Predict the major product(s) of the following allylic bromination reactions.
(b)

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Identify the allylic position: In the given structure, locate the allylic position, which is the carbon atom adjacent to the double bond. This is the position where bromination will occur.

Understand the role of NBS: N-bromosuccinimide (NBS) is used for allylic bromination. It selectively brominates the allylic position due to the formation of a radical intermediate.

Generate the allylic radical: Under heat, the allylic hydrogen is abstracted, forming an allylic radical. This radical is stabilized by resonance, allowing the radical to delocalize over the allylic system.

Draw resonance structures: Illustrate the resonance structures of the allylic radical. This helps in understanding the possible sites for bromination, as the radical can be stabilized at multiple positions.

Predict the major product: Based on the resonance structures, determine the most stable radical position for bromination. The major product will be formed by the addition of bromine at this position, resulting in the allylic bromide.

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Allylic Bromination

Allylic bromination is a reaction where a bromine atom is introduced at the allylic position of an alkene. This position is adjacent to the double bond, allowing for resonance stabilization of the radical intermediate formed during the reaction. N-bromosuccinimide (NBS) is commonly used as the bromine source in the presence of heat or light to facilitate the formation of the allylic radical.

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Mechanism of Allylic Bromination.

Resonance Stabilization

Resonance stabilization involves the delocalization of electrons across adjacent atoms, which stabilizes reactive intermediates like radicals. In allylic bromination, the allylic radical formed can be stabilized by resonance, where the unpaired electron is shared between the allylic carbon and the adjacent double bond, leading to multiple resonance structures and increased stability.

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The radical stability trend.

Regioselectivity in Radical Reactions

Regioselectivity refers to the preference for a chemical reaction to occur at one location over others in a molecule. In allylic bromination, the reaction tends to occur at the most stable allylic position, often influenced by the stability of the resulting radical. The major product is typically formed at the site where the radical is most stabilized by resonance and hyperconjugation.

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