The effect of α-cyclodextrin on the oxidation of formic acid by aqueous bromine: inhibition despite overall rate increase

Abstract

A strong 1:1 inclusion complex is formed (K_α= 4 690 dm³ mol^–1) between Br₂ and α-cyclodextrin (cd). The reaction kinetics between HCOOH and Br₂(aq)([H⁺]= 0.1 mol dm^–3) with bromide present in large excess, have been studied potentiometrically both in the presence and absence of cd. It is shown that Br₂ with cd is about half as reactive as aqueous Br₂, and it would appear from the activation parameters that Br₂ should be released from inclusion during the rate-limiting step. However, despite the stabilization of Br₂ by cd, addition of the latter makes the experimental rate constant increase. The reason for this apparent contradiction is that non-reactive tribromide is transformed by cyclodextrin into mildly reactive Br₂cd. The rate constants (25 °C, I= 0.2 mol dm^–3) for the oxidation of HCOO^– by Br₂ and Br₂cd are 27 and 15 dm³ mol^–1 s^–1, respectively.