Kinetic versus Thermodynamic Control in the Addition of HBr to 1,3-Butadiene

Kinetic versus Thermodynamic Control in the Addition of HBr to 1,3-Butadiene 

One of the interesting peculiarities of the reaction of 1,3-butadiene with HBr is the effect of temperature on the products. If the reagents are allowed to react briefly at —80°C, the 1,2-addition product predominates. If this reaction mixture is later allowed to warm to 40° C, however, or if the original reaction is carried out at 40° C, the composition favOTs the 1,4-addition product. 

This variation in product composition reminds us that the most stable product is not always the major product. Of the two products, we expect I-bromo-2-butene (the 1,4-product) to be more stable, since it has the more substituted double bond. This prediction is supported by the fact that this isomer predominates when the reaction mixture is warmed to 40°C and allowed to equilibrate. 

Reaction-energy diagram for the second step of the addition of 1,3 butadiene to HBr. The allylic carbocation (center) can react at either of its electrophilic carbon atoms. The transition state (4) leading to 

2- addition has a lower energy than that leading to the 1,4 product, so the 

---