1,4-Addition is an electrophilic addition reaction of conjugate dienes.
eg: Two electrophilic addition reactions could occur between 1,3-butadiene (1) and hydrogen chloride.
In Reaction 1, the net reaction is addition of a hydrogen atom to C-1 and a chlorine atom to C-4 in 1. Hence, Reaction 1 is called 1,4-addition and its product (2) 1,4-adduct.
In Reaction 2, the net reaction is addition of a hydrogen atom to C-1 and a chlorine atom to C-2 in 1. Hence, Reaction 2 is called 1,2-addition and its product (3) 1,2-adduct.
The regioselectivity of the overall reaction depends on the temperature. At low temperature (eg: –78 °C), the major product is 3; at high temperature (Δ), it is 2. The carbon-carbon double bond in 2 is more highly substituted than the one in 3, so 2 is more stable than 3. That the less stable 3 is the major product at low temperature implies that at low temperature the system is under kinetic control and 3 is the faster-forming product. That the more stable 2 is the major product at high temperature means the system is under thermodynamic control.