We demonstrate in this work by theory and experiment that benzaldehyde hydrotrioxide (PhC(O)OOOH), the intermediate most likely formed in the low-temperature ozonation of benzaldehyde, is too unstable to be detected by NMR (1H, 13C, and 17O) spectroscopy in various organic solvents at temperatures ≥ −80 °C and that its previous detection must have been erroneous. Several plausible mechanisms for the formation of this polyoxide were explored by using density functional theory. We found that the formation of the hydrotrioxide involves the facile 1,3-dipolar insertion of ozone into the C−H bond (ΔH = 11.1 kcal/mol) in a strongly exothermic process (ΔHR = −57.0 kcal/mol). The hydrotrioxide then quickly decomposes in a second concerted, exothermic reaction involving an intramolecular H transfer to form benzoic acid and singlet oxygen (O2(1Δg)) (ΔH = 5.6 kcal/mol), ΔHR = −14.0 kcal/mol). The equilibrium is thus expected to be shifted toward the products; therefore, this intermediate cannot be observed experimentally. Peroxybenzoic acid, still another major reaction product formed in the ozonation reaction, is formed as a result of the surprising instability of the RC(O)O−OOH bond (ΔHR = 23.5 kcal/mol), generating HOO• and benzoyloxyl radicals. Both of these radicals can then initiate the chain autoxidation reaction sequence-the abstraction of a H atom from benzaldehyde to form either a benzoyl radical and HOOH or a benzoyl radical and benzoic acid. Because only very small amounts of HOOH were detected in the decomposition mixtures, the recombination of the benzoyl radical with the HOO• radical (ΔHR = −80.7 kcal/mol) appears to be the major source of peroxybenzoic acid. A theoretical investigation of the mechanistic possibility of the involvement of still another intermediate, a cyclic tetraoxide (tetraoxolane) formed as a primary product in the 1,3-dipolar cycloaddition of ozone to the carbonyl group of the aldehyde, revealed that the tetraoxide is a 'real' molecular entity with the five-membered ring adopting an envelope conformation. The tetraoxide is destabilized by 7.0 kcal/mol relative to the reactant complex, and the transition state for its formation is 17.4 kcal/mol above the reactant complex, which, although accessible under the reaction conditions, is not expected to be competitive with the reaction generating the hydrotrioxide.
- primary reaction intermediates
Cerkovnik, J., Plesnicar, B., Koller, J., & Tuttle, T. (2009). Hydrotrioxides rather than cyclic tetraoxides (tetraoxolanes) as the primary reaction intermediates in the low-temperature ozonation of aldehydes. the case of benzaldehyde. Journal of Organic Chemistry, 74(1), 96-101. https://doi.org/10.1021/jo801594n