Conversion Mechanisms of Nitroxyl Radical (TEMPO),Oxoammonium Cation,and Hydroxylamine in Aqueous Solutions: Two-Dimensional Correlation Ultraviolet–Visible Spectroscopy

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.