Mishra, K. K.

Sinha, B. P.

English

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Indian Journal of ChemistryVol. 15A, December 1977, pp. 1079-1081Kinetics of Oxidation of z-Mercaptoethylamine Hydrochloride by2,6-Dichlorophenolindophenol in Methanol-Water MediumB. P. SINHA & K. K. MISHRADepartment of Postgraduate Studies & Research in Chemistry, University of j abalpur, ]abalpurReceived 29 September 1976; revised 23 February 1977; accepted 18 July 1977The kinetics of oxidation of 2-mercaptoethylamine hydrochloride by 2,6-dichlorophenol-indophenol have been Investtgated in methanol-water medium (88% vjv), and in the presence ofalkali. The reaction shows zero order in thiol hydrochloride and second order with respect to2,6-dichlorophenolindophenol. The rate increases linearly with the [alkali] in the range investi-gated. Neither of the reaction products, viz. the disulphide or the leuco-dye, affects the rate.Similarly, the addition of neutral electrolytes also does not influence the rate. Activation para-meters have been determined and a plausible reaction scheme has been suggested,To follow the course of many biochemical redoxreactions 2,6-dichlorophenolindophenol, a well-known oxidation-reduction indicator, has oftenbeen used. The redox properties of this dye havebeen studied extensively'. Basford and Huennekens-also employed this compound in the assay of anumber of thiols with quite reliable results. Thispaper deals with the kinetics of oxidation of2-mercaptoethylamine hydrochloride with this oxi-dant and also suggests the tentative course of the re-action. However, it would be relevant to add thatlike ferricyanide=U ion used earlier for the oxidationof a variety of thiols including the present one, thischromophoric compound too possesses several advan-tages and the kinetics of oxidation can be followedcolorimetrically in a convenient manner.Materials and Methods2-Mercaptoethylamine hydrochloride (designatedas RSH) of 99% purity was obtained as a gift fromMis Evans Chemetics, USA. Its solution was pre-pared in doubly distilled water, standardized againstiodine solution-" and stored under N2 atmosphere.The solution of 2,6-dichlorophenolindophenol (denot-ed as 'In' and referred hereafter as indophenol)was prepared in redistilled methanol (BDH, LR).The solution was prepared afresh for each run,since the light and temperature both affect thestability=. The disulphide was prepared by oxidizingthe thiol hydrochloride with ferric alum-s. Theleuco-dye was prepared by bleaching indophenolby passing S02 gas15 and later boiling-off the solutionto remove excess S02' This solution was alsostored under N2 atmosphere. All other reagentswere either of BDH (analar) or E. Merck's (GR)grade.The reaction was studied colorimetrically bymeasuring the depletion in the concentration of2,6-dichlorophenolindophenol with time. For thispurpose, a Klett-Summerson photoelectric colori-meter fitted with filter No. 59 (spectral range565-630 nm) was employed. The use of this filteris justified as indophenol has maximum absorbanceat 620 nm (t'max 2x 104 cm2 mole:") while thedisulphide and the leuco-dye respectively showmaximum absorbance at 240 and 284 nm. Thesespectra were recorded on a Unicam spectrophoto-meter SP 500 using silica cells with a light pathof I em. As the Beer's law is obeyed in the entireconcentration range of indophenol under investi-gation, the concentration of indophenol was readfrom a calibration curve. The runs were madeunder N2 atmosphere though it was observed thatfor the duration of the experiments described, itsabsence did not affect the rate.Results and DiscussionThe stoichiometry was determined analyticallyas well as spectrophotometric ally. In the formercase, the amount of indophenol consumed by thiolhydrochloride was determined while in the latter,the amount of disulphide formed was determinedemploying the relationship D = eci at the wave-length of its maximum absorbance. These experi-ments lead to a stoichiometry of 2:1 in accordancewith Eq. (1) .2RSH+In -+ RSSR+H2In ... (1)The formation of the disulphide, when stoichio-metrv is 2: 1 has also been suggested by Basfordand Huennekens'',The order of the reaction with respect to oxidantwas first determined bv Ostwald's isolation method.As stated earlier, all the runs were made in 88%(v/v) methanol-water medium. It may also beadded that the runs followed at this compositionof the system showed good reproducibility. Theseruns gave a second order dependence in indophenol,which was evident from second order plots shownin Fig. 1. This was further confirmed by vari'tHoff's differential method (Fig. 2). It would, how-ever, be seen from Fig. 1 that the magnitude ofrate coefficients depends upon the initial [indo-phenol]. It was found that the index of inverseproportionality is 1·5. Incidentally, this observation1079INDIAN J. CHEM., VOL. 15A, DECEMBER 1977''0l8·0I6'C~..I,~'"x,- i_I':' I104~-(;).'-Ii:JL ....l.. "-- J........'0Fig. 1 - Second order plots in the oxidation of 2-mercapto-ethylamine hydrochloride (RSH) by 2,6-dichlorophenolindophenol at different initial [oxidant] {[RSH] = 1·5X to-3M; [NaOH] = 2 X to-2M; [oxidant], = 10X to-5(curve 1). 9'33 X 10-0 (curve 2), 8·67 X 10-0 (curve 3), 8 X 10-6(curve 4), 7'33 X 10-5 (curve 5) and 6·67 X 10-oM (curve 6);CHaOH = 88% (v/v); and temp.= 25°}145/.•.CD0725 0-8505 + log cFig. 2 - Confirmation of second order dependence on[oxidant] by van't Hoff's differential method1080is similar to that made earlier by Shrivastavavwhile using this oxidant in the oxidation of someother thiols. In the next series of experiments, theconcentration of thiol hydrochloride was increasedfrom ten to forty-folds in excess over indophenol,while keeping [indophenol] ~ (Lx 1O-4M) and othervariables unchanged. The rate was found to beindependent of [thiol] (Table 1) which suggestszero order in this reactant. Keeping in view thefast nature of the reaction, the values of k2 givenin Table 1 may be taken to be reasonably con-stant.The reaction rate is found to exhibit a lineardependence on the [alkali] as revealed by the linearplot of log k2 versus log [OH-] with a slope ofunity. As the addition of neutral electrolytesdid not produce any change in the rate over awide concentration range (0·020 to 0·053M), it wasnot deemed necessary to maintain the ionic strengthof the reaction system at any constant value inthese runs.The reaction products, viz. 2,2'-diaminorthyl-disulphide dihydrochloride (the disulphide) and theleuco-dve also did not influence the rate whenadded "externally in a series of runs (Table 2).This implies that these products are not involvedin the mass law term.Arrhenius equation was found to be applicableand the slope of the Arrhenius plot gave the valueof energy of activation as 13·86 kcal/mole. Theentropy of activation was calculated employingEyring's equation and found to be -4,41 e.u.Mechunism=: In the light of the results obtainedabove, the tentative reaction Scheme 1 is suggested.k,In + OH-~InOH-s:k.InOH- + In ...• C (reactive intermediate)fastC + RSH - ...• reaction productsScheme 1... (2}... (3}... (4)TABLE 1- RATE DATA FOR THE OXIDATION OF2-MERCAPTOETHYLAMINE HYDROCHLORIDE BY INDOPHENOL{[In]=t·OO X to-1M; [NaOH]=2·00 X to-2M; CHaOH=88%{v/v}; fL = 0·020M; temp.e- 25°}[RSH] X 108M »; litre [RSH] X loaM k2' litremole'? seel mole? see'?1'502·002'503·004·0036·7239·5437·3838·4937·14TABLE 2 - RATE DATA IN PRESENCE OFREACTION PRODUCTS{[RSH] = t'5x1Q-sM; [In] = l'0xtQ-'M; [NaOH] = 2·0xl0-2M; CHaOH = 88% iv/v}; fL = 0·020M; temp.= 25°}00975 [H2In]Xt06M 2h2' litre mole'? see 39·90438·72638·38837·89[RSSR] X 10'Mh2' litre mole'?see'?14t·59542·19240·14340·21441·76SINHA & MISHRA: KINETICS OF OXIDATION OF 2-MERCAPTOETHYLAMINEIn Scheme 1 formation of InOH- has beenpresumed in one of the rate determining steps. Thispresumption is similar to that made by Amis and La-Mer16 in the case of alkaline fading of certain phtha-lein and sulphopht haleine dyes. Further,2-mcrcapto-ethylarnine hydrochloride (HS.CH2·CH2·NH2·HCI)molecule has been presumed to participate in thereaction as HS.CH2.CH2.NH2 and is represented asRSH. This also appears justified in view of thenature of the medium and the fact that the disso-ciation of sulphydryl (-SH) group is reportedto be very small (K = 1·995 X 10-11 at 25°)17. Sinceas per stoichiometry step (4) cannot alone give the.reaction products it appears probable that thisin turn produces other reaction intermediates whichthrough a sequence of fast steps lead finally to thereaction products, viz. the disulphide and the leu co-dye. The participation of one such reaction inter-mediate is at least partly demonstrated by thecapability of this system to initiate the polymeriza-tion of vinyl acetate.For deriving the ratereaction Scheme 1 if onetreatment to InOH-, itsgiven by Eq. (5):[InOH-] = k][In][OH-]k_l+k2[In]On this basis the rate of reaction vvould be givenby Eq. (6):d[In] 2k1k2[In]2[OH-]- (it = k_1+k2[In]On dividing Eq. (5) by , k-l" we getd[In] 2k2K[InJ2[OH-]- lit = l+~[In]k-lwhere K = (kl/k_l) and is the equilibrium constantof step (2).Under the condition when k2 [In]/k_l~l, Eq.(7) becomes Eq. (8)._ d[In] = 2k KlIn]2[OH-]dt 2"law on the basis ofapplies the steady-stateconcentration would be... (6)... (7)... (8)Ior_ d~;] = k'[In]2[OH-]where k' = 2k2K. At a constant COHo] Eq. (8) canbe further simplified to Eq. (9)_ d[In] = k"[In]2 (9)dt ...where k" = k'[OH-] and is the observed pseudo-second order rate coefficient. The assumption that(k2/k_l)[In]~1 appears reasonable, firstly because[In] is fairly small (,-.,10-4M) and secondly, under theconditions described k2/k_l is also expected to beless than unity. The proposed mechanism satis-factorily explains all the observed kinetic featuresexcepting the inverse proportionality between therate and the initial [indophenol]. In this context amention of the addition reactions of quinones18-20would be useful as this may at least qualitativelyaccount for this effect. According to Hadler et al.21thiols undergo addition reactions with 2,6-dichloro-phenolindophenol and form l,4-addition product (II)(species II). This product may again react withan another indophenol molecule and result in aconjugate of the dye (III) and the leuco-dye.A species of the type III is expected to berelatively inert as compared to indophenol as wasalso suggested by Hadler et al.21. Consequently,on increasing the initial [indophenol] the sidereaction leading to the formation of III may becomeprominent, and this would consequently produce theretardation in rate.-AcknowledgementOne of us (K.K.M.) is grateful to the UGC, NewDelhi, for the award of a research fellowship.... (5)References1. BAXENDALE, J. H. & LEWIN, S., Trans. Faraday Soc.,42 (1946), 126.2. BASFORD, R. B. & HEUNNEKENS, F. M., J. Am. chem.Soc., 77 (1955), 3873.3. BOHNING, J. J. & WEISS, K. J., I Am. chem, Soc., 82(1960), 4724.4. KOLTHOFF, I. M., MEEHAN, P. J., TSAO, M. S. & CHOt.Q. W., I phys. Chem., 66 (1962), 1233.5. MEEHAN, E. J., KOLTHOFF, 1. M. & KAKIUCHI. H., J.phys. Chem., 66 (1962), 1238.6. KAPOOR, R. C., KACHHWAHA, O. P. & SINHA, B. P.,I phys. Chem., 73 (1969), 1627.7. KACHHWAHA, O. P., SINHA, B. p. & KAPOOR, R. C.,Indian I Chem., 8 (1970), 806.8. KAPOOR, R. C., CHOHAN, R. K. & SINHA, B. P., I phys.Chem., 75 (1971), 2036.9. SINHA, B. P., KAPOOR, R. C. & KACHHWAHA, O. P.,Indian I Chem., 10 (1972), 499.10. CHOHAN, R. K., SIl-OHA,B. P. & KAPOOR, R. C., I phys.Chem., 76 (1972), 3641.11. CHOHAN, R. K., KAPOOR, R. C. & SINHA, B. P., IndianI cu«, 11 (1973), 759.12. KRAMER, H., I ass. agr. Chern., 3S (1952), 285.13. LOJANDER, W., Chem. Abstr., 35 (1941), 50565•14. LEUSSING, D. L. & KOLTHOFF, 1. M., I electrochem,Soc., 100 (1953), 334.15. SHRIVASTAVA, A. K., Kinetic studies on oxidation ofsome organic sulphur compounds, Ph.D. thesis, Univer-sity of jabalpur, 1972.16. AMIS, E. S. & L.-\MER. V. K., I Am. chem, Soc., 61(1939), 90S.17. ASHTHANA, M., KAPOOR, R. C. & NIGAM, H. L., Electro-chim. Acta, 11 (1966), 1587.18. SNELL, j. M. & WEISSBERGER, A., I Am. chem. Soc.,61 (1939), 450.19. SCHUBERT, M., J. Am. chem. Soc., 69 (1947). 712.20. WILGUS (III). H. S., FRAUENGLASS, E., JONES, E. T. &PORTER, R. F., J. org. Chem., 29 (1964), 594.21. HADLER, I. M., ERWIN, 1\1. j. & LARDY, H. A., I Am.chem. Soc., 85 (1963), 458.1081

Kinetics of Oxidation of 2-Mercaptoethylamine Hydrochloride by 2,6-Dichlorophenolindophenol in Methanol-Water Medium

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Kinetics of Oxidation of 2-Mercaptoethylamine Hydrochloride by 2,6-Dichlorophenolindophenol in Methanol-Water Medium

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