Textbook Question
In the following molecules, identify the carbon where the radical is most likely to form in the first propagation step.
(c)
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11. Radical Reactions
Allylic Bromination
5:41 minutesProblem 32a
Textbook Question
Predict the products of the following allylic halogenation reactions.
(a) 
Verified step by step guidance1
Identify the allylic position in the given molecule. The allylic position is the carbon atom adjacent to a double bond.
Recognize that allylic halogenation typically involves the substitution of a hydrogen atom at the allylic position with a halogen atom, often using reagents like N-bromosuccinimide (NBS) in the presence of light or heat.
Consider the resonance stabilization of the allylic radical formed during the reaction. The allylic radical can delocalize its unpaired electron over the π system, leading to multiple resonance structures.
Determine the most stable resonance structure of the allylic radical, as this will guide the formation of the major product. The stability is often influenced by the degree of substitution at the allylic position.
Predict the final product by replacing the hydrogen atom at the allylic position with the halogen atom, taking into account the most stable resonance structure and the regioselectivity of the reaction.
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Video duration:
5mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Allylic Halogenation
Allylic halogenation is a reaction where a halogen atom is introduced at the allylic position of an alkene. This position is adjacent to a carbon-carbon double bond. The reaction typically involves the use of N-bromosuccinimide (NBS) in the presence of light or heat, which generates a radical that abstracts a hydrogen atom from the allylic position, allowing the halogen to attach there.
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The general mechanism of Allylic Halogenation.
Radical Mechanism
The radical mechanism is a chain reaction process involving initiation, propagation, and termination steps. In allylic halogenation, the initiation step generates radicals, often through the homolytic cleavage of a halogen molecule. Propagation involves the radical reacting with the substrate to form a new radical, which continues the chain reaction. Understanding this mechanism is crucial for predicting the products formed during the reaction.
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03:28The mechanism of Radical Polymerization.
Resonance Stabilization
Resonance stabilization is a concept where the stability of a molecule is increased due to the delocalization of electrons across multiple atoms. In allylic systems, the allylic radical formed during halogenation is stabilized by resonance, as the unpaired electron can be delocalized over the π system. This stabilization influences the regioselectivity of the reaction, often leading to multiple possible products depending on the structure of the starting material.
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03:43The radical stability trend.
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