Indian Journal of ChemistryVol. 18A. September 1979. pp.225-227

Kinetics of Oxidation of Malonic Acid by Acid Dichromate

N. K. SARAN, M. N. DASH & R. C. ACHARYA*Department of Chemistry, Ravenshaw College, Cuttack 753 003

Received 31 January 1979; accepted 21 March 1979

The oxidation of malonic acid by acid dichromate in aqueous acetic acid medium has been found to be first orderwith respect to both [oxidant] and [substrate]. Complex formation between the oxidant and the substrate has beenindicated. The acid dependence shows first order with respect to [sulphuric acid] and beyond two molar acid concen-tration the order is about 3. Participation of both HCrO~ and Cr20~- has been inferred. The effect of dielectricconstant of the medium and ionic strength on the rate indicate the reaction to be of ion-ion type. Rate increase by theaddition of Mn(ll) salt has been discussed. A reaction path consistent with the results of oxidation and productanalysis has been suggested.

THE oxidation of a number of aliphatic dibasicacids has been studied with Cr (VI)1-3. Adetailed study on the oxidation of an aliphatic

dicarboxylic acid containing active methylene grouphas not been attempted. As a part of our programmeon the oxidation of organic compounds containingactive methylene groups, this paper incorporates ourkinetic results on Cr (VI) oxidation of malonic acid.

Materials and MethodsMalonic acid (MA) and diethyl malonate (DM)

were of Riedel grade. Glacial acetic acid (GR, S.Merck) was used as the solvent. Doubly distilledwater was used for preparing different compositionsof solvent mixture. All other chemicals used were ofAR or GR grade.

Kinetic measurement - Equal volumes of Cr (VI)solution (containing sulphuric acid or perchloricacid) and substrate (in glacial acetic acid) solutionof known concentrations were brought to the sametemperature within ± 0.1° and then mixed. Theionic strength of the medium was adjusted by addingrequisite quantity of potassium bisulphate in the caseof sulphuric acid solution and sodium perchlorate incase of perchloric acid solution. The course of the re-action was followed by withdrawing a known volumeof the reaction mixture at definite intervals of time intoa flask containing known excess of ferrous ammoniumsulphate and then back titrating the excess of Fe2+ion with standard potassium dichromate solution.

The kinetic runs were carried out under psuedo-first order conditions and the first order rate con-stant was calculated from the slope of the linear plotof log [oxidant], against time. These plots werelinear for about 70 % of the reaction. The duplicaterate measurements were reproducible to ± 3 %.

Stoichiometry and product analysis- The resultsof the experiment on stoichiometry indicated that onemole of substrate was consumed per mole of theoxidant. The end products were identified to beformic acid and CO2, The presence of tartronicacid and glyoxalic acid was indicated by spot tests'.

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The overall reaction may be represented by empiricalequation (1)

C804H4+Cr2Q~- +8H+ ~ 2 Cr+3+HCOOH + 2C02 + 5H20 ... (1)

Results and Discussion

Effect of varying [substrate] and [oxidant] - Theorder with respect to [Cr (VI)] and [substrate] wasobserved to be one each. Second order rate constants(k2 X I03M-1sec-1) calculated from the slopes of thelinear plots of kObs vs [substrate] are 0.96, 2.75,3.99, 8.51 at 35°,40°,45° and 50° respectively.

Although, the rate at which Cr (VI) disappearsfollows a first order rate law, the rate constants showa decreasing trend with increase in initial [Cr (VI)](Table 1). Such a behaviour is, however, notuncommon with Cr (VI) oxidations". The decreasingtrend in the rate constants may be due to the hydrol-ytic equilibrium between Cr20~- and HCrO-;;:. In viewof this, [HCrO-;;:] has been calculated for each initialconcentration of Cr (VI) using the value" of Kh= 1.6 X10-2 at 35°. The ratios between the observed rate and[HCrO-;;:] do not show constancy (Table I) indicatingthat HCrO-;;: is not the only oxidising species respon-sible for this oxidation. The observed rate data fitinto the expression(2)

- d(C~c;I)]= a [HCrO~] + b [HCrO-;;:]2 ... (2)

TABLE 1 - DEPENDENCEOF REACTIONRATE ON [Cr(VI)]{[H2S04] = 2M; solvent: 50% HOAc-H20 (v/v); ",=2.05M

[MA] = 2x 10-1 M ; temp. =35°} /

[Cr(Vl)] X 103 kl X 105 [HCrO~x 108 Ratef[HCrO~M See-I M

0.8331.001.251.502.00

2.4132.1802.0881.9281.918

2.642.422.372.242.32

0.7600.901.101.291.65

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fINDIAN J. CHEM., VOL. 18A, SEPTEMBER 1979,

The plot of ratej[HCrO~] versus [HCrO~] is linearfrom which the values of constants a and b were com-puted to be 3.2 X 10-5 and 7.5 X 10-3 respectively.This is indicative of the participation of both HCrO~and Cr20~- in tile oxidation process. Similar viewshave also been expressed' by Bakore et al,' in theoxidation of p-methoxyacetophenone by Cr(VI).

The plots of Ijkobs vs Ij[subs'trate] at. differenttemperatures are linear and db not pass 'throughthe origin but show definite intercepts at Ijkotsaxis. This indicates kinetic evidence for complexformation between the oxidant and the substrate.Condu tance measurement confirms 1 : 1 complexformation under the experimental conditions. .Fromthe intercepts and slopes of the above plots,the values of kd [the rate constant for the -dispropor-tionation of Cr (VI) - malonic acid-complex] and thevalues of K (the equilibrium constant which is a meas-ure of the stability of the complex) have been calculatedrespectively at different temperatures. The valuesof K (litre mol ") are 4.8, 4.0, 3.5 and 2.1 at 35°,40°,45~, .and 50° respectively. !::,H and !::,S associatedwith the complex formation are -13.1 kcaljmol and-39.3, e.u. respectively. Similarly, the values of rateconstants for the dispropertionation of the complex(Kd X 104sec") are 2, 5, 6.66, 10 :0, 40.0 respectivelyat the above temperatures. !::,Et and !::,St for thedecomposition process are 36.4 kcaljmol and 40.7e.u. respectively. These data show that the complexhas a rigid structure and it is formed by an exothermicprocess. Further, the tendency to form the complexduring oxidation process seems to decrease withincreasing temperature.

The thermodynamic parameters calculated from thelinear Arrhenius plot between kobs and IjT for MAand DM in the temperature range 30°-50° arepresented in Table 2. The anions like CIO~ andHSO~ increase the tendency of the Cr (VI)species to accept electrons from a reducing agent.Since CIO~ is more electron withdrawing thanHSO~, the rate of oxidation of malonic acid inHCI04 medium is found to be faster than in sulphuricacid medium. The rate of oxidation of diethyl malo-nate is slower than the rate of oxidation of malonicacid in perchloric acid medium. The slow rate ofoxidation of the ester may be explained by suggestingthat the electron withdrawing inductive (-1) effectof COOEt group is less than-COOH group and thereis possibility of steric effect of ethyl group whichhinders the formation of Cr(VI)-ester complexand thus reduces the rate of oxidation of ester.;

Acid dependence- The rate of oxidation increases. with increase in [H2S04]- The plot of log kobs

versus 'log [H2S04] gives .two intersecting lines,the linear 'plot bel-ow 2M acid passes through theorigin -with a slope of "unity and that above 2M,gives a definite intercept on log [H2S04l. axis with aslope of about '3.

The dissociation constant of malonic acid is Of theorder of 10-3 under experimental temperature rangeand this value being small, the 'organic acid remainsundissociated in the presence of high concentrationof mineral acid. Rather there is a possibility ofits being protonated due to intense inductive -effectof one of the carboxylic groups. This is further _

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supported by the fact that the rate constant is a linearfunction of [H2S04], However, second protonationof malonic acid is not favourable from electrostaticconsiderations. It is therefore, reasonable to assumethat monoprotonated malonic acid takes part inthe oxidation. This view has also been expressed bySenet et al. (loc. cit).

Effect of solvent composition on rate - The rate ofthe 'reaction increases with increase in acetic acidcontent of the solvent mixture (Table 3). The plot of;logkous vs IjD 'is linear with a positive slope up to60 % acetic acid. Thereafter, the increase in rate is veryrapid. Dependen.ce of rate on the dielectric constantof the medium suggests the possibility of ion-ioninteraction or ion-dipole interaction. First orderdependence on [H2S04] does not show the involve-ment of a positively charged Cr (VI) species. Hence,the possibility of the reaction between a positivelycharged Cr(VI)-species and malonic.acid may be ruledout under the experimental conditions. Consideringthe reaction between monoprotonated malonic acidHCrO~ -and the distance of closest approach (r)between the reactants has been calculated from theslope of the above plot and found to be 8,6A. Thisvalue is not far from the molecular dimensions.

Effect of varying ionic strength - An increase inionic strength by adding potassium hydrogen sul-phate decreases the rate. (Table 4). The plot of (log k-log ko) versus x/ fL indicates that the reaction is betweentwo oppositely charged reactants". It is difficult

TABLE2 - THERMODYNAMICPARAMETERS

Substrate t,E:f: t,H:f: log)oA -t,St t,G:f:(W)kcal/rnol kcal/mol e.u, kcal/mol

MAin H2SO. 22.2 21.6 10.57 10.5 24.8MA in HCIO. 18.0 17.3 8.97 11.0 23.4DM in HCI04 20.7 20,2 9.92 15.6 24.8

TABLE 3 - DEPENDENCEOF RATE ON SOLVENTCOMPOSITION{[Cr(VI)] = 1X 10-3M; [MA]=2x 10-~M; fL = 2.05 M;

[H2S04] = 2M; temp. = 35"]

HOAc% k,Xl05 103/D HOAc% k,xl0· 103/D(v/V) sec-1 (v/v) sec-1

304050

1.452.022.18

17.9822.3826.04

31.737.545.0

607080

3.559.27

26.66

TABLE4 - DEPENDENCEOF RATE ON IONIC STRENGTHAND[Mn2+]

{[Cr(VI)] = 1X 10-3M; [MA] = 2x 1O-2M; [H2S04] = 2M;temp. = 35°; solvent = HOAc - H20 50% (v/v)} /

[KHSO.]X 10 k'LX 10· ...;;- [Mn2+] X 103 k,x 1~M sec-1 M sec-1

0 2.20 1.415 0 2.201.0 2.12 1.783 0.833 5.072.0 1.95 2,001 1.0 6.283.0 1.80 2.237 2.0 9.154.0 1.65 2.2455.0 1.54 2.647

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SARAN et al. : OXIDATION OF MALONIC ACID

to interpret these data since there are several effectiswhich may cause a change in rate with changing ionc,strength. For example, the acid chromate-dich-romate equilibrium constant is a function of the ionicstrength. Increase in ionic strength will favour theformation of the dichromate ion and this will decreasethe rate of the reaction-".

Effect of added Mn2+-Mn2+ is generally known toretard the rate of oxidations of organic and inorganjssubstrates by Cr (VI) but in some cases it is found toaccelerate'! the rate of oxidation. The observed rateconstant has been found to increase linearly withincreasing [Mn (II)] (Table 4) in accordance withthe empirical relation, kobs = 2.3XI0-5 + 0.034[MnH]. The exp-erimental value, of kobs, in theabsence of [Mn2+) is 2.2 X 10-5 sec! and itagrees quite well with the extrapolated valueof 2.3 X 10-5 seel when [Mn2+]=0. Mn(ID is knownto form a metal chelate-" with malonic acidand it is oxidised by Cr (VI) to give a chelatehaving Mn (IID which is not stable at acidity belowSM. Under our experimental condition of acidity(2M), Mn (III)- malonic acid complex is expected toget reduced to Mn(H) quickly by abstracting electronfrom the substrate resulting in a rapid rate ofreaction. A similar explanation has also been offeredby Sengupta and Sarkar-''.

Nature of Cr (VI) species- It is not easy to decide.the exact Cr (VI) species present in the solutionsince it is involved in a series of protolytic and hyd-rolytic equilibria e.g.

Cr20~- + H20 ~ 2HCrO~HCrO-; + H+ ~ H2Cr04

HCrO-; + 2H+ ~ HCrOt + H20. In addition to the above species HCrO~ and HCrOt

are known to combine with HSO-; in sulphuric acidto form Cr (VI) species'v-", CrSO~- and HCr03•

HS04• Since the reaction has been carried outin acetic acid medium, Cr(VI) dissolved in highpercentage of acetic· acid may also exist in the form ofan acetyl chromate ion, AcOCrO';. Since the presentoxidation is being carried out in 2M sulphuric acid,Cr (Vlj-species involved in the reaction seem to beHCrO-; and Cr20~-. Solvent dependence on rate indi-cates a superior oxidising species beyond 60 % aceticacid in the mixture. It may be acetyl chromate ionas acetyl group would increase the electron acceptingpower of Cr (VI). Rapid increase in rate beyond 2Msulphuric acid may be explained by assuming theinvolvement of a neutral but more active Cr (VI)species like HCr03• HS04•

Mechanism - The formation of free radical (nopolymerisation of acrylonitrile) was not indicated in

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the initial stage. However, polymerization of acry-lonitrile and reduction of mercuric chloride weredetected towards the later part of the oxidation. Theseobservations show that free radical is not formedin the rate-determining step and Cr (VI) appears tobehave as a two electron oxidant. The intermediateCr (IV) species is assumed to get reduced to Cr (III)by one-electron change in view of the positive freeradical tests. In consideration of the above discussions,stoichiometry and products formed, the mechanismshown in scheme 1 is suggested for the oxidation ofmalonic acid by acid dichromate.

MA + H+ ~ MAH+2e

MAH+ + Cr(VI) ~ Complex --+ Tartronic acid +Cr(IV). . ~w2e

Tartronic acid + Cr(Vl) --+ Glyoxalic acid + Cr(lV) + CO.Ie

Glyoxylic acid + Cr(IV) --+ Formic acid + 'COOH +Cr([Il)Ie

-cooa + Cr(IV) --+ CO2 + Cr(TII)

Scheme 1

AcknowledgementOne of the authors (N. K. S) is thankful to the UGC,

New Delhi for the award of a research fellowship.

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