Kinetics and mechanism of oxidation of hydroxylamine by tetrachloroaurate(III) ion

Abstract

The oxidation of H₂NOH is first-order both in [NH₃OH⁺] and [AuCl₄ ⁻]. The rate is increased by the increase in [Cl⁻] and decreased with increase in [H⁺]. The stoichiometry ratio, Δ[NH₃OH⁺]/Δ[AuCl₄ ⁻], is ≈1. The mechanism consists of the following reactions.

$$\begin{gathered} {\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} \mathop \leftrightharpoons \limits^{K_{\text{a}} } {\text{NH}}_{\text{2}} {\text{OH}} + {\text{H}}^ + {\text{ (i)}} \hfill \\ {\text{AuCl}}_{\text{4}}^ - + {\text{NH}}_{\text{2}} {\text{OH}}\xrightarrow{k}{\text{AuCl}}_{\text{4}}^ - + {\text{HNO}} + {\text{2Cl}}^ - + 2{\text{H}}^ + {\text{ (ii)}} \hfill \\ {\text{AuCl}}_{\text{4}}^ - + {\text{NH}}_{\text{3}} {\text{OH}} + {\text{Cl}}^ - \xrightarrow{{k_1 }}{\text{AuCl}}_{\text{2}}^ - + {\text{HNO}} + 3{\text{Cl}}^ - + 3{\text{H}}^ + {\text{ (iii)}} \hfill \\ {\text{2HNO}}\xrightarrow{{{\text{fast}}}}{\text{N}}_{\text{2}} {\text{O + H}}_{\text{2}} {\text{O (iv) }} \hfill \\\end{gathered}$$

The rate law deduced from the reactions (i)–(iv) is given by Equation (v) considering that [H⁺] ≫ K _a.

$$k_{{\text{obs}}} /[{\text{H}}_{\text{2}} {\text{NOH}}]_{\text{o}} = k_2 = (kK_{\text{a}} [{\text{H}}^ + ]^{ - 1} + k_1 [{\text{Cl}}^ - ]){\text{ (v)}}$$

The reaction (iii) is a combination of the following reactions:

$$\begin{gathered} {\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} + {\text{AuCl}}_{\text{4}}^ - \xrightarrow{{{\text{fast}}}}\left[ {{\text{AuCl}}_{\text{4}}^ - ..{\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} } \right]_{{\text{precursor}}}^\ddag {\text{ (a)}} \hfill \\ \left[ {{\text{AuCl}}_{\text{4}}^ - ..{\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} } \right]_{{\text{precursor}}}^\ddag + {\text{Cl}}^ + \xrightarrow{{k_1 }}\left[ {{\text{Cl}}^ - \ldots {\text{AuCl}}_{\text{4}}^ - ..{\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} } \right]_{{\text{succesor}}}^\ddag {\text{ (b)}} \hfill \\ \left[ {{\text{Cl}}^ - \ldots {\text{AuCl}}_{\text{4}}^ - ..{\text{NH}}_{\text{3}} {\text{OH}}^{\text{ + }} } \right]_{{\text{succesor}}}^\ddag \xrightarrow{{{\text{fast}}}}{\text{AuCl}}_{\text{2}}^ - + {\text{HNO}} + 3{\text{Cl}}^ - + 3{\text{H}}^ + {\text{ (c)}} \hfill \\\end{gathered}$$

The activation parameters for the reactions (ii) and (iii) are consistent with an outer-sphere electron transfer mechanism.