alkaline medium with a reduction potential of 1.74 V 1. Jayaprakash Rao et al.

Transcription

1 Diperiodatoargentate(III) (DPA) is a powerful oxidizing agent in alkaline medium with a reduction potential of 1.74 V 1. Jayaprakash Rao et al. have used diperiodatoargentate(iii) as an oxidizing agent for the kinetics of oxidation of various organic substrates 2,3. They normally found that the order with respect to both oxidant and substrate concentration was unity and H - ion concentration was found to enhance the rate of reaction. It was also observed that they did not arrive at the possible active species of DPA in alkali, and on the other hand they proposed mechanisms by generalizing the DPA as [Ag(HL)L] (x+1)- active species, where L is a periodate with an uncertain number of protons and HL is a protonated periodate of an uncertain number of protons. However, Kumar et al. made an effort to give evidence for the reactive form of DPA in the large scale of alkaline ph 4,5. In the present investigation, we have obtained the evidence for the reactive species of DPA in alkaline medium. In view of the strong versatile nature of Ag(III) species, studies on the kinetics of oxidation of various organic and inorganic substrates have attracted attention even in the late 20 th century. Among the various species of Ag(III), Ag(H) 4-, diperiodatoargentate(iii), and ethylene bis (biguanide) (EBS), silver(iii) diverted the maximum attention of researchers because of their relative stability. The stability of Ag(H) 4- is very sensitive toward traces of dissolved oxygen and other impurities in the reaction medium, and this had not attracted much attention. However, the two forms of Ag(III), DPA and EBS, are considerably stable 2,6. DPA is used in highly alkaline medium, and EBS is used in highly acidic medium. 26

2 6-Aminopenicillanic acid (6-APA) is an integral β-lactam compound of the various penicillins. Penicillins consist of the heterocyclic group which consisting of a thiazolidine ring (with 5 members including one sulphur atom) fused with a β-lactam ring (with 4 members), which are distinguished from each other by the nature of the side chain attached to the amine group in position 6 through a peptide link. The structural formula of 6-APA is: H 2 N H H S CH 3 CH 3 N H It is a non-hygroscopic crystal that decomposes at 209 o C and is a key intermediate in the production of many commercial β-lactum antibiotics. It is produced by enzymetic hydrolysis of penicillin G 7. 6-APA has been chosen as the subject of this study because of their large sales volume in Europe and the United states 8. Several studies have indicated that this antibiotic was practically non biodegradable and have the potential to survive waste water treatment. These properties can be expected to lead to the persistence of this compound in the environment and potential for bioaccumulation 9,10. This necessitates the development of various advanced oxidation processes for the transformation of 6-APA in water. Some of the advanced oxidation processes used have their own drawbacks. Chlorination may create and leave disinfection byproducts, and ozonation can form carcinogenic bromate ion by reacting with bromide present in water. The photo catalytic process in the presence of Ti 2 is expensive and time consuming. 27

3 The literature survey reveals that there are no reports on mechanistic studies of 6-amino penicillanic acid oxidation by diperiodatoargentate(iii). Thus 6- aminopenicillanic acid has been selected as a substrate in order to explore its oxidation by diperiodatoargentate(iii) in alkaline medium and to check the reactivity of 6-amino penicillanic acid toward diperiodatoargentate(iii). The present investigation aimed to investigate the kinetics of redox chemistry of the Ag(III) in such media and to arrive at a possible mechanism. EXPERIMENTAL Materials All the chemicals used were of reagent grade and double distilled water was used throughout the work. The 6-aminopenicillanic acid solution was prepared by dissolving it in double distilled water. KN 3 (AR) and KH (BDH) were used to maintain ionic strength and alkalinity respectively of the reaction. Aqueous solution of AgN 3 was used to study the product effect, Ag (I). A stock standard solution of I 4 was prepared by dissolving a known weight of KI 4 (Riedel-de Haen) in hot water and used after 24h. Its concentration was ascertained iodometrically at neutral ph maintained by using phosphate buffer 11. Preparation of diperiodatoargentate(iii)(dpa) DPA was prepared by oxidizing Ag (I) in the presence of KI 4 as described elsewhere 12. The complex was characterized from its U.V. spectrum, which exhibited three peaks at 216, 255 and 362 nm. These spectral features 28

4 were identical to those reported earlier. This diperiodatoargentate(iii) complex is found to be diamagnetic with the following structure. H H I H Ag H I H H It is believed that the periodate acts as a bidentate ligand and contributes to the stabilization of Ag(III). The magnetic moment study reveales that the complex is diamagnetic. This is in good agreement with literartue 12. The compound prepared was analyzed 13 for silver and periodate by acidifying a solution of the material with HCl, recovering and weighing the AgCl for Ag and titrating the iodine liberated when excess KI was added to the filtrate for I - 4. Thus the obtained diperiodatoargentate(iii) was dissolved in water and used for the required concentration of diperiodatoargentate(iii) in the reaction mixture. Instruments used a) For kinetic measurements, a Peltier Accessory (temperature control) attached Varian CARY 50 Bio UV-vis spectrophotometer (Varian, Victoria-3170, Australia) was used. b) For product analysis, a Nicolate Impact -410 FTIR (Thermo, U.S.A.), 300 MHz 1 H NMR spectrometer (Bruker, Switzerland), and LC-MS (Agilent 1100 series API 2000) mass spectrometer ionization technique were used. 29

5 Kinetic studies The kinetic measurements were performed on a Varian CARY 50 Bio UV vis spectrophotometer. The kinetics was followed under pseudo-first order condition where [6-APA] >>[DPA] at 25 ± 0.1 ºC, unless specified. The reaction was initiated by mixing the DPA with 6-aminopenicillanic acid solution which also contained required concentration of KN 3, KH and KI 4. The progress of reaction was followed spectrophotometrically at 360 nm by monitoring decrease in absorbance of DPA with time. The application of Beer s law has been verified and found that ε = ±100 dm 3 mol -1 cm -1 at 360 nm (Fig.II (i) (p.31)). It was also verified that there is a negligible interference from other species present in the reaction mixture at this wavelength. An example run is given in Table II (i) (p.32). The pseudo-first order rate constants k obs,were determined from the log(absorbance) versus time plots (Fig.II (ii) (p.33)). The plots were linear up to 80% completion of reaction and good first order kinetics were observed. The rate constants were reproducible to within ± 5%. Since periodate is excess in the reaction mixture, the possibility of oxidation of 6-aminopenicillanic acid by periodate in alkaline medium at 25 C was verified and found that there was no significant reaction under the experimental conditions. The effect of dissolved oxygen on the rate of reaction was verified by preparing the reaction mixture and following the reaction in an atmosphere of nitrogen. No significant difference between the results obtained in the presence and absence of nitrogen. 30

6 ptical density Fig. II (i) Verification of Beer lamberts law for diperiodatoargentate(iii) concentrations at 360 nm in 0.02 mol dm -3 alkali at 25 o C [DPA] x 10 5 mol dm -3 31

7 Table II (i) xidation of 6-aminopenicillanic acid by diperiodatoargantate (III) in aqueous alkaline medium at 25 o C Example run [DPA] = x 10-5 [6-APA] = x 10-4 [KH] = 0.02 I = 0.04 / mol dm -3 Time (min) ptical density (360 nm) [DPA] x 10 5 (mol dm -3 )

8 2+log(absorbance) Fig. II (ii) First order plots for the oxidation of 6-aminopenicillanic acid by diperiodatoargantate (III) in aqueous alkaline medium at 25 o C [DPA] x10 5 mol dm -3 =1) ; 2) 3.0; 3) ; 4) 7.0; 5) (5) (4) (3) (2) (1) Time(min) 33

9 RESULTS Stoichiometry and product characterization Different sets of reaction mixtures containing varying ratios of DPA to 6-aminopenicillanic acid in presence of constant amount of H -, KN 3 in reaction were kept for 3 h in a Kjeldahl flask under nitrogen atmosphere. The remaining concentration of DPA was estimated by spectrophotometrically at 360 nm. The results indicated 1:1 stoichiometry for the reaction as given in equation (4). The main reaction products were identified as 2-Formyl-5,5 -dimethyl thiazolidine 4-carboxylic acid and Ag (I). H 2 N N S CH 3 CH 3 H - [Ag(H 3 I 6 ) 2 ] + + H - + H 2 + HC N H C 2 NH 3 H + S CH 3 CH 3 H + Ag(I) [H 3 I 6 ] (1) 2- The oxidation product, 2-Formyl-5, 5-dimethyl thiazolidine 4-carboxylic acid was characterized by LC-ESI-MS, FTIR and 1 H NMR spectral studies. LC-ESI-MS analysis was carried out using reverse phase high performance liquid chromatography (HPLC) system with a phenomenes C-18 column, UV/Visible detector and series mass analyzer. 12 µl of acidified reaction mixture was injected. The mobile phase consisted of 10 mm ammonium acetate ph 3.0(eluent A) and acetic acid (eluent B) at a flow rate of 1 ml/min. Gradient elution was run to separate the substrate and reaction products. LC-ESI-MS analysis indicated the presence of main products with 34

10 molecular ions of m/z at and 90% yield of the product is obtained (Fig. II (iii) (p.36)). The product 2-Formyl-5,5-dimethyl thiazolidine 4- carboxylic acid is further confirmed by its characteristic IR (Fig. II (iv) (p.37)) and 1 H NMR (Fig. II (v) (p.38)) spectrum. The disappearance of the sharp band (peak) at 1773 cm -1 due to ketonic carbonyl in 6-aminopenicillanic acid, appearance of aldehydic carbonyl at 1707 cm -1 and appearance of new secondary amine (- NH) band at 3002 cm -1 confirms the product. Further hydroxyl group (-H) observed around 3430 cm -1 in 6-aminopenicillanic acid, remains in the product (Fig. II (iv) (p.37)). The 1 H NMR (DMS) spectrum shows, two singlets at 8.48 and 8.09 δ ppm corresponding to carboxylic acid and aldehydic protons respectively. A multiplet is observed in the region of 4.42 & 4.45 ppm due to C2 and C4 protons, and a singlet at 2.85 ppm corresponds to NH which is D 2 exchangeable. Remaining two methyl protons are resonated as a singlet at 1.90 ppm (Fig. II (v) (p.38)). All these observations proved the formation of 2- Formyl-5,5-dimethyl thiazolidine 4-carboxylic acid as a major product. Reaction orders The reaction orders were determined from the slope of log k obs versus log (concentration) plots by varying the concentrations of one reactant at a time, keeping all other concentrations and conditions constant. Effect of [DPA] The DPA concentration was varied in the range x 10-5 to x 10-4 mol dm -3. The linearity and almost parallelism of the plots of log (absorbance) versus time up to 80% completion of the reaction (Fig. II (ii) (p.33)) indicate 35

11 Fig. II (iii) Chromatogram and LC - ESI - MS spectra of the product, 2-Formyl-5,5 dimethyl thiazolidine 4-carboxylic acid obtained during the oxidation of 6- aminopenicillanic acid by diperiodatoargentate(iii) in aqueous alkaline medium at 25 o C 36

12 Fig. II (iv) FT-IR spectra of the product 2-Formyl-5,5-dimethyl thiazolidine 4- carboxylic acid obtained during the oxidation of 6-aminopenicillanic acid by diperiodatoargentate(iii) in aqueous alkaline medium at 25 o C 37

13 Fig. II (v) 1 H NMR spectrum of the product, 2-Formyl-5,5-dimethyl thiazolidine 4- carboxylic acid obtained during the oxidation of 6-aminopenicillanic acid by diperiodatoargentate(iii) in aqueous alkaline medium at 25 o C 38

14 first order with respect to DPA concentration. Constant values of rate constant at different DPA concentrations (Table II (ii) (p.40)) also confirmed the order with respect to DPA concentration is unity. Effect of [6-APA] The 6-APA concentration was varied in the range x 10-4 to x 10-3 mol dm -3 at 25 0 C. The k obs values increased with the increase in 6-APA concentration (Table II (ii)(p.40)) and found from the plot of log k obs versus log[6-apa], an apparent less than unit order dependence on 6-APA concentration(fig. II (vi) (p.41)) was observed. Effect of [alkali] The effect of alkali on the reaction rate was studied in the range of 4.0x10-3 to 4.0x10-2 mol dm -3. The rate constants increased with increasing H - ion concentration (Table II (ii)(p.40)) and the order with respect to H - ion concentration was found to be less than unity (Fig. II (vi) (p.41)). Effect of [periodate] Periodate concentration was varied from x 10-5 to x10-4 mol dm -3 at constant DPA, 6-APA concentrations and at constant ionic strength. It was observed that the rate constants decreased by increasing I 4 - concentration (Table II (ii)(p.40)). Effect of added products Initially added products, Ag (I) and 2-Formyl-5,5-dimethyl thiazolidine 4-carboxylic acid did not have any significant effect on the rate of reaction. 39

15 Table II (ii) Effect of variation of DPA, 6-APA and alkali concentrations on the oxidation of 6-APA by alkaline diperiodatoargantate (III) at 25 0 C, I = 0.04/ mol dm -3 [DPA] x 10 5 [6-APA] x 10 4 [H - ] x 10 2 [I 4 - ] x10 5 k obs x10 3 k cal x10 3 (mol dm -3 ) (mol dm -3 ) (mol dm -3 ) (mol dm -3 ) (s -1 ) (s -1 )

16 Fig. II (vi) rder with respect to 6-aminopenicillanic acid and H - concentrations on the oxidation of 6-aminopenicillanic acid by DPA in aqueous alkaline medium at 25 0 C logk obs log[6-APA] logk obs log[H - ] 41

17 Effect of ionic strength and dielectric constant of the medium The effect of ionic strength was studied by varying the potassium nitrate concentration in the range 0.02 to 0.32 mol dm -3 at constant concentration of DPA, 6-APA & alkali. It was found that ionic strength of the medium had no significant effect on the rate of reaction. The ionic strength was varied to know which type of species was involved in the reaction (ion-dipole, two ions having the same charge etc.). Dielectric constant of the medium, D was studied by varying the tert-butyl alcohol at constant concentration of DPA, 6-APA & alkali. The dielectric constants of the reaction medium at various composition of t-butyl alcohol-water (v/v) were calculated by the following equation, D = V 1 D 1 + V 2 D 2 where D 1 and D 2 are dielectric constants of water and t-butyl alcohol as 78.5 and 10.9 at 25 0 C and V 1 and V 2 are volume fractions respectively. It was found that dielectric constant of the medium had no significant effect on the rate of reaction. Polymerisation Study The intervention of free radicals was examined as follows: The reaction mixture, to which a known quantity of acrylonitrile scavenger had been added initially, was kept in an inert atmosphere for 1 h. Upon diluting the reaction mixture with methanol, precipitate resulted, suggesting the involvement of free radical in the reaction. 42

18 Effect of temperature The activation parameters for the reaction were studied by using linear regression analysis (also known as the method of least square). In generalised notation, the formula for the straight line is y = ax + b. The most tractable form of linear regression analysis assumes that values of the independent variables x are known without error and that experimental error is manifested only in values of the dependent variable y. Most sets of kinetic data approximate this situation, in as much as times of observation are more accurately measurable than the chemical or physical quantities related to reactant concentrations. The straight line selected by common linear regression analysis is that which minimizes the sum of the squares of the derivations of the y variable from the line. The slope a and intercept b parameters for the above equation can be calculated by linear regression analysis by any of several mathematically equivalent but different looking experiments. Most familiar are, Slope: a = Intercept: b = n x y x y n x 2 ( x) 2 y x 2 x xy n x 2 ( x ) 2 where n is the number of data points and the summation are for all data points in the set. These data were subjected to least square analysis. The influence of temperature on the rate of reaction was studied at 15, 20, 25and 30 0 C. As the temperature increases the rate of reaction also increases. The rate constants (k), of the slow step of Scheme 1 were obtained from the slopes and the intercepts of the plots of 1/k obs versus 1/[6-APA], 1/k obs 43

19 versus [H 3 I 2-6 ] and 1/k obs versus 1/[H - ] plots at four different temperatures. The values of k are given in Table II (iii)(p.46). The energy of activation was evaluated from the plot of log k(y * calc) versus 1/T as shown in Fig. II (vii)(p.45), from which the activation parameters were calculated and are tabulated in Table II (iii)(p.46)). The activation energy of the reaction was calculated by E a = x R x slope The Arrhenius factor A was calculated by, loga = log k + E a /2.303RT The entropy of activation was calculated by, ΔS # = log k log T + E a /4.576 T where, k is in sec -1, temperature in Kelvin and E a in calories. The enthalpy of activation was calculated by, ΔH # = E a RT and the free energy of activation was calculated by, G # = ΔH # - T S # DISCUSSIN A literature survey 12 reveals that the water soluble diperiodatoargentate(iii) (DPA) has a formula [Ag(I 6 ) 2 ] 7- with square planar configuration, similar to a diperiodatocopper(iii) complex with two bidentate periodate ligands to form a planar molecule. When used in alkaline medium, it is unlikely to exist as [Ag(I 6 ) 2 ]

20 Fig. II ( vii) Effect of temperature on the oxidation of 6-aminopenicillanic acid by diperiodatoargantate (III) in aqueous alkaline medium / T X log k ( y* cal)

21 Table II (iii) Effect of temperature on the oxidation of 6-aminopenicillanic acid by diperiodatoargantate (III) in aqueous alkaline medium with respect to slow step of Scheme 1 Temperature (K) 288 1/T x 10 3 (K) (x) k x 10 2 (dm 3 mol -1 s -1 ) 0.36 log k (y) log k ( y* cal ) Activation parameters Parameters E a (kj mol -1 ) ΔH # (kj mol -1 ) ΔS # (JK -1 mol -1 ) ΔG # (kj mol -1 ) log A Values 23±1 20±1-222±5 86±3 ±0.1 46

22 Depending on the ph of the solution, periodate exists in various protonated forms and is given in the equilibria (2)- (4). H 5 I 6 H 4 I H + K 1 = 5.1 x 10-4 (2) H 4 I 6 - H 3 I H + K 2 = 4.9 x 10-9 (3) H 3 I 6 2- H 2 I H + K 3 = 2.5 x (4) Periodic acid, H 5 I 6, exists in acid medium and at ph=7 as H 4 I 6 -. Thus, under the present experimental conditions, the main species of periodic acid are expected to be H 3 I 6 2- and H 2 I At higher alkali concentrations, periodate also tends to dimerize 1. However,the formation of dimerised species is negligible under conditions employed for kinetic studies. n the contrary, the authors 2,3 in their recent studies proposed the diperiodatoargentate(iii) as [Ag(HL) 2 ] x-. This can be ruled out by considering the alternative form 14 of I - 4 at ph > 7 which is in the form H 3 I 2-6 or H 2 I 3-6. Hence, DPA could be considered as [Ag(H 3 I 6 ) 2 ] - or [Ag(H 2 I 6 ) 2 ] 3- in alkaline medium. So, under the present experimental conditions, diperiodatoargentate(iii) may be regarded as [Ag(H 3 I 6 ) 2 ] -, which is also the case in earlier work 15. During the experimental study it has been observed that, as the H - ion concentration increases, the rate of reaction also increases and the added periodate retards the reaction rate. The reaction is first order dependency in DPA and fractional order dependency in 6-APA concentration. Based on the experimental results the following mechanism (Scheme 1) has been proposed. 47

23 - - [Ag(H 3 I 6 ) 2 ] + H K 4 [Ag(H 2 I 6 ) (H 3 I 6 )] 2- +H 2 K 5 [Ag(H 2 I 6 ) (H 3 I 6 )] 2- +2H 2 [Ag (H 2 I 6 ) (H 2 ) 2 ] + [H 3 I 6 ] 2- [Ag(H 2 I 6 ) (H 2 ) 2 ] + H 2 N S CH 3 N CH 3 H K 6 Complex (C) Complex(c) slow k. CH S CH 3 CH N 3 H H + C 2 + NH 3 + [Ag(H)] + + [H 3 I 6 ] 2- + H +. CH H N S CH 3 CH 3 H fast + [ Ag (H) ] + - H Ag(I) + HC H N S CH 3 CH 3 CH + H 2 Scheme 1 In the prior equilibrium step 1, the [H - ] deprotonates the DPA to give a deprotonated diperiodatoargentate(iii). In the second step, displacement of a ligand, periodate, takes place to give free periodate which is evidenced by decrease in the rate with increase in periodate concentration (Table II (ii)(p.40)). It may be expected that a lower Ag(III) periodate species such as monoperiodatoargentate(iii) (MPA) is more important in the reaction than the DPA. The inverse fractional order in [H 3 I 2-6 ] might also be due to this. In the pre-rate determining stage, this MPA combines with a molecule 6-APA to give 48

24 a complex, which decomposes in a slow step to give the free radical derived from 6-APA and Ag(II) species. This free radical of 6-APA combines with Ag (II) species in a fast step to give an 2 Formyl-5, 5-dimethyl thiazolidine 4- carboxyllic acid. 6-APA-DPA complex (C) is formed by chelation of MPA molecule at the NH 2 group of 6-APA. As oxygen is more electronegative compared to nitrogen, nitrogen donates lone pair of electron to monoperiodatoargentate(iii). The Michaelis-Menten plot (Fig. II (ix) (p.54) proved the complex formation between DPA and 6-APA, which explains the less than unit order in 6-APA concentration. Spectroscopic evidence for the complex formation between oxidant and substrate was obtained from UV-vis spectra of 6-APA ( 10-4 mol dm -3 ), DPA ( 10-5 mol dm -3 ), [H - ] ( 10-2 mol dm -3 ), and a mixture of both. A hypsochromic shift of about 4 nm from 359 nm to 355 nm in the spectra of DPA was observed (Fig.II (viii) (p.50)). Such complex formation between substrate and oxidant has been reported in the literature 15. The probable structure of MPA and complex(c) are given below H I H Ag H 2 H 2 H I H H 2 N Ag N H 2 H 2 S CH 3 CH 3 CH monoperiodatoargentate (III) Complex(C) 49

25 Fig. II(viii) UV spectrum for the complex formation between 6-aminopenicillanic acid and diperiodatoargentate(iii) at 25 0 C [DPA] = x 10-5 ; [6-APA] = x 10-4 ; [H - ] = 0.02 I = 0.04/ mol dm -3 50

26 From Scheme 1 the following rate law ( 10 ) can be derived as followes: Rate = -d[dpa] dt = k [Complex (C)] [ H 3 I 2-6 ] The total [DPA] can be written as = k K 4 K 5 K 6 [6-APA] [H - ] [Ag( H 3 I 6 ) 2 ] 2- (5) [DPA] T = [DPA] f +[Ag (H 3 I 6 )(H 2 I 6 )] 2- + [Ag (H 2 I 6 ) (H 2 ) 2 ] + [Complex(C) ] = [DPA] f + K 4 [DPA] f [H - ] + K 5[Ag(H 2 I 6 )(H 3 I 6 )] 2- [H 3 I 6 2- ] + K 4K 5 K 6 [DPA][H - ][6-APA] [H 3 I 6 2- ] = [DPA] f + K 4 [DPA] f [H - ] + = [DPA] f 1 + K 4 [H - ] + The free [DPA] is given by, K 4 K 5 [DPA] f [H 3 I 2-6 ] K 4 K 5 [H - ] + [H 3 I 2-6 ] K 4 K 5 K 6 [DPA][H - ][6-APA] + [H 3 I 2-6 ] K 4 K 5 K 6 [H - ][6-APA] [H 3 I 2-6 ] [DPA] T [H 3 I 2-6 ] [DPA] f = [H 3 I 2-6 ] +K 4 K 5 K 6 [H - ] [6-APA] +K 4 K 5 [ H - ] + K 4 [H 3 I 2-6 ][H - ] (6) where T and f stands for total and free concentrations respectively. Similarly, total [H - ] can be calculated as, [H - ] T = [H - ] f + [Ag (H 3 I 6 )(H 2 I 6 )] 2- + [Ag (H 3 I 6 ) (H 2 ) 2 ] K 4 K 5 [H - ] [DPA] = [H - + ] f + K 4 [H - ] [DPA] (7) [H 3 I 2-6 ] In view of low concentration of DPA used, the second and third terms in the above equation(7) is neglected, and [H - ] T H - ] f (8) Similarly, [6-APA] T [6-APA] f (9) 51

27 Substituting equations (6),(8) and( 9) in equation (5) and omitting T and f we get, Rate = _ d[dpa] dt k K 4 K 5 K 6 [DPA] [6-APA] [H - ] = [[H - ] [H 3 I 2-6 ] + K 4 [H - ][ H 3 I 2-6 ] + K 4 K 5 [H - ] + K 4 K 5 K 6 [H - ][6-APA] (10) R Rate [DPA] = k obs = k K 4 K 5 K 6 [6-APA] [H - ] (10) [H 3 I 2-6 ] + K 4 [H - ][ H 3 I 2-6 ] + K 4 K 5 [H - ] + K 4 K 5 K 6 [H - ][6-APA] Equation(10) can be rearranged into the following form which is suitable for verification 1 k obs = [ H 3 I 6 2- ] kk 4 K 5 K 6 [H - ][6-APA] [ H 3 I 2-6 ] + + kk 5 K 6 [6-APA] 1 kk 6 [6-APA] + 1 k (11) According to equation (11) other conditions being constant, the plots of 1/k obs versus [H 3 I 6 ] 2-, 1/[H - ], and 1/[6-APA]should be linear and are found to be so( Fig.II (ix) (p. 54 ). From the intercepts and slopes of such plots, the reaction constants K 4, K 5, K 6, and k were calculated as 0.7 dm 3 mol -1, mol dm -3, dm 3 mol -1, and s -1, respectively at 25 0 C. The values of K 4 and K 5 obtained are also in agreement with earlier literature 16. These constants were used to calculate the rate constants and compared with the experimental k obs values and found to be in reasonable agreement with each other, which fortifies Scheme 1. The equilibrium constant K 4 is far greater than K 5. This may be attributed to the greater tendency of DPA to undergo deprotonation compared to the formation of hydrolyzed species in alkaline 52

28 medium. The negligible effect of ionic strength and dielectric constant on the rate explains qualitatively the reaction between negatively charged ion and a neutral molecule 17. The thermodynamic quantities for the different equilibrium steps, in Scheme 1 can be evaluated, thus: 6-APA, periodate and hydroxide ion concentrations as given in Table II (ii) (p. 40) were varied at four different temperatures. The slopes and intercepts of the plots 1/k obs versus 1/[ 6-APA], 1/k obs versus [H 3 I 2-6 ], and 1/k obs versus 1/[H - ] ( Fig. II (ix)(p.54)) lead the values of K 4, K 5 and K 6 (Table II (iv)(p.55). Van t Hoff plot was made for the variation of K 4 with temperature i.e., log K 4 versus 1/T, and the values of the enthalpy of reaction ΔH, entropy of reaction ΔS, and free energy of reaction ΔG were calculated and are given in Table II (iv)(p.55). In the same manner, K 5 and K 6 values were calculated at different temperatures and the corresponding values of thermodynamic quantities are given in Table II (iv)(p.55). A comparison of the ΔH value (78.9 kj mol -1 ) from K 4 with that of ΔH value (20.2 kj mol -1 ) of rate limiting step supports that the reaction before the rate determining step is fairly slow as it involves high activation energy 18. In first, equilibrium step, only active species of DPA are involved, where as in the slow step, the activated complex is involved, hence this step is taken as rate determining step. The values of ΔS (-222 JK -1 mol -1 ) and ΔH (20.2 kj mol -1 ) were both favorable for electron transfer processes. The favorable enthalpy was due to release of energy on solution changes in the transition state. The low 53

29 Fig. II (ix) Verification of rate law (10) in the form of (eq 11) for the oxidation of 6-APA by diperiodatoargentate(iii) at different temperatures.(a) 1/k obs versus 1/[6- APA] (b)1/k obs versus 1/[H - ] and (c) 1/k obs versus [H 3 I 2-6 ] 1/kobs s C 20 0 C 25 0 C 30 0 C /[6-APA] dm 3 mol C 1/kobs s C 25 0 C 30 0 C /[H - ] dm 3 mol -1 1/kobs s C 20 0 C 25 0 C 30 0 C [H 3 I 6 2- ] mol dm -3 54

30 Table II (iv) Effect of temperature on first, second and third step of Scheme 1 Temp. (K) K 4 ( dm 3 mol -1 ) K 5 x10 3 ( mol dm -3 ) K 6 x 10-3 (dm 3 mol -1 ) Activation parameters with respect to K 4, K 5 and K 6 Quantities for K 4 for K 5 for K 6 H (kj mol -1 ) S (J K -1 mol -1 ) G (kj mol -1 )

31 value of the enthalpy of activation obtained might be due to the involvement of prior equilibrium steps 19a, as given in Scheme 1. The high negative value of ΔS suggests that the intermediate complex is more ordered than the reactants 19b. The observed modest enthalpy of activation and a higher rate constant for the slow step indicates that the oxidation presumably occurs via an inner-sphere mechanism. This conclusion is supported by earlier observations 20,21. CNCLUSIN Based on the strong oxidizing capability of DPA, advanced oxidation transformation of 6-APA in aqueous alkaline medium has been studied. Among various species of DPA in alkaline medium, monoperiodatoargentate(iii) (MPA),[Ag(H 2 I 6 )(H 2 ) 2 ] is considered as active species. Rate constant of slow step and other equilibrium constants involved in the mechanism are evaluated and activation parameters with respect to slow step of reaction were computed. The values of ΔS and ΔH were both favorable for electron transfer processes. xidation presumably occurs via an inner-sphere mechanism. The overall sequence described here is consistent with product, mechanistic, and kinetic studies. 56

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