engleski

Influence of the medium on the reaction rate of the t-butoxy radical with iron(II).

Tert-butoxyl radicals (t-BuO·) were generated by homolytic photodecomposition of di-tert-butylperoxide (1) using ruby laser flashes at 347 nm. The reaction of t-BuO· radicals with Fe2+ (3) was studied under pseudo-first order conditions: t-BuOOt-Bu 2 t-BuO· (1) t-BuO· products (2) t-BuO· + Fe2+ t-BuO- + Fe3+ (3) The quantum yield F(Fe3+) was determined by measuring the absorbance of Fe3+ ion as [FeCl]2+ complex at 360 nm 2 ms after the flash. According to the equation (4) derived from the reaction scheme, 1/F(Fe3+) = 1/F( t-BuO·) {1+ k0/(k3[Fe2+])} (4) the rate constant k3 was obtained from the relative rate constant kr (kr=k0/k3) and the value of k0; the latter refers to the overall rate of the competing disappearance of t-BuO· radicals from the system (reaction 2), including the highest contribution of b-cleavage. The rate constant k0 was determined using diphenylmethanol instead of Fe2+. The known rate constant of the reaction of t-BuO· radical with diphenylmethanol giving ketyl radicals (6.9 ´ 106 dm3 mol-1 s-1) was applied. The quantum yield of ketyl radicals was determined by measuring the maximum of absorbance at 535 nm. At acid concentration of 0.023 mol dm-3 HCl the rate constant k3 = 3.4 ´ 108 dm3 mol-1 s-1 was determined. It was found that the relative rate constant increased with an increase of the hydrochloric acid concentration which has been ascribed to the lower stability of t-BuO· radical at higher acidity of the medium.The effect of polarity of the medium on the reaction rate (3) was also determined. Decreasing k3 in media of increasing polarity were explained by increasing of the b-scission rate of t-BuO· radical with increasing polarity of the medium.

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