Buta-1,3-diene reacts with 1 mol of HBr at 40 ${}^{\circ }$C under thermodynamic control. The major product is

Protonation at C-1 gives the allylic cation $\mathrm{CH}{}_3-\mathrm{CH}{}^{+}-\mathrm{CH}=\mathrm{CH}{}_2$ with charge shared by C-2 and C-4. At the higher temperature the more stable product, which has the more substituted (disubstituted) double bond, predominates. This is the 1,4-adduct $\mathrm{CH}{}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_2\mathrm{Br}$, 1-bromobut-2-ene; the 1,2-adduct 3-bromobut-1-ene is the kinetic product.