.

(••) Tributyltin hydride (Bu₃SnH) is often used as a 'radical carrier' in radical reactions. Which bond would you expect to be weaker, Sn ― H or C― H? How might this relate to radical stability? Explain your answer.

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Assessments 11.57–11.61 should be answered in order. Each question should be used to help you answer the next. At the end of these assessments, you should have learned a new skill.

(••••) Haloalkanes can be reduced to alkanes using radical reactions involving Bu₃SnH and a catalytic amount of AIBN. Suggest a mechanism for this reaction. [Pay close attention to the bonds formed and broken in developing your mechanism.]

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(•••) If a small amount of a moderately nonpolar poisonous compound was added to a pond, why would it be safer to drink the water than it would be to eat the fish that live there?

Predict the major monohalogenation product(s) of the following reactions. Indicate whether you think the reaction will be selective and justify your position.

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(••) Predict the product(s) of the following halogenation reactions. Only one equivalent of the halogen is used in each case. If the reaction proceeds selectively, indicate this by only drawing the major product. If the reaction is not selective, draw all possible products.

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(••) Predict the product(s) of the following halogenation reactions. Only one equivalent of the halogen is used in each case. If the reaction proceeds selectively, indicate this by only drawing the major product. If the reaction is not selective, draw all possible products.

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We have studied the following reactions in previous chapters. For each, (i) indicate which reaction sheets they should appear on, (ii) show the best structure to use to represent them, and (iii) write the notes you could put in the margin so that the mechanism is implied.

(b) Radical halogenation of an alkane

Provide a mechanism for the chlorination of cyclohexane. Be sure to include initiation, propagation, and three possible termination steps.

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(•••) For the following reaction, answer questions (a)–(d).

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(a) Give an arrow-pushing mechanism, including the initiation step and two propagation steps.

Predict the major products of the following alkane halogenation reactions. [The number of products shown ignores the formation of racemic mixtures.]

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Assessments 11.57–11.61 should be answered in order. Each question should be used to help you answer the next. At the end of these assessments, you should have learned a new skill.

(•) Other molecules can be used as initiators in radical reactions. One such molecule is 2, 2' -azobisisobutyronitrile (AIBN). Show an arrow-pushing mechanism that rationalizes the formation of the following radical species. What are the driving forces of this reaction?

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Assessments 11.57–11.61 should be answered in order. Each question should be used to help you answer the next. At the end of these assessments, you should have learned a new skill.

(••••) Cyclizations can be carried out under radical conditions involving Bu₃SnH and a catalytic amount of AIBN. Suggest a mechanism for this reaction. [Pay close attention to the bonds formed and broken in developing your mechanism. It may be helpful to number the carbons.]

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a. What alkane, with molecular formula C5H12, forms only one monochlorinated product when it is heated with Cl2?

Explain why the rate of bromination of methane decreases if HBr is added to the reaction mixture.

How many alkyl halides are obtained from monochlorination of the alkanes in Problem 4 if stereoisomers are included?

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c. How many monochlorination products would be obtained if all stereoisomers are included?

What is the major product obtained from treating an excess of each of the following compounds with Cl2 in the presence of ultraviolet light at room

temperature? Disregard stereoisomers.

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In the presence of an acid catalyst, acetaldehyde forms a trimer known as paraldehyde. Because it induces sleep when it is administered to animals in

large doses, paraldehyde is used as a sedative or hypnotic. Propose a mechanism for the formation of paraldehyde.

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a. Propose a mechanism for the following reaction:

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How many alkyl halides are obtained from monochlorination of the alkanes in Problem 4 if stereoisomers are included?

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a. How many monochlorination products can be obtained from the radical chlorination of methylcyclohexane? Disregard stereoisomers.

Write the steps for formation of tetrachloromethane (CCl4) from the reaction of methane with Cl2 + hv.

A possible alternative mechanism to that shown in Problem 47 for the monochlorination of methane involves the following propagation steps:

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How do you know that the reaction does not take place by this mechanism?

If cyclopentane reacts with more than one equivalent of Cl₂ at a high temperature, how many dichlorocyclopentanes would you expect to obtain as products?

Show how you would prepare cyclopentene from each compound.

c. cyclopentane (not by dehydrogenation)

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong.

Explain how the experimental evidence disproves each mechanism.

a.

CL2 + hv —> CL2* (an “activated” form of Cl2)

Cl2* + CH4 —> HCl + CH3Cl

a. Propose a mechanism for the free-radical chlorination of ethane,

CH3—CH3 + CL2 hv —> CH3—CH2Cl + HCl

a. Draw the structure of the transition state for the second propagation step in the chlorination of methane.

⋅CH3 + Cl2 —> CH3Cl + Cl⋅

Show whether the transition state is product-like or reactant-like and which of the two partial bonds is stronger.

When healthy, Earth’s stratosphere contains a low concentration of ozone

(O3) that absorbs potentially harmful ­ultraviolet (UV) radiation by the cycle shown at right.

Chlorofluorocarbon refrigerants, such as Freon 12 (CF2Cl2), are stable in the lower atmosphere, but in the stratosphere they absorb high-energy UV radiation to generate chlorine radicals.

The presence of a small number of chlorine radicals appears to lower ozone concentrations dramatically.

The following reactions are all known to be exothermic (except the one requiring light) and to have high rate constants.

Propose two mechanisms to explain how a small number of chlorine ­radicals can destroy large numbers of ozone molecules.

Which of the two mechanisms is more likely when the concentration of chlorine atoms is very small?

Cl—O—O—Cl hv—> O2 + 2Cl⋅

Cl—O⋅ + O —> O2 + Cl⋅

Cl⋅ + O3 —> Cl—O⋅ + O2

2 Cl—O⋅ —> Cl—O—O—Cl

Using the BDEs in [TABLE 4-2] <IMAGE> (page 167), 

c. Suggest two reasons why iodine does not react well with methane.

Alkoxy radicals (R—O•) 

are generally more stable than alkyl 

(R⋅)radicals. 

1. Write an equation showing an alkyl free radical (from burning gasoline) abstracting a ­hydrogen atom from tert-butyl alcohol, (CH3)3COH. 

2. Explain why tert-butyl alcohol works as an antiknock additive for gasoline

Using cyclohexane as one of your starting materials, show how you would synthesize the following compounds.

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When exactly 1 mole of methane is mixed with exactly 1 mole of chlorine and light is shone on the mixture, a ­chlorination reaction occurs.

The products are found to contain substantial amounts of di-, tri-, and tetrachloromethane, as well as ­unreacted methane.

b. How would you run this reaction to get a good conversion of methane to CH3Cl? Of methane to CCl4?

a. Write the propagation steps leading to the formation of dichloromethane (CH2Cl2) from chloromethane.

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism.

b. CH4 + hv —> •CH3 + H•

•CH3 + Cl2 —> CH3Cl + Cl•

Cl• + H• —> HCl

a. Propose a mechanism for the free-radical chlorination of ethane, CH3—CH3 + CL2 hv —> CH3—CH2Cl + HCl

b. Calculate ΔH° for each step in this reaction.

c. Calculate the overall value of ΔH° for this reaction.

For each alkane, 1. draw all the possible monochlorinated derivatives.

a. Cyclopentane

b. Methylcyclopentane

2. For each alkane, determine whether free-radical chlorination would be a good way to make any of these monochlorinated derivatives. Will the reaction give mostly one major product?

a. Cyclopentane

b. Methylcyclopentane

b. Explain why free-radical halogenation usually gives mixtures of products.

(•••) FROM THE LITERATURE The following transformation was found to occur in areas with large NO₂ emissions. Suggest a mechanism for the reaction (J. Phys. Chem. 2013, 117, 14132–14140). [Hint: Use the fishhook arrows associated with radical reactions.]

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The human body can excrete drugs and other exogenous molecules by converting them into polar, water-soluble compounds by a reaction similar to the autoxidation described in Section 11.6. Why are the following drug candidate molecules susceptible to oxidation by this pathway?

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b. For each reaction, show which stereoisomers are obtained

1. NBS/∆/peroxide

2. Br2/CH2Cl2

a. What is the major product of the reaction in Problem 7 when the alkane reacts with Cl2 instead of with Br2? Disregard stereoisomers.

Radical addition to alkenes is not effective for the synthesis of iodo- and chloroalkanes. Using your knowledge of the mechanism of this reaction, along with bond dissociation energies, explain why the radical additions of HI and HCl are not effective. (Assume ∆H = 65 kcal/ mol for the C―C π bond.)

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Show how the following compounds could be prepared from 2-methylpropane:

a. 2-bromo-2-methylpropane