Free radical addition reactions of conjugated dienes

• 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.

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⋅

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.