The photocatalytic decomposition of chloroform by tetrachloroaurate(III)

Abstract  

Near-UV irradiation of solutions of (Bu₄N)AuCl₄ in aerated ethanol-stabilized chloroform causes the continuous decomposition of chloroform, as evidenced by the production of many equivalents of HCl and peroxides. At the outset of irradiation, most of the AuCl₄ ⁻ is reduced to AuCl₂ ⁻, but the reduction stops and is reversed. The same experiments done in ethanol-free chloroform cause chloroform decomposition only until the irreversible reduction of the gold is complete. In deoxygenated ethanol-free chloroform, irreversible reduction to AuCl₂ ⁻ is accompanied by the formation of HCl and CCl₄, while the main decomposition products in deoxygenated ethanol-stabilized chloroform are HCl and C₂Cl₆. It is proposed that, in ethanol-free chloroform, photoreduction of AuCl₄ ⁻ begins with the concerted elimination of HCl from an association complex of CHCl₃ with AuCl₄ ⁻, and that ethanol suppresses { \textCHCl₃ ·\textAuCl₄ ^- } \{ {\text{CHCl}}_{3} \cdot {\text{AuCl}}_{4}^{ - } \} complex formation, leaving a slower radical process to carry out the photoreduction of AuCl₄ ⁻ in ethanol-stabilized chloroform. In the presence of oxygen, the radical process causes a build-up of CCl₃OOH, which reoxidizes AuCl₂ ⁻ to AuCl₄ ⁻ and allows the photodecomposition of CHCl₃ to continue indefinitely.