Kinetics and mechanism of oxidation of primary alcohols by pyridinium hydrobromide perbromide

Abstract

Oxidation of nine primary aliphatic alcohols b y pyridinium hydrobromide perbromide (PH PB) in aqueous acetic acid leads to the formation of the corresponding aldehydes. The reaction is first order with respect to PHPB. Michaelis-Menten-type kinetics are observed with respect to the alcohol. The oxidation of [I ,I -2H2]ethanol (MeCD,OH) exhibits a substantial kinetic isotopic effect. The effect of solvent composition indicates that the transition state is more polar than the reactants. The reaction is susceptible t o both polar and steric effects of the substituents. A mechanism involving transfer of a hydride ion from the alcohol t o the oxidant has been proposed.Pyridinium hydrobromide perbromide (PHPB) has been extensively used in synthetic organic chemistry as a brominating reagent and as an oxidant.'72 There are not many reports on the mechanistic aspects of reactions of PHPB.3.4 Gnanadoss and Vijayalaxmi4 reported the kinetics of the oxidation of some primary aliphatic alcohols and attempted a correlation of reactivity with Charton's steric parameter, V.5 However, they did not determine the order of reaction with respect to the alcohol and, in its absence, the correlation analysis is of doubtful significance. We have been interested in the study of kinetics and mechanism of oxidations by oxidants containing halogens. In this article, we report the kinetics of oxidation of nine aliphatic primary alcohols in aqueous acetic acid. Attempts have been made to correlate structure and reactivity in the reaction. Mechanistic aspects are discussed.Paper 2/044 1 7 J

Kinetics and mechanism of the oxidation of substituted benzaldehydes by hexamethylenetetramine-bromine

“…The rate constant for the decomposition of the complex k 2 , however, showed considerable variation as a function of substituent in the reductant. Similar observations have been previously recorded in the oxidation of benzyl alcohols [13] and mandelic acids [14] by ceric ammonium nitrate, aliphatic alcohols [4] and diols [5] by HABR, substituted benzaldehydes by bis(2,2Ј-bipyridyl) copper(II) permanganate [15] and aliphatic alcohols by pyridinium fluorochromate [16] and pyridinium hydrobromide perbromide [17].…”

Kinetics and mechanism of oxidation of aliphatic alcohols by tetrabutylammonium tribromide

“…A nonlinear transition state, implied in the synchronous mechanism, is supported by the relatively low magnitude of the kinetic isotope effect (k H /k D ≈ 3⋅1). The oxidation of benzyl alcohol by PHPB 21 also was reported to involve a nonlinear transition state and the kinetic isotope effect was found to be ≈ 3⋅5. The mechanism depicted in scheme 1 accounts for all the observed results.…”

Kinetics and mechanism of the oxidation of some neutral and acidic α-amino acids by tetrabutylammonium tribromide

“…An intermediate complex may be formed by the interaction between the non-bonded pairs of electrons of the carboxylicoxygen and tribromide ion. The formation of a similar complex has been postulated in the oxidation of alcohols 15 by pyridinium hydrobromide perbromide (PHPB) also. The formation of moderately stable complexes is supported by the values of thermodynamic parameters also.…”