The presence of a double bond in an alkene plays a crucial role in the electrophilic addition reaction with hydrogen bromide HBr . The double bond consists of a sigma bond and a pi bond, with the pi bond being weaker and more exposed to electrophiles. This makes alkenes more reactive than alkanes, which only have single bonds.In the electrophilic addition reaction of an alkene with HBr, the alkene's double bond acts as a nucleophile, attacking the electrophilic hydrogen atom in HBr. The mechanism behind this reaction can be explained in the following steps:1. Polarization of HBr: The bond between hydrogen and bromine is polar, with bromine being more electronegative than hydrogen. This causes a partial positive charge on the hydrogen atom + and a partial negative charge on the bromine atom - .2. Nucleophilic attack: The electron-rich double bond of the alkene attacks the electrophilic hydrogen atom + of HBr, forming a new sigma bond between the alkene carbon and the hydrogen atom. Simultaneously, the electrons in the H-Br bond move to the bromine atom, resulting in the formation of a bromide ion Br- .3. Carbocation formation: The breaking of the double bond in the alkene and the formation of a new bond with the hydrogen atom lead to the formation of a carbocation intermediate. The carbocation is a positively charged carbon atom that is electron-deficient and highly reactive.4. Nucleophilic attack by bromide ion: The negatively charged bromide ion Br- acts as a nucleophile and attacks the carbocation, forming a new sigma bond between the carbon and the bromine atom.5. Product formation: The electrophilic addition of HBr to the alkene is now complete, resulting in the formation of a new alkyl halide with a single bromine atom added to the molecule.In summary, the presence of a double bond in an alkene allows for the electrophilic addition reaction with hydrogen bromide. The mechanism involves the polarization of HBr, nucleophilic attack by the alkene, carbocation formation, nucleophilic attack by the bromide ion, and product formation.