The addition of hydrogen halide to conjugated dienes is the classic example involving the interplay of kinetics and thermodynamics in determining the nature and extent of reaction yield. Addition to conjugated dienes makes frequent appearance on the MCAT.
When hydrogen halide is reacted with a conjugated diene at low temperatures, direct addition (1,2 addition) is favored, but at room temperature and above, conjugate addition (1,4 addition) is favored. This occurs because the 1,2 product is kinetically favored but the 1,4 product is thermodynamically favored.
Why is one pathway kinetically favored and the other thermodynamically favored? The activation energy is greater for the conjugate addition pathway (1,4 addition), so it occurs more slowly than 1,2 addition. In other words, 1,2 addition is kinetically favored. However, the product of 1,4 addition contains an internal double bond, which is lower energy and the 1,2 adduct. Therefore, 1,4 addition is thermodynamically favored.
When the addition of hydrogen halide to conjugated diene is carried out under conditions such that the products may equilibrate (higher temperature), the product composition no longer reflects the relative rates of formation but tends to reflect relative stabilities. The 1,4 adduct is the major product. Under these conditions, if the 1,2 adduct does form, it is only temporary. The reagents reform by equilibrium and proceed to the 1,4 adduct.