Adsorption-parallel catalytic waves of cinnamic acid in hydrogen peroxide-tetra-n-butylammonium bromide-acetate system

The mechanism of the adsorption-parallel catalytic wave of cinnamic acid (C 5H5-CH = CH-COOH) in acetate buffer (pH = 4.0)-H2O2-tetra-n-butylammonium b romide (Bu4N . Br) solution was studied by the linear-sweep polarography, c yclic voltammetry and digital simulation approach. Experimental results ind icate that the reduction mechanism of cinnamic acid is ECdimE' process, in which the C =C double bond of cinnamic acid first undergoes 1e, 1H(+) reduc tion to produce an intermediate free radical C6H5-(CH)-H-.-CH2-COOH(E), the n the further reduction of the free radical in 1e,1H(+) addition (E') occur s simultaneously with a dimerization reaction between two free radicals (C- dim). Bu4N . Br enhances the polarographic current of cinnamic acid and shi fts the peak potential to positive direction. The enhancement action of Bu4 N Br is due to the adsorption of cinnamic acid induced by Bu4N+ species. In addition, H2O2 causes the parallel catalytic wave of cinnamic acid. The me chanism of the catalytic wave is EC' process because H2O2 oxidizes the free radical of cinnamic acid to regenerate the original C =C bond(C'), prevent ing both the further reduction and the dimerization of the free radicals. T he apparent rate constant kt of the oxidation reaction is 1.35 x 10(2) mol . L-1 . s(-1). A new class of catalytic waves for organic compounds, the ad sorption-parallel catalytic waves upon the dual enhancement action of both the surfactant and oxidant, has been presented.