Carbocation Stability Hierarchy

Steps:

• Protonate 2-methylbut-2-ene (CH₃-C(CH₃)=CH-CH₃) with H⁺ from HBr, adding to either C2 or C3.
• Addition to C3 forms a tertiary carbocation at C2 (attached to three carbons: two CH₃ and one CH₂CH₃).
• Addition to C2 forms a secondary carbocation at C3 (attached to two carbons: C2 and CH₃).
• Compare stabilities: tertiary carbocation is more stable than secondary due to greater hyperconjugation and inductive stabilization from alkyl groups.

Why A is correct:

• A depicts the tertiary carbocation, which follows Markovnikov's rule favoring the more substituted, stable intermediate.

Why the others are wrong:

• B shows the secondary carbocation, less stable with fewer alkyl groups for stabilization.
• C represents an incorrect primary carbocation structure, highly unstable and not formed here.
• D illustrates a rearranged or erroneous intermediate, not matching the reaction pathway.

Final answer: A