Step 1: Understanding the Concept:
Electrophilic addition of H-Br to alkenes proceeds via a carbocation intermediate. Carbocations can undergo rearrangement (like hydride or methyl shifts) to reach a more stable state before the final nucleophilic attack.
Step 2: Detailed Explanation:
The starting material is 3-methyl-1-butene: \( CH_3-CH(CH_3)-CH=CH_2 \).
Step 1: Protonation. \(H^+\) adds to the terminal \(CH_2\) (Markovnikov's rule) to form a secondary (\(2^\circ\)) carbocation: \( CH_3-CH(CH_3)-\overset{+}{C}H-CH_3 \).
Step 2: Rearrangement. This \(2^\circ\) carbocation is adjacent to a tertiary carbon. A 1,2-hydride shift occurs to form a more stable tertiary (\(3^\circ\)) carbocation: \( CH_3-\overset{+}{C}(CH_3)-CH_2-CH_3 \).
Step 3: Nucleophilic Attack. \(Br^-\) attacks the \(3^\circ\) carbocation to form the final product: 2-bromo-2-methylbutane.
The question asks for the "first step" (which produces the stable \(3^\circ\) cation after rearrangement) and the "final product".
Step 3: Final Answer:
The final product is 2-bromo-2-methylbutane, formed via a tertiary carbocation intermediate.