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Kinetics and mechanism of oxidation of diphenyl sulfoxide by bispyridinesilver(I) dichromate.

INTRODUCTION

Bispyridinesilver(I) dichromate (BPSDC), has been used as an mild and selective oxidant in synthetic organic chemistry (Firouzabadi et al., 1984). Cr(VI) reagents have been proved to be versatile reagents capable of oxidizing almost all the oxidisible functional groups (Rihter and Masnori, 1988; Anitha Kothari et al., 2005; Bhandari et al., 2001).

Mo(VI) catalysis of perborate oxidation of dimethyl and dibenzylsulphoxide has been carried out by (Karunakaran and Venkatramanan, 2006). Kinetics and mechanism of oxidation of sulphides by chromium(VI) complexes have already been reported (Chellamani and Alhaji, 1999; Dhariwal et al., 2006). Only few reports are available about the kinetics and mechanism of oxidation of sulphoxide with chromium(VI) complexes. However the kinetics and mechanism of oxidation of diphenyl sulphoxide by BPSDC has not been reported. Hence we investigated the oxidation of diphenyl sulphoxide by bispyridinesilver(I) dichromate in aqueous acetic acid medium and the corresponding mechanistic aspects are discussed in this research paper.

MATERIALS AND METHODS

'Bispyridinesilver(I) dichromate was prepared by standard method (Firouzabadi et al., 1984). Diphenyl sulphoxide (Lancaster), sodium perchlorate, perchloric acid and other chemicals are all of AnalaR grade and used as such.

All the kinetic reactions were carried out under pseudo-first order conditions in 60% aqueous acetic acid medium at 308 K unless other wise mentioned. The course of the reaction was followed by spectrophotometric method. In this method a known volume of BPSDC is transferred to the reaction mixture at zero time and shaken well. The rate of disappearance of BPSDC was followed by monitoring the decrease in absorption of BPSDC at 347 nm employing ELICO-BL-198 UV-visible spectrophotometer with a variable temperature accessory.

Product analysis and stoichiometry

DPSO (0.2 mol [dm.sup.-3]) and BPSDC (0.1 mol [dm.sup.-3]) were mixed and kept at room temperature for 24 h. The product was extracted with chloroform and dried over anhydrous sodium sulphate. The chloroform layer was evaporated and the product of this reaction was identified as diphenyl sulphone.

Reaction mixture containing an excess of oxidant [BPSDC] over DPSO in the presence of perchloric acid were kept at room temperature for the sufficient length of time. Estimation of unreacted oxidant showed that one mole of DPSO consumed by one mole of BPSDC.

RESULTS AND DISCUSSION

The kinetic results for the oxidation of DPSO by BPSDC along with the experimental conditions are given in Tables 1 and 2. The oxidation of DPSO by BPSDC is first order with respect to [oxidant] as well as [DPSO]. A plot of log [k.sub.obs] versus log [DPSO] is linear with a slope of 0.97. In order to know whether this reaction is acid catalysed or not, the reaction has been carried out in presence of a mineral acid (perchloric acid) and the rate constants were measured. The rate constant increases with the addition of perchloric acid suggesting that the reaction is acid catalysed. The plot of log [k.sub.obs] versus log [HCl[O.sub.4]] was linear with slope of 2.7 (r = 0.980) indicating a complex dependence on the concentration of [H.sup.+] (Kabir-ud-Din et al., 2004) The rate of the reaction has been found to increase with the decrease in the dielectric constant of the medium. Addition of neutral salt (sodium perchlorate) has slightly decreasing the rate of the reaction shows that ion and a neutral molecule may be involved in the rate-determining step (Kabilan et al., 1995). Also there is no visible polymerization occurs due to the added acrylonitrile. Added [Mn.sup.2+] retarded the rate of the reaction shows that the absence of free radical pathway mechanism (Kabilan et al., 1995).

The reaction has been conducted at three different temperatures and the activation parameters [DELTA][H.sup.#] and [DELTA][S.sup.#] were calculated.

[DELTA][H.sup.#] = 22.96 kJ [mol.sup.-1] and [DELTA][S.sup.#] = -240 J [K.sup.-1] [mol.sup.-1]

MECHANISM

In the aqueous acetic acid medium the oxidant BPSDC dissociates to give pyridinium ions and chromate ions. The literature survey reveals that HCr[O.sub.4.sup.-] is the active oxidizing species in the Cr(VI) oxidation reactions (Mangalam and Meenakshisundaram, 1991). In the light of all the experimental results obtained in the present investigation and the evidences from other investigations the following mechanistic steps have been proposed.

[FORMULA NOT REPRODUCIBLE IN ASCII]

ACKNOWLEDGEMENT

The authors wish to express their sincere thanks to Prof. K. Pandiarajan, Head of the Department of Chemistry, Annamalai University and Prof. S. Kabilan, Department of Chemistry, Annamalai University, for their keen interest and encouragement in the study.

REFERENCES

(1.) Anitha Kothari, Seema Kothari and Banerji, K.K. (2005). Indian J. Chem. Vol. 447; p. 2039.

(2.) Bhandari, A., Sharma, P.K. and Banerji, K K. (2001). Indian J. Chem. Vol. 40A; p. 470.

(3.) Chellamani, A. and Alhaji, N.M.I. (1999). Indian J. Chem. Vol. 38A; p. 888.

(4.) Dhoriwal, N., Yajurvedi, D. and Sharma, P.K. (2006). Indian J. Chem. Vol. 45A; p. 1158.

(5.) Firouzabadi, H., Sardarian, A. and Gharibi (1984). Synth. Commun. Vol. 14; p. 89.

(6.) Kabilan, S., Pandiarajan, K., Krishnasamy, K. and Sankar, P. (1995). Int. J. Chem. Kinet. p. 443.

(7.) Kabir-ud-Din, Abu Mohammad and Zaheer Khan (2004). Indian J. Chem. p. 2178.

(8.) Karunakaran, C. and Venkatramanan, R. (2006). Catal. Commun. Vol. 7; p. 236.

(9.) Mangalam, G. and Meenakshisundaram, S.P. (1991). J. Indian Chem. Soc. Vol. 68; p. 77.

(10.) Rihter, B. and Masnori, J. (1988). J. Chem. Soc. Chem. Commun. p. 35.

K. Krishnasamy, * [1] V. Venkateswaran [2], M. Shanmugam [3] and J. Dharmaraja [1]

[1] Department of Chemistry, Annamalai University, Annamalainagar 608 002, India.

* Author for correspondence: E-mail: krishbala56@yahoo.co.in

[2] Department of Chemistry, Erode Arts College, Erode-638 005, India.

[3] Department of Chemistry, Muthayammal College of Arts and Science, Rasipuram 637 408, India.
Table--1: Effect of the concentration of DPSO, BPSDC, perchloric acid,
solvent composition and temperature on reaction rates

[BPSDC] [DPSO] [HCl[O.sub.4]] Solvent AcOH-
[10.sup.3] mol [10.sup.2] mol [10.sup.1] mol [H.sub.2]O
[dm.sup.-3] [dm.sup.-3] [dm.sup.-3] (% v/v)

0.50 8.0 7.7 60-40
1.00 8.0 7.7 60-40
1.50 8.0 7.7 60-40
2.00 8.0 7.7 60-40
1.00 4.0 7.7 60-40
1.00 5.0 7.7 60-40
1.00 6.0 7.7 60-40
1.00 7.0 7.7 60-40
1.00 8.0 7.7 60-40
1.00 9.0 7.7 60-40
1.00 8.0 5.5 60-40
1.00 8.0 6.6 60-40
1.00 8.0 7.7 60-40
1.00 8.0 8.8 60-40
1.00 8.0 9.9 60-40
1.00 8.0 7.7 50-50
1.00 8.0 7.7 70-30
1.00 8.0 7.7 75-25
1.00 8.0 7.7 80-20
1.00 8.0 7.7 60-40
1.00 8.0 7.7 60-40
1.00 8.0 7.7 60-40
1.00 8.0 7.7 60-40

[BPSDC] Temp. K [k.sub.obs] x
[10.sup.3] mol [10.sup.4]
[dm.sup.-3] [s.sup.-1]

0.50 308 2.26
1.00 308 2.33
1.50 308 2.37
2.00 308 2.34
1.00 308 1.17
1.00 308 1.43
1.00 308 1.86
1.00 308 2.04
1.00 308 2.33
1.00 308 2.52
1.00 308 1.26
1.00 308 1.71
1.00 308 2.33
1.00 308 4.46
1.00 308 5.52
1.00 308 1.42
1.00 308 5.56
1.00 308 11.94
1.00 308 29.74
1.00 303 2.04
1.00 308 2.33
1.00 313 2.83
1.00 318 3.20

Table--2: Effect of variation of [MnS[O.sub.4]], [NaCl[O.sub.4]] and
[Acrylonitrile] on the reaction rates

[DPSO] = 8.0 x [10.sup.-2] mol [dm.sup.-3]; [BPSDC] = 1.00 x
[10.sup.-3] mol [dm.sup.-3]; [HCl[O.sub.4]] = 7.7 x [10.sup.-1] mol
[dm.sup.-3]; Solvent = 60-40 (v/v AcOH-[H.sub.2]O);
Temperature = 308 K

[MnS[O.sub.4]] [NaCl[O.sub.4]] [Acrylonitrile] [k.sub.obs] x
 [10.sup.3] [10.sup.1] mol [dm.sup.-3] [10.sup.4]
mol [dm.sup.-3] mol [dm.sup.-3] [s.sup.-1]

 0.5 - - 1.39
 1.0 - - 1.32
 1.5 - - 1.27
 2.0 - - 1.17
 2.5 - - 0.87
 - 1.0 - 2.03
 - 2.0 - 1.99
 - 3.0 - 1.91
 - 4.0 - 1.86
 - 5.0 - 1.59
 - - 0.5 1.37
 - - 1.0 1.37
 - - 1.5 1.28
 - - 2.0 1.27
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Author:Krishnasamy, K.; Venkateswaran, V.; Shanmugam, M.; Dharmaraja, J.
Publication:Bulletin of Pure & Applied Sciences-Chemistry
Date:Jul 1, 2006
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