16.8:
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
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.
16.8:
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.
At lower temperatures, the two products are not in equilibrium. Under these conditions, the product distribution depends on the relative rates at which they are formed, and the reaction is said to be kinetically controlled. Since the 1,2-adduct is formed faster, it is the favored product.
As the reaction mixture is warmed up, the products coexist in equilibrium. In this scenario, the product distribution depends on their relative stabilities, and the reaction is said to be under thermodynamic control. Since the 1,4-adduct is thermodynamically more stable, it predominates at higher temperatures.