Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions

Problem 61

Chapter 10, Problem 61

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.]

Verified step by step guidance

Begin by understanding the role of AIBN (azobisisobutyronitrile) in the reaction. AIBN is a radical initiator that decomposes upon heating to form nitrogen gas and two 2-cyanoprop-2-yl radicals. These radicals are crucial for initiating the radical chain reaction.

The radical chain reaction starts with the generation of a radical from Bu₃SnH. The 2-cyanoprop-2-yl radical abstracts a hydrogen atom from Bu₃SnH, forming a Bu₃Sn radical.

The Bu₃Sn radical then abstracts the bromine atom from the alkyl bromide substrate, generating an alkyl radical. This step is key as it sets the stage for the cyclization process.

The alkyl radical undergoes intramolecular cyclization by attacking the double bond, forming a new carbon-carbon bond and generating a new radical at the terminal carbon of the double bond.

Finally, the newly formed radical abstracts a hydrogen atom from another molecule of Bu₃SnH, completing the cyclization and regenerating the Bu₃Sn radical, which can continue the chain reaction. The final products are the cyclized compound and Bu₃SnBr.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Radical Reactions

Radical reactions involve species with unpaired electrons, known as radicals, which are highly reactive. These reactions typically proceed through three main steps: initiation, propagation, and termination. In the context of cyclization, radicals can facilitate the formation of cyclic structures by promoting bond formation between carbon atoms, often leading to the closure of rings.

Mechanism of Cyclization

Cyclization refers to the process of forming a cyclic compound from a linear precursor. Understanding the mechanism involves identifying the specific steps and intermediates that occur during the reaction. In radical cyclization, the mechanism often includes the generation of a radical species that attacks another part of the molecule, leading to the formation of new bonds and the closure of the ring.

Catalysis in Organic Reactions

Catalysis involves the use of a substance that increases the rate of a chemical reaction without being consumed in the process. In this case, AIBN acts as a radical initiator, facilitating the generation of radicals necessary for the cyclization reaction. Understanding how catalysts influence reaction pathways and mechanisms is crucial for predicting the outcomes of organic reactions.

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Related Practice

Textbook Question

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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Textbook Question

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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Textbook Question

A halogenation intended to make compound A formed B instead.

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Textbook Question

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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Textbook Question

A halogenation intended to make compound A formed B instead.

(a) Suggest a mechanism for the intended formation of A.

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Textbook Question

A halogenation intended to make compound A formed B instead.

(b) Suggest a mechanism that rationalizes the formation of B.

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