Kinetics and mechanisms of redox reactions in aqueous solution. Part 7. Decarboxylation of aliphatic acids by aquasilver (II) ions
Abstract
The overall reaction 2AgII+ RCO2H → 2AgI+ 2H++ CO2+ products has been studied at different acidities (1.0 ⩽[HClO4]⩽ 4.0 and I= 4.0 mol dm–3) and temperatures (283 ⩽T⩽ 303 K). The rate slightly increases (18%) with increasing [AgI] within the range investigated. The rate-limiting reactions are the electron transfers from the carboxylic acids to Ag2+(aq) and [Ag(OH)]+(aq). The decrease in rate with increasing [H+] is considered to be due to the formation of the unreactive RCO2H2+ species as a result of protonation of the carboxylic acids. This assumption is more plausible than an earlier one favouring carboxylate anion as the reactive entity in solutions of acid at between 1.0 and 4.0 mol dm–3 HClO4 because KcKa where Kc and Ka are respectively the protonation and dissociation constants of the carboxylic acid. The rate constants with Ag2+(aq) and [Ag(OH)]+-(aq) may be correlated with Taft σ* values giving ρ values of –10.7 ± 0.7 and –9.8 ± 0.4 respectively. The thermodynamic parameters corresponding to the rate-limiting electron transfer to Ag2+(aq) and [Ag(OH)]+(aq) are reported.