The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule.

With conjugated systems like 1,3-butadiene, the addition of one equivalent of HBr yields a mixture of products: 1,2 and 1,4-addition products. As shown below, the mechanism involves the addition of H⁺ across one of the double bonds of the conjugated diene to form a resonance stabilized allyl cation. This is followed by the nucleophilic attack of Br⁻ at either carbon of the allyl cation bearing a positive charge to form the 1,2 or the 1,4-addition product.

The ratio of the products formed depends on the reaction temperature. Low temperature favors the 1,2-adduct, whereas the 1,4-adduct is preferentially formed at higher temperatures.