A laser flash photolysis kinetic study of reactions of the Cl2? radical anion with oxygenated hydrocarbons in aqueous solution

Abstract

A laser photolysis–long path laser absorption (LP-LPLA) experiment has been used to determine the rate constants for H-atom abstraction reactions of the dichloride radical anion (Cl2−) in aqueous solution. From direct measurements of the decay of Cl2− in the presence of different reactants at pH = 4 and I = 0.1 M the following rate constants at T = 298 K were derived: methanol, (5.1 ± 0.3)·104 M−1 s−1; ethanol, (1.2 ± 0.2)·105 M−1 s−1; 1-propanol, (1.01 ± 0.07)·105 M−1 s−1; 2-propanol, (1.9 ± 0.3)·105 M−1 s−1; tert.-butanol, (2.6 ± 0.5)·104 M−1 s−1; formaldehyde, (3.6 ± 0.5)·104 M−1 s−1; diethylether, (4.0 ± 0.2)·105 M−1 s−1; methyl-tert.-butylether, (7 ± 1)·104 M−1 s−1; tetrahydrofuran, (4.8 ± 0.6)·105 M−1 s−1; acetone, (1.41 ± 0.09)·103 M−1 s−1. For the reactions of Cl2− with formic acid and acetic acid rate constants of (8.0 ± 1.4)·104 M−1 s−1 (pH = 0, I = 1.1 M and T = 298 K) and (1.5 ± 0.8) · 103 M−1 s−1 (pH = 0.42, I = 0.48 M and T = 298 K), respectively, were derived. A correlation between the rate constants at T = 298 K for all oxygenated hydrocarbons and the bond dissociation energy (BDE) of the weakest C-H-bond of log k2nd = (32.9 ± 8.9) − (0.073 ± 0.022)·BDE/kJ mol−1 is derived. From temperature-dependent measurements the following Arrhenius expressions were derived: k (Cl2− + HCOOH) = (2.00 ± 0.05)·1010·exp(−(4500 ± 200) K/T) M−1 s−1, Ea = (37 ± 2) kJ mol−1 k (Cl2− + CH3COOH) = (2.7 ± 0.5)·1010·exp(−(4900 ± 1300) K/T) M−1 s−1, Ea = (41 ± 11) kJ mol−1 k (Cl2− + CH3OH) = (5.1 ± 0.9)·1012·exp(−(5500 ± 1500) K/T) M−1 s−1, Ea = (46 ± 13) kJ mol−1 k (Cl2− + CH2(OH)2) = (7.9 ± 0.7)·1010·exp(−(4400 ± 700) K/T) M−1 s−1, Ea = (36 ± 5) kJ mol−1 Finally, in measurements at different ionic strengths (I) a decrease of the rate constant with increasing I has been observed in the reactions of Cl2− with methanol and hydrated formaldehyde. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 169–181, 1999