Conversion Mechanisms of Nitroxyl Radical (TEMPO), Oxoammonium Cation, and Hydroxylamine in Aqueous Solutions:...

Oxidation reactions of alcohols have been of interest due to their broad applications in different fields. Oxoammonium cation (TEMPO⁺) of 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) is a high-potential oxidant for the selective oxidation of primary alcohols, with hydroxylamine (TEMPOH) as a side product. TEMPO or TEMPO⁺ has been widely applied for various reactions. However, the conversion mechanisms among TEMPO, TEMPO⁺, and TEMPOH are not well understood and remain controversial, due to complications in the direct observation of the reactions. In this work, two-dimensional correlation (2D-COS) UV-visible (UV-Vis) spectroscopy is applied to examine the correlations between the characteristic bands of each species, to obtain insights into the complete reaction mechanisms. Series of dynamic UV-Vis spectra of solutions under different external perturbations (as a function of reaction time) were recorded and used in the generation of 2D-COS synchronous and asynchronous maps. The key UV-Vis band assignments are as follows: 250 nm and 400 nm for TEMPO, 290 nm and 480 nm for TEMPO⁺, and 200 nm and 315 nm for TEMPOH. The results indicate that the conversion between TEMPO and TEMPOH in acidic solution is a reversible process, which reaches an equilibrium state after two hours. However, the reaction becomes irreversible after three hours, due to a higher degree of irreversible protonation of TEMPOH to form TEMPOH-H⁺. Fast conversion from TEMPO to TEMPO⁺ is observed when sodium hypochlorite co-oxidant is added. The synproportionation-disproportionation also reaches an equilibrium. However, there is no evidence of the conversion from TEMPOH to TEMPO⁺ under the reaction conditions. At high reaction temperature, the formation of TEMPOH occurs first from TEMPO⁺ decomposition, followed by TEMPO decomposition. These detailed mechanisms are beneficial in designing the optimum process conditions for the oxidation of specific alcohols.