Indian Journal of ChemistryVol. 19A, June 1980, pp. 544-546

Kinetics & Mechanism of Mn(II)-catalysed & Uncatalysed Oxidationof Oxalic Acid by Acid Bromate

Ch. SANJEEVA REDDY, VJJAYA LAXMlt & E. V. SUNDARAM·Department of Chemistry, University College, Kakatiya University, Warangal 506009

Received 26 May 1979; revised 20 November 1979; accepted 7 January 1980

The kinetics of oxidation of oxalic acid by bromate ion in binary solvent mixture of acetic acid-water in thepresence of sulphuric acid has been studied in detail. In the uncatalysed oxidation the reaction is first order each inbromate, sulphuric acid and oxalic acid. There is no stable complex formation between the oxidant and the substrate,whereas in Mn(ll)-catalysed oxidation the order with respect to bromate is found to be two. Kinetic evidence forcomplex formation between the oxidant and the substrate has also been obtained. In the presence of catalyst the re-action follows a free radical mechanism. Based on the kinetic data the possible mechanisms for uncatalysed and cata-lysed reactions are suggested.

THE oxidation of oxalic acid has been studiedwith different oxidants like Mn(VI)1-3, TI(III)4,Ce(IV)5 V(V)6,7 in acid medium. No work

seems to have been reported on the oxidation ofoxalic acid by potassium bromate. In the presentpaper we report. the oxidati?n of oxalic acid. bybromate ion. It IS worth notmg that the reductionproduct, bromide ions, le~ds to t~e t:ormation ?fbromine which interferes 10 the OXIdatIOn of oxalIcacid due to its oxidizing property and hence mercuricacetate has been used for masking the bromide ions".Added mercuric acetate does not interfere with thekinetics.

Materials and MethodsAll the chemicals used were (BDH) AnalaR grade

reagents and these were further purified by recrystal-lization or by distillation. The stock solutions wereprepared by dissolving the weighed amounts of thesubstances in conductivity water.

The reaction was followed by withdrawing aliquotsof the reaction mixture at known intervals of timeand quenching the reaction by pouring in excess ofpotassium iodide solution. The liberated iodine wasestimated by titrating against standard thiosulphatesolution. Oxalic acid was kept ten times in excessand hence pseudo-first order rate constants wereobtained from the plots of log (a-x) against time.The second order rate constants were obtained bydividing the pseudo-first order rate constant by[substrate].

The reaction mixture containing known excessof bromate was kept at 45° until there was no changein bromate concentration. The remaining bromatewas estimated and it was observed that 3 moles ofsubstrate were used for 1 mole of bromate. Thestoichiometry (3 : 1) was also obtained by keepingoxalic acid in excess and estimating the remaining

tPresent address: Department of Chemistry, L. B. College,Waranga!.

544

oxalic acid by potassium permanganate. The stoi-chiometry remained same in Mn(II)-catalysed oxida-tion also.

Results and DiscussionUncatalysed reaction - The order of reaction with

respect of [bromate] was determined by studyingthe reaction with five different initial concentrationsof bromate and measuring the initial rates. Theplot oflog initial rates versus log [oxidant] was linearwith a unit slope, indicating the reaction to be offirst order.

The order of reaction with respect to [oxalic acid]was found to be one from the study of the reactionwith different initial concentrations of oxalic acid(Table 1). When Ijkob8 was plotted against 1([oxalic acid), a linear plot passing through the originwas obtained indicating that there is no stable complexformation between the oxidant and the substrate.

The effect of varying [sulphuric acid) on the reactionrate was also studied (Table 1). The plot of log kversus log [H~S04) is linear with a unit slope indicat-ing the involvement of a proton. The bromate ionforms bromic acid in acid medium. Since bromicacid is a powerful oxidising agent, the reaction issupposed to occur between a protonated bromateion (HBrOa) and oxalic acid.

The effect of dielectric constant was studied byvarying the composition of acetic acid. The dielec-tric constants of different acetic acid-water mixtureat 30° were obtained by approximate validity method",A good linear plot was obtained when log k was

plotted against [~ - I ] rather than L]«. According

to Kirkwood equation for dipole-dipole reaction

the plot of log k against [: - 1 ] should be linear

with a negative slope. Hence this observation is inaccordance with the proposed mechanism involv-

SANJEEVA REDDY et al. : BROMATE ION OXIDATION OF OXALIC ACID

TABLE 1 - EFFECTOFVARYINGCONCENTRATIONSOFOXAL'CAC'D, SULPHURICACID ANDMn(II) IONSONTHE RATEOFREACTION

{[BrOl; = 1.0 X 10-"M; Acetic acid

Effect of [oxalic acid]

30% (v/v); [Hg(CH.C00)2] = 1.0 X lO-'M; temp. = 30°}

Effect of [H,SO.] Effect of [Mn(II)]

{[C20.H2]=0.0IM;[H.SO.]=1.0M}

([Mn(II)] = 1.0 x lO-cM; [H2SO.] = 1.0M} {[CIO,H.] = O.OlM; [Mn(JJ)] = 1.0xlQ-'M}

lC.O.H.]xlOS k.xlO" k'. [H2SO.] k.x102 r,M litre mol=! sec=! litre mol-I sec-I M litre mol-1 sec+' litre mol=' sec-1

[Mn(II)] X 1()I k:« x 10M litre mol"? see-I

1 0.877 0.392 0.5 0.122 2 3.342 1.835 0.641 1.0 0.877 0.390 4 4.054 3.315 0.933 1.5 1.375 8 4.496 5.055 1.100 2.0 1.535 5.750 10 4.378 6.825 1.460 3.0 2.303 19.250 12 5.00

10 8.530 4.0 3.175 16 5.5212 1.600 4.6 3.750

k. and k'. are rate constants of uncatalysed and catalysed reactions respectively.

ing the reactive species as HBr03 and oxalic acidmolecule.

The reaction was studied at different temperaturesand from the Arrhenius plot, different thermodynamicparameters, viz. 6.Et, 6.Ht, 6.st and 6.Gtand log PZ were evaluated and found to be 88.345K J mol<, 85.825 K J mol-I, - 31.37 J deg=rnol'<,SS.314 K J mol-l and 11.15 respectively for uncata-lysed reaction and 77.302 K J mol-I, 74.783 K Jmol:", - 35.785 J deg" mol">, 86.630 K J mol-l and10.97 respectively for catalysed reaction.

From the observed data a possible mechanismfor uncatalysed reaction as shown in Scheme I issuggested. The Br(III) formed further reacts in thefast steps to produce bromide ion as final product,the stoichiometry being 3: I.

KlH+ + BrO; ••• HBrO.

COOH k,HBrO. + I ->- 2CO. + H20 + HBrO.

COOH

Scheme 1

The rate expression obtained is

_ d[8~O;] = Klk2 [BrO;] [H+] [oxalic acid]II

which explains all the observed facts.

Catalysed reaction - In the presence of Mn(II) asa catalyst the interesting features observed were (i)The reaction followed free radical mechanism in thepresence of Mn(II). The presence of free radicalswas confirmed by adding acrylonitrile when precipi-tate was formed. This clearly indicated one electrontransfer steps, probably involving Mn(III) or Mn(IV);{ii) The order of reaction with respect to bromate wasfound to be two. The order in bromate was deter-mined by measuring different initial rates and plottinglog rate against log [bromate]. The plots of I/(a - x).against time were also linear .. The slopes (ps~~d.o-second order constants) being independent of initial

bromate concentration. This clearly indicated thattwo bromate molecules were involved in the ratedetermining step.

The order of reaction with respect to Mn(IJ) wasfound to be fractional « 1), (Table 1) and the orderwith respect to [H2S04] was two indicating the invol-vement of two protons.

The order of reaction with respect to oxalic acidhas been found to be fractional « 1), (Table 1) anddefinite intercept was obtained when llkobs was plott-ed against l/[oxalic acid] indicating the formation ofa complex between oxidant and substrate. In theabsence of Mn(lI) there was no complex formationbetween the oxidant and the substrate. Hence thecomplex might be formed between the bromic acidand the Mn(II)-oxalic acid complex (Scheme 2)rather than between bromic acid and oxalic acid,the complexing centre being Mn(II). The dissocia-tion of the c~mplex is the rate determining step.Based on the kinetic data the following mechanismas shown in Scheme 2 is suggested for Mn(II)-catalys-ed oxidation.

SCHEME 2

545

The rate law can be expressed as

INDIAN J. CHEM., VOL. 19A, JUNE 1980

dfBrO;]dt

K;K2Kak'2 [H+J2 [Mn2+] [C204H21 [BrO";p1 + 2KiK2Ka [HrOa] [H+]2 [C204H2] [MnH]

which agrees well with the observed data.

From this rate expression one can observe that thepseudo-second order rate constants should decreasewith the increase in [bromate]. However in thepresent work the order with respect to bromate isfound to be nearly 2.0. The effect of [bromate]on second order rate constants seems to be lesswhereas in the results on oxidation of malonic acid(to be published) the order with respect to bromate isfound to be less than two (1.5) and in that case thepseudo-second order rate constant decreases with[BrO;] as required by above expression.

546

AcknowledgementOne of the authors (V.L) thanks the CSIR, New

Delhi for the award of a senior research fellowship.The author (Ch.S.R.) thanks Kakatiya University forthe award of a university fellowship.

References1. NOYES, R. M. & TRANS, N. Y., Acad. Sci., 13 (1951), 314_2. ABEL, E., Mh. Chem., 83 (1952), 695.3. LADBURY, J. W. & CULLIS, C. E, Chem. se«. 58 (1958),

403.4. SRINIVASAN, V. S. & VENKATASUBRAMANAIAN, N., Indian

J. Chem., 11 (1973), 702.5. KANSAL, B. D. & NEPAL SINGH., J. Indian chem, Soc.,

LV (1978), 304; 618.6. JONES, J. R. & WATERS, W. A., J. chem., Soc., (1961),

4757.7. KURlHARI, H. & NOZAKI, T., J. chem. Soc. Japan, 83

(1962), 708.8. VIJAYA LAXMI & SUNDARAM, E. V., J. Indian chem. Soc.

LV (1978), 567.9. VENKATASUBRAMANIAN,N., J. scient. Ind. Res., 20B (1961).

541.