Abstract
The kinetics of oxidation of catechol and a series of catechol derivatives by vanadium(V) have been studied by stopped flow at 25° and ionic strength 1.0 M (C104-) in acidic media (0.2-1.0 M HC104). The rate law is -d[ligand]/dz = kK2 [ V( V) ]2 [ligand]/(1 + A2[V(V)]) where “ligand” is catechol or one of its derivatives, k is given by k = k\ + k2 [H+], and K2 = [complex]/[V(V)] [ligand]. The mechanism most consistent with the data and chemistry of this system is the reversible formation of a complex between vanadium(V) and the reductant, followed by rate determining reaction of this complex and V(V). The redox rate constants were found to be [H+] dependent indicating that two parallel paths are impor- tant for the rate limiting step. The respective rate constants are the following (reductant, k\ in units of M~l sec-1, k2 in units of M-2 sec-1): catechol, 23.8 ± 2.4, 71.7 ± 3.3; pyrogallol, 1200 ± 90, 1140 dt 140; 1,2,4-benzenetriol, 6080 ± 180, 775 ± 280; L-dopa cation, 11.6 ± 1.6, 31.2 ± 2.3; epinephrine cation, 10.2 ± 1.0, 26.2 ± 1.6; gallic acid, 58.6 ± 2.2, 53.3 ± 2.9. These results are discussed in terms of the Marcus cross-reaction equation for outer-sphere electron transfer reactions.