Acid-Catalyzed Ring Opening of Furan in Aqueous Solution

The mechanism and energetics of furan decomposition via the opening of the five-membered ring structure in dilute acid solution were investigated using density functional Car–Parrinello molecular dynamics combined with metadynamics simulations. Diffusion of an acidic proton from the aqueous medium leading to the formation of a protonated furan is found to be the rate-limiting step of the ring-opening process, with protonation at the Cα position being 7 kcal mol–1 less activated than that at the Cβ position. Protonation at the Cα position results in the formation of 2,5-dihydro-2-furanol or 2,3-dihydro-3-furanol via nucleophilic attack by a solvent molecule. Subsequent protonation of the furanols at the ring oxygen initiates the opening of the furanic ring, leading to the formation of 4-hydroxy-2-butenal.