Recall that the addition of HBr to 1,3-butadiene gives 1,2- and 1,4-addition products.
The product ratio varies with temperature, with the 1,2-adduct dominating at low temperatures and the 1,4-adduct dominating at high temperatures. But why?
Let's examine the energy diagram.
The first step of the mechanism involves protonation of the diene to form a resonance-stabilized allylic cation and a bromide anion.
Next, the carbocation can pass through two different transition states to form the two products.
The transition state for the 1,2-adduct bears a positive charge on a secondary carbon, rendering it more stable and lower in energy.
Consequently, the 1,2-adduct is formed faster and is called the kinetic product. In comparison, the 1,4-adduct is more stable and is known as the thermodynamic product.
At low temperatures, the reaction is irreversible, and the product distribution depends on the relative rates. So, the 1,2-adduct dominates.
At elevated temperatures, the reaction becomes reversible. There is sufficient energy for the 1,2-adduct to ionize back to the carbocation and form the more stable 1,4-adduct as the major product.
