Ion pairing and reactivity of the alkali-metal derivatives of diethyl 5- bromopentylmalonate

Abstract

The effect of added tetraethylammonium and alkali-metal (Li, Na, K) bromides on the rate of cyclisation of the anion derived from diethyl 5-bromopentylmalonate has been studied over a wide concentration range in 99% aqueous Me₂SO at 25.0 °C. Whereas the rate of cyclisation is unaffected by the tetraethylammonium salt, it is significantly retarded by the alkali-metal salts. These rate-depressing effects are quantitatively accounted for on the basis of a reaction scheme involving independent kinetic contributions from the free reactant anion and from the cation-paired reactant anion. A self-consistent kinetic analysis has allowed determination of both the reactivities of the associated species and the ion-pairing equilibrium constants. The results show that the latter vary in the order Li⁺ > Na⁺ > K⁺, showing that the ion-pairing tendency is dominated by coulombic interactions. Consistently, the opposite trend is observed with the ion-pair reactivities, indicating a greater reduction of the nucleophilicity of the nucleophilic carbon by the stronger interaction with the smaller cations in the cation-associated species.