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

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 less than or equal to X-W less than or equal to 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,DeltaH(op)(double dagger) and DeltaS(op)(double dagger), were dependent on the solvent composition. In fact, a good linear correlation of a plot of DeltaH(op)(double dagger) vs. DeltaS(op)(double dagger), 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.