Hello! I would appreciate some help with these questions, thank you so much!! Ring expansion and carbocation rearrangement .png
step by step.
Protonation of the alkene (electrophilic addition)
• In H₂SO₄/MeOH, the alkene π bond is the nucleophile and grabs H⁺.
• Proton adds in the Markovnikov direction to give the more stable carbocation (a cyclobutyl-carbinyl cation).
Fast ring expansion (carbocation rearrangement)
• A C–C bond from the adjacent cyclobutane migrates (1,2-shift) to the positively charged carbon.
• This converts the strained 4-membered ring into a 5-membered ring and places the positive charge on the new cyclopentyl carbon.
• Driving force: relief of cyclobutane ring strain and formation of a more stabilized carbocation (the classic cyclobutyl-carbinyl ⇌ cyclopentyl rearrangement).
Nucleophilic attack by methanol
• MeOH (the solvent, present in large excess and far more nucleophilic here than HSO₄⁻) attacks the planar carbocation to form a C–O bond.
• The oxygen bears a positive charge in this "oxonium" intermediate.
Deprotonation (regeneration of the acid catalyst)
• Another MeOH molecule acts as a base and removes the extra proton from oxygen.
• This neutralizes the oxonium, giving the ether product and regenerating H⁺ (acid-catalyzed addition overall).
Mechanism of carbocation rearragement .jpg
Net reaction: "hydro-alkoxylation" of the alkene by MeOH under acid catalysis to give a methyl dicyclopentyl ether.
• The key feature is the carbocation rearrangement (ring expansion) that occurs before nucleophilic capture; without it you'd retain a 4-membered ring, but the system preferentially expands to a 5-membered ring because it is both less strained and better at stabilizing the cation.
• Stereochemistry: attack occurs on a planar carbocation, so if a stereocenter were formed it would come from either face; here the final product (a monosubstituted cyclopentane) is not stereogenic.