The largely reversible, light-induced tautomerization of 2-nitrotoluene (1) to the quinonoid aci-nitro tautomer aci-1 was studied by flash photolysis as a benchmark for comparison with the widely used nitrobenzyl phototriggers (`caged compounds'). The pH-rate profile for the decay of aci-1 in aqueous solution exhibits downward curvature at pH 3 – 4, which is attributed to pre-equilibrium ionization of the nitronic acid aci-1 to its anion 1− (pKa=3.57). Two regions of upward curvature, at pH ca. 6 and <0 (H0≈−1), each indicate a change in the reaction mechanism. The elementary reactions that dominate between the curved regions are assigned on the basis of kinetic isotope effects and the observation of general acid catalysis: Hydronium ions regenerate 2-nitrotoluene by C-protonation of 1− in the pH range of 0 – 6, and H2O is the proton source at pH>6. A hird, irreversible Nef-type isomerization of aci-1 prevails in highly acidic solutions (pH<0). The equilibrium constant for the thermal tautomerization of 1 to aci-1 is estimated as pKT=17.0±0.2 based on kinetic data.
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Schwörer et al. (2001) studied this question.