v W Bhagwat, Mona Pipada, S B Jonnalagadda+ & Brijesh Pare#*

Transcription

1 Indian Journal of Chemistry Vol. 42A, July 2003, pp Kinetics and mechanism of C 16 T AB catalyzed oxidation of methylene violet by chloramine-t in acidic medium v W Bhagwat, Mona Pipada, S B Jonnalagadda+ & Brijesh Pare#* School of Studies in Chemi stry, Vikram Un iversity Ujj ain Received 25 February 2002; revised 6 February 2003 Kinetics and mechanism of uncatalyzed and C'6TAB catalyzed oxidation of methylene.violet (3-amino-7-(diethyamino)-5-phenyl phenazinium chloride) (MV+) by chloramine-t in acidic media has been kinetically studied using spectrophotometry. With excess concentrations of other reactants, the reaction rate follows pseudofirst order kinetics with respect to methylene vi olet. The uncatalyzed reaction has first order dependence on chloramine-t and zero order dependence on H+ concentrations (in the range l.ox to 6.0x M). The reaction is catalyzed by cetyltrimethyl ammonium bromide, a cationic surfactant, even before the cmc. A bathochromic shift is the evidence of dyesurfactant interaction. The pre-micellar kinetics has been rati onalized in the light of Piszpiwicz positive co-operativity. Positive cooperatively index (n = 2.7) has been computed. The order in [Cn is unity. Variation of ionic strength and the initial additi on of p-toluenesulphonamide have no influence on the reaction rate. The rate of depletion of the dye decreases with the increase in ph from 2 to 6. On the basis of product analysis a perti nent mechani sm is proposed. Organic dyes are extensively used in textile industry for dying nylon, wool, silk, and cotton. After process completion, the effluent containing the dyes are discharged in water bodies and thus these dye molecules enter into the ecosystem. Most of the synthetic dyes are recalcitrant, mutagenic and carcinogenic and hence cause adverse effect on human health I. Therefore, to treat this coloured wastewater, containing dyes, is a necessary aspect for human welfare and sustainable environment. Increasing concern about colour in the effluent is leading to worldwide efforts to develop more efficient colour removing process. Most of the processes used for the treatment of wastewater containing dyes are physical, chemical and biological. Therefore, oxidation of methylene violet (MY+) by chloramine-t has been carried out in order to optimize the reaction +University of Durban-Westville, Durban (South Africa) # Madhav Science College, Vikram University, Ujjain (India) conditions to develop more efficient colour removing chemical oxidation method. MY+, a water-soluble dye of phenazine class, has a sharp absorption peak in visible region and is an ideal substrate for photometric monitoring. Chloramine-T (p-me-c 6 JL-S02NCINa 1.5 H20) abbreviated as CAT is the most widely studied oxidane. The oxidative depletion of dye is catalyzed by the cationic detergent, C I6 TAB. Hence, this study. The addition ofc I 6 TAB to the dye solution shifts its absorbance maxima from 560 nm to 580 nm. This spectra] shift due to dye-surfactant interaction leads to the possibility of an analytical method for the determination of cationic surfactant which is discussed elsewhere 3. Experimental The chemicals used were of analar grade and doubly distilled water was used throughout to prepare the solutions. An aqueous solution of CAT (E. Merck) was prepared and standardised periodically by the iodometric method and preserved in an amber coloured bottle until further use. Aqueous solution of MY+ (Aldrich) was prepared by dissolving g of MY+ in 100 rnj water. Working solution was prepared by appropriate dilution of the stock solution. Stock solutions of the detergent was prepared by dissolving g of C I 6 TAB in 50% acetic acid. Stock solution of HCI0 4 and H 2 S0 4 were prepared by diluting the calculated volume (from specific gravity) of acid with distilled water and finally its concentration was determined by titrating it against standard NaOH solution using phenolphthalein as indicator. Kinetic procedure In all the experiments the pseudo-first order kinetics with respect to methylene violet was monitored at 560 nm, using systronix UY -vis spectrophotometer, but the addition of C I6 TAB shifts this to 580 nm. No interference from other reagents, intermediates or products was observed at 560 nm. Beer's law was valid for the measurement under the experimental conditions considered. The total initial volume of the reaction mixture was always kept 10 ml and at 25.0 ± 0.10c. Reagent solutions were mixed in the orders requisite volumes of methylene violet, sulphuric acid, C I6 TAB, plus water or other reagents,

2 NOTES 1645 where necessary. Separately thermally equilibrated solution of CAT was added to commence the reaction. After vigorous mixing, solution was transferred to spectrophotometer cell. In all the experiments, the reactions were followed up to two half lives. A constant ionic strength of the reaction mixture was maintained by adding required amount of sodium sulphate solution. Stoichiometry and product analysis Methylene violet (250 mg1250 ml water), perchloric acid (5.0 mol dm 3/lOO ml) and CAT (500 mg/150 ml) were mixed for the product analysis. After 24 hr, the organic components were ether extracted, dried and concentrated using rotary evaporator under low pressure. Using benzene: ethyl acetate mixture (8:2) as an eluent, preliminary studies were carried out by thin layer chromatography. A distinct single spot was obtained 4. The IR spectra of the sample showed strong absorption bands at cm l, 1652 cm- I and 1733 cm 1 indicating the presence of conjugated carboxylic acid. Absence of intense absorption peak between cm- I showed the absence of N-O stretching. Thus, oxidation at the nitrogen atom and formation of N oxide in the heterocyclic ring is unlikely. Based on the IR spectra, major product was identified as quinoxaline dicarboxylic acid (7-amino-9-phenyl quinoxaline-2,3-dicarboxylic acid). This is consistent with the information on oxidation of phenazine compounds. With mild oxidising agents such as H20 2 gives a monooxide or dioxide, while in the presence of strong oxidising agents like KMn04, quinoxalinedicarboxylic acid is reported as product 5. Studies on the further characterization of reaction products are in progress. Stoichiometry was determined using varying ratios of the oxidant to MV+ at 25 C after 24 hr incubation. The absorbance at 560 nm was measured and residual CA T was determined iodometrically using standard sodium thiosulphate as titrant and potassium iodidestarch as an indicator. The mole ratio of (number of moles of the oxidant consumed per mole of MV+) was calculated. Methylene violet and oxidant react as follows: MV+ + 4ArS02NHCl + 4H20 = p+ + 4ArS02NH2. + CH 3 CN +2CH 3 CIhOH + 4HCI... (J) p+ = 7-amino-9-phenyl quinoxaline-2,3-dicarboxylic acid and ArS02NH2 is p-toluenesulphonamide (PTS). Results and discussion In a typical kinetic run, for the uncatalyzed reaction ([CAT] 2.0x I 0-4 mol dm- 3, [H+]2.0x I 0-2 mol dm- 3 and [MV+] 2.0xlO- 5 mol dm-\ a plot of log A versus time was linear, indicating that reaction follows pseudo first order kinetics. The order in [MV+] is unity. The mean pseudo first order rate constant, ko' was found to be (3.03 ± 0.2)xlO- 2 S- I. FigureJ depicts that there is a initial fast drop. Pseudo-first order rate constants were calculated from the linear portions of log A versus time profile. Order with respect to CAT and hydrogen ion concentrations At constant ionic strength and [H+] = 1.0 mol dm 3 when 10 4 [CAT] was varied from 2.0 to 7.0 mol dm- 3, 10 2 ko' varied from 3.03 to S- I. The plots of log ko' versus log [CAT] were linear with slopes 0.99 (R 2 = 0.986), suggesting that the reaction is first order in [CA T]. The reaction has been found to be fast initially. This initial drop period of 60 seconds extends from 2% to 46% with the increase in the CAT concentration from 1.0x lo-4 mol dm- 3 to 6.0xI0-4 mol dm- 3. The cause of this fast initial drop could be the formation of dichloramine-t (OCT). But with the progress of the reaction ArS02NH2 concentration accumulates which in turn results in suppress of OCT formation as shown in Eqs (2 and 3). Consequently, linearity is obtained. Pseudo-first order constants were calculated from the linear portions of log A versus time plots.... (2)... (3) Varying [H2S0 4 ] and [HCI04] in the range 1.0x lo 2 to 6x lo- 2 mol dm- 3 does not affect the reaction rate '" <t: -0.4 CD Time (min) Fig. I-Plot of loga versus time

3 1646 INDIAN J CHEM, SEC A, JULY 2003 appreciably showing that the reaction rate does not depend upon the concentration of acid in this range. Dependence of rate on suifactant concentration Addition of aqueous solution of C'6TAB to the solution of methylene violet does not produce turbidity and a clear solution was obtained. The spectral studies revealed a bathochromic shift from 560 nm to 580 nm. This provides an evidence for the interaction between methylene violet and the C'6TAB. In this case the dye molecule and the surfactant molecule both are cationic in nature, thus obviously the cause of interaction is hydrophobic that overweighs the electronic repulsion. This interaction results in penetration of dye molecules into the micelles. This interactive localization of the reacting species in the relatively small volume of the pseudomicellar phase compared to the bulk solution leads to a large increase in the effective concentration and the observed rate (in terms of moles per unit time per litre of the entire solution) increases accordingly. In the present case 10 2 ko' increases from 2.35 sol in the absence to sol with the increase in cetyltrimethylammonium bromide-concentration from l.ox IO s to 6.0x 1O- 5 mol dm-) even before the cmc of the surfactant. The plot of kif versus [C'6TAB] is linear. The cmc has been determined conductometrically. A plot of specific conductance versus [C'6TAB] gives the cmc value 6.1 x 10-4 while the reported cmc is 9.2xl0--4 mol dm-) at 25 C. Reports are available that the catalysis below cmc i.e. premicellar catalysis is also feasible suggesting that the substrate promotes micellization of the cationic surfactant or that the small aggregates of the detergent exits below the cmc and that they catalyse the reaction. A similar substrate promoted micellization has been observed in the deacylation of p-nitrophenyl benzoate. Further, there are extensive evidences from other systems that (i) external agents can promote micellisation; (ii) some aggregation of detergents occur below cmc; and (iii) these small aggregate can be catalytically active 6. The catalysis by C'6TAB has, therefore, been treated by a theme proposed by Piszpiwicz 7. This scheme includes the decomposition of substrate-micelle complex into the free components, resulting in Eq. (4). log [kobs- krjkm - kobs] = n log [D]- Iog kd... (4) From Eq. 4 the plot of log [kobs-kr/km -kobs] versus log [0] should be linear with a slope = n, called the index of co-operativity. Values of n ranges from 1 to 6. In the present case n = 2.3 indicating a positive cooperativity i.e.induced interaction of the additional substrate molecule due to the interaction of the micelle with the first substrate molecule. The physical basis for micellar catalysis involves several contributing factors. First, there is the effect of the micellar environment on the rate-controlling step in the reaction mechanism. The relative free energies of the reactant(s) and/or the transition state can be altered when the reaction takes place in the micellar phase instead of the bulk water. This concept is reminiscent of catalysis by an enzyme, and many initial studies of rates in micellar systems focussed on this possibility. However, further studies have shown that this effect is often rather small and cannot account for the very large rate changes in many micellar systems. A more important consideration is the localization of the reacting species in the relatively small volume of the micelles compared to the bulk solution. This leads to a large increase in the effective concentration and the observed rate (in terms of moles per unit time per litre of the entire solution) increases accordingly8. Effect of chloride ion variation The role of chloride ion In the kinetics and mechanism of the oxidation reactions by CAT is very crucial. The reaction rate has been found to be increased by the addition of chloride ions. In the absence of perchloric acid the reaction rate increased from l.0ix1o- 2 sol to 6.90x 1O- 2 sol on addition of chloride ions as NaCI. The plot of log ko' versus log [Cn is linear with unit slope (R 2 = 0.98). Thus, the order with respect to [Cn is one. In the presence of perchloric acid the value" of slope is less than one i.e This is in accordance with the previous findings as in the case of oxidation of arginine 9 where the order in [Cn was fractional (0.22 to 0.64) and the fraction increased with increase in [acid]. At low ph the oxidation of chloride ion to chlorine may be possible by protonated CAT as Ch(g) + 2e- ~ 2 cr is V. At higher ph due to decrease in oxidation potential of CAT it may not oxidise chloride to chlorine. Effect of PTS and ionic strength Addition of PTS, one of the reaction products, from 1.0x1O-4 to 6.0x 10-4 mol dm-) at constant CAT and MV+ does not affect the reaction rate significantly. Under conditions, [MV+] = 2.0x1O- 5 mol dm-)' [H+] =

4 NOTES xlO 2 mol dm- 3, [CAT] = 2.0xl0-4 mol dm- 3, the initial ionic strength of the reaction mixture was The variation of ionic strength from 0.05 to 0.30 mol dm- 3 by the addition of sodium sulphate, does not affect reaction rate, suggesting that a neural molecule is participating in the rate determining step. Effect of ph The reaction was studied in the ph range of 2 to 6. The rate of MY+-CAT reaction decreases with the increase in ph due to formation of ocr at high ph while HOCI is more reactive. Furthermore, probably with increase in ph, protonation of HOCI is forming H20Cl+. Thus, there seems to be a competition between HOCI protonation and MY+ oxidation by HOCl. Moreover, oxidation potential of CAT decreases with increase in ph. Oxidation potential at ph = 0 is 1.55 while at ph = 7 it is 0.9(ref.1O). Detection of free radicals Tests performed using olefinic monomers were negative indicating the absence of free radical formation in the mixtures of MY+ and CAT. Mechanism Chloramine-T behaves as a strong electrolyte in aqueous solution as given in the Eq. (5) suggested that at higher acid concentration (PH < 2.8) ArS02NHCl is further protonated, Eq. (10).... (10) Thus, the possible oxidizing species in acidic CAT are ArS02NHCl (acid form of CAT), ArS02NCh (dichloramine-t) and HOCl. The hypochlorous acid may be regarded as the main oxidizing species for the reaction. Since in the present study the product characterized is a dicarboxylic acid, oxygen transfer has taken place. This also supports that HOCI is the main oxidizing species. In our studies, the effect of added PTS and neutral salts was negligible. A direct reaction is proposed between CAT and H+ to give ArS02NHCl, which leads to the formation of HOCI on hydrolysis, (Eq. 9). As the addition of PTS had no significant effect on the rate, the reverse of step-9 was considered negligible because of the fast interaction of HOCI with the substrate 10. To account for the experimental observations the mechanism In Scheme 1 is proposed:... (11)... (5) The anion ArS02NCr gets protonated in an acidic solution to give N-chlorotoluene-p-sulphonamide (CAT) as... ( 12)... (6) Thus, CAT exists as a free acid (ArS02NHCl) in acidic media. This free acid undergoes disproportionation or hydrolysis according to Eqs (7-9)... (7) ArS02NCl2 + H20 <=> ArS02NHCl + HOCI... (8)... (9) The disproportionation of ArS02NHCl in to p toluenesulphonamide (PTS) and dichloramine T(DCT) was studied and it was observed that ArS02NHCl is the predominant reacting species of CAT in feebly acidic media (PH 6-7). It was later ~ N...:: -0 -::7 N".;:;: _ O H 2 HOCI + 211,0 ~ 0" + ~ H,C-H,C, ('Y +v + H,C-H,C~NAc-o ~N = 0 6~~.r. 61~) + 2CH,CH,OH + 2HCI... ( 13) where p+ = 7 -amino-5-phenyl quinoxaline-2,3- dicarboxylic acid. Scheme 1 In presence of chloride ion With the increase in concentration of chloride ions the reaction rate increases. The order of reaction with respect to chloride ion has been found to be one. This is because of the fact that in the presence of cr ions

5 1648 INDIAN J CHEM, SEC A, JULY 2003 there is a fonnation of molecular chlorine and consequently HOCI: ArS02NHCI + cr <=}ArS02NHCl..... cr... (14) ArS02NHCl..... cr +H+ <=}ArS02NH2 + Clz... (15)... (16) Suifactant catalysed mechanism The scheme 2 is proposed for the catalysis by C I6 TAB. Step 17 depicts the surfactant monomers in equilibrium with the globules (micelles). Then the interaction between. the dye molecules and the micelle takes place resulting in increase in the effective concentration of the substrate, followed by the attack of HOCI to give product. r=~/ 1MV1... (23) where the pseudo first-order rate constant for uncatalyzed reaction, ~' = ~ [CAT]. In the presence of cr ions this rate law will take the fonn as... (24) In presence of the catalyst, the oxidation reaction proceeds through both uncatalyzed and catalyzed pathways. Therefore, Eqs25 and 26 represent rate of depletion of MY+ in presence of catalyst under excess [CAT] and acid concentrations:... (25) where Icc' = Icc [CAT] = k" [MY+] where k" = {~' + Icc' [C I6 TAB]}... (26) fast P HOCI + 2 IhO p+ + 2 CH 3 CH20H + 2 HCl... (21) Scheme 2 *Bathochromic shift is the spectroscopic evidence of association of the dye with the detergent. The rate equation for the uncatalyzed reaction between MY+ and CA T can be represented by Eq. (22). -d[my+]/dt = ~ [MY+] [CAT]... (22) when [CAT] is in excess, Eq. (22) reduces to Eq. (23) Since Eq. (25) holds good, a plot of the observed pseudo-first order rate constant in presence of catalyst, k" versus [C I6 T AB] has been found to be linear. References 1 Park J & Shore J, J Soc Dyers Colour, 100, (1984) Venkatesha B M, Ananda S & Mahadevappa D S, Int J chern Kinet, 27, (1995) Bhagwat V W, Mona Pipada, Jonnalagadda S B & Brijesh Pare, J Korean chern Soc (accepted) Fritz Feigl, Spot test in organic chemistry, (1996), Elderfield R C, Heterocyclic compounds (John Wiley, New York), 1957, Vol 6. 6 Raghavan. P S, Srinivasan Vangalur S & Venkatasubrarnanian N, Indian J Chern, 21 B, (1982) Piszkiewicz Dennis, JAm chern Soc, 99 (1977) Zyka J, Instrumentation in analytical chemistry. Vol Il (Ellis Horwood Limited, West Sussex, England) (1994) pp Gowda BT & Vijayalaxmi Rao R, Indian J Chern, 25 A (1986) 908. IO Agrawal M C & Upadhyay S K, J sci ind Res, 49 (1990) 13.