Reactions of some simple α- and β-hydroxyalkyl radicals with Cu2+ and Cu+ ions in aqueous solution. A radiation chemical study

Abstract

Using pulse radiolysis, rate constants have been measured for the reactions of the hydroxyalkyl radicals ĊH₂OH, ĊH₃CHOH, (CH₃)₂ĊOH, ĊH₂CH₂OH and ĊH₂C(CH₃)₂OH with Cu^I and Cu^II ions in aqueous solutions. For Cu^II the reaction was followed by observing the rate of formation of Cu^I which was measured as the Cu^I—acrylamide or Cu^I—ethylene complexes.

For the reaction of the α-hydroxyalkyl radicals with Cu^II the rate constants decrease in the order primary > secondary > tertiary and have values of the order of 10⁸ dm³ mol^–1 s^–1. These results are consistent with the reaction proceeding via organocopper complexes, but these complexes have not been directly observed. The β-radicals react more slowly than the α-radicals and ethylene oxide is produced in the oxidation of ĊH₂CH₂OH by Cu²⁺.

Rate constants for the reaction of the α- and β-radicals with Cu⁺ are estimated to be ∼ 10¹⁰ dm³ mol^–1 s^–1, and for the reaction of the β-radicals with the Cu^I—ethylene complex are estimated to be ∼5 × 10⁷ dm³ mol^–1 s^–1. Organocopper intermediates in the reactions with Cu⁺ have been observed spectrophotometrically and absorb in the region 350–550 nm. The β-radicals oxidise Cu^I to Cu^II and are themselves reduced to the corresponding olefin. The Cu^I—α-radical complexes decompose to Cu⁺ and the radical which then reduces the Cu²⁺ present. Mechanisms for these reactions are proposed.

Rate constants have also been obtained for the addition of the radicals (not ĊH₂CH₂OH) to acrylamide (∼3 × 10⁷ dm³ mol^–1 s^–1) and ĊH₂CH₂OH to ethylene (∼2 × 10⁶ dm³ mol^–1 s^–1).