Gupta, Kalyan K. Sen

Gupta, Shipra Sen

Sen, Pratik Kumar

English

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Indian Journal of ChemistryVol. 15A, December 1977, pp. 1082-1085Kinetics of Chromic Acid Oxidation of Thioglycollic, Thiolactic &Thiomalic AcidsKALYAN K. SEN GUPTA, SHIPRA SEN GUPTA & PRATIK KUMAR SENDepartment of Chemistry, Jadavpur University, Calcutta 700032Received 13 July 1976; accepted 1 February 1977Chromic acid oxidation of thfoglycolltc, thiolactic and thiomalic acids has been studiedspectrophotometrically in sodium acetate-acetic acid buffer. Thiolactic and thiomalic acids areoxidized in an identical manner whereas thioglycotlic acid undergoes oxidation by a differentmechanism. The thermodynamic parameters for the reactions are: thiogtycotllc acid AEt = 11·4(kcal mol-I), AS;= -15·6 (cal mol-l deg-l); thiolactic acid AE; = 4·8 (kcal mol-I), AS; = -47·7(cal mol-l deg-l); thiomalic acid AEt = 4·8 (kcal rriol'<), AS; = -51·6 (cal mol-l deg-l)_ Theformation of free radical intermediate has been demonstrated in the oxidation of thioglycollicacid whereas experimental evidence does not indicate the presence of free radicals in the oxida-tion of thiolactic and thiomalic acids.IT is known that thiol group is very sensitivetoward reaction with oxygen) and the oxidationof a thiol to its disulphide is a process offundamental importance in biochemistry. Consider-able amount of work has been done'' on theoxidation of cysteine, thioglycollic acid and gluta-thione by oxygen. Eager and Winkler" studied thekinetics of the oxidation of RSH (R = n=C4H9,n=CSH17 and n=C12H25) by potassium perdisulphatein acetic acid-water medium. The kinetics of the oxi-dation of thioglycollic acid by ferricyanide have alsobeen reported+", The oxidation of some reducingsulphur compounds by Ce(IV)6, Co(III)7 and V(V)8have also been studied. All these metal ions whichare one equivalent oxidants produce free radicals insolution and finally radical dimerization occursleading to the formation of stable products. Themechanism of the oxidation of cc-mercapto acids bychromic a.cid has not received any attention. Thepresent publication is concerned with an a.ttemptto explain the kinetics of the oxidation of thio-glycollic (TGA), thiolactic (TLA) and thiomalic(TMA) acids by chromic acid.Materials and MethodsInorganic reagents were of BDH (AR) or E. Merck(GR) grades. Freshly prepared solutions of thio-glycollic (Riedel), thiolactic (Fluka, AG) and thio-malic (Koch-Light) acids were used for kinetic runs.Sodium perchlorate was prepared by the neutraliza-tion of perchloric acid with caustic soda. Deuteriumoxide was supplied by Bhabha Atomic ResearchCentre (Trombay) and was claimed to have 99%deuterium content. All solutions were made indoubly distilled water. Sodium acetate-acetic acidbuffer was employed throughout the experiment.Kinetic measurements - All kinetic studies weredone by following the decrease in absorbance at350 nm using stoppered cells of 1 ern path lengthin a Beckman DU model spectrophotometer. The1082cell compartment was thermostated at desiredtemperatures. The maximum uncertainty in tem-perature was ± 0·5°. The air-free reactants pre-viously equilibrated to bath temperature were mixed,flushed with nitrogen and optical densities notedat definite time intervals. <x-Mercapto acids anddisulphides do not show any detectable absorbanceat this wavelength. The reactions were carried outunder the condition in which [substrateJ~[Cr(VI)J.Generally 6-8 experimental points were taken ineach run. The plots (log OD versus time) gavegood linearity over 90% of reaction and the pseudo-first order rate constants (.k]) were reproducible towithin ± 5% accuracy.Stoichiometry and products- The stoichiometry wasdetermined in excess substrates and the substrateswere oxidized according to Eq. (1)6RSH+2Cr(VI) --+ 3RSSR+2Cr(III)+6H+ ... (1)The products of the oxidation were identified asfollows. Substance (1 g) was treated with calculatedamount of Cr(VI) such that [substrateJ~[Cr(VI)].The reaction mixture was extracted with etherand the latter was removed. The residue wastreated with toluene, refluxed for 1 hr and con-centrated. The infrared spectra of the reactionproducts were found to be identical with thesamples prepared by the oxidation of the respective<x-mercapto acids by Ce(IV)6.ResultsEffect of varying reactant concentrations - Theresults are presented in Tables 1 and 2. It hasbeen observed that the reaction is independent ofionic strength over the range 0'2-1'OM (NaCl04).Despite this independence, the ionic strength wasadjusted to l·OM. The pseudo-first order rateconstant decreases with the increase in gross [Cr(VI)],suggesting that HCrO:;-is the main oxidizing species.The [HCr04J has been calculated using the relationSEN GUPTA et at.: KINETICS OF Cr(VI) OXIDATION OF IX-MERCAPTOACIDSsignificantly less than one. The slopes of log kivs pH plots are identical (0,4) in the cases ofTLA and TMA whereas the slope is different (0,2)for TGA. This is in agreement with the observationmade earlier- in thiol oxidation that the rate isnot a simple function of [H+J.Solvent isotope effect - The rates of oxidation havebeen measured in both water and deuterium oxide.The ratios, kD,o/kH,o have been calculated to be0·90, 1·2 and 1·2 for the oxidation of TGA, TLAand TMA respectively. It has earlier been shown-"that for any proton-catalysed reaction, kD:O/kH,olies between 2·0 and 2·5. This solvent isotope effectwould favour all reactions involving molecularchromic acid for the equilibrium (2) is shifted by'deuterium oxide in the forward direction yieldinghigher concentration of H2Cr04 than what could bepresent in water.HCr04+H30+ ~ H2Cr04+H20 ... (2}The solvent isotope effects which have been ob-served in the oxidations of cc-mercapto acids furthersuggest that HCr04 is the reacting oxidizing species.Effect of varying temperature en the rate - Thepseudo-first order rate constants have been deter-mined at different temperatures and the specificreaction rates have been calculated frcm the relationsk - . kt k ki ki3 - [TGA]2' 2= [TGA] or [TMAjThe plots of logarithm of specific reaction ratesagainst reciprocal of absolute temperature are linear(Fig. 1). A least square fit of Each plot was usedto evaluate energy of activations. Values of pzwhere p is the probability or steric factor and Zthe collision number at different temperatures werecalculated by equating experimentally obtained rateconstants with pZe-E/R:T. Entropies of activationwere then computed frcm the relation, pZ =en(RT)jNh fXP b.St/R (where 11,= 2 for thioJactic andthiomalic acids and n = 3 for thioglycollic acid).The results are record ed in Table 3.TABLE1 - EFFECTOF V.~RYING[OXIDANT]ONTHEPSEUDO-FIRSTORDERRATECONSTANTAT [L = 1·0.1\1[TGA] = 2'Oxl0-3M; pH = 5·48; TE~IP.= 20°[Cr(VI)] x 104M[HCrO.] x 104Mkl x 103 (see-l)keorr x 103 [sec"}0·8331·5501·360·7291'6663·041'280'7012·504-451·150·6473·335·780·9970·5754·1657·070·9740·575[TLA] = 2·0 x 10-3M; pH = 5·18; TEMP.= 30°[Cr(VI)] x 104M 0·833 1'70 2·50 3·30 4·10[HCrO.] x 104M 1·57 3·11 4·46 5·75 6·98kl x 103 (see:") 1·82 1·79 1·67 1·56 1·52keorr x 103 (see-l) 0·966 1-03 0·936 0·895 0·891[TMA] = 3·93xl0-3M; pH = 5·16; TEMP.= 30°[Cr(VI)) x 104M 0·883 1·56 2·33 3·13 3·92[HCrO.] x 104111 1·57 2·85 4·16 5-46 6'736ki x 103 (see-l) 2·71 2·63 2·48 2'35 2·15keorr x 103 (sec<) 1-43 1·44 1·39 1·26 1'25described earlier". The dissociatoin constants Kdand K~ of the reactionsKa . Kd2HCr04 ~ Cr20~-+H20 and HCr04 ~ H++CrO;jha ve been taken from results of Espenson andKinneylO and Tong and Kingll respectively. The(k1rCr(VI)]).values of keorr where keorr= are given[HCrO:!]in Table 1. The quotients, k1/[substrate] have beencalculated which indicate that the order with respectto both [TLA] and [TMA] is unity whereas theorder with respect to [TGA] is different. However,the quotients, ki/[TGA]2 are found to be constantindicating that the order with respect to TGA istwo. The average values of kl/[TGA]2, kd[TLA]and kl/[TMA] in six experiments have been calculat-ed to be 145 ± 4 (Iitre- mol-2 seel), 0·87 ± 0·03(litre mol-l sec") and 0·45 ± 0·03 (litre mol"! sec+)at 200, 400 and 350 respectively.Effect of varying pH on the rate - The pH ofeach solution ([1. = 0·2M) was adjusted by theaddition of buffer. Over the pH range 4·7-6·1,the plot of log kl versus pH is linear with a slopeDiscussionThe results obtained indicate that while TLA andTMA are oxidized by a similar mechanism, altogethErTABLE2 - EFFECTOF VARYING[SUBSTRATES]ONTHEPSEUDO-FIRSTORDERRATECONSTANT[Cr(VI)J = 3'33x10-IM; pH = 5·68; TEMP.=20°[TGA] x 103Mkl x 103 (see:")kl!(TGA] (litre mol-l sec+)ktf[TGA]2 [litre'? mol" sec+)1·00·1490·149149·001·50·3220'215143·002·00·5680·284142.002·50·9110'364145·003·0 4·01'33 2·360'443 0'59()148·00 147·002'0 2·51-68 NO8'40 8·806·0 7·02·82 3·244'70 4·631083[Cr(VI)] = l'66x10-4M; pH = 5'60; TEMP.=40°[TLA] x 102Mki x 103 (sec <)kl/[TLA] x 10 (litre mol-I sec+)0·250·228'800·50·4418'821·00·9938·831·51'298'60[er(VI)] = 1·66 x lO-IM; pH = 5·60; TEMP.=35°[TMA] x 103Ms, x 103 (see-I)kl/[TMA] x 10 (litre mol-l see-l)2·00·864·34·01·754·374·701·984·215'502'494·541085-0.1 -0~I .. '" (3)2~ I.I1C .II<". "".2 .2-0.2 -Q.5(1)2.62]252.1INDIAN J. CHEM., VOL. 15A, DECEMBER 1977(2) (3)o -0,5(1 )3.355.1 3.15 3.2 3.25 3.3 3.4~ .103Fig. 1 - Plots of logarithm of specific rate constants against reciprocal of absolute temperatures [(1) Thioglycollic acid;(2) thiolactic acid; and (3) thiomalic acid. 0, experimental points, and A, least square points]TABLE 3 - KINETIC EXPRESSIONS AT CONsTANT [H+], ORDER WITH RESPECT TO [H+] ANDACTIVATION PARAMETERS OBTAINED IN THE OXIDATIONS OF SOME IX-MERCAPTO ACIDS ANDTHEIR CORRESPONDING OXYGEN ANALOGUESAcid -d [Cr(VI)]ldt n E (kcal mol-I) -llSt Reference(cal deg-l mol")Thioglycollic ka [Cr(VI)] [TGA]I 0·2 11-4 15·6 This workThiolactic k'/, [Cr(VI)] [TLA] 0·4 4·8 47·7 doThiomalic kl [Cr(VI)] [TMA] 0·4 4·8 5106 doGlycollic kK [Cr(VI)] [GA] 0·4 13·7 25·7 15l+K [GA]Lactic k. [Cr(VI)] [LA] 1·0 8·4 37·3 16Malic k. [Cr(VI)] [MA] 1·0 8'3 37'3 16n = order with respect to [H+J. GA, LA and MA denote glycollic, lactic and malic acids respectively. k and K denotedisproportionation constant and equilibrium constant of 1: 1 complex formed by the interaction of GA and Cr(VI) asK "follows: GA + Cr(VI) ~ complex ->- products.different mechanism seems to operate in the TGAoxidation. This is in agreement with the observa-tions made earlier15,16 in the oxidation of corres-ponding oxygen analogues such as lactic, malic andglycollic acids by chromic acid (Table 3). Theactivation parameters (Table 3) obtained in theoxidations of thiolactic and thiomalic acids arealmost identical but the values are widely differentfor thioglycollic acid. This suggests that thio-glycollic is oxidized by a different mechanism.However, a comparison of the magnitude of thelast dissociation (RSH ~ RS-+H+) shows that PKvalues13,14 are very high (10·64-10·67) and sincedisulphide is produced in excess substrate it isobvious that -SH group provides the reaction site.In solutions where an excess of Cr(VI) is present,1084the stoichiometry is found to increase considerablywith time. This indicates that further oxidation ofthe disulphides to give sulphoxides and sulphones-takes place. When Cr(VI) is added to excess,substrate a readily distinguishable reddish browncolour appears rapidly and then disappears. Chro-mium(VI) has also been shown to form complexeswith some sulphur compounds and evidence forCr(VI)-sulphur bonding-? has been demonstrated.The oxidation of TGA by chromic acid can beexplained considering that 1:1 complex is formedby the interaction of TGA and HCrD4 followed bythe reaction of 1:1 complex with another substratemolecule to give 1: 2 complex in which two sulphuratoms are attached symmetrically on two sides ofhexacoordinated Cr(VI). The 1: 2 complex decom-SEN GUPTA et a.. KINETICS OF Cr(VI) OXIDATION OF IX-MERCAPTO ACIDSposes slowly to give two RS' radicals and Cr(IV) Acknowledgement(Eq. 3) followed by the dimerization of free radicals Thanks are due to the CSIR, New Delhi, for theto give disulphide. The existence of free radicals inthe TGA oxidation has been proved by the poly- award of a senior research fellowship (to S.S.G.)and to the UGC, New Delhi, for the award of amerization of acrylamide. Chromium(IV) may dis-proportionate to give chromium(V) or react rapidly junior research fellowship (to P.R.S.).with Cr(VI) to give chromium(V). The latter: Referencesreacts with another substrate to give RS+ whichthen reacts with RSH to give disulphide.Although in earlier reports4,6,7 it has been pointedout that disulphide is formed by the radicaldimerization, the formation of disulphide also by thereaction between RS+ and RSH cannot be totallyignoreds,onR-S-Cr-S-R ~ 2RS'+Cr(IV)IIoThe intermediate 1:1 complexes which are formedwith thiolactic and thiomalic acids readily decomposeto give RS+ and Cr(IV) (Eq. 4) followed by thereaction of RS+ and RSH to give disulphide asmentioned earlier. It may be mentioned that theoxidation of TLA and TMA does not involve freeradicals.oUR-S-Cr-OH ~ RS++Cr(IV)1/oThe entropies of activation obtained in the casesof thiolactic and thiomalic acids are well within thevalues for many bimolecular reaction-", On the otherhand, much lower negative entropy of activationfor the oxidation of TGA suggests that the reactiontakes place by a different mechanism.••• (3)1. CECIL, R. & MCPHEE, J. R., Adv. protein chem., 14(1959), 255.2. KHARASH, N., Organic sulphur compounds, Vol. 1(Pergamon Press, Oxford), 1961, 97.3. EAGER, R. L. & WINKLER, C. A., Can. J. Res., 26B(1948), 527.4. KAPOOR, R. C., KACHHWAHA,O. P. & SINHA, B. P.,J. phys. cu«, 73 (1969), 1627.5. KACHHWAHA,O. P., SINHA, B. P. & KAPOOR, R. C.,Indian J. cu-«; 8 (1970), 806. •6. HILL, J. & McAULEY, A., J. chem. Soc., (A) (1968), 156.7. HILL, J. & McAULEY, A., J. chem. Soc., (A) (1968),2405.8. PICKERING, W. F. & McAULEY, A., J. chem. Soc., (A)(1968), 1173.9. SEN GUPTA,K. K., SEN GUPTA, S. & CHATTERJEE,H. R.,J. inorg. nucl. Chem., 38 (1976), 549.10. ESPENSON, J. H. & KINNEY, R. J., Inorg, Chem., 10(1971), 1376.11. TONG, J. Y. & KING, E. L., J. Am. chem. Soc., 75 (1953),6180.12. BEST, P. A., LITTLER, J. S. & WATERS, W. A., J. chem.Soc., (1962), 822.13. LARSON, E., Z. anorg, allgem. Chem., 172 (1928), 375.14. CHENEY, G. E., FERHANDO,Q. & FREISER, J., J. phys.Chem., 63 (1959), 2055.15. SEN GUPTA, K. K., SEN GUPTA, S., SARKAR, T. &CHATTERJEE, A. K., Indian J. Chem., 13 (1975),1024.16. BAKORE, G. V. & NARAIN, S., J. chem, Soc., (1963),1419.17. MUIRHEAD, K. A. & HAIGHT, G. P., Inorg, Chem., 12(1973), 1116.18. FROST, A. A. & PEARSON,R. G., Kinetics and mechanism(John Wiley, New York), 1969, 128.1085

Kinetics of Chromic Acid Oxidation of Thioglycollic, Thiolactic & Thiomalic Acids

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Kinetics of Chromic Acid Oxidation of Thioglycollic, Thiolactic & Thiomalic Acids

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