See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Electrophilic Addition Mechanism. When HBr reacts with an alkene in the absence of peroxides, the reaction proceeds via an electrophilic addition mechanism (Markovnikov addition). Step 1 - Identify the mechanism: In the absence of peroxides, HBr adds to alkenes via ionic (electrophilic addition) mechanism, NOT radical addition. Step 2 - Identify the rate-determining step: The mechanism proceeds in two steps: (i) The pi electrons of the alkene attack the electrophilic H of HBr. The proton (H+) acts as the electrophile and adds to the alkene in a slow, rate-determining step, forming a carbocation intermediate. (ii) The bromide ion (Br-), a nucleophile, rapidly attacks the carbocation in a fast step. Step 3 - Characterize the first step: The first step is the addition of an electrophile (H+) to the alkene. This step is slow because it involves breaking the relatively strong H-Br bond and forming a high-energy carbocation intermediate, making it the rate-determining (slow) step. Why other options fail: - (a) 'fast addition of an electrophilic' - incorrect because the electrophilic addition step is slow (rate-determining), not fast. - (c) 'fast addition of a nucleophilic' - incorrect because the first step involves an electrophile (H+), not a nucleophile, and it is slow. - (d) 'slow addition of a nucleophile' - incorrect because the first step involves an electrophile (H+), not a nucleophile (Br- adds in the second, fast step). Therefore, the correct answer is B.