title: The role of low levels of water in the electrochemical oxidation of α-tocopherol (vitamin E) and other phenols in acetonitrile.
abstract: The phenol, α-tocopherol, can be electrochemically oxidised in a -2e(-)/-H(+) process to form a diamagnetic cation that is long-lived in dry organic solvents such as acetonitrile and dichloromethane, but in the presence of water quickly reacts to form a hemiketal. Variable scan rate cyclic voltammetry experiments in acetonitrile with carefully controlled amounts of water between 0.010 M-0.6 M were performed in order to determine the rate of reaction of the diamagnetic cation with water. The water content of the solvent was accurately determined by Karl Fischer coulometric titrations and the voltammetric data were modelled using digital simulation techniques. The oxidation peak potential of α-tocopherol measured during cyclic voltammetry experiments was found to shift to less positive potentials as increasing amounts of water (0.01-0.6 M) were added to the acetonitrile, which was interpreted based on hydrogen-bonding interactions between the phenolic hydrogen atom and water. Several other phenols were examined and they displayed similar voltammetric features to α-tocopherol, suggesting that interactions of phenols with trace amounts of water were a common occurrence in acetonitrile. The H-bonding interactions of α-tocopherol with water were also examined via NMR and UV-vis spectroscopies, with the voltammetric and spectroscopic studies extended to include other coordinating solvents (dimethyl sulfoxide and pyridine).
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: Ying Shan
authors.initials: YS
authors.lastname: Tan
authors.forename: Shanshan
authors.initials: S
authors.lastname: Chen
authors.forename: Wan Mei
authors.initials: WM
authors.lastname: Hong
authors.forename: Jia Min
authors.initials: JM
authors.lastname: Kan
authors.forename: Edwin Swee Hee
authors.initials: ES
authors.lastname: Kwek
authors.forename: Shi Yu
authors.initials: SY
authors.lastname: Lim
authors.forename: Zhen Hui
authors.initials: ZH
authors.lastname: Lim
authors.forename: Malcolm E
authors.initials: ME
authors.lastname: Tessensohn
authors.forename: Yinlu
authors.initials: Y
authors.lastname: Zhang
authors.forename: Richard D
authors.initials: RD
authors.lastname: Webster
mesh_terms: D000097:Acetonitriles / Q000737:chemistry*; D002084:Butylated Hydroxytoluene / Q000031:analogs & derivatives / Q000737:chemistry; D003198:Computer Simulation; D003408:Cresols / Q000737:chemistry; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D004563:Electrochemistry; D006860:Hydrogen Bonding; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D014867:Water / Q000032:analysis / Q000737:chemistry*; D024502:alpha-Tocopherol / Q000737:chemistry*
chemical_list: D000097:Acetonitriles; D003408:Cresols; D010636:Phenols; D011725:Pyridines; D012997:Solvents; D014867:Water; D002084:Butylated Hydroxytoluene; C010915:2,6-di-tert-butyl-4-hydroxymethylphenol; C019584:2,4,6-tri-tert-butylphenol; C505148:2,4,6-trimethylphenol; D024502:alpha-Tocopherol; C023666:pyridine; D004121:Dimethyl Sulfoxide; C032159:acetonitrile