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.