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Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Capitolo 10, Problema 32d

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

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1
Identify the allylic position in the given molecule. The allylic position is the carbon atom adjacent to the double bond. In this structure, the allylic position is the carbon atom next to the double bond on the left side.
Understand the role of NBS (N-bromosuccinimide) in the reaction. NBS is used for allylic bromination, which selectively replaces a hydrogen atom at the allylic position with a bromine atom.
Consider the reaction conditions. The presence of light indicates a radical mechanism, which is typical for allylic bromination with NBS.
Generate the allylic radical. The reaction begins with the abstraction of an allylic hydrogen atom by a bromine radical, forming an allylic radical.
Predict the product. The allylic radical will react with a bromine molecule to form the allylic bromide. The bromine atom will attach to the allylic position, resulting in the formation of the allylic brominated product.

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Allylic Halogenation

Allylic halogenation is a reaction where a halogen is introduced at the allylic position of an alkene. This position is adjacent to the double bond, allowing for resonance stabilization of the intermediate radical. The reaction typically involves a radical mechanism, initiated by light or heat, and is useful for functionalizing alkenes.
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The general mechanism of Allylic Halogenation.

N-Bromosuccinimide (NBS)

N-Bromosuccinimide (NBS) is a reagent commonly used for allylic bromination. It provides a controlled source of bromine radicals, which selectively react at the allylic position. NBS is preferred for its ability to minimize over-bromination and its effectiveness in radical reactions, especially under light or heat conditions.
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Radical Mechanism

The radical mechanism involves the formation of radical intermediates, which are highly reactive species with unpaired electrons. In allylic halogenation, radicals are generated by the homolytic cleavage of bonds, often initiated by light. These radicals propagate the reaction by abstracting hydrogen atoms and forming new bonds, leading to the substitution at the allylic position.
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The mechanism of Radical Polymerization.