Kinetics of Oxidation of Curcumin and Analogues of Curcumin by V(V) in Acidic Medium and Effect of Substituents
摘要
Kinetics of oxidation of Curcumin and analogues of Curcumin by V(V) has been studied under pseudo-first order and second order conditions in acid media using aqueous ethanol as solvent in the temperature range 303–333 K. It is observed that the increase in both [acid] and [V(V)] increased the oxidation reaction rate indicated it is second order reaction. The effect [Curcumin] was variable: at higher concentrations (from 2.5 × 10−5 to 2 × 10−4 mol.dm−3) the reaction rate decreased with increasing [Curcumin] where at lower concentrations (from 2 × 10−6 to 2.5 × 10−5 mol.dm−3) the rate increased with increasing [Curcumin] wherein it gave fractional order, this observation indicated steric hindrance at higher concentrations from curcumin. Moreover, increase the temperature showed increase in the rate where on the other hand, decrease ethanol% increased the rate. The products have been identified by mass spectra as ferulic acid and guaiacol, when V(V) was taken 10 times excess to Curcumin. The mechanism was proposed by imagining attack of V(V) molecules on >C=O group of Curcumin, it substantiates the products detected. Solvent effect studies revealed decrease in dielectric constant of solvent mixture (ethanol + water) decreased the rate constant of the reaction and a linear plot was obtained for lnk and 1/D, with positive slope indicating ionic interactions between positive ion and dipole. The rates of the reaction are changed by electron releasing and electron withdrawing substituents. Linear Hammett’s plots are obtained for various substituents at para position of Curcumin and analogues of Curcumin in the benzene rings; the reaction constant obtained is −1.4996 indicating that electron donating substituents increase the rate of the reaction.