The addition reaction between HBr hydrogen bromide and propene an alkene is an example of an electrophilic addition reaction. The mechanism involves two main steps and follows Markovnikov's rule, which explains its regioselectivity.Here's the step-by-step mechanism:1. Electrophilic attack on the double bond:The alkene propene has a double bond, which consists of a sigma bond and a pi bond. The pi bond has a high electron density, making it nucleophilic. HBr, on the other hand, is a polar molecule with the hydrogen atom being partially positive + and the bromine atom being partially negative - . The electron-rich double bond of propene attacks the partially positive hydrogen atom of HBr, forming a new C-H bond. This results in the breaking of the H-Br bond, and the bromide ion Br- is released as a leaving group.2. Nucleophilic attack by the bromide ion:In this step, a carbocation intermediate is formed. According to Markovnikov's rule, the hydrogen atom will add to the carbon with the least number of hydrogen atoms in the alkene, resulting in the more stable carbocation. In the case of propene, the hydrogen atom will add to the terminal carbon, forming a secondary carbocation at the central carbon. The bromide ion Br- , which is a good nucleophile, will then attack the carbocation, forming a new C-Br bond. This results in the formation of 2-bromopropane as the major product.The regioselectivity of this reaction is due to the stability of the carbocation intermediate. A more stable carbocation will form preferentially, and in the case of propene, the secondary carbocation is more stable than the primary carbocation. This is in accordance with Markovnikov's rule, which states that in the addition of a polar reagent HX to an alkene, the hydrogen atom will add to the carbon with the greater number of hydrogen atoms, and the halogen X will add to the carbon with the fewer hydrogen atoms. This rule helps predict the major product in such addition reactions.