The kinetics of oxidation of four vicinal diols, four nonvicinal diols, and one of their monoethers by pyridinium bromochromate (PBC) have been studied in dimethyl sulfoxide. The main product of oxidation is the corresponding hydroxyaldehyde. The reaction is first-order with respect to each the diol and PBC. The reaction is acid-catalyzed and the acid dependence has the form:The oxidation of [1,1,2,2-2 H 4 ]ethanediol exhibitedobs a primary kinetic isotope effect ( at ). The reaction has been studied in 19 k /k ϭ 6.70 298 K H D organic solvents including dimethyl sulfoxide and the solvent effect has been analyzed using multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical transition state in the rate-determining step. A suitable mechanism has been proposed.
The oxidation of benzyl alcohol involves transfer of a hydride ion to a benzyltrimethylammonium dichloroiodate±zinc dichloride complex in the rate-determining step.Benzyltrimethylammonium polyhalides are widely used as halogenating reagents in synthetic organic chemistry. 1,2 We have recently reported the oxidation of phosphorus oxyacids by benzyltrimethylammonium dichloroiodate (BTMACI). 7 Here, we describe kinetics of oxidation of benzyl alcohol and thirty-two monosubstituted benzyl alcohols by BTMACI, in glacial acetic acid in the presence of zinc chloride. Mechanistic aspects are discussed.BTMACI was prepared by the reported method 1 and its purity was checked by an iodometric method. BTMACI is only slightly soluble in acetic acid at room temperature. However, addition of zinc chloride renders it readily soluble in acetic acid. The reactions were carried out under pseudo-®rst-order conditions by maintaining a large excess of the alcohol over BTMACI. The solvent was glacial acetic acid. The reactions were carried out in the presence of zinc chloride and were followed by monitoring the decrease in [BTMACI] iodometrically. The pseudo-®rst-order rate constant, k obs , was evaluated from the linear plots of log[BTMACI] vs. time. The experimental third-order rate constant, k 3 , was determined from the relationship:The oxidation of alcohols results in the formation of the corresponding aldehydes. The overall reaction may be represented as eqn. (1).
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