Abstract
The reactions between CrII 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 CrIIvia groups other than the N-oxide and the relative stabilities of the free-radical intermediates.
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