Kinetics and Mechanism of Oxidation of Vanillin by Permanganate in Neutral Medium and the Effect of Different Transition Metal Ion Catalysts

Research Article

Austin Chem Eng. 2016; 3(1): 1026.

Kinetics and Mechanism of Oxidation of Vanillin by Permanganate in Neutral Medium and the Effect of Different Transition Metal Ion Catalysts

Fawzy A1,2*, Zaafarany IA2, Althagafi I2, Alfahemi J2 and Morad M2

1Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt

2Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia

*Corresponding author: Fawzy A, Associate Professor, Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia; Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt

Received: March 25, 2016; Accepted: April 01, 2016; Published: April 04, 2016


The kinetics of oxidation of vanillin (VAN) by permanganate ion in neutral medium has been investigated spectrophtometrically. The stoichiometry of the reaction was found to be 3:2 (VAN: MnO4 -). The oxidation reaction exhibited a first order dependence in [MnO4 -] and less than unit order dependence with respect to vanillin concentration. Addition of small amounts of Ag(I), Co(II) and Ru(III) catalysts increased the oxidation rate and the catalytic efficiency increased in the order: Ru(III) > Ag(I) > Co(II). The proposed oxidation mechanism involves formation of a 1:1 intermediate complex between vanillin and permanganate ion in pre-equilibrium step. The final oxidation product of vanillin was identified by both spectral and chemical analysis as vanillic acid. The appropriate rate law was deduced. The reaction constants involved in the different steps of the mechanism were evaluated. The activation parameters associated with the rate constant of the slow step of the proposed mechanism along with the thermodynamic quantities of the equilibrium constant have been evaluated and discussed.

Keywords: Vanillin; Permanganate; Neutral medium; Oxidation; Kinetics; Mechanism


Vanillin is a phenolic aldehyde, which is an organic compound including aldehyde, hydroxyl and ether. It is the primary component of the extract of the vanilla bean. Vanillin is a very popular flavouring reagent in the food industry and is widely used in the synthesis of drugs such as Aldomet, L-Dopa (I) and Trimethaprin 2A. It has been also used as a chemical intermediate in the production of pharmaceuticals and other fine chemicals. Furthermore, it is used in the preparation of perfume and as a catalyst in various polymerization reactions. If the hydroxyl group in vanillin is protected, it undergoes oxidation to vanillic acid [1]. Kinetics of oxidation of vanillin has been studied previously in alkaline media by few reagents, such as hexacyanoferrate(III) [2], diperiodatoargentate(III) [3], bismuth(V) [4], diperiodatonickelate(IV) [5] and periodate catalyzed by ruthenium(III) [6], and in acid media by cerium(IV) [7].

Permanganate ion is an efficient oxidant in acid, neutral and alkaline media [8-14] which still remains as one of the most important, eco-friendly and powerful multi-electron oxidants employed in the kinetic studies [15]. The mechanism of oxidation by this multivalent oxidant depends not only on the substrate but also on the medium used for the study. During oxidation by permanganate ion, it is evident that the Mn(VII) species in permanganate is reduced to various oxidation states in different media

Transition metal ions, M(Y), where M is the metal and Y is its valance, have been widely employed as homogenous catalysts for oxidation of organic and inorganic substrates by reaction pathways such as formation of complexes with the reactants, oxidation of a substrate, or the formation of free radicals [16-28]. The mechanistic study of the catalyzed reactions is considered an important research field due to the role played by metals in biological systems.

The present study deals with the oxidative behavior of permanganate ion with vanillin in neutral medium and study the catalytic effect of some metal ions with different valences, namely Ag(I), Co(II) and Ru(III). We aim in the present to establish the optimum conditions affecting such oxidation, to examine the catalytic activity of the investigated metal ions, and finally to elucidate a plausible oxidation mechanism on the basis of the obtained kinetic and spectral results.



All reagents used in this investigation were from Merck or Sigma. A stock solution of vanillin was prepared afresh by dissolving the appropriate amount of the sample (S.D. Fine Chem.) in the required volume of distilled water. Solution of potassium permanganate was prepared and standardized as reported earlier [29]. Other chemicals were of analytical grade and their solutions were prepared by dissolving requisite amounts of the samples in double-distilled water.

Kinetic measurements

Kinetic runs were performed under pseudo-first order conditions with a large excess of vanillin over permanganate. The reactions temperature (25°C) was controlled within ±0.1°C. The progress of the reaction was followed by monitoring the decrease in the absorbance of permanganate ion, as a function of time, at λ = 526 nm, its absorption maximum, on a thermostatted Shimadzu UV-VIS-NIR-3600 doublebeam spectrophotometer.

First order plots, ln(absorbance) – time plots, were straight lines for more than two half-lives completion of the reaction, and the observed first order rate constants (kobs) were calculated as the slopes of such plots. Average values of at least three kinetic measurements of the rate constant were taken. The rate constants were reproducible to within 2-3%. The reaction orders with respect to the reactants were determined from the plots of log kobs versus log (conc.). Some kinetic runs were performed under purified nitrogen and compared with those taken under air, and the results were the same. Thus, dissolved oxygen did not affect the oxidation rate.


Stoichiometry and product analysis

Different sets of reaction mixtures containing varying ratios of permanganate to vanillin were mixed in neutral medium at constant temperature then were kept for about 24 hours. Estimation of the remaining permanganate concentrations was performed spectrophotometrically. The results confirm that the stoichiometry is 3:2 (vanillin: permanganate) which holds by the following equation 1,