When 1,3-butadiene undergoes electrophilic addition reaction with hydrogen chloride HCl , the major product formed is 3-chloro-1-butene. This reaction follows the regioselectivity rule, which is explained by the resonance stabilization of the allylic carbocation intermediate. The HCl adds to the double bond, with the hydrogen attaching to the less substituted carbon C1 and the chlorine attaching to the more substituted carbon C2 , resulting in the formation of 3-chloro-1-butene.