Mechanism of the oxidation of hydrazoic acid by tetrachloroaurate(III) ion

Abstract

The oxidation of hydrazoic acid in perchloric acid in the absence of added chloride under pseudo first-order conditions ([HN 3 ] » [AuCl 4 -]) is first order in [Au(III)]. Michaelis-Menten type of dependence (linear plots of k obs -1 vs [HN 3 ] -1 ) is observed with respect to [HN 3 ]. The k obs is independent of ionic strength and the plot between k obs -1 and [H + ] is linear. The inner-sphere mechanism is consistent with the formation of an axial complex (K = 25 dm 3 mol -1 ) between AuCl 3 (HO) -ion and HN 3 prior to its rate determining decomposition (k = 0.0182 s -1 ). It is inferred that the free radicals N 3• do not oxidise Au(II). The reaction becomes outer-sphere in the presence of added Cl -ions which are inferred to form a cage around the hydronium ion surrounding the AuCl 4 -ions. The penetration of N 3 -through the cage is rate controlling and within the cage, the electron transfer from N 3 -ion to AuCl 4 -is fast. The value of the rate determining constant k 2 is 0.547 dm 3 mol -1 s -1 and the equilibrium constant K Cl for the cage formation is 5 dm 3 mol -1 at 25°C. It is calculated that the minimum HN 3 concentration required before the reaction exhibits zeroorder dependence in HN 3 is 0.31 mol dm -3 when [H + ] = 0.18 mol dm -3 at 25°C.