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.