Mathur, P. C.

Srivastava, S. S.

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indian Journal of ChemistryVol. ISA, September 1977, pp. 788-790Kinetics & Mechanism of Cu(lI)-catalysed Oxidation of Tartaric Acidby Alkaline Hexacyanoferrate (III)S. S. SRIVASTAVA & P. c. MATHURDepartment of Chemistry, Agra College, AgraReceiued 23 February 1976; revised 13 December 1976; accepted 9 February 1977Cu(II) ion-catalysed oxidation of tartaric acid by hexacyanoferrate(III) has been investigatedin alkaline medium. The reaction rate has first order dependence in the oxidant,tartaric acid,sodium hydroxide and the catalyst. A mechanism based on the formation of an intermediateCu(II)-tartrate complex and its subsequent oxidation to Cu(III)-tartrate complex by hexacyano-ferrate(III) has been suggested. The mechanism is substantiated by the results of a separatekinetic study of the oxidation of such intermediate complex which has been synthesized.Positive salt effect has been observed. Activation energy and entropy of activation have beenevaluated as 6·15 kcal/mole and -53·6 e.u. respectively.THE kinetics of oxidation of a number oforganic and inorganic compounds by hexa-cyanoferrate(III) has been studied by anumber of workersl'". In many cases the uncatalysedreactions are slow and hence some suitable catalystsuch as Os(VIII)5-S has been employed. RecentlyWilson et al.&-ll have investigated Cu(II)-catalysedoxidation of cystine and related thiols and hydroxyl-amine by hexacyanoferratc(III). The present paperdeals with kinetics of oxidation of tartaric acidby hexacyanoferrate(III) in the presence of Cu(II)ion in alkaline medium.Materials and MethodsThe stock solution of potassium hexacyano-ferrate(III) (AR, BDH) was prepared in doublydistilled water and standardized iodornetrically'".All other chemicals used were of AR grade and theirsolutions were prepared in doubly distilled water.The solution of sodium hydroxide was always stan-dardized before use. Aqueous solution of 1: 1Cu(II)-tartrate complex was preparedI3 by dis-solving equimolar amounts of tartaric acid andcopper sulphate in sodium hydroxide solution atpH~9.The kinetics of the reaction was followed bvestimating the decrease in [hexacyanoferratetl Hj]with time iodometrically. The method gave repro-ducible results. The results are interpreted interms of initial rate of the reaction to avoid theinterference due to reaction products. These rateswere obtained from the slopes of hexacyanoferrate-(III) reacted versus time plots for the first 10%of the reaction.Results and DiscussionI t was found that six moles of hexacvanoferrate-(III) are consumed per mole of tartaric-acid for itscomplete oxidation, which takes several days forcompletion to give formic acid, identified by chromo-tropic acid test14 and CO2,788The kinetics of the reaction was studied with,equimolar [oxidant] and [substrate] (O·OIM) at[alkali]= O'4M and [catalyst] = 2·5x 1O-4M at fivedifferent temperatures. It was observed that thedisappearance of hexacyanoferrate(III) followed asecond order path. The activation parameterscalculated are: temperature coefficient = 1'35;energy of activation = 6·15 kcal/mole ; frequencyfactor = 5·56 X 102 sec+: and entropy of activation=-53·6 e.u.The reaction rate has first order dependenceon [hexacyanoferrate(III)] at lower concentration(up to O·OIM) but deviates at higher concentrations.A plot of 1/[Fe(CN)~-J vs IjIR is linear, implyingthat the rate of reaction with respect to oxidantis governed by Eq. (1)R = A CFe[Cl\l~-1+BCFe[CN1~-The constants A and B have been evaluatedas 1·66xl0-2 and 16·6 respectively from the slopeand intercept of the linear plot. The reactionrate is first order with respect to tartaric acid upto 6·6 X 1O~3M concentration beyond which it fallsand has a first order dependence with respect to[alkali]. The reaction rate is directly proportionalto [Cu(II)], indicating first order dependence incatalyst. Hexacyanoferrate(II) has a retarding effecton the reaction rate.The reaction rate increases with an increase inthe ionic strength of the reaction mixture by theaddition of salts like potassium chloride and potas-sium sulphate (Table 1). Further, the absence ofvinyl polymerization ruled out the possibility ofa radical mechanism.Mechanism - It is generally known that tran-sitional metal ions with 3d9 structure are not easilyoxidized by mild oxidants but complex formationwith a suitable ligand provides favourable condi-tions for their oxidation to higher oxidation state.Hence, a mechanism involving the participation... (1)SRIVASTAVA & MATHUR: KINETICS OF OXIDATION OF TARTARIC ACID BY Fe(CN)~-plex in the rate-controlling step-4. The positivesalt effect and a negative value of entropy lendssupport to step-4 as rate determining one. Thereversible nature of this step is supported by a pro-nounced negative effect of hexacyanoferrate(II) onthe reaction rate. The Cu(III)-tartrate complex isthen attacked by OH- to give Cu(I) and intermediateproducts (step-5). Cu(I) is then rapidly oxidizedto Cu(II) by Fe(CN)~- ion (step-S). The inter-mediate products are oxidized in a fast step to finalproduct formic acid by a series of reaction (step-7).The above mechanism is similar to that proposedby 'Nilson (Ioc. cit.) for the Cu(II)-catalysed oxi-dation of thiols by hexacyanoferrate (III). Recentlythe existance of Cu(III) ion in alkaline mediumhas been reported by Jaiswal and Yadav'". Suchspecies has also been postulated in the Cu(II)-catalysed oxidation of tartaric acid by peroxidi-sulphate-".This mechanism finds further support from theresults of a separate kinetic study of the oxidationof the synthesized Cu(II)-tartrate complex byalkaline hexacyanoferrate(III), which reveals thatthe rate of oxidation of this complex has a firstorder dependence on [complex], [oxidant] and[alkali]. This justifies the validity of steps (4)and (5) of the proposed mechanism.Rate law - Applying steady state treatment tothe various intermediates and necessary approxi-mations for the initial stages of the reaction, thefinal rate term can be written in the form of Eq. (8)TABLE 1 -- EFFECT OF ADDED SALT ON THEREACTION RATEUFe(CN)!-] = [Tartaric acid] = 1 x lO-2M,[NaOHJ = 4·0 X lO-IM, [CuSO.] = 2·5 x 10-'M, temp. 40°}lO[salts]M Ionic strength JiI[ 10' IR M min-Inil 0·49 1·36KCI1·0 0·59 1·942·0 0·69 2·45K.SO.1·0 0·79 2·552·0 1·09 3·62of coordination complex between Cu(II) and tar-crate ion has been postulated as shown in Scheme 1.The alkali dependence of the reaction rate suggestsan attack of a OH- ion on tartrate ion resultingin the formation of a trivalent anion of tartaricacid (step-2). This step is essential prior to complexformation as has been reported in literaturet'' thatin tartrate-Cu(II) system one of the hydrogensof the alcoholic group is also replaced at high pH.This anion then forms a 1 : 1 coordination complexwith Cu(II) ion by a reversible equilibria (step-S),which in turn is oxidized to a Cu(III)-tartrate com-coo-ICHO}-t _ k II +OH;;:::-~CHOH kI -Icoo-coo--ICHO-I +HOCHOH ]Icoo-'- -- -- - --- 111coo-ICHQ-ICHOH,coo- r JyOO-l~HO _ CuCHOW'"hoo---J- ----- - - --1]1coo-~HOHCOO +1F.(CNI~- +20H~2HCOO-+C02+HP+2F'(CNI;~-J{very tast not included on rot. lowderivotion I_Scheme I-d ](Cp-CFe(CN):-CCu(II)COH-TtCFC(CN);'- = --1+J('C-;'e[CN1~ ... (8)whereK = 2klk21l3 and K' = !!'}-k_1k_2CH,O k-2On reversing Eq. (8)1 1=d--- = KCp-CFe[CN1'.-CCu[II1COH-(it- CFe[CNll-,41K'+--------K CT,-CCu[II1COH-Eq. (9) demands that a plot between 1jIR and1j[Fe(CN)~-] should be linear. This has actuallybeen observed, thus justifying the validity of ratelaw Eq. (8). The values of the constants K andK' have been evaluated from the slope and interceptof linear plot which on substituting in Eq. (8) givethe rate law (10)-d CFe[CN1'- =--= 16·66 X 103CFc[CN1;-Cp-COH-CCu[IIldt • 1+ 16'66CFe[CN1:-... (9)... (7)... (10)The validity of the rate law is further demons-trated by calculating the rate of reaction fromEq. (10) at different concentrations of the reactantsand comparing these values with those observedexperimentally. Table 2 incorporates the observedand calculated values of initial rates under identicalconditions at 40° which shows a considerable degreeof agreement between the two values justifyingthe validity of rate law and hence the mechanism.789INDIAN J. CHEM., VOL. 15A, SEPTEMBER 1977TABLE 2 - OBSERV'ED AND CALCULATED REACTION RATES{[Tartaric acid] = 1'OxlO-2M; [NaOH] = 4'Ox10-1M;temp. 40°}10S[Fe(CN):-] 104[CuSO.] IR mole litre" 1M M min ? x10'Obs. Calc.20·0 2·5 2·50 2'5010·0 2·5 1·45 1·435·0 2·5 0·66 0·774·0 2·5 0·60 0·622·5 2·5 0·36 0·392·0 2·5 0·31 0·3210·0 10·0 5·00 5·7110·0 5·0 2·50 2·8510'0 1·66 1-11 0·9510·0 1·25 0·67 0·71AcknowledgementThanks are due to Dr O. P. Bansal for his keeninterest in the work and Agra College authoritiesfor providing necessary facilities. Thanks are alsodue to the UGC, New Delhi, for financial assistance.790References1. THYAGARAJAN,B. S., Chern. Rev., 58 (1958), 439.2. SANT, B. R. & SANT, B. S., Talanta, 3 (1960), 261.3. SOLYMOS~,F., T'alanta, 4 (1960), 211.4. WILSON, L R., Rev. pure appl. Chern. A ustr. , 16 (1966),103.5. MEHROTRA,U. S. & MUSHRAN,S. P., J. Indian chem,. Soc., 45 (1968), 526.6. SINGH,V. N., SINGH, H. S. & SAXENA,B. B. L., J. Am.chem, Soc., 91 (1969), 2643.7. MUSHRAN, S. P., JINDAL, V. K. & AGARWAL,M. C.,J. chem, Soc., A (1971), 622.8. MATHUR, P. C. & SRIV'ASTAV'A,S. S., J. Indian chem.Soc., 53 (1976), 576.9. BRlDGART,G. J., FULTER, M. W. & WILSON, L R., J.chem, Soc., Dalton Trans., (1973), ) 274.10. BRlDGART,G. J. & WILSON, L R., J. chem, Soc., DaltonTrans., (1973), 1281.11. BRlDGART, G. J., WATER, W. A. & WILSON, L R., J.chem, Soc., Dalton Trans., (1973), 1582.12. VOGEL, A. L, A text book of quantitative inorganic ana-lysis (John Wiley, New York), 1951, 356.13. TrKHONOV',A. S. & BEL'SKAYA, V. P., Shoruik StateipoObschei Khim Akad Nauk USSR, 2 (1953), 1211.14. FEIGL, F, Spot test in organic analysis (Elsevier, NewYork), 1960, 368.15. BOBTELSKY,M. & JORDON, J., J. Am chem, Soc., 67(1945), 1824.16. JAISWAL, P. K. & YADAV, K. L., Indian]. cu-«,(1973), 837.17. AGARWAL,S. C. & SAXENA, L. K., Agra Univ. Jour.(Science), 23 (1974), 53.

Kinetics & Mechanism of Cu(II)-catalysed Oxidation of Tartaric Acid by Alkaline Hexacyanoferrate (III)

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Kinetics & Mechanism of Cu(II)-catalysed Oxidation of Tartaric Acid by Alkaline Hexacyanoferrate (III)

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