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A kinetic and mechanistic study on the reduction of dithiazone by stannous chloride in micellar system.

Introduction

The similarity between micelles and cell membranes has increased the interest in catalysis of organic reactions by micelles formed by the aggregation of surfactant molecules, which is used as models for enzyme-catalyzed reactions [1-3]. Azo dyes are well known for the analytical applications [4,5]. They also have strong pharmacological activities [6,7]. Many reducing agents such as sodium sulphite, hydrazine, titanous chloride, lithium aluminium hydride, stannous chloride [8,9], etc. can bring about the reduction of azo compounds. Ogawa et al studied about the reduction of azo compounds and found that sodium sulphite reduces them to hydrazo derivatives and then gradually to amines, whereas Sn[Cl.sub.2] reduces azo compounds directly to amines [10]. Sn[cl.sub.2] is a reducing agent which is used widely in solution where [Sn.sup.2+] ion

([Sn.sup.4+] + [2e.sup.-] [??] [Sn.sup.2+]) [11] is the active species for reduction purpose. Micelle forming surfactants in aqueous medium have been found to cause significant rate enhancements and inhibitions of several organic and inorganic reactions [12,13]. The transfer of solutes from water to the micelles is a diffusion process and hence much faster than most chemical reactions. Micelles equilibrate rapidly with monomeric surfactant and hydrophobic solutes, and are partitioned between bulk solvent and the micelles. Hence, in micelles the reactions may occur at higher rates than in bulk solvent [14]. Our previous paper was on the kinetics of the reduction of dithiazone by sulphite ions [15] in micellar media and now we have attempted to explore the use of micellar media for the kinetics of the reduction of azo compounds by stannous chloride in aqueous medium.

Experimental procedure Materials required

Dithiazone, stannous chloride, acetic acid, sodium acetate, disodium hydrogen phosphate and sodium dihydrogen phosphate were of AR grade. All the solutions were prepared in double distilled water. Ionic strength was maintained constant with NaCl for the reaction in absence of CTAB and showed an increasing effect in presence of CTAB. The reactions showed no effect or in other words remained constant in presence of SDS.

Kinetic measurements

The reactions were followed spectrophotometrically (Shimadzu 160A) by observing decrease in absorbance of dithiazone at 603.5nm. All the kinetic runs were recorded at room temperature. Calculations and analysis of the kinetic data were carried out using a computer. The product identified was an amine by IR spectroscopy, and also the literature reveals that Sn[Cl.sub.2] reduces the azo compounds directly to amines [10]. The progress of the reactions was monitored by the decrease in absorbance upto two half-lives.

Results and Discussions

Kinetic studies in the absence of surfactants

The reduction of dithiazone was carried out in phosphate buffer of pH 7.0. The kinetic runs were carried out under pseudo-first order conditions keeping the concentration of stannous chloride much higher than that of substrate, dithiazone. A decrease in absorbance of the substrate with time was noted at 603.5nm. The dependence of the reduction rate on the substrate concentration was examined by monitoring the kinetic runs at varying dithiazone concentrations in the range of 0.5 x [10.sup.-4] - 2.0 x [10.sup.-4]M (Table 1).

Similarly the rate of reduction on the Sn[Cl.sub.2] concentration was examined by monitoring the kinetic runs at varying Sn[Cl.sub.2] in the range of 0.8 x [10.sup.-3] - 4.0 x [10.sup.-3]M (Table 1). Added NaCl influence on the rate of reduction of dithiazone was also studied by varying concentration of NaCl (Table 1). In the absence of surfactants the effect of added NaCl was negligible. Dependence of the rate of reduction of dithiazone on [[H.sup.+]] was examined in the pH range of 3.0-10.3. The rate of the reaction increased slightly with increase in pH and decreased (Table 1).

Kinetic studies in presence of surfactants

Kinetics of reduction of dithiazone by stannous chloride in aqueous medium was studied in presence of cationic surfactant (CTAB) and anionic surfactant (SDS). The effect of the [dye], [Sn[Cl.sub.2]] and pH on the rate of reduction of the substrate was studied in presence of 0.1M CTAB (Table 2). Increase in Sn[Cl.sub.2] concentration increased the rate linearly in the presence of CTAB, indicating the first order dependence on Sn[Cl.sub.2] also. Effect of ionic strength was found to be higher in the case of presence of CTAB. Hence the salt effect is larger in the case of reduction reaction in the presence of CTAB. Effect of cationic micelles on the rate of reduction of dithiazone by Sn[Cl.sub.2] was studied by varying the CTAB concentration in the range 0.1 x [10.sup.-3] to 10 x [10.sup.-3] (Fig.1). Similarly, effect of anionic surfactant SDS on the rate of reduction of dithiazone by Sn[Cl.sub.2] was studied by varying the concentration in the range of 0.1 x [10.sup.-3] to 10 x [10.sup.-3] (Fig.1). After this concentration, the reaction was completely inhibited. The reaction exhibited first order dependence on the substrate and Sn[Cl.sub.2] concentrations.

[FIGURE 1 OMITTED]

The observed rate law under experimental conditions may be given as

-d [dithiazone]/dt = k [S] [Sn[Cl.sub.2]] (1)

According to the Scheme1, the rate of disappearance of dithiazone is given by,

Rate = [k.sub.1] [S] [Sn[Cl.sub.2]] + [k.sub.2] [S[H.sup.+]] [Sn[Cl.sub.2]] (2)

The total concentration of the substrate can be expressed as

[[S].sub.T] = [S] [S[H.sup.+]] (3)

The concentration of S and S[H.sup.+] in terms of [S.sub.T] and the protonation constant ([K.sub.P]) can be given as

[S] = [[S.sub.T]]/1 + [K.sub.P][[H.sup.+]] (4)

[S[H.sup.+]] = [[S.sub.T]][K.sub.P][[H.sup.+]]/1 + [K.sub.P][[H.sup.+]] (5)

Substituting for [S[H.sup.+]] and [S] in equation (2), the rate of the reaction can be expressed as,

Rate = [k.sub.1] + [k.sub.2][K.sub.P][[H.sup.+]]/1 + [K.sub.P][[H.sup.+]] [[S.sub.T]][Sn[Cl.sub.2]] (6)

The observed rate law is consistent with the rate equation. Hence on comparing equation (6) and equation (2), it yields,

k' = [k.sub.1] + [k.sub.2] [K.sub.P] [[H.sup.+]]/1 + [K.sub.P][[H.sup.+]] (7)

Equation (7) explains the rate dependence on [H.sup.+] and the reaction proceeds predominantly through the path involving S (Scheme 1).

[ILLUSTRATION OMITTED]

Effect of cationic surfactant

The rate of catalytic reduction of the substrate in CTAB (at 0.1M) was found to be nearly 100 times higher than without CTAB. The substrate induced micellisation or formation of premicellar aggregates are formed since catalysis takes place well below the reported CMC of CTAB (9.2 x [10.sup.-4]). The concentration effect of CTAB on the rate of reduction at pH 7 is shown in the Fig.3.

[FIGURE 3 OMITTED]

This catalytic effect can be understood on the basis of electrostatic and hydrophobic interactions between micelles of CTAB, dithiazone and stannous chloride. Hydrophobicity of dithiazone is responsible for its incorporation into the micellar pseudophase. The reason for inhibition is also the same. Enhanced concentration of both dithiazone and stannous chloride at the micellar interface leads to the catalysis of reaction.

The fundamental process in micellar catalysis or inhibition is the counter ion binding with the micelle. They provide an unusual medium affecting the rate of reaction. Depending upon the electrical charge on their head groups, the micelles can either attract the reactive ions or repel them. Thus the reactions are catalyzed or inhibited as the micelles bring the solubilised substrate and reactive ions together.

The above argument can be supported by the facts that increase in added [[Cl.sup.-]], shows a positive effect of CTAB catalyzed reaction than in non-micellar medium. To study the effect of added counter ions on the CTAB catalyzed reaction, kinetic runs were performed by varying the concentration of NaCl in the range 0.015-0.035M.

The proposed mechanism, Scheme 1 is consistent with the interesting observation of involving two paths, protonated substrate and the non-protonated substrate. The electrostatic attraction between S[H.sup.+] and CTAB micelles shows that the binding between them has been effective than between the substrate and CTAB micelles.

Effect of pH on the CTAB catalyzed reduction of dithiazone by stannous chloride was studied in the range of 3.31-8.02, which showed that the pH increased to a maximum and then decreased (Fig.2).

[FIGURE 2 OMITTED]

Effect of anionic surfactant

Anionic micelles of SDS have complete inhibitory effect (Fig.1). The reason behind this must be the SH+ binding to SDS micelles. Both electrostatic and hydrophobic forces favor binding but Sn[Cl.sub.2] approaches to the micelle bound S[H.sup.+] where electrostatic repulsions becomes favorable. The observed rate in the presence of very low SDS concentration may be due to the small reaction taking place in aqueous phase.

Quantitative treatment of micellar catalysis

Analysis of the data of [k.sub.[psi] Vs [CTAB] plots have been carried out in terms of models for micellar catalyzed reactions which were reported earlier. Micellar catalysis comprises of three main steps: (i) substrate binding to the micelle. (ii) Actual chemical transformation in the micelle and (iii) release of products. A pseudo phase model was developed by Menger and Portnoy [16]. According to this model, the variation of the rate constant with surfactant is based on the assumption that substrate is distributed between the aqueous and micellar-phases as given in Scheme 1. It can also be represented as Scheme 2:

[ILLUSTRATION OMITTED]

Where [D.sub.n] is the concentration of micellised surfactant. The observed rate constant ([K.sub.[psi]]) according to Menger and Portnoy, is given by

[K.sub.[psi]] = [k.sub.w] + [k.sub.m][K.sub.s]([C.sub.d] - CMC)/ 1 + [K.sub.s] ([C.sub.d] - CMC) (1)

This model leads to the following relationship for micellar catalysis:

1/[k.sub.w] - [k.sub.[psi]] = - 1/[k.sub.w] - [k.sub.m] + 1/([k.sub.w] - [k.sub.m])[K.sub.s] [[D.sub.n]]

The above equation predicts that a plot of [([k.sub.w] - [K.sub.[psi]]).sup.-1] versus [[[D.sub.n]].sup.-1] ([D.sub.n] = [D] - CMC) should be linear (Fig.4). This model helps in the determination of binding constant [K.sub.s] in the micellar phase. From the slope and intercept of the plot [K.sub.s] and [k.sub.m] can be calculated. The [K.sub.s] values of the substrate in the aqueous medium in the presence of CTAB were found to be 359 [M.sup.-1] from the Fig.4.

Piszkiewicz's model: Theoretically by making certain simplifications and assuming that only one substrate is incorporated into the micelle and that the aggregation number N of the micelle is independent of the substrate. On the basis of these assumptions Piszkiewicz [17] proposed a model for micellar catalyzed reaction similar to the Hill model [18] of enzyme kinetics. This model is applicable especially at low surfactant concentrations. In this model the assumption is that "n" number of surfactant molecules (D) and substrate (S) aggregate to yield the catalysis aggregate [D.sub.n]S which then reacts to yield the product (P). This is represented by the following Scheme 3.

[ILLUSTRATION OMITTED]

From the Menger and Portnoy model the Ks value for the reduction of dithiazone was 359 [M.sup.-1]. Raghavan and Srinivasan justified [19] the model given in Scheme 3 to the bimolecular reaction under investigation considering the arguments put forward. A similar idea was given by Romsted [20]. In the model represented in Scheme 3, D, S and N refer to the detergent monomer, substrate and the nucleophile, respectively, while DnS and DnSN refer to the binary and ternary complexes, respectively. The products are assumed to result from the reactions in aqueous medium as well as surfactant medium. The assumption of Romsted was that in cases, where nucleophile resides predominantly in Stern layer of the micelle, Menger and Portnoy treatment for unimolecular reaction should hold good for bimolecular reaction also.

Conclusion

The interpreted results based on this study shows that the micelle substrate interactions are specific and depend on both electrostatic and hydrophobic forces. The investigation suggests that rate of reduction of azo compounds by Sn[Cl.sub.2] carried out in the aqueous medium is faster in the presence of cationic micelles. The utility of micellar media for preparative and analytical purposes is expected to be enhanced, due to these studies. This catalytic effect of micelles on the redox reactions of azo compounds helps in the development of several methods to reduce the pollution in waste waters of industries dealing with azo dyes.

Bibliography

[1] Fendler, J. H.; Fendler, E. S. Catalysis in micellar and macromolecular systems; Academic: New York, 1975.

[2] Rosen, M. Surfactants and Interscience. New York, 1978.

[3] Rafiquee, M. D. Z.; Shah R. A.; Kabir-ud-Din; Khan, Z. Kinetics of the Interaction of Cd(III)-histidine complex with ninhydrin in absence and presence of cationic and anionic micelles. Int J Kinet 1997, 29:13.

[4] Isa, R. M.; Ghonium, A. K.; Dessouk, H. A.; Mustafa, H. M. J. Chem. Soc., 1984, 61, 286.

[5] Lurieju, S. Hand Book of Analytical Chemistry; Mir Publishers, Moscow, 1367, 196.

[6] Gaind, K.N.; Gulati, S. K. Indian J. Pharm., 1996, 28, 272.

[7] Goodman, L.S.; Gilman, A. The Pharmacological Basis of Therapeutics; McMillan: New York, 1970, 111.

[8] Brown, H. C.; Weissman, P. M. J. Am. Chem. Soc., 1965, 87, 5614.

[9] Brown, H. C.; Weissman, P. M.; Yoon, N. M. J. Am. Chem. Soc., 1966, 88, 1458.

[10] Ogawa, T.; Shibata, K.; Yotome, C.; Takase, Y.; Kogyo Kagakuzasshi, 1971, 74, 720.

[11] Harrison, G. P.; Haylett, B. J.; King, J. T. Inorg. Chim. Acta, 1983, 75, 265-270.

[12] Frendler, E. J.; Frendler, J. H. Adv. Phys. Org. Chem., 1970, 8, 27.

[13] Cordes, E. H.; Dunlop, R. B. Acc. Chem. Res., 1969, 2, 329.

[14] Bunton, C. A. Prog. Solid Chem., 1970, 8, 239; Micellar Reactions. Chap. IV. Application of Biomedical Systems in Chemistry: Part II, ed. J. Bryan Jones, Wiley, New York, 1976.

[15] Radjarejesri, S; Sarada, N. C. J. Indian Chem. Soc., 2007, 84, 554.

[16] Romsted, L. S. Micellisation, Solubilisation and Microemulsions; Vol. 2, ed. K. L. Mittal, Plenum Press: New York, 1977.

[17] Menger, F. M.; Portnoy, C. E. (1967) On the chemistry of reactions proceeding inside molecular aggregates; J. Am. Chem. Soc., 89: 4698.

[18] Piszkiewicz, D. J. Am. Chem. Soc., 1977, 99, 1550.

[19] Hill, A. V. (1910), The possible effects of the aggregation of the molecules of haemoglobin on its dissociation curve; J. Physiol 40: 4.

[20] Raghavan, P. S.; Srinivasan, V.S. Proc Indian Acad Sci (Chem Sci)., 98 (1987) 199.

S. Radjarejesri (a) *, (b) and N. C. Sarada (b)

(a) * Department of Chemistry, Sona College of Technology, Salem, Tamil Nadu, India

E-mail: uma.seshayer@gmail.com

(b) Chemistry Division, School of Science & Humanities, V.I.T. University, Vellore, Tamil Nadu, India

Email: ncsarada@yahoo.com
Table 1: Effect of [Sn[Cl.sub.2]], [dithiazone], [[Cl.sup.-]] and pH
on the rate of reduction of dithiazone by Sn[Cl.sub.2] in aqueous
medium in absence of CTAB at room temperature at pH = 7.0
[Sn[Cl.sub.2]] = 1.6 x [10.sup.-3]M [dithiazone] = 2 x [10.sup.-4]M

[dithiazone]   k x [10.sup.3],   [Sn[Cl.sub.2]]   k x [10.sup.3],
[10.sup.4],M     [s.sup.-1]       [10.sup.3],M      [s.sup.-1]

    0.5             0.018             0.8             0.0134
    0.8             0.02              1.6             0.0247
     1             0.0208             2.4             0.0422
    1.5            0.0212             3.6              0.053
     2              0.022              4               0.065
    2.2             0.023

[[Cl.sup.-]]   k x [10.sup.3],         pH               k x
[10.sup.2],M     [s.sup.-1]                        [10.sup.5], M

    0.5             0.047             3.31              2.0
    1.5             0.084             4.09             2.44
     2              0.098             5.32              3.0
    2.5             0.091             7.0              2.05
     3              0.094             8.02             1.89
    3.5             0.096             10.3              1.7
    4.0             0.097

Table 2: Effect of [Sn[Cl.sub.2]], [dithiazone], [[Cl.sup.-]] and pH
on the rate of reduction of dithiazone by Sn[Cl.sub.2] in aqueous
medium in presence of CTAB at room temperature at pH = 7.0
[Sn[Cl.sub.2]] = 1.6 x [10.sup.-3] M [dithiazone] = 2 x [10.sup.-4]M

[dithiazone]   k x [10.sup.3],   [Sn[Cl.sub.2]]   k x [10.sup.3,
[10.sup.4],M     [s.sup.-1]       [10.sup.3],M      [s.sup.-1]

    0.5              4.5              0.8              1.5
    0.8               5               1.6              3.7
     1               5.4              2.4              4.3
    1.4              5.5              3.6              6.1
    1.5              4.1               4               7.4
    1.7              5.8
    2.0              6.8

[[Cl.sup.-]]   k x [10.sup.3],         pH              k x
[10.sup.2],M     [s.sub.-1]                       [10.sup.3], M

    0.5              1.5              3.31             0.64
    1.5               3               4.09             1.17
     2               3.7              5.32             0.8
    2.5              5.4              7.0              0.7
     3               6.2              8.02             0.6
    3.5              6.7
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Author:Radjarejesri, S.; Sarada, N.C.
Publication:International Journal of Applied Chemistry
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2010
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