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11. Radical Reactions

Allylic Bromination

11. Radical Reactions

Allylic Bromination

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The general mechanism of Allylic Halogenation.

The general mechanism of Allylic Halogenation.

Johnny Betancourt

The products of Allylic Chlorination.

The products of Allylic Chlorination.

Johnny Betancourt

Mechanism of Allylic Bromination.

Mechanism of Allylic Bromination.

Johnny Betancourt

Predict the product(s) of the following reaction.

Predict the product(s) of the following reaction.

Johnny Betancourt

Practice this topic

Multiple Choice

Predict the major, organic product of the following reaction.

Multiple Choice

Which reaction is best suited to form the following product?

Textbook Question

(•••) THINKING AHEAD The halogenation of an alkane when there is an alkene present in the molecule does not proceed with the regioselectivity you might expect. Using principles similar to those developed in this chapter, rationalize the formation of A as the only product. We study this reaction further in Chapter 8.

Textbook Question

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products: 2,3-dimethylbut-2-ene

a. Give a mechanism for this reaction, showing how the two products arise as a consequence of the resonance-stabilized intermediate.

Textbook Question

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products: b. The bromination of cyclohexene using NBS gives only one major product, as shown on the previous page. Explain why there is no second product from an allylic shift.

Textbook Question

Using 1,2-dimethylcyclohexene as your starting material, show how you would synthesize the following compounds. (Once you have shown how to synthesize a compound, you may use it as the starting material in any later parts of this problem.) If a chiral product is shown, assume that it is part of a racemic mixture.

Textbook Question

(••) Predict the major product(s) of the following allylic bromination reactions. (a)

Textbook Question

(••) Predict the major product(s) of the following allylic bromination reactions. (c)

Textbook Question

(•••) Suggest a mechanism for the following reactions. (c)

Textbook Question

(••) Identify the two different alkenes that undergo allylic halogenation to prepare the compound shown.

Textbook Question

In the following molecules, identify the carbon where the radical is most likely to form in the first propagation step. (c)

Textbook Question

In the following molecules, identify the carbon where the radical is most likely to form in the first propagation step. (d)

Textbook Question

In the following allylic radicals, identify the carbon where the new C― Br bond is most likely to form in the second propagation step. (b)

Textbook Question

In the following allylic radicals, identify the carbon where the new C― Br bond is most likely to form in the second propagation step. (c)

Textbook Question

Predict the products of the following allylic halogenation reactions. (a)

Textbook Question

Predict the products of the following allylic halogenation reactions. (c)

Textbook Question

(••••) Provide the mechanism of the radical reactions shown. (b)

Textbook Question

Draw the products of the following reactions, including all stereoisomers: e.

Textbook Question

Draw the products of the following reactions, including all stereoisomers: f.

Textbook Question

How many allylic substituted bromoalkenes are formed from the reactions in Problems 17 if stereoisomers are included?

Textbook Question

What is the major product of the following reactions? Disregard stereoisomers: d.

Textbook Question

How many allylic substituted bromoalkenes are formed from the reactions in Problems 18 if stereoisomers are included?

Textbook Question

Propose a mechanism to account for the products formed in the following reaction:

Textbook Question

When N-bromosuccinimide is added to hex-1-ene in CCl4 and a sunlamp is shone on the mixture, three products result. (a) Give the structures of these three products. (b) Propose a mechanism that accounts for the formation of these three products

Textbook Question

Predict the products of the following reactions. (c) 2-methylpropene + NBS, light

Textbook Question

What reagent would you use to brominate the allylic carbon of cyclohexene?

Assessments 11.62–11.65 should be answered in order.

(•••) Predict the product of the following benzylic bromination reactions.

(a) <IMAGE>

Textbook Question

(••) Predict the major product(s) of the following allylic bromination reactions.

(b) <IMAGE>

Textbook Question

(••) Predict the major product(s) of the following allylic bromination reactions.

(d) <IMAGE>

Textbook Question

Two products are formed when methylenecyclohexane reacts with NBS? Show how each is formed. Disregard stereoisomers.

<IMAGE>

Textbook Question

a. Draw the major product(s) of the reaction of 1-methylcyclohexene with the following reagents, disregarding stereoisomers:

1. NBS/∆/peroxide

2. Br2/CH2Cl2

Textbook Question

What is the major product of the following reactions? Disregard stereoisomers:

a. <IMAGE>

Textbook Question

What is the major product of the following reactions? Disregard stereoisomers:

b. <IMAGE>

Textbook Question

a. How many stereoisomers are formed from the reaction of cyclohexene with NBS?

Textbook Question

b. How many stereoisomers are formed from the reaction of 3-methylcyclohexene with NBS?

Textbook Question

Give a mechanism to explain the two products formed in the following reaction.

3-methylbut-1-ene <IMAGE> NBS, hv—>

not rearranged <IMAGE> + rearranged <IMAGE>

Textbook Question

Propose mechanisms to account for the observed products in the following reactions.

(a) <IMAGE> 1. NBS, 2. hv—> <IMAGE> + <IMAGE>

Textbook Question

Show the products you expect when each compound reacts with NBS with light shining on the reaction.

(c) <IMAGE>

Textbook Question

A student adds NBS to a solution of 1-methylcyclohexene and irradiates the mixture with a sunlamp until all the NBS has reacted.

After a careful distillation, the product mixture contains two major products of formula C7H11Br.

(a) Draw the resonance forms of the three possible allylic free radical intermediates.

(b) Rank these three intermediates from most stable to least stable.

(c) Draw the products obtained from each free-radical intermediate.

Textbook Question

In the presence of a small amount of bromine, the following light-promoted reaction has been observed.

<IMAGE> + Br2 hv—> <IMAGE> + <IMAGE> + HBr

b. Explain why only this one type of hydrogen atom has been replaced, in preference to any of the other hydrogen atoms in the starting material.

Textbook Question

Propose mechanisms to account for the observed products in the following reactions.

(b) <IMAGE> NBS, hv—> <IMAGE> + <IMAGE>

Textbook Question

Show the products you expect when each compound reacts with NBS with light shining on the reaction.

(a) <IMAGE>

(b) <IMAGE>

Textbook Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction: Cyclohexane

+ trace Br2 hv—> 3-bromocyclohexene

+ HBr a. Propose a mechanism for this reaction.

Textbook Question

When methylenecyclohexane is treated with a low concentration of bromine under irradiation by a sunlamp, two substitution products are formed. (a) Propose structures for these two products. (b) Propose a mechanism to account for their formation.

Textbook Question

Predict the products of the following allylic halogenation reactions.

(d) <IMAGE>

Textbook Question

In the following allylic radicals, identify the carbon where the new C― Br bond is most likely to form in the second propagation step.

(a) <IMAGE>

Textbook Question

In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

Cyclohexane <IMAGE> + trace Br2 hv—>

3-bromocyclohexene <IMAGE> + HBr

d. Explain why cyclohexene reacts with bromine much faster than cyclohexane, which must be heated to react.

Textbook Question

In the presence of a small amount of bromine, the following light-promoted reaction has been observed.

<IMAGE> + Br2 hv—> <IMAGE> + <IMAGE> + HBr

a. Write a mechanism for this reaction. Your mechanism should explain how both products are formed. (Hint: Notice which H atom has been lost in both products.)

Textbook Question

The fact that allylic halogenation results in formation of the most stable alkene suggests that it is under thermodynamic control [see Chapter 5 for a review.] Thus, the second propagation step must be reversible. Suggest an arrow-pushing mechanism by which the less stable allylic halide might equilibrate to the more stable allylic halide.

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