Sulphuric acid-catalysed formation of hemiacetal from glyoxal and ethanol

We examine the reaction of ethanol with glyoxal to form hemiacetal by means of the high-level G4(MP2) procedure. In this reaction, an intermolecular proton transfer is coupled with the formation of a covalent C-O bond between the two molecules. We find a novel catalytic reaction mechanism in which an H₂SO₄ catalyst reduces the barrier height from ΔH^‡₂₉₈ = 140.2 to 16.3 kJ mol^-1. It is well established that H₂SO₄ can effectively catalyse intramolecular proton transfers. This letter shows that H₂SO₄ can catalyse an intermolecular proton transfer that is coupled with a covalent bond formation.