Kinetics and mechanism of the reaction of Cr(ii) aqua ions with benzoylpyridine N-oxide

Abstract

Aqueous chromium(II) ions, Cr_aq²⁺, react with benzoylpyridine oxide (BPO) much more rapidly than with other pyridine N-oxides previously explored. The kinetics were studied under pseudo-first order conditions with either reagent in excess. Under both sets of conditions, the major kinetic term exhibits first order dependence on limiting reagent, and second order dependence on excess reagent, i.e.k_Cr = k₂^Cr[BPO][Cr_aq²⁺]² (excess Cr_aq²⁺), and k_BPO = k₂^BPO[Cr_aq²⁺][BPO]² (excess BPO), where k₂^Cr = (6.90 ± 0.27) × 10⁴ M⁻² s⁻¹ and k₂^BPO = (3.32 ± 0.28) × 10⁵ M⁻² s⁻¹ in 0.10 M HClO₄. The rate constant k₂^Cr contains terms corresponding to [H⁺]-independent and [H⁺]-catalyzed paths. In the proposed mechanism, the initially formed Cr_aq(BPO)²⁺ engages in parallel oxidation of Cr_aq²⁺ and reduction of BPO. The latter reaction provides the basis for a convenient new preparative route for the BPO complex of Cr(III).