Textbook Question
What is the major product of the following reactions? Disregard stereoisomers:
a.
11. Radical Reactions
Allylic Bromination
5:12 minutesProblem 50
Textbook Question
Give a mechanism to explain the two products formed in the following reaction.
Verified step by step guidance1
Step 1: Recognize the reaction conditions. The reaction involves 3-methylbut-1-ene treated with NBS (N-bromosuccinimide) under light (hv). This indicates a radical bromination reaction, specifically at the allylic position due to the stability of allylic radicals.
Step 2: Initiation step. The light (hv) causes homolytic cleavage of the N-Br bond in NBS, generating a bromine radical (Br•). This radical is highly reactive and initiates the reaction.
Step 3: Formation of the allylic radical. The bromine radical abstracts a hydrogen atom from the allylic position of 3-methylbut-1-ene, forming an allylic radical. The allylic radical is stabilized by resonance, creating two possible resonance structures.
Step 4: Reaction of the allylic radical with bromine. Each resonance structure of the allylic radical reacts with another bromine molecule (Br2, generated from NBS) to form two different products. One product corresponds to the non-rearranged structure, while the other corresponds to the rearranged structure due to the resonance stabilization.
Step 5: Explain the rearranged product. The rearranged product arises because the allylic radical can shift its position via resonance, allowing bromine to attach at a different carbon atom. This leads to the formation of the rearranged brominated product.
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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, typically using a brominating agent like N-bromosuccinimide (NBS) under light (hv) conditions. This process involves the formation of a radical intermediate, allowing for the substitution of hydrogen atoms at the allylic position with bromine, leading to the formation of different products based on the stability of the intermediates formed.
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Mechanism of Allylic Bromination.
Radical Mechanism
The radical mechanism involves the generation of free radicals, which are highly reactive species with unpaired electrons. In the context of allylic bromination, the reaction proceeds through three main steps: initiation (formation of radicals), propagation (reaction of radicals with the substrate), and termination (combination of radicals). Understanding this mechanism is crucial for predicting the products formed, including both rearranged and non-rearranged structures.
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Rearrangement in Organic Reactions
Rearrangement in organic chemistry refers to the structural reorganization of atoms within a molecule during a reaction. In the case of allylic bromination, the formation of a more stable radical can lead to the rearrangement of the product. This concept is essential for understanding why two different products are formed: one that retains the original structure and another that has undergone rearrangement to achieve greater stability.
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