Kinetics of oxidation of ethanol, isopropyl alcohol, and benzyl alcohol by [ethylenebis(biguanide)]silver(III) cation in aqueous perchloric acid solutions

Abstract

In aqueous acid solutions, the complex [Ag(H₂L)]³⁺(H₂L = ethylenebis(biguanide)] quantitatively oxidises ethanol, isopropyl alcohol, and benzyl alcohol to the corresponding carbonyl products at moderate rate. The experimental rate law is –d[complex]/dt=(k₁+k₂/[H⁺])[alcohol][complex]. At 40 °C and I= 1.0 mol dm^–3, 10⁵k₁, (dm^–3 mol^–1 s^–1) and 10⁶k₂(s^–1) values are: EtOH, 4.2 ± 0.3 and 2.8 ± 0.1; PrⁱOH, 7.5 ± 0.5 and 4.8 ± 0.2; PhCH₂OH, 95 ± 6 and 32.5 ± 2 respectively. The upper limit for the rate of reaction of [Ag(H₂L)]³⁺ with methanol, t-butyl alcohol, ethylene glycol, and glycerol is the rate of autodecomposition of [Ag(H₂L)]³⁺ and is less than 10% of the rate of oxidation of EtOH, PrⁱOH, or PhCH₂OH. Deprotonation of the alcohols assisted by axial co-ordination to [Ag(H₂L)]³⁺ is suggested as a source of the inverse acid dependence. The whole reaction course is e.s.r. silent and a rate-determining C–H bond cleavage in the –CHOH group appears reasonable. Compared to other metal-ion oxidants, [Ag(H₂L)]³⁺ appears to be the most selective reagent for PhCH₂OH.