Reaction Kinetics in Ionic Liquids as Studied by Pulse Radiolysis: Redox Reactions in the Solvents Methyltributylammonium Bis(trifluoroniethyl-sulfonyl)imide and N-Butylpyridinium Tetrafluoroborate

Published

April 1, 2002

Author(s)

D Behar, Pedatsur Neta, C R. Schultheisz

Abstract

Rate constants for several reduction and oxidation reactions were determined by pulse radiolysis in three ionic liquids and compared with rate constants in other solvents. Radiolysis of the ionic liquids methyltributylammonium bis(trifluoromethylsulfonyl)imide (R₄NNTf₂), N-butylpyridinium tetrafluoroborate (BuPyBF₄), and N-butyl-3-methylpyridinium hexafluorophosphate (BuPicPF₆) leads to formation of solvated electrons and organic radicals. In R₄NNTf₂ the solvated electrons do not react rapidly with the solvent and were reacted with several solutes, including CCl₄, benzophenone, and quinones. In the pyridinium ionic liquids the solvated electrons react with the pyridinium moiety to produce a pyridinyl radical, which, in turn, can transfer an electron to various acceptors. The rate constant for reduction of duroquinone by the benzophenone ketyl radical in R₄NNTf₂ is considerably lower than that measured in water but the difference is due mainly to the high viscosity of the ionic liquid. Rate constants for electron transfer from the solvent-derived butylpyridinyl radicals in BuPyBF₄ and BuPicPF₆ to several compounds are lower than those measured in water and in 2-PrOH but are significantly higher than the diffusion-controlled rate constants estimated from the viscosity, suggesting an electron hopping mechanism through solvent cations. Electron transfer between methyl viologen and quinones takes place much more slowly in BuPyBF₄ than in water or 2-PrOH and the direction of the electron transfer is also solvent dependent. The driving force reverses direction on going from water to 2-PrOH and is intermediate in the ionic liquid. Radiolysis of ionic liquid solutions containing CCl₄ and O₂ leads to formation of CCl₃O₂ radicals, which oxidize chlorpromazine (ClPz) with rate constants near 1 10⁷ L mol^-1 s^-1. In the absence of CCl₄, other peroxyl radicals are formed which also may oxidize ClPz. The contribution of these peroxyl radicals was noticeable in R₄NNTf₂, and was accounted for, but it was negligible in BuPyBF₄ and BuPicPF₆. Some of the rate constants were also measured at various temperatures (20 to 75 ^oC) and the activation energies of the processes were determined. The absolute rate constants in the ionic liquids are considerably lower than those found in aqueous solutions and close to those determined in alcohols. On the other hand, the experimental rate constants in the ionic liquids and in water are somewhat close to the respective diffusion-controlled limits in these solvents while the values in alcohols are much slower than diffusion-controlled. From all the observations in this study it appears that the ions of the ionic liquids may be highly associated and yet many electron transfer reactions take place relatively rapidly.

Citation

Journal of Physical Chemistry A

Volume

106

Issue

No.13

Pub Type

Journals

Keywords

ionic liquids, pulse radiolysis, rate constants, redox reactions, solvent effect

Citation

Behar, D. , Neta, P. and Schultheisz, C. (2002), Reaction Kinetics in Ionic Liquids as Studied by Pulse Radiolysis: Redox Reactions in the Solvents Methyltributylammonium Bis(trifluoroniethyl-sulfonyl)imide and N-Butylpyridinium Tetrafluoroborate, Journal of Physical Chemistry A (Accessed April 25, 2024)