Free radical addition reactions of conjugated dienes

Free radical addition reactions Definition

• Free radical addition reactions provide alternative pathways to product formation in conjugated dienes, differing from electrophilic addition.
• These reactions proceed via a radical mechanism and can lead to unique regioselective products.

The steps involved in this process are outlined in detail below:

1. Initiation Step

   1. The reaction begins with the initiation step, where a peroxide initiator (ROOR) is used to generate radical species.
   2. This occurs through homolytic cleavage of the peroxide bond, producing two alkoxy radicals (RO·).
   3. ROOR → 2RO⋅

2. Propagation Steps

   • The propagation phase consists of two key steps, involving the interaction of the generated radicals with the reactants to form the product progressively.

   • Propagation Step 1

     • The first propagation step involves the reaction of an alkoxy radical (RO·) with hydrogen bromide (HBr), leading to the abstraction of a hydrogen atom and the formation of a bromine radical (Br·).
     • RO⋅+HBr → ROH + Br⋅ 

   • Propagation Step 2

     • Next, the bromine radical reacts with 1,3-butadiene, specifically adding to one of the terminal carbons.
     • This action generates a new carbon-centered radical intermediate at the other terminal carbon.
     • Br⋅ + H2C=CH-CH=CH2 → H2C-CH=CH-CH2⋅-Br

3. Termination Step

   • The reaction concludes with the termination step, where the carbon-centered radical reacts with another molecule of hydrogen bromide (HBr).
   • This step produces the final product, 3-bromo-1-butene, and a bromine radical, potentially initiating another cycle of the propagation steps.
   • H2C-CH=CH-CH2⋅-Br + HBr → H2C-CH=CH-CH2CH2Br

Outcome

• The product of this free radical addition reaction, 3-bromo-1-butene, demonstrates how the nature of the initiating radical species and the mechanism of reaction can significantly influence the structure of the final product.
• It is noteworthy that the major product of this reaction differs from that typically obtained through electrophilic addition to 1,3-butadiene, which often results in 1,4-addition products due to the stability of the resulting conjugate addition products.

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