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Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 11 - Properties and Synthesis of Alkyl Halides: Radical ReactionsProblem 62b
Chapter 10, Problem 62b

A halogenation intended to make compound A formed B instead.

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

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1
Identify the starting compound and the intended product. The starting compound is an alkylbenzene with two different hydrogen atoms labeled Ha and Hb. The intended product A is the bromination at the position of Ha.
Recognize the actual product formed, which is compound B. This indicates that bromination occurred at the position of Hb instead of Ha.
Consider the mechanism of radical halogenation. In the presence of Br2 and heat, a radical chain reaction is initiated. The first step involves the homolytic cleavage of Br2 to form two bromine radicals.
Analyze the stability of the radicals formed by abstraction of Ha and Hb. The radical formed by abstraction of Hb is more stable due to hyperconjugation and possibly resonance stabilization with the benzene ring, making it the preferred site for bromination.
Propose the mechanism: The bromine radical abstracts the hydrogen atom Hb, forming a more stable secondary radical. This radical then reacts with another Br2 molecule to form the brominated product B, completing the radical chain reaction.

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Key Concepts

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

Halogenation

Halogenation is a chemical reaction that involves the addition of halogen atoms (such as chlorine or bromine) to a compound. This process can occur through various mechanisms, including electrophilic addition or radical substitution, depending on the nature of the substrate and reaction conditions. Understanding halogenation is crucial for predicting the products formed when halogens react with organic compounds.
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Halogenation Mechanism

Reaction Mechanism

A reaction mechanism is a step-by-step description of the pathway by which reactants transform into products during a chemical reaction. It includes details about the intermediates formed, the transition states, and the energy changes involved. Proposing a mechanism for the formation of compound B from A requires knowledge of the specific steps and conditions that lead to the observed product.
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Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to occur at one location over another in a molecule, leading to the formation of specific isomers. In halogenation reactions, regioselectivity can significantly influence the outcome, as the halogen may add to different positions on the substrate. Understanding regioselectivity is essential for rationalizing why compound B was formed instead of the expected compound A.
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Related Practice
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

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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Textbook Question

A halogenation intended to make compound A formed B instead.

(c) Given the two mechanisms you drew, why might B have formed selectively?

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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.

(d) Without looking it up, would you expect C–Ha or C–Hb to have the lower bond-dissociation energy?

Textbook Question

Predict the product of the following benzylic bromination reactions.

(a)

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