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Free radical addition reactions of conjugated dienes

simpal95
2 min read

  • Free radical addition reactions of conjugated dienes, such as 1,3-butadiene, can offer alternative pathways to different products compared to electrophilic addition reactions.

  • These reactions proceed through the generation and reaction of radical species.

  • An illustrative example of this type of reaction is the addition of hydrogen bromide (HBr) to 1,3-butadiene in the presence of a peroxide initiator, which leads to the formation of 3-bromo-1-butene.

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

1. Initiation Step

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

1. 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⋅RO⋅ + HBr → ROH + Br⋅

 2. 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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