Hydroxyl radical reactions with halogenated ethanols in aqueous solution: Kinetics and thermochemistry

Laser flash photolysis combined with competition kinetics with SCN- as the reference substance has been used to determine the rate constants of CH radicals with three fluorinated and three chlorinated ethanols in water as a function of temperature. The following Arrhenius expressions have been obtained for the reactions of OH radicals with (1) 2-fluoroethanol, k(1) (T) = (5.7 +/- 0.8) x 10(11) exp((-2047 +/- 1202)/T) M-1 s(-1), (2) 2,2-difluoroethanol, k(2)(T) = (4.5 +/- 0.5) x 10(9) exp((-855 +/- 796)/T) M-1 s(-1), (3) 2,2,2-trifluoroethanol, k(3)(T) = (2.0 +/- 0.1) x 10(11) exp((-2400 +/- 790)/T) M-1 s(-1), (4) 2-chloroethanol, k(4)(T) = (3.0 +/- 0.2) x 10(10) exp((-1067 +/- 440)/T) M-1 s(-1), (5) 2, 2-dichloroethanol, k(5)(T) = (2.1 +/- 0.2) x 10(10) exp((-1179 +/- 517)/T) M-1 s(-1), and (6) 2,2,2-trichloroethanol, k(6)(T) (1.6 +/- 0.1) x 10(10) exp((-1237 +/- 550)/T) M-1 s(-1). All experiments were carried out at temperatures between 288 and 328 K and at pH = 5.5-6.5. This set of compounds has been chosen for a detailed study because of their possible environmental impact as alternatives to chlorofluorocarbon and hydrogen-containing chlorofluorocarbon compounds in the case of the fluorinated alcohols and due to the demonstrated toxicity when chlorinated alcohols are considered. The observed rate constants and derived activation energies of the reactions are correlated with the corresponding bond dissociation energy (BDE) and ionization potential (IP), where the BDEs and IPs of the chlorinated ethanols have been calculated using quantum mechanical calculations. The errors stated in this study are statistical errors for a confidence interval of 95%. (C) 2008 Wiley Periodicals, Inc.