Kinetics of oxidation of vanadium(IV) by alkyl hydroperoxides in acidic, aqueous solution

Abstract

The title reactions produce VO[sub 2][sup +] and products derived from [beta]-scission of alkoxy radicals, largely acetone, ethane, and methane (tert-butyl hydroperoxide) or ethane, ethylene, and n-butane (tert-amyl hydroperoxide). The minor amounts of methane from the first reaction and the excess amount of ethane from the second suggest a small contribution from the oxidation of VO[sup 2+] by [sm bullet]CH[sub 3] and [sm bullet]C[sub 2]H[sub 5], respectively. The kinetic dependence of the main reaction upon [H[sup +]] is consistent with the formation of an intermediate OVOOR[sup +] present at steady-state concentrations. This intermediate produces VO[sub 2][sup +] and RO[sm bullet] in a rate-limiting step. The yield of VO[sub 2][sup +] is stoichiometric in the t-AmOOH reaction, whereas only about half this amount of VO[sub 2][sup +] is produced from t-BuOOH. Although chloride ions affect neither reaction, Br[sup [minus]] changes the stoichiometry, products, and kinetics of the t-BuOOH reaction, but not those of the t-AmOOH reaction. These differences are accounted for by the relative reactivities of the intermediate alkoxy radicals, RO[sm bullet] toward [beta]-scission versus bromide oxidation. Addition of alcohols with [alpha]-H's (i.e., CH[sub 3]OH, C[sub 2]H[sub 5]OH, (CH[sub 3])[sub 2]CHOH) does not affect VO[sub 2][sup +] production from t-AmOOH butmore » does prevent VO[sub 2][sup +] formation from t-BuOOH because VO[sub 2][sup +] is reduced by the alcohol radical formed in the reaction between t-BuO[sm bullet] and ROH. Several models are suggested, all of which feature [beta]-scission of t-BuO[sm bullet] and t-AmO[sm bullet]. The data pertaining to products and stoichiometry suggest an interaction or complexation of VO[sub 2][sup +] by t-BuOOH (but not t-AmOOH), which is supported by product yields, ion-exchange isolation, and spectroscopic evidence.« less