Textbook Question
Provide the mechanism of the radical reactions shown.
(b)
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11. Radical Reactions
Allylic Bromination
5:48 minutesProblem 31d
Textbook Question
What is the major product of the following reactions? Disregard stereoisomers:
d. 
Verified step by step guidance1
Step 1: Recognize the reagent and reaction conditions. NBS (N-Bromosuccinimide) in the presence of heat (Δ) and peroxide is used for allylic bromination. This reaction selectively replaces a hydrogen atom at the allylic position with a bromine atom.
Step 2: Identify the allylic positions in the given molecule. The molecule is a substituted cyclohexene with two methyl groups. The allylic positions are the carbon atoms adjacent to the double bond.
Step 3: Determine the most reactive allylic position. The allylic position with the most stable radical intermediate will be favored. In this case, the allylic position adjacent to the methyl group is more stable due to hyperconjugation and inductive effects.
Step 4: Predict the major product. Bromine will replace the hydrogen atom at the most reactive allylic position, resulting in the formation of an allylic bromide.
Step 5: Disregard stereoisomers as instructed. The major product will be the allylic bromide formed at the most stable allylic position, without considering stereochemistry.
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Video duration:
5mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Allylic Bromination
Allylic bromination is a reaction where bromine is introduced at the allylic position of an alkene or alkane. This process typically involves the use of N-bromosuccinimide (NBS) in the presence of a radical initiator, such as peroxide, under heat. The reaction proceeds via a radical mechanism, where the allylic hydrogen is abstracted, forming a resonance-stabilized radical that can then react with bromine.
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Mechanism of Allylic Bromination.
Radical Mechanism
A radical mechanism involves the formation and reaction of free radicals, which are highly reactive species with unpaired electrons. In the context of allylic bromination, the reaction begins with the generation of radicals through the homolytic cleavage of the N–Br bond in NBS, followed by the abstraction of hydrogen from the allylic position. This mechanism is crucial for understanding how the product distribution is influenced by the stability of the resulting radicals.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In allylic bromination, the stability of the radical intermediates determines the major product. More stable radicals, such as those formed at more substituted allylic positions, are favored, leading to a specific major product in the reaction. Understanding regioselectivity is essential for predicting the outcome of the reaction.
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