The butadiene hydrocyanation can, as in the process developed by Du Pont in 1951, take place indirectly by chlorination of butadienc. Initially, the chlorination was carried out in the liquid phase; today it is usually conducted in the gas phase at 200-300°C without catalyst. A mixture of 3,4-dichloro- 1 -butene and cis- and trans- I ,4-dichloro-2-butene is obtained with a selectivity of about 96%: