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Kinetics and Mechanism of Uncatalyzed and Selenium Dioxide Catalyzed Oxidation of Nicotinic Acid Hydrazide by Bromate

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Kinetics and Mechanism of Uncatalyzed and Selenium Dioxide Catalyzed Oxidation of Nicotinic Acid Hydrazide by Bromate Indian Journal of Chemical Technology Vol. 20, January 2013, pp. 70-76 Kinetics and mechanism of uncatalyzed and selenium dioxide catalyzed oxidation of nicotinic acid hydrazide by bromate R S Yalgudre & G S Gokavi* Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India Received 27 June 2011; accepted 23 August 2012 The uncatalyzed and selenium dioxide catalyzed oxidation of nicotinic acid hydrazide, (NIH) by bromate has been studied in hydrochloric acid medium. The –NH2 of hydrazoic moiety and pyridine nitrogen of the NIH forms protonated species which are involved in two ion pair complexes with the oxidant in prior equilibria. In case of uncatalyzed reaction the complex with the protonated hydrazoic moiety decomposes to give corresponding acyl diimide intermediate while that of the pyridine nitrogen decreases the rate of reaction. In presence of selenium dioxide as catalyst, the NIH reduces the catalyst to H2SeO2 species which is oxidized by the oxidant to complete its catalytic cycle. The product of the reaction is found to be nicotinic acid and there is no intervention of any free radicals. A rate law derived for both the reactions satisfy the kinetic data obtained and UV-spectrophotometer examination of the reaction mixture also support the mechanisms proposed. Keywords: Bromate, Catalysis, Nicotinic acid hydrazide, Selenium dioxide The ligands containing either a hydrazone or organic substrates is slow which requires a catalyst5,7 hydrazine moiety are found to exhibit antibacterial to inititiate the reaction. The catalyst of the present activity1 and such activity is also enhanced by study is selenium dioxide which is a mild and complexation2 of hydrazides with metal ions of the selective oxidant8 used in synthetic organic chemistry. first transition series. Therefore, aroylhydrazones are Due to its mild nature, selenium dioxide is also used used as ligands in inorganic coordination chemistry. as a catalyst for various organic transformations8 in Further, hydrazones containing pyridine ring are also presence of cooxidants like hydrogen peroxide. Such utilized as analytical reagents for transition and catalysis include oxidation of amines9, anilines10, lanthanide ions due to their high sensitivity3 towards alkenes11, aldehydes12 and Bayer-Villiger13 reactions. these metal ions. Nicotinic acid hydrazide is one such Therefore, the activity of selenium dioxide will be pyridine containing hydrazide which is an anologue of facilitated by the presence of another cooxidant like isoniazid, the anti tuberculosis drug. These hydrazides bromate. In continuation of our earlier work14,15, in are also found to effectively inhibit4 the this study the oxidation of nicotinic acid hydrazide by myeloperoxidase enzyme activity. Hydrogen peroxide bromate is investigated kinetically to know the is generated during digestion of pathogens in presence probable pathway of its oxidative degradation. of myeloperoxidase enzyme, which, in turn, reacts with chloride ion to produce hypochlorous acid. The Experimental Procedure role of hydrazides is to react with the intermediates Materials and methods like hypochlorous acid, thus inhibiting the tissue Double distilled water was used throughout the damage at the sites of inflammation. In synthetic work. All the chemicals used for experiments were of organic chemistry, hydrazides are used as starting reagent grade. The stock solution of KBrO3 was materials for preparation of esters, amides5 and N-N- prepared by dissolving KBrO3 (BDH) in water and diacylhydrazines6 in presence of various oxidizing standardized iodometrically. The solution of nicotinic agents. acid hydrazide(SD fine) was prepared by dissolving The oxidant of the present study, bromate, is also a requisite amount in water. The ionic strength was strong oxidizing agent but its rate of oxidation of maintained using sodium perchlorate and to vary hydrogen ion concentration HCl (BDH) was used. ____________ *Corresponding author. Acetic acid and acrylonitrile were used directly as E-mail: [email protected] received to study the effect of solvent polarity on the YALGUDRE & GOKAVI: KINETICS & MECHANISM OF UNCATALYZED & SeO2 CATALYZED OXIDATION OF NIH 71 reaction medium and free radical formation shows peak at 123. The m.p. of the recrystallized respectively. The solution of catalyst selenium (IV) product is found to be 236oC (lit m. p. 236-237oC16). was obtained by dissolving selenium dioxide (SD From the GCMS analysis and the m. p. determination fine) in distilled water. the product of the reaction is confirmed to be nicotinic acid for both uncatalyzed and catalyzed reactions. Procedure and kinetic measurements Therefore, the stoichiometry of the reaction is found to The reaction was studied under pseudo-first-order be two moles of oxidant per three moles of the conditions keeping hydrazide concentration large hydrazide as shown in following equation: excess over that of oxidant (KBrO3) at constant o - - temperature of 25.0 ± 0.1 C. The reaction was 2BrO3 + 3RCONHNH2 2Br + 3RCOOH+3H2O + 3N2 initiated by mixing the previously thermostated ... (1) solutions of the oxidant, substrate and catalyst which where R = C5H5N also contain the required amount of hydrochloric acid, potassium chloride and distilled water. The reaction Results and Discussion was followed by titrating the reaction mixture for Effect of reactants concentration unreacted oxidant iodometrically and the rate The reaction was carried out under pseudo-first- constants were determined from the pseudo-first-order order conditions keeping the concentration of plots of log [oxidant] against time. The pseudo-first- nicotinic acid hydrazide large excess at a constant order plots were linear for more than 90% of the HCl concentration (0.1 mol dm-3) and at a constant reaction and rate constants were reproducible within ionic strength of 0.5 mol dm-3 (Table 1). The pseudo- ±6% for both uncatalyzed and selenium dioxide first-order plots are found to be linear on varying the catalyzed reactions. concentration of oxidant between 0.5 × 10-3 and 5.0 × 10-3 mol dm-3, keeping the concentration of nicotinic Stoichiometry and product analysis acid hydrazide (NIH) constant at 1.0 × 10-2 mol dm-3 The stoichiometry of bromate oxidation predicts (Table 1) for both uncatalyzed and catalyzed either Br2 or HOBr as the product of reaction but the reactions, indicating that the order in oxidant hydrazides can be very easily oxidized by both of 4 them in acidic solutions due to the oxidation potential Table 1Effect of [bromate], [NIH] and [H+] on the uncatalyzed of HOBr or Br2 as 1.34 and 1.07 V respectively. The and selenium dioxide catalyzed oxidation of nicotinic acid test for formation of bromide ion was carried out in hydrazide by bromate at 298 K -3 5 -3 sulphuric acid solution instead of hydrochloric acid, [I = 0.5 mol dm and [SeO2] ×10 = 1.0 mol dm ] for both uncatalyzed and catalyzed reactions, by 3 2 + 4 4 0[BrO3]×1 [NIH] ×10 [H ]×10 kuncat×10 kcat×10 adding silver nitrate to the reaction mixture after mol dm-3 mol dm-3 mol dm-3 s-1 s-1 completion of the reaction. The precipitation of silver bromide confirms the formation of bromide ion as one 1.0 0.5 1.0 1.0 8.70 of the product of the reaction. Therefore, the product 1.0 0.8 1.0 0.88 8.20 1.0 1.0 1.0 0.70 7.70 of the reaction under the present experimental 1.0 2.0 1.0 0.57 6.10 conditions is bromide ion. It is also noticed during the 1.0 5.0 1.0 0.31 2.80 kinetic studies and the stoichiometric analysis that no 1.0 7.0 1.0 0.21 2.30 bromine is evolved, further confirming the bromide 0.5 1.0 1.0 0.71 7.80 ion as the only product. 0.6 1.0 1.0 0.70 78.0 Further, in 10 mL of 0.6 mol dm-3 hydrochloric acid 0.8 1.0 1.0 0.71 7.71 1 mmol nicotinic acid hydrazide (0.1371 g) was 1.0 1.0 1.0 0.71 7.72 dissolved. To the resulting solution 2 mmol (0.3349 g) 2.0 1.0 1.0 0.70 7.83 of KBrO was added in presence of 1.0 × 10-5 mol dm-3 3.0 1.0 1.0 0.70 7.81 3 5.0 1.0 1.0 0.71 7.82 selenium dioxide for catalyzed reaction. The reaction o 1.0 1.0 0.4 0.27 1.01 mixture was stirred at 25 C for 2 days in case of 1.0 1.0 0.8 0.38 4.02 uncatalyzed reaction and 1 day for catalyzed reaction. 1.0 1.0 1.0 0.71 7.81 The respective nicotinic acid separated was filtered and 1.0 1.0 2.0 4.0 30.1 recrystallized form ethanol-water mixture. The GCMS 1.0 1.0 3.0 8.0 68.3 analysis of the solution of the product in methanol 1.0 1.0 4.0 16 180.0 72 INDIAN J. CHEM. TECHNOL., JANUARY 2013 concentration is unity. The effect of nicotinic acid Table 2Effect of temperature and activation parameters of hydrazide was studied by varying the concentration of uncatalyzed and catalyzed oxidation of nicotinic acid hydrazide by nicotinic acid hydrazide between 5.0 × 10-3 and 7.0 × bromate -2 -3 3 -3 -3 10 mol dm keeping all other concentrations [BrO3] ×10 = 10[NIH] = 10[HCl] = 0.1 mol dm , I = 0.5 mol dm and × 5 -3 constant (Table 1). The pseudo-first-order rate [SeO2] 10 = 1.0 mol dm ] constants(kobs) are found to decrease as the Temp.,K 293 298 303 313 4 -1 concentration of nicotinic acid hydrazide increases for kuncat ×10 , s 0.46 0.70 1.0 2.5 4 -1 both uncatalyzed and catalyzed reactions.
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