Reduction of Arenediazonium Salts by Hydroquinone. Kinetics and Mechanism for the Electron-Transfer Step

Article abstract of DOI:10.1021/jo00249a021

Arenediazonium tetrafluoroborate salts undergo facile electron transfer reaction with hydroquinone in aqueous phosphate-buffered solutions containing the hydrogen donor solvent acetonitrile.Reaction rates are the first-order in hydroquinone and arenediazonium ion concentrations, and they exhibit inverse first order dependence on the hydrogen ion concentration over the pH range of 1.0-9.5.Benzoquinone and arene are the principial products, but arylation of acetonitrile and benzoquinone are competitive in a reaction process that exhibits a 2:1 stoichiometric relationship between the arenediazonium ion salt and hydroquinone.Rate constants for reduction of substituted benzediazonium ions by the monoanion of hydroquinone correlate with ? constants yielding a ρ value of 3.5.Reactions performed in deuterium oxide show kinetic isotope effects that reflect the solvent isotope effect on the acidity constant for hydroquinone, and levels of isotope incorporation by deuterium abstraction from deuterium oxide and/or acetonitrile-d3 demonstrate that neither water nor hydroquinone are hydrogen atom donors to intermediate aryl radicals.Reduction of arenediazonium ions involves a rate-limiting single electron transfer from the monoanion of hydroquinone followed by a rapid single electron transfer from the semiquinone intermediate to the diazonium ion.Application of Marcus theory provides calculated rate constants for electron transfer from the hydroquinone monoanion to arenediazonium ions.These rate constants, together with the absence of evidence for a diazo ether intermediate and rate constants for diazotate formation, suggest that electron transfer occurs by an outer-sphere mechanism.