Solvent Effects on The reactivity Trends of some Coumarin Derivatives

Kinetic results, rate laws and reaction mechanisms were established for the base hydrolysis of 7-hydroxycoumarin (HC), 7-hydroxycoumarin-4-acetic acid (HCA), 6-nitrocoumarin (NC), 6-nitrocoumarin-3-carboxylic acid (NCC), 7-dimethylamino-4-methylcoumarin (DMAC), 7-diethylamino-4-methylcoumarin (DEAC) and coumalic acid (COU) in aqueous solution, water-methanol and water-acetone mixtures. The kinetics of the base hydrolysis followed a rate law with k_obs= k₂[OH^-] for all coumarin derivatives under study except 6-nitrocoumarin-3-carboxylic acid (NCC)  which followed a rate law with k_obs= k₁+ k₂[OH^-]. The kinetic data leaded to Gibbs free energies of activation (the change in activation barrierδ _m∆G^#).

The solubilities of the non-charged compounds, HC, NC, DMAC and DEAC in water-methanol and water-acetone mixtures were established and their transfer chemical potentials were calculated. Moreover, the solubilities of calcium and lanthanum salts of HCA, NCC and COU in water-methanol mixtures were established and the transfer chemical potentials of their anions were calculated. Furthermore, the solubilities of cerium and lanthanum salts of HCA, NCC and COU in water-acetone mixtures were established and the transfer chemical potentials of their anions were calculated.

Solvent effect on reactivity trends of the investigated coumarin derivatives have been analyzed into initial and transition state components by using the transfer chemical potentials of the reactants and the kinetic data of the studied compounds.

            The decrease in the observed rate constant values (k_obs) of the base hydrolysis of HC, NC, DMAC and DEAC with increasing of methanol% or acetone% is dominated by the initial state (IS), while the decrease in the observed rate constant values (k_obs) of the base hydrolysis of HCA^-, NCC^- and COU^- with increasing of methanol% or acetone% is dominated by the transition state (TS). 

            The observed enhancement of the rate constant values of the studied coumarin derivatives in acetone co-solvent mixtures compared with the corresponding values in methanol co-solvent mixtures would be ascribed to the destabilization of the hydrophilic OH^- ions in acetone more than in methanol. Also this may be due to the steric effect which results from the size of solvent molecules that allowing the OH^- ions to penetrate the large acetone molecules more than the small methanol molecules.    The activation parameters of the base hydrolysis reaction are evaluated and discussed by carrying out the base hydrolysis of the studied coumarin derivatives in water-methanol and water-acetone mixtures at different temperatures. The high negative values of entropy of activation (∆S^#­) support the proposal mechanism, i.e. the investigated reaction takes place via the formation of an intermediate complex. Moreover, these values refer to the rigidity and stability of the intermediate complex .Thus, the ring opening of the intermediate complex would be the rate controlling step. Also the relatively high free energy of transfer (∆G^#­) indicate that the slow step is the ring opening of the established intermediate complex and many vibrational degrees of freedom have been transformed into translations.     The straight lines obtained from the isokinetic plots (∆S^≠ versus ∆H^≠) of the studied base hydrolysis reaction in water-methanol and water-acetone mixtures provide that the base hydrolysis of the studied coumarin derivatives followed one mechanism, i.e. the same rate determing step (RDS).