Kinetics and Mechanism of Oxidation of Coumarin and Substituted Coumarins by Chloramine-b in Hydrochloric Acid Medium

Abstract

The kinetics of oxidation of coumarin (see Equation (1)) and four substituted coumarins, viz. 7-methoxycoumarin, 7-ethoxycoumarin, 7-hydroxycoumarin and 7-nitrocoumarin, to the corresponding o-hydroxycinnamic acids by chloramine-B (CAB) have been studied at 298 K. Under comparable experimental conditions, the reaction rate shows a first-order dependence each upon [CAB]oand [coumarin]o, and individually less than unit order dependence on [H+] and [Cl-]. Addition of benzenesulfonamide retards the reaction rate and the dielectric effect is positive. Variation of ionic strength of the medium does not influence the rate but the rate increases with increase in dielectric constant of the medium. The solvent isotope effect was studied using D2O. The reaction was studied at different temperatures and activation parameters have been evaluated. The stoichiometry of the reaction was found to be 1:1 and the oxidation products of coumarins were identified as their corresponding o-hydroxycinnamic acids. The rate of oxidation of coumarins increases in the order: 7-hydroxycoumarin > 7-ethoxycoumarin > 7-methoxycoumarin > coumarin > 7-nitrocoumarin. The rates satisfactorily correlate with the Hammett γ relationship and the reaction constant ρ is -0.04, which signifies that electron-donating groups enhance the rate and the electron-withdrawing group retards the rate. An isokinetic relationship was observed with β = 348 K, which showed the reaction to be enthalpy controlled. A mechanism consistent with the experimental results is proposed in which the rate-determining step is the formation of an intermediate complex between the substrate and molecular chlorine. The related rate law has been deduced and the decomposition constants were evaluated.