Kinetic studies on the peroxyoxalate chemiluminescent reaction: imidazole as a nucleophilic catalyst
The peroxyoxalate system undergoes one of the most efficient chemiluminescence reactions and is the only one supposed to involve an intermolecular chemically induced electron exchange luminescence (CIEEL) mechanism, with proven high efficiency.
In this work we report kinetic results on the reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide, catalysed by imidazole (IMI), in the presence of chemiluminescence activators. The kinetics were followed by measurement of the intensity of light emission and the 2,4,6- trichlorophenol (TCP) release, observed as an absorption change. From the dependence of the observed rate constants on the concentrations of TCPO, imidazole, hydrogen peroxide and activator, we attribute rate constants to three elementary steps in the proposed simplified mechanistic scheme. The initial step consists of attack of the nucleophilic imidazole on TCPO. A bimolecular (k1(2)= 1.4 ± 0.1 dm3 mol–1 s–1) and a trimolecular [k1(3)=(9.78 ± 0.08) 102 dm6 mol–2 s–1] rate constant can be attributed to this step. The imidazolide subsequently suffers imidazole-catalysed peroxide attack, leading to a peracid derivative; trimolecular rate constants k2(3)=(1.86 ± 0.06)104 dm6 mol–2s–1 and k2(3)′=(8.7 ± 0.2) 103 dm6 mol–2 s–1 can be obtained for this step from the peroxide and the imidazole dependence, respectively. The cyclization of the peracid affords the reactive intermediate, which we believe to be 1,2-dioxetanedione; a rough estimate of k3∼ 0.2 s–1(at [IMI]= 1.0 mmol dm–3) is obtained for this step. Finally, the interaction of the activator with the reactive intermediate, probably involving the CIEEL sequence and leading ultimately to light emission, is extremely fast and cannot be observed kinetically.