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)
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11. Radical Reactions
Allylic Bromination
5:49 minutesProblem 46e
Textbook Question
Draw the products of the following reactions, including all stereoisomers:
e. 
Verified step by step guidance1
Step 1: Recognize the reaction type. The reaction involves NBS (N-Bromosuccinimide) in the presence of heat (Δ) and peroxide. This is a radical bromination reaction that specifically targets allylic positions in the molecule.
Step 2: Identify the allylic positions in the given molecule. The allylic position is the carbon atom adjacent to the double bond. In this case, the allylic positions are the carbons directly connected to the double bond in the cyclohexene ring.
Step 3: Generate the allylic radical intermediate. The peroxide initiates the formation of a bromine radical, which abstracts a hydrogen atom from the allylic position, forming an allylic radical. This radical is stabilized by resonance.
Step 4: Draw the resonance structures of the allylic radical. The radical can delocalize over the π-system of the double bond, creating multiple resonance structures. These resonance structures indicate the possible sites for bromine addition.
Step 5: Add bromine to the allylic radical. Bromine will attach to the allylic position, forming the brominated product. Include all possible stereoisomers if the allylic position is chiral or if multiple resonance structures lead to different products.
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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 added to the allylic position of an alkene or alkane, typically using N-bromosuccinimide (NBS) in the presence of a peroxide. This reaction proceeds via a radical mechanism, where the allylic hydrogen is abstracted, forming a resonance-stabilized radical that can then react with bromine to form the brominated product.
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Mechanism of Allylic Bromination.
Radical Mechanism
A radical mechanism involves the formation and reaction of free radicals, which are species with unpaired electrons. In the context of allylic bromination, the reaction starts 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, leading to the formation of a new carbon-bromine bond.
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Stereoisomerism
Stereoisomerism refers to the existence of compounds with the same molecular formula and connectivity but different spatial arrangements of atoms. In the case of allylic bromination, the formation of stereoisomers occurs due to the possibility of forming different configurations at the chiral centers created during the reaction, necessitating the consideration of all possible stereoisomers in the final product.
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