Mechanism of the Catalytic Deperoxidation of tert‐Butylhydroperoxide with Cobalt(II) Acetylacetonate

Abstract

The Co(II)/Co(III)-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation. In the first step of the so-called Haber-Weiss cycle, alkoxyl radicals and Co(III)-OH species are generated upon the reaction of the Co(II) ion with ROOH. The catalytic cycle is closed upon the regeneration of the Co(II) ion through the reaction of the Co(III)-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals. Herein, the deperoxidation of tert-butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane. Kinetic information extracted from experiments, together with quantum-chemical calculations, led to new mechanistic hypotheses. Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical-chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals. This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 μM at 333 K. However, at higher cobalt concentrations, a remarkable decrease of the rate is observed. The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto overlooked chain termination of two Co(III)--OH species. The direct competition between the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous deperoxidation system.

Low temperature oxidation of linseed oil: a review

“…As already well recognised in this review, oxidation of linseed oil involves peroxyl chemistry, displaying importance to other industrial applications, such as that described by Turra et al (Turra et al 2010). These authors reported the mechanism of the decomposition of tertbutylhydroperoxides catalysed by cobalt(II) acetylacetonate (in cyclohexane).…”

“…The rate of this deperoxidation decreases at higher cobalt concentration, for instance at 333 K, the rate decreases once the concentration of Co(acac) 2 exceeds 100 μM (Turra et al 2011). The proposed explanation invokes the termination reaction of the Co III -OH to form a stable bis (μ-hydroxo) dimer, Co-(OH) 2 -Co, (Turra et al 2010) as illustrated in the following Scheme 5:…”

“…Alkoxyl radicals arise as the main products of the self reaction of tertiary peroxyl radical reactions (reaction 22), while the minority pathway comprises the termination reaction (23), with about 10% of the ROO radicals consumed through reaction (24) (Turra et al 2010). Alkoxyl radical produced by reaction (17) can interact with the cyclohexane (CyH) solvent (20) or with the hydroperoxide (21), however, reaction (21) consumes the majority of the alkoxyl radicals (Turra et al 2010).…”

“…Alkoxyl radical produced by reaction (17) can interact with the cyclohexane (CyH) solvent (20) or with the hydroperoxide (21), however, reaction (21) consumes the majority of the alkoxyl radicals (Turra et al 2010).…”

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

“…A recent study by Hermans and co-workers comprising both computational and kinetic studies of the autoxidation of cyclohexane by means of UV-Vis spectroscopy suggested that the dominant species is Co II . 153 Additionally, considering a more detailed view of eq.6 in the case of alkanes (either linear, branched or cyclic) this step can lead to the expected partial oxidation products alcohol and ketone at once:…”