The electrophilic addition of hydrogen bromide HBr to 1,3-butadiene involves a two-step reaction mechanism with the formation of an intermediate carbocation. Here's the step-by-step mechanism:Step 1: Electrophilic attack on the double bondThe nucleophilic pi electrons of the double bond in 1,3-butadiene attack the electrophilic hydrogen atom of HBr. This results in the formation of a new C-H bond and the breaking of the H-Br bond. The bromide ion Br- is released as a leaving group, and a carbocation intermediate is formed at the allylic position carbon 2 .Step 2: Nucleophilic attack on the carbocationThe bromide ion Br- , which was released in the first step, acts as a nucleophile and attacks the carbocation at the allylic position carbon 2 . This leads to the formation of a new C-Br bond and the final product, 3-bromo-1-butene.It's important to note that there is another possible intermediate in this reaction, which is the 1,2-addition product. However, due to the resonance stabilization of the allylic carbocation formed in the 1,4-addition pathway, the 1,4-addition product 3-bromo-1-butene is the major product in this reaction. This phenomenon is known as the regioselectivity of 1,3-butadiene and is an example of the kinetic versus thermodynamic control of reactions.