Photoinduced Oligomerization of Aqueous Pyruvic Acid

Abstract: The 320 nm-band photodecarboxylation of aqueous pyruvic acid (PA), a representative of the α-oxocarboxylic acids widely found in the atmospheric aerosol, yields 2,3-dimethyltartaric (A) and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic (B) acids, rather than 3-hydroxy-2-oxobutanone as previously reported. A and B are identified by liquid chromatography with UV and ESI-MS detection, complemented by collisionally induced dissociation and ²H and ¹³C isotope labeling experiments. The multifunctional ether B gives rise to characteristic δ ∼ 80 ppm ¹³C NMR resonances. Product quantum yields are proportional to [PA](a + [PA])^-1 in the range [PA] = 5−100 mM. CO₂(g) release rates are halved, while A and B are suppressed by the addition of >1.5 mM TEMPO. A and B are only partially quenched in air-saturated solutions. These observations are shown to be consistent with an oligomerization process initiated by a bimolecular reaction between ³PA* and PA producing ketyl, CH₃Ċ(OH)C(O)OH, and acetyl, CH₃C(O)·, radicals, rather than by the unimolecular decomposition of ³PA* into 1-hydroxyethylidene, ³HO(CH₃)C:  (+CO₂), or [CH₃C(O)· + ·C(O)OH] pairs. A arises from the dimerization of ketyl radicals, while B ensues the facile decarboxylation of the C₈β-ketoacid formed by association of acetyl radicals with the ketyl radical adduct of PA. Since the radical precursors to A and B are scavenged by O₂ with a low probability per encounter (k_sc ∼ 1 × 10⁶ M^-1 s^-1), PA is able to accrete into multifunctional polar species in aerated aqueous media under solar illumination. 

M. I. Guzman, A. J. Colussi, and M. R. Hoffmann. Photoinduced Oligomerization of Aqueous Pyruvic Acid. J. Phys. Chem. A 2006, 110, 10, 3619–3626, https://doi.org/10.1021/jp056097z