Another Unprecedented Wieland Mechanism Confirmed: Hydrogen Formation from Hydrogen Peroxide, Formaldehyde, and...

In 1923, Wieland and Wingler reported that in the molecular hydrogen producing reaction of hydrogen peroxide with formaldehyde in basic solution, free hydrogen atoms (H^. ) are not involved. They postulated that bis(hydroxymethyl)peroxide, HOCH₂ OOCH₂ OH, is the intermediate, which decomposes to yield H₂ and formate, proposing a mechanism that would nowadays be considered as a "concerted process". Since then, several other (conflicting) "mechanisms" have been suggested. Our NMR and Raman spectroscopic and kinetic studies, particularly the determination of the deuterium kinetic isotope effect (DKIE), now confirm that in this base-dependent reaction, both H atoms of H₂ derive from the CH₂ hydrogen atoms of formaldehyde, and not from the OH groups of HOCH₂ OOCH₂ OH or from water. Quantum-chemical CBS-QB3 and W1BD computations show that H₂ release proceeds through a concerted process, which is strongly accelerated by double deprotonation of HOCH₂ OOCH₂ OH, thereby ruling out a free radical pathway.