Recall that the electrophilic addition of a hydrogen halide like HBr to an alkene follows Markovnikov's rule, forming an alkyl bromide.

In comparison, a conjugated diene such as 1,3-butadiene has two double bonds, both equally suited to react with HBr, resulting in two products.

The products are often called 1,2- and 1,4-adducts, where the numbers indicate the mode of addition of the electrophilic reagent.

The first step of the mechanism involves the addition of the proton to one of the terminal carbons giving an allylic carbocation intermediate, which is resonance stabilized. So, it will be favored over the relatively unstable primary carbocation.

In the second step, the bromide ion can attack the allylic intermediate at either carbon bearing the positive charge.

Nucleophilic attack at the second carbon results in the 1,2-addition product, whereas attack at the fourth carbon yields the 1,4-addition product.

The product distribution varies with temperature, with the 1,4-adduct favored at higher temperatures and the 1,2-product at lower temperatures.

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.

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