Kinetics and mechanism of the chromium(II) reduction of pyridine N-oxides

Abstract

The reactions between Cr^II and 15 substituted pyridine N-oxides have been studied in 1 mol dm^–3 HClO₄ and found to follow the stoicheiometry (i) and the rate law d[Cr³⁺]/2dt=k[Cr²⁺][X-pyo]. The reactions are suggested, 2Cr²⁺+ X-pyo + 2H⁺→ 2Cr³⁺+ X-pyo + H₂O (i), to proceed by the stepwise reduction of pyridine N-oxide molecules via a free-radical path. The results are indicative of two distinct types of behaviour: compounds with co-ordinating substituents in the ortho or para position (k > 0.3 dm³ mol^–1 s^–1 and ΔH^‡ < 6 kcal mol^–1) and all other compounds (k < 2 × 10^–3 dm³ mol^–1 s^–1 and ΔH^‡= 11–13 kcal mol^–1). These differences are discussed in terms of the ability of the pyridine N-oxides to co-ordinate to Cr^IIvia groups other than the N-oxide and the relative stabilities of the free-radical intermediates.