Kinetics and mechanisms of redox reactions in aqueous solution. Part 7. Decarboxylation of aliphatic acids by aquasilver (II) ions

Abstract

The overall reaction 2Ag^II+ RCO₂H → 2Ag^I+ 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 [Ag^I] 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 K_cK_a where K_c and K_a 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.