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.

Medienart:

E-Artikel

Erscheinungsjahr:

2021

Erschienen:

2021

Enthalten in:

Zur Gesamtaufnahme - volume:75

Enthalten in:

Applied spectroscopy - 75(2021), 3 vom: 04. März, Seite 325-335

Sprache:

Englisch

Beteiligte Personen:

Le, Tu Phuong Pham [VerfasserIn]
Opaprakasit, Pakorn [VerfasserIn]

Links:

Volltext

Themen:

2D-COS
Hydroxylamine
Journal Article
Oxidation mechanisms
Oxoammonium
TEMPO
Two-dimensional correlation analysis
UV–Vis
Ultraviolet–visible spectroscopy

Anmerkungen:

Date Revised 16.03.2021

published: Print-Electronic

Citation Status PubMed-not-MEDLINE

doi:

10.1177/0003702820961097

funding:

Förderinstitution / Projekttitel:

PPN (Katalog-ID):

NLM316010677