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 1Effect 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 2Effect 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. The effect kcat ×10 , s 4.91 7.73 9.44 19.0 of catalyst concentration was studied between the Uncatalyzed Catalyzed -6 -5 -3 concentration range 1.0 × 10 and 5.0 × 10 mol dm Ea, kJ mol-1 64.9 ± 0.5 50.5 ± 0.6 and the plot of kcat against [catalyst] is found to be ∆G#, kJ mol-1 94.7 ± 0.5 89.6 ± 0.6 linear, indicating an order of unity in [catalyst]. ∆H#, kJ mol-1 61.5 ± 0.5 47.6 ± 0.6 −∆S#, JK-1 mol-1 111.3 ± 4 140.8 ± 5 Effect of hydrogen ion concentration The effect of hydrogen ion was studied in order to and 313 K. The activation parameters ∆H#, ∆G# and understand the nature of reactant species present in ∆S# along with corresponding pseudo-first-order rate + -2 the solution. The [H ] was varied between 4.0 × 10 constants are given in Table 2. -3 + and 0.4 mol dm (Table 1). Increasing [H ] accelerates the rate of reaction and the order in [H+] is Test for free radical intervention found to be about 1.9 for uncatalyzed and about 2.3 In order to understand the intervention of free for catalyzed reaction as determined from the plot of radicals in the reaction the reaction was studied in + log kobs against log [H ]. presence of added acrylonitrile for both uncatalyzed and catalyzed reactions. It is found that there is no Effect of ionic strength induced polymerization of the acrylonitrile in both the The effect of ionic strength was studied keeping reactions, as there was no formation of the precipitate × [NIH], [KBrO3], [catalyst] and [HCl] constant at 1.0 due to polymerization of acryllonitrile and also it does -2 -3 -3 -3 -5 -3 10 mol dm , 1.0 × 10 mol dm , 1.0 × 10 mol dm not affect the rate of the reaction. -3 and 0.1 mol dm respectively. Sodium perchlorate was used to vary the ionic strength. The rate of Mechanism of the reaction and the rate laws uncatalyzed reaction decreases with increasing ionic Uncatalyzed reaction -3 -3 + strength from 0.05 mol dm to 0.5 mol dm , while The order in [H ] is found to be 1.9 for uncatalyzed that of selenium dioxide catalyzed reaction remain reaction indicating two prior protonation equilibria. unaffected. Potassium bromate is a strong electrolyte and in - aqueous solution it exists as BrO3 which is also a Effect of solvent polarity strong acid thus its protonation would not be possible The effect of solvent polarity on both the under the experimental conditions. Another possibility uncatalyzed and catalyzed reactions was studied by of explaining the presence of protonation prior varying percentage of acetic acid from 2% to 40% v/v equilibria is the involvement of an induction period. keeping nicotinic acid hydrazide and bromate The bromate oxidations of one-electron oxidants are -2 -3 concentration, constant at 1.0 × 10 mol dm and also found to involve induction period17 due to initial -3 -3 1.0 × 10 mol dm respectively. The dielectric hydrogen ion dependent reduction of bromate constants of the reaction mixture were calculated by according to the equilibrium shown in following using the D values for pure solvents. It is found that equation: the decrease in the dielectric constant of the medium - + increases the rate of the reaction for both uncatalyzed Red + BrO3 + 3H Ox + HBrO2 + H2O … (2) and catalyzed reactions. The plots of log kobs against (1/D), where D is the dielectric constant, are found to But in the present investigation there is no such induction period which is also not observed by be linear with negative slopes. 18 Thomson during oxovanadium(IV) oxidation by Effect of temperature bromate. Therefore, the hydrogen ion dependence of The effect of temperature on both the uncatalyzed the reaction is not due to the equilibrium (2). Other and catalyzed reactions was studied at 293, 298, 303 possible protonation in the present system would be YALGUDRE & GOKAVI: KINETICS & MECHANISM OF UNCATALYZED & SeO2 CATALYZED OXIDATION OF NIH 73

that of the nicotinic acid hydrazide(NIH). There are expected to increase as the [reactants] increase. The two possible protonation sites in nicotinic acid decrease in the values of kuncat would indicate hydrazide analogous to that of isoniazid19, the formation of two complexes between the reactants, pyridine nitrogen and –NH2 group of the hydrazide. one of them does not decompose while the other The pK of the pyridine nitrogen is reported19 to be decomposes to give products. The oxidant in the 1.8 and that of the –NH2 group is 3.5. Since the present study is an anion which can undergo an ion- reaction is carried out in acidic medium the nicotinic pair complex with positively charged sites of the acid hydrazide will be present in the diprotonated substrate, nicotinic acid hydrazide. In acidic medium form. The protonation equilibria of both the sites can the active species of the nicotinic acid hydrazide is 2+ be represented using the following equations: NIH3 as shown in equilibrium (3) which contain protonated pyridine nitrogen as well as protonated – NH2 group of the hydrazide moiety. The ion-pair complex between the protonated –NH2 group leads to the further reaction while that with protonated pyridine nitrogen does not undergo further reaction. Such stable tetra alkylammonium salts of bromate22-24 have been prepared and used for various synthetic applications. Therefore, the pyridinium salt of the bromate in the present study is quite stable and does not undergo any oxidative transformation, thus converting the oxidant into an inactive form. In order to understand the interaction between the reactants, the UV-VIS spectra of nicotinic acid hydrazide in presence of bormate in acidic medium was Further, the π →π∗ and n→π* transitions of the 20 investigated. It is observed that the intensity of the hydrazide are observed between 225-260 nm and absorbance between 200 nm and 300 nm increases as 270-290 nm respectively and both these transitions the bromate ion concentration increases21 indicating are sensitive to the pH of the solution. Therefore, to the complex formation between the two. The get further information regarding protonation the UV- mechanism of the reaction is shown in Scheme 1 in VIS spectra of nicotinic acid hydrazide was examined terms of active species of the reactants. The rate law in presence of HCl. The spectrum of aqueous solution for the uncatalyzed reaction and the corresponding of nicotinic acid hydrazide shows peak at 264 nm but -3 expression for the kuncat can be represented by Eq. 5 in presence of 0.1 mol dm HCl the peak shifts at and Eq. 6 respectively. The following Eqs (5) and (6) 268 nm with increase in intensity and also a new peak are derived by considering the protonation equilibria at 216 nm is observed analogous to that observed for 21 of the nicotinic acid hydrazide and the formation of isonicotinic acid hydrazide . It is also observed that ion pair complexes as shown in Scheme 1: there are two isosbestic points at 228 and 246 nm, + 2 - + 2 + indicating the existence of three absorbing, NIH, Rate = k1K3[H ] [BrO3 ][NIH]/([H ] + K1[H ] + + 2+ 21 NIH2 and NIH3 species in the solution . Since, the K1K2)(1 + (K3+K4)[NIH]) ... (5) + order in [H ] is more than unity, the diprotonated 2+ + 2 + 2 + NIH3 of the nicotinic acid hydrazide is the active kuncat = k1K3[H ] /([H ] + K1[H ] + K1K2) (1 + species in the reaction. (K3+K4)[NIH]) ... (6) The mechanism of the uncatalyzed reaction involves interaction between diprotonated nicotinic The decrease in the kuncat values as the 2+ acid hydrazide, NIH3 , and bromate in a prior concentration of nicotinic acid hydrazide increases is equilibrium forming a complex which further due to the formation of an inert ion-pair complex decomposes to give the products. The kuncat values are between the protonated pyridine nitrogen and the found to decrease as the [NIH] increases while the bromate ion. The reported values of equilibrium -2 3 -1 values remain almost constant as the [oxidant] constants K1 and K2 are 1.58 × 10 dm mol and increases. Since the reaction proceeds with the 3.16 × 10-4 dm3 mol-1 respectively. The values are interaction of both the reactants the values kuncat are small and thus if the denominator of Eq (6) is 74 INDIAN J. CHEM. TECHNOL., JANUARY 2013

Fig. 1Plots of (1/kuncat) and (1/kcat) against [NIH] 2 - 3 5 [[NIH] × 10 = [BrO3 ] × 10 = [HCl] ×10 = [SeO2] × 10 =1.0 mol dm-3 and I = 0.5 mol dm-3]

protonation equilibria are due to that of nicotinic acid hydrazide as shown in Eqs (3) and (4), and bromate ion does not take part in any of the hydrogen ion dependent reaction as explained earlier. Therefore, the third protonation is due to either the catalyst itself or any intermediates of the catalyst generated during the reaction. Selenium dioxide in aqueous solution exists as selenous acid and H SeO , the step wise dissociation 2 3 Scheme 1Uncatalyzed oxidation of nicotinic hydrazide (NIH) constants of this acid are as shown below: by bromate + - H2SeO3 K5 H + HSeO3 … (7) + 2 neglected the plot of kuncat against [H ] is expected to 2- be linear. Such a plot is found to be linear without any HSeO3- K6 H+SeO3 … (8) intercept thus verifying the derived rate law Eq. (6) on The dissociation constant25 for first equilibrium is the basis of Scheme 1. Further, according to Eq. (6) -3 -3 2.4 × 10 mol dm whereas that of the second the plot of (1 / k ) against [NIH] is also fund to be uncat dissociation constant is 4.76 × 10-9 mol dm-3. The linear with an intercept (Fig. 1), thus supporting the values of dissociation constants indicate that first mechanism predicted. deprotonation occurs, to a significant extent, in The reaction shown in Scheme 1 involves formation aqueous acidic solutions but due to very low value, of two ion-pair complexes and the complex 1 and the second deprotonation does not occur under decomposes in a two-electron transfer slow step the acidic condition of the present study. Therefore, generating acyl diimide as intermediate. Further, the under the present conditions of the reaction selenium nucleophilic attack of water molecule on the carbonyl dioxide exists as selenous acid which will be in carbon of acyl diimide intermediate gives nicotinic equilibrium with the HSeO - and the catalysis by [H+] acid and another intermediate NH=NH. The fast 3 indicates selenous acid as the reactive species. oxidation of NH=NH by the HOBr will complete the Hydrazides are very good reductants with reduction observed stoichiometry of the reaction. 4 potential of benzoic acid hydrazide is reported to be Selenium dioxide catalyzed reaction 0.19 V and our preliminary experiments in the The hydrogen ion dependence of the selenium absence of bromate indicates that when selenium dioxide catalyzed reaction is about 2.3 an order more dioxide is allowed to react with nicotinic acid than that of the uncatalyzed reaction. Therefore, three hydrazide, red colloidal selenium is precipitated protonation prior equilibria are taking part in the which on standing turns grey. But in presence of mechanism of the catalyzed reaction. Two of the oxidant no such precipitation is observed. Therefore, YALGUDRE & GOKAVI: KINETICS & MECHANISM OF UNCATALYZED & SeO2 CATALYZED OXIDATION OF NIH 75

the mechanism of the reaction involves reduction of + 3 k2 K7 [H ] [H2 SeO 3 ][NIH] [BrO− ] the selenium dioxide by hydrazide converting it into Rate = 3 … (9) ([H+ ] 2 + K [H+ ]+K K )(K +[H+ ]) an intermediate species which is then oxidized back 1 1 2 5 by bromate thus completing the catalytic cycle. During reduction of selenium dioxide by olefins26 in k K [H+ ][HSeO 3 ] k = 2 7 2 3 … (10) acetic acid- acetic anhydride it has been proposed that cat ([H+ ] 2 +K [H + ]+K K )(K +[H + ]) - - 1 1 2 5 HSeO2 is generated. Therefore, HSeO2 can be considered as the intermediate produced after the The decrease in the rate of reaction as [NIH] reduction of selenous acid by hydrazide. Further, 27 increases is due to the ion pair formation of bromate bromate has been used as an analytical volumetric 2+ with [NIH3 ] as explained in case of uncatalyzed reagent for the titration of small amount of colloidal - reaction. Then on substituting [BrO3 ] by considering selenium. In such titration the colloidal selenium is ion pair formation and substituting in Eq. (9), we get oxidized to selenous acid and further oxidation to the final rate law Eq. (10). The expression for kcat will selenic acid by bromate does not takes place. be given by Eq. (11): Therefore, considering the kinetic data obtained and the reported results, the mechanism of hydrazide + 3 k2 K[H7 ][HSeO2 3 ] oxidation is initiated by the reduction of selenous acid kcat = + 2 + + - ([H ] +K1 [H ]+K1 K 2 )(K 5 +[H ])(1+(K3 +K 4 )[NIH] to HSeO2 . Since the reaction is carried out in acidic - medium the intermediate, HSeO2 , is in the form of … (11) H2SeO2 which is oxidized by bromate in a fast step to selenous acid. The comparison of the pseudo-first-order rate The selenium dioxide mechanism of nicotinic acid constants of both uncatalyzed and catalyzed reactions hydrazide by bromate can be summarized as in indicates that the uncatalyzed reaction occurs to the Scheme 2. The corresponding rate law is given by negligible extent in presence of catalyst. Therefore, Eq. (9) and the expression for kcat by Eq. (10) as while deriving Eq. (11) the contribution of the shown below: uncatalyzed reaction is neglected. The derived

Scheme 2Selenium dioxide catalyzed oxidation of nicotinic hydrazide (NIH) by bromate 76 INDIAN J. CHEM. TECHNOL., JANUARY 2013

Eq. (11) for the pseudo-first-order rate constant based Acknowledgement on the mechanism of Scheme 2 explains the order of One of the authors (RSY) gratefully acknowledges more than two in [H+] and an order of unity in the University Grants commission, New Delhi for the [catalyst]. The equilibrium constants K1 and K2 are award of teacher fellowship under FIP- UGC-XI plan. small and [H+] is also small, therefore as an approximation if we neglect the denominator of Eq. References 1 Carcelli M, Mazza P, Pelizzi C, Pelizzi G F & Zani F, (11) then it is possible to verify the equation after + 3 J Inorg Biochem, 57 (1995) 43. rearranging, and by plotting kcat against [H ] this is 2 Gudasi K B, Patil M S, Vadavi R S, Shenoy R V, Patil S A & found to be linear (Fig. 1) with an intercept, thus Nethaji M, Spectrochimica Acta Part A, 67 (2007)172. 3 Galic N, Peric B, Kojic-Prodic B & Cimerman Z, J Mol verifying the rate [Eq. (11)]. The plot of 1/ kcat against [NIH] is also found to be linear, further supporting the Struct, 559 (2001) 187. 4 Amos R I J, Gourlay B S, Schiesser C H, Smith J A & Yates proposed mechanism. B F, Chem Commun, (2008) 1695.

The increase in ionic strength of the reaction 5 Kocevar M, Mihorko P & Polanc S, J Org Chem, 60 (1995) decreases the rate of uncatalyzed reaction, while the 1466. 6 Kulkarni P P, Kadam A J, Desai U V, Mane R B & rate of the catalyzed reaction remains unaffected. The - Wadgaonkar P P, J Chem Res(S), 2000, 184. uncatalyzed reaction occurs between BrO3 and 7 Reddy C S & Kumar T V, Transition Met Chem, 32 (2007) 2+ NIH3 , thus decreasing the rate with increase in ionic 246. strength while the catalyzed reaction involves neutral 8 Maity A C, Synlett, 2008, 465. H SeO species of the catalyst. The test for free 9 Shunichi M & Tatsuki S, Tetrahedron Lett, 28 (1987) 2383. 2 3 10 Christin G, Beate P, Elisabeth I & Karola R, Synthesis (2008) radicals is found to be negative for both uncatalyzed 1889. and catalyzed reactions, therefore the reaction 11 Manktala R, Dhillon R S & Chhabra B R, Indian J Chem, proceeds without any intervention of free radicals. 45B (2006) 1591. This observation is also supported by the product 12 Halina W, Monica B, Krystian K & Jacek M, Tetrahedron, 57 (2001) 9743. analysis in which corresponding nicotinic acid is the 13 Guzman J A, Mendoza V, Gracia E, Garibay C F, OlivaresL only product obtained. The N-N–diacylhydrazine Z & Maldonado L A, Synth Commun, 25 (1995) 2121. would have been obtained15 along with nicotinic acid 14 Kadam S D, Supale A R & Gokavi G S, Z Phys Chem, as a result of free radical intervention in the reaction. 222 (2008) 635. Colloidal selenium is not formed in any of the 15 Shewale S A, Phadkule A N & Gokavi G S, Int J Chem Kinetics, 40 (2008) 151. catalyzed kinetic runs although it has been obtained in 16 Handbook of Chemistry and Physics, 67th edn (CRC Press, absence of oxidant. Hence, it is assumed that the Florida, USA), 1987. H2SeO2 formed during the reduction of selenous acid 17 Mohammad A, Transition Met Chem, 28 (2003) 345. is oxidized by bromate in a fast step. 18 Thomson R C, Inorg Chem, 10 (1971) 1892. 19 Wheate N J, Vora V, Anthony N G & McInnes F J, J Incl The increase in relative permittivity of the reaction Phenom Macrocycl Chem, 68 (2010) 359. medium with acetic acid increases the rate of both 20 Anoussakdise G, Ristos M & Uri C, Candian J Chem, uncatalyzed and catalyzed reactions and the plots of 51 (1973) 811. log k against (1/D)( D = dielectric constant of the 21 Yalgudre R S & Gokavi G S, Ind Eng Chem Res, 51 (2012) obs 5135. medium) are linear with a negative slope. The charge 22 Grancicova O, React Kinet Catal Lett, 94 (2008) 99. separation in the transition state formed and its larger 23 Nath U, Das S S, Deb D & Das P J, New J Chem, 28 (2004) size increases its stability28 in the medium of higher 1423. relative permittivity, thus increasing the rate of 24 Das S S, Nath U, Deb D & Das P J, Synth Commun, 34 (2004) 2359. reaction with increase in acetic acid content. The 25 Lurie Ju, Handbook of Analytical Chemistry (MIR # negative value of ∆S can be ascribed to the lesser Publishers, Moscow), 1971. degree of freedom formerly available to the reactants. 26 Trachtenberg E N, Nelson C H & Carver J R, J Org Chem, The moderate value of enthalpy of activation is due to 35 (1970) 1653. 27 Coleman W C & McCrosky C R, Ind Eng Chem Anal, the electron transfer process. The activation energy of 9 (1937) 1458. the uncatalyzed reaction is higher than that of the 28 Amis E S, Solvents Effect on Reaction Rates and Mechanism catalyzed reaction as expected. (Academic Press, New York), 1996.