Enthalpy–entropy compensation effect in the chalcone formation from naringin in water–ethanol mixtures

Abstract

The isomerisation equilibrium between the flavanone and chalcone forms of the naturally occurring flavonoid naringin (7-rhamnoglucosyl-4′,5-dihydroxyflavanone) has been studied in water–ethanol mixtures in the presence of NaOH. The variation of the observed pseudo-first order rate constant for the equilibrium reaction, k_obs, and the equilibrium composition were determined as a function of the base concentration (−4.0 < log[NaOH] < −2.4) and the water molar fraction (0.03 ≤ X_W ≤ 1.00) of the solvent mixture. The variation of the ring opening k_op, and the cyclisation k_cy, rate constants with the base concentration and solvent composition indicated that the isomerisation reaction is mediated by a carbanion intermediate. The temperature effect on k_op and k_cy showed that only the activation enthalpy and entropy changes for the ring-opening reaction, ΔH_op^‡ and ΔS_op^‡, were dependent on the solvent composition. In fact, a good linear correlation of a plot of ΔH_op^‡vs. ΔS_op^‡, indicate the existence of an enthalpy–entropy compensation effect. This result was associated with changes in the balance of hydrogen-bonding interactions between the intermediate carbanion and its solvation sphere as solvent composition was modified.