Effect of Medium Acidity on the Rate of Oxidative Functionalization of Hydrocarbons in Sulfuric Acid Solutions

Abstract

The effect of medium acidity on the rate constant (k) of the reactions of alkanes (AlkH), cycloalkanes (c-AlkH), and arenes (ArH) in sulfuric acid solutions with peroxy compounds (H2O2 or (NH4)2S2O8), complexes (Pd(II), Pt(III), Hg(II)), acids (H2CrO4, HNO3, H2SO4, HVO3), 1-adamantanol (AdOH), and formaldehyde (CH2O) was analyzed using the equation log k = C0 – m0H0, where Н0 is the Hammett acidity function, C0 is the intercept on the ordinate, and m0 is the slope of the line in the coordinates log k – H0. All the reactions can be divided into three groups depending on the slope value (1 ~ m0 ≤ 3.4) in accordance with the degree of protonation of the main species: single protonation (H2O2 or (NH4)2S2O8, Pd(II), Pt(III)) (group 1); protonation followed by dehydration (AdOH, H2CrO4, HNO3, H2SO4, as well as HVO3, and CH2O in reactions with ArH in 57–65% and 60–80% H2SO4, respectively) (group 2); and more profound double or triple protonation (Hg(II), HVO3, and CH2O in reactions with AlkH and c-AlkH in 84–90% and 88–93% H2SO4, respectively) (group 3). For the active oxidant species in the HVO3–H2SO4 system and in H2SO4 itself, the thermochemical characteristics of their reactions with 2,3-dimethylbutane were evaluated using the DFT B3LYP/6-311G(d,p) quantum-chemical method, and the possibilities of alternative reaction mechanisms were analyzed.