“KINETICS AND MECHANISTIC STUDY OF OXIDATION OF NICOTINIC HYDRAZIDE BY WAUGH-TYPE ENNEAMOLYBDOMANGANATE (IV) IN PERCHLORIC ACID MEDIUM”

The kinetics of oxidation of the nicotinic hydrazide in acidic medium was studied spectrophotometricaly at 468 nm. The reaction between nicotinic hydrazide and enneamolybdomanganate (IV) in aqueous acidic medium was studied spectrophotometrically at a constant ionic strength of 0.3 mol dm at 27±0.2C exhibits 1:1 stiochiometry, Nicotinic hydrazide : [MnMo9O32] 6− under pseudo-first order condition. The main oxidative products were identified by spot test, FT-IR, NMR, Melting Point, LC-MS. The effect of [H] ion and ionic strength of the reaction medium have been investigated. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed. A mechanism related to this reaction is proposed. Key wards: Kinetics, Mechanism, Nicotinic hydrazide, Enneamolybdomanganate (IV). *Corresponding author: Sunil G. Shankarwar, Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad-431004, Maharashtra, India. E-mail: shankarwar_chem@yahoo.com Copyright: ©2016 Sunil G. Shankarwar. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. INTRODUCTION Polyoxometalates (POM) have been studied in an extraordinarily broad range of areas including electrochemistry, catalysis, optics and medicine [1-3]. Many potential medicinal applications have been reported, such as anti-tumoral and anti-viral applications [4]. The diverse capability of (POM) as an important doubly function reagent (oxidative and catalytic properties) has been demostrated with respect to many organic reactions [5]. This is particularly so with heteropolyanions containing higher valent transition ions as hetero atoms. In contrast Mn (IV) is present in a number of biologically important compounds [6]. The oxidant used in the present study [MnMo9O32] 6− is also strong oxidizing agent in acidic medium [7,8]. In the present study a two-electron transfer complimentary reaction between Waugh type enneamolybdomanganate (IV) and nicotinic hydrazide is investigated. {(NH4)6[MnMo9O32]8H2O} is a Waugh-type heteropolyacid salt in which manganese is coordinated to six oxygen atoms in a slightly distorted octahedral arrangement. Each of these oxygen atom is coordinated to three molybdenum atoms and each molybdenum is also surrounded by six oxygen atoms in a much distorted octahedral geometry. International Journal of Industrial Chemistry and Biotechnology Page: 1 Available online at www.ijicab.com Sunil G. Shankarwar et al Int. J.Ind.Chem and Biotec Copyrights@2016 It can oxidize organic as well as inorganic substrates [9]. Literature survey reveals that the kinetics of oxidation of nicotinic hydrazide has been studied using selenium dioxide [10] Thallium (III) oxidants [11] in acidic medium. In view of the potential pharmaceutical importance of nicotinic hydrazide and lack of literature on the mechanism of its oxidation it is of immense interest to follow the oxidation kinetics of nicotinic hydrazide by [MnMo9O32] 6− in perchloric acid medium. Nicotinic acid hydrazide or pyridine-3-carbohydrazide commercially known as nicotinic hydrazide (NIH). Recent studies have shown that nicotinic acid hydrazide could be considered as anti-inflammatory, analgesic agent [12] and as a novel pharmacophore in the design of anticonvulsant drug [13]. Nicotinic acid hydrazide is one of such pyridine containing hydrazide which is an analogue of isoniazid the antituberculosis drugs. MATERIALS AND METHODS Chemicals All chemicals used were of analytical reagent grade and double distilled water was used throughout the work. A solution of nicotinic hydrazide was prepared by dissolving a known amount of recrystallized sample in double distilled water. The purity of nicotinic hydrazide sample was checked by comparing its FT-IR spectrum with literature data and with its melting point160-163C. The cobalt complex [MnMo9O32] was prepared by the reported method [14] as follows 50 gm of ammonium molybdate was dissolved in 200 ml of water, excess of hydrogen peroxide as oxidant was added to it and the resultant solution was heated to 95C. To this hot solution, 5 gm of MnSO4.H2O in 50 ml of water was added slowly with constant stirring. The resultant orange-red colored solution was boiled for 10 min and quickly filtered and cooled. The orange-red colored crystals were recrystallized thrice from hot (70C) water. The solution of (NH4)6[Mn Mo9O32] was standardized by treating known amount of its solution with excess of As III and back titrating As. The oxidation state of hetero atom was also confirmed to be four by iodometric method and standardized spectrophotometrically using Shimadzu UV-1800 spectrophotometer at wavelength 468nm[7-8]. The ionic strength was maintained using NaClO4 .HClO4 and NaCl AR grade. Kinetic measurements The kinetic measurements were performed on a ShimadzuUV-1800 UV-Visible spectrophotometer. The reaction between [MnMo9O32] 6− and nicotinic hydrazide was followed under pseudo-first order condition where as nicotinic hydrazide was always in excess over [MnMo9O32] 6− at a constant ionic strength of 0.3 mol dm in acidic medium at constant temperature 27±0.2C. The reaction was initiated by mixing the [MnMo9O32] and nicotinic hydrazide solutions, which also contains required concentration of HClO4 and NaClO4. The reaction was followed by measuring the absorbance of [MnMo9O32] 6− at 468 nm as a function of time. Beer’s law was tested for [MnMo9O32] 6− between the concentration 1.0×10 and 5.0×10 mol dm (ε468 = 360±2 mol dm −3 cm ) under the experimental conditions [7-8]. Spectrophotometric measurements The UV-Visible spectra of [MnMo9O32]complex and the reaction mixture were recorded by using ShimadzuUV-1800 UV-Visible spectrophotometer between the wavelength range 400-650 nm. The spectrum of the reaction mixture at various time intervals is shown in (Fig-1) Stoichiometry and product Analysis The stoichiometry was studied by keeping concentration of [MnMo9O32] 6− 1.0 × 10mol dm constant and varying the concentration of nicotinic hydrazide from 0.1× 10 to 0.5× 10 mol dm with constant concentration of 0.5 mol dm HClO4. The reactants were equilibrated at 27±0.2C for 24 hours in a nitrogen atmosphere. The progress of the reaction was followed by measuring the absorbance at 468 nm. The stoichiometry was found to be one mole of [MnMo9O32] 6− per mole of nicotinic hydrazide after completion of the reaction The reaction product was extracted with ethyl acetate and recrystallized from aqueous alcohol. International Journal of Industrial Chemistry and Biotechnology Page: 2 Available online at www.ijicab.com Sunil G. Shankarwar et al Int. J.Ind.Chem and Biotec Copyrights@2016 The purity was checked by melting point. The main oxidative product was identified by spot test and FT-IR spectrum which showed a band at (ύ) 1698 cm due to >C=O stretching of acid and broad band at (ύ)3071 cm due to O-H stretching. H NMR spectrum (400MHz DMSO), shows peaks δ 13.12 (s 1H), acidic OH (D2O exchanged) 9.12 (s 1H), 8.29 (d 1H), 8.77 (d 1H) and 7.49 (dd 1H). Mass spectrum of product indicates the presence of nicotinic acid by molecular ion peak at m/z (M) 124 amu. RESULTS AND DISCUSSION Reaction order The reaction orders have been determined from the slopes of log kobs versus log concentration plots by varying the concentrations of nicotinic hydrazide, [MnMo9O32] 6− and HClO4 in turn while keeping other conditions constant. The concentration of [MnMo9O32] was varied in the range 0.1×10 to 2.0×10 mol dm at fixed nicotinic hydrazide, [H] and ionic strength. The non-variation in the pseudo first order rate constant at various concentrations of [MnMo9O32] 6− indicates the order in [MnMo9O32] as unity (Table1). The substrate nicotinic hydrazide concentration was varied in the range 0.2×10 to 2.0×10 mol dm at 27±0.2C keeping all other reactants concentration and conditions constant. The apparent order in nicotinic hydrazide was found to be less than unity under the experimental conditions. The effect of increasing concentration of perchloric acid on the reaction rate at constant concentrations of nicotinic hydrazide and [MnMo9O32] at constant ionic strength was studied. The rate constants were found to be increased with the increase in acid concentration and the order in acid was less than unity (Table1). Effect of hydrogen ion concentration The effect of hydrogen ion concentration on the rate of reaction was studied at constant nicotinic hydrazide, [MnMo9O32] 6− at ionic strength 0.3 mol dm at 27±0.2C. The effect of hydrogen ion concentration on the reaction was studied in order to understand the nature of reactant species present in the solution. The concentration of [H] ion was varied between 0.01 to 1.0 mol dm keeping all other concentrations constant. It was found that the reaction rate increases on increase in concentration of [H] ions as shown in (Table 1). The order of [H] ion was found to be 0.64 as determined from the plot of log kobs versus log [H + ] ion. Effect of ionic strength The effect of ionic strength was studied by varying sodium perchlorate concentration in the reaction mixture. The ionic strength of the reaction medium was varied from 0.05 to 1.5 mol dm, with all other reactant concentrations and other conditions being constant. It was found that as ionic strength increased, the rate of reaction decreased [15]. The plot of log kobs versus I 1/2 was linear with negative slope. Effect of solvent polarity The relative permittivity effect was studied by varying the percentage of acetonitrile from 10 to 40 % v/v in the reaction mixture with all other conditions being constant. The relative permittivities of the reaction mixtures were computed from the values of the pure solvent. It was f