KINETICS AND MECHANISM OF HYDROGEN-PEROXIDE OXIDATION OF CHROMONE-3-CARBOXALDEHYDES IN AQUEOUS ACID AND MICELLAR MEDIA

Oxidation of chromone-3-carboxaldehyde (CCA) and substituted analogues by H2O2 has been carried out in aqueous acid (HCl and H2SO4) and mice llar media. Reaction kinetics indicated order in [CCA] as well as [H2O 2] to be unity while it is a Fraction (1 > n > 0) in [acid]. Reaction rates were found to be faster in the solvents of low-dielectric consta nt (D). Added salt (KCl or (NH4)(2)SO4) increased the rate of oxidatio n marginally. On the basis of observed linearity of Amis plot and marg inal positive ait effect, protonated CCA (enol form of CCA, a cation) and H2O2 (neutral molecule) were considered as reactive species in the rate limiting step. Reaction rates were found to be enhanced signific antly in anionic and nonionic micellar (sodium dodecylsulfate (SDS) an d Triton X-100 (Tx), respectively) media. However, cationic micelles [ cetyl trimethyl ammonium bromide (CTAB)] indicated marginal retardatio n effect. Effect of anionic and cationic micelles has been interpreted in terms of electrostatic interactions, while that of nonionic micell es in terms of hydrophobic interactions. Structure-reactivity correlat ions have been interpreted by Hammett's equation. Negative ''rho'' (re action constant) values indicated cationic transition state. (C) 1996 John Wiley & Sons, Inc.