The electrophilic addition reaction of HBr with an alkene is a two-step process that involves the formation of a carbocation intermediate. The mechanism proceeds as follows:1. Protonation of the alkene: The alkene's double bond acts as a nucleophile, attacking the electrophilic hydrogen atom of the HBr molecule. This results in the formation of a carbocation intermediate and a bromide ion Br- .2. Nucleophilic attack by the bromide ion: The negatively charged bromide ion acts as a nucleophile and attacks the carbocation, forming a new C-Br bond. This results in the formation of the alkyl bromide product.The reactivity of the alkene affects both the rate and regioselectivity of the reaction. Alkenes with more substituted double bonds i.e., more alkyl groups attached to the carbon atoms involved in the double bond are generally more reactive in electrophilic addition reactions. This is because the alkyl groups can donate electron density to the double bond, making it more nucleophilic and thus more reactive towards electrophiles like HBr.The regioselectivity of the reaction is governed by Markovnikov's rule, which states that the electrophile in this case, the hydrogen atom of HBr will add to the less substituted carbon atom of the double bond, while the nucleophile the bromide ion will add to the more substituted carbon atom. This results in the formation of the more stable carbocation intermediate, as more substituted carbocations are generally more stable due to hyperconjugation and inductive effects.For example, consider the electrophilic addition of HBr to 2-butene CH3CH=CHCH3 . According to Markovnikov's rule, the hydrogen atom will add to the less substituted carbon atom the one with only one alkyl group attached , while the bromide ion will add to the more substituted carbon atom the one with two alkyl groups attached . This results in the formation of 2-bromobutane CH3CHBrCH2CH3 as the major product.In contrast, if the reaction were to occur with 1-butene CH2=CHCH2CH3 , the hydrogen atom would still add to the less substituted carbon atom the one with no alkyl groups attached , while the bromide ion would add to the more substituted carbon atom the one with one alkyl group attached . This would result in the formation of 1-bromobutane CH3CH2CHBrCH3 as the major product.In summary, the reactivity of the alkene in electrophilic addition reactions with HBr is influenced by the degree of substitution of the double bond, with more substituted alkenes being more reactive. The regioselectivity of the reaction is governed by Markovnikov's rule, which predicts the formation of the more stable carbocation intermediate and thus the major product of the reaction.