Reaction kinetics in the ionic liquid methyltributylammonium bis(trifluoromethylsulfonyl)imide.: Pulse radiolysis study of •CF3 radical reactions

The reactions of trifluoromethyl radicals with pyrene, phenanthrene, crotonic acid, and 2-propanol (2-PrOH) in the ionic liquid methyltributylammonium bis(trifluoromethylsulfonyl)imide (R4NNTf2) are studied by pulse radiolysis. Radiolysis of R4NNTf2 leads to formation of solvated electrons and organic radicals, including (CF3)-C-.. The solvated electrons do not react rapidly with the solvent and are reacted with CF3Br to produce additional (CF3)-C-. radicals. The rate constants for addition of (CF3)-C-. radicals to pyrene and phenanthrene are determined to be (1.1 +/- 0.1) x 10(7) L mol(-1) s(-1) and (2.6 +/- 0.4) x 10(6) L mol(-1) s(-1), respectively. By competition kinetics, the rate constant for reaction of (CF3)-C-. radicals with crotonic acid is determined to be (2.7 +/- 0.4) x 10(6) L mol(-1) s(-1), and the reaction is predominantly addition to the double bond. Competition kinetics with 2-PrOH in the absence of CF3Br gives a rate constant of (4 +/- 1) x 10(4) L mol(-1) s(-1) for H-abstraction from 2-PrOH, but in the presence of CF3Br, the rate constant cannot be determined because a chain reaction develops through the reduction of CF3Br by the (CH3)(2)COH radical. The rate constants for reactions of (CF3)-C-. radicals in acetonitrile solutions are slightly higher, by a factor of 2.3 for pyrene and phenanthrene and a factor of 1.3 for crotonic acid. The rate constant for pyrene in aqueous acetonitrile (30% water) solutions is 4 times higher than that in the ionic liquid. Rate constants for H-abstraction and addition reactions of (CF3)-C-. radicals in the ionic liquid are of the same order of magnitude as in water and acetonitrile, whereas electron-transfer reactions were reported to be one or more orders of magnitude slower in ionic liquids than in water or alcohols.