Kinetic and Thermodynamic Study of Proton Transfer Reactions of 1‐Hydroxy‐2,2‐Dinitroethane: Establishing a Predictive Relationship between Tautomeric and Acidity Constants in Water

Kinetic and thermodynamic analysis of 1-hydroxy-2,2-dinitroethane nitronate reveals very low acidity constants for O- (pKaNO2H$p K_{\text{a}}^{\left(\text{NO}\right)_{2} \text{H}}$ = 1.67) and C-protonation (pKaCH$p K_{\text{a}}^{\text{CH}}$ = 3.78) in water. Marcus theory modeling indicates that NO₂ resonance stabilization significantly retards protonation. Original linear correlation between tautomeric (pK _N) and acidity constants (pKaCH$p K_{\text{a}}^{\text{CH}}$) in nitroalkanes is established, enabling prediction of previously inaccessible nitronic acid pKaNO2H$p K_{\text{a}}^{\left(\text{NO}\right)_{2} \text{H}}$ values and providing deeper insight into protonation behavior.

The proton transfer reactions of the nitronate anion derived from 1-hydroxy-2,2-dinitroethane 1 with HCl and carboxylic acid buffers in aqueous solution at 25 °C are investigated through a comprehensive kinetic and thermodynamic analysis. The reaction mechanism is elucidated, allowing the determination of acidity constants for both O-protonation (pKaNO2H$p K_{\text{a}}^{\left(\text{NO}\right)_{2} \text{H}}$ = 1.67) and C-protonation (pKaCH$p K_{\text{a}}^{\text{CH}}$ = 3.78), which are among the weakest reported for nitroalkanes in water to date. The intrinsic rate constant (log k _o = 1.60), determined using the Marcus approach, is markedly lower than typical values reported for nitrile protonation, reflecting the exceptional resonance stabilization of the conjugate base by the strongly electron-withdrawing NO₂ groups. Most importantly, by combining reported data from the literature with the results obtained in this study, a fundamental and predictive relationship between the tautomeric equilibrium constants (pK _N = pKaCH$p K_{\text{a}}^{\text{CH}}$-pKaNO2H$p K_{\text{a}}^{\left(\text{NO}\right)_{2} \text{H}}$) and the acidity constants (pKaCH$p K_{\text{a}}^{\text{CH}}$) of nitroalkanes in water has been established. This unprecedented linear correlation allows to estimate the acidity constants of the nitronic acids of four nitroalkanes in water, which were previously experimentally inaccessible. To the best of knowledge, this work is among the first to exploit such an approach, representing a significant advancement in understanding the structure–reactivity relationships governing the protonation of carbon and oxygen sites in nitroalkanes.

Zum Volltext