Besides hydrogen halides, halogens are another class of electrophilic reagents that react with conjugated dienes to form 1,2- and 1,4-addition products.

Here, the numbers designate the mode of addition of the electrophile.

When 1,3-butadiene is treated with bromine, one of the π bonds attacks the electrophilic center on the polarized halogen, displacing a bromide ion and forming a bromonium ion intermediate.

Next, the displaced bromide ion attacks the intermediate at one of the carbons, causing the ring to open.

Between C1 and C2, C2 is the secondary carbon that can stabilize the positive charge better. So, it is the preferred site for a nucleophilic attack by the bromide ion, giving the 1,2 -adduct.

Alternatively, an attack at C4 moves the double bond to the adjacent position, displacing the bromonium ion at C2 and forming the 1,4 -adduct.

Lastly, the 1,4-dihalide is dominant at higher temperatures, whereas the 1,2-dihalide is favored at lower temperatures.