Indian Journal of Chemistry Vol 35A, May 1996, pp. 408-415 Oxidative mechanism of glycolic acid and formaldehyde by acid bromate: A critical study Ch Sanjeeva Reddy" & T Vijaya Kumar Department of Chemistry, Kakatiya University, Warangal 506 009 Received 6 September 1995; revised 16 November 1995 Kinetics and mechanism of oxidation of glycolic acid (GA) by acid bromate (free from the autoca- talytically generated molecular bromine oxidation) is studied in detail. The reaction exhibits first order each in [GAl and [oxidant] and second order in [acid]. Reaction rates are very much susceptible to change in dielectric constant (e) of the medium. The reaction exhibits solvent isotope effect (1.8 ± 0.01 at 303 K) but primary isotope effect is absent. The rate determining step is the esterification of glycolic acid with acid bromate, followed by a fast disproportionation of the ester through a C - C bond clea- vage to yield formaldehyde. This product undergoes oxidation with a faster rate than glycolic acid en- ding up with formic acid, inert towards further oxidation. An independent study of oxidation of for- maldehyde is also made and results are correlated 'fith the observations of the later part of the reac- tion. Related activation parameters are evaluated and discussed. An attempt is also made to compare the similar oxidations of other substrates and oxidants. Glycolic acid (GA) being a bifunctional com- pound, its oxidation can proceed by many possi- hie routes. Rocek and co-workers' in the GA oxi- dation by chromic acid, demonstrated that Cr(VI) acted as a three electron oxidant. In a similar study with permanganate, Sengupta et al? noticed that the oxidation involves the rupture of C- C bond without proceeding through usual steps viz., alcohol - carbonyl compound -- carboxylic acid, in which Mn(IV) was the active species rather than Mn(VII). A kinetic isotope effect of 5.8 was ob- served by Banerji" in the pyridinium chloroch- romate-GA reaction. The reaction rate increased by substituting alkyl group in HOCH 2 - COOH and hydride ion transfer was assumed in the rate determining step. In the study of oxidation by manganese(III) sulphate, Khamrui et at: summa- rised that the mechanism involved rapid and re- versible formation of Mn(III)-GA complex fol- lowed by its decomposition to yield a transient free radical. Similar mechanism was also for- warded by Mehta and coworkers" with aquo-man- ganese(III). On the otherhand, a transient 1:1 cyc- lic complexation between GA and Mn(III) pyro- phosphate, with the stability constant of 44.0 mol dm - 3, which was followed by a C - C bond clea- vage in the rate determining step to yield pro- ducts was proposed by Rao and Gandhi". The ox- idative kinetics of the same substrate by Ce(IV) was studied by a number of workers. Grover and Gupta", made a comparative kinetic study of sub- stituted glycolic acids by eerie ions in sulphuric acid medium. Catalytic activity by manganese(II) was also noticed in these investigations. In a simi- lar study with Ce(IV) as an oxidant, Dayal et al. R and later Prasad and Choudary? postulated C-C bond cleavage as the rate determining step. Am- zad et al.1O and Calvaruso et al." studied the oxi- dation of a-hydroxy acids including GA by Ce(IV) in perchloric acid medium and established an innersphere mechanism involving a thermody- namically more stable and kinetically more reac- tive complex. In the Cu(II)-catalysed oxidation of GA by vanadium(V) in sulphuric acid medium, Rao et al.'? suggested prior formation of a com- plex between copper(II) and GA, which got oxi- dised by V(V) to give an aldehyde as an end pro- duct. A similar study was also undertaken by Choudhari and Prasad':'. Cobalt(III) perchlorate'v " and cobalt(III) sulphate'? were extensively used for the oxidation of different types of organic sub- strates including GA. A general pattern in all these oxidations included the formation of a short lived intermediate complex which subsequently disproportionated in the rate determining electron transfer step. Similar studies using acid bromate as an oxi- dant are lacking and hence the present work. In
Transcript
Indian Journal of ChemistryVol 35A, May 1996, pp. 408-415
Oxidative mechanism of glycolic acid and formaldehyde by acid bromate:A critical study
Ch Sanjeeva Reddy" & T Vijaya Kumar
Department of Chemistry, Kakatiya University, Warangal 506 009
Received 6 September 1995; revised 16 November 1995
Kinetics and mechanism of oxidation of glycolic acid (GA) by acid bromate (free from the autoca-talytically generated molecular bromine oxidation) is studied in detail. The reaction exhibits first ordereach in [GAl and [oxidant] and second order in [acid]. Reaction rates are very much susceptible tochange in dielectric constant (e) of the medium. The reaction exhibits solvent isotope effect (1.8 ± 0.01at 303 K) but primary isotope effect is absent. The rate determining step is the esterification of glycolicacid with acid bromate, followed by a fast disproportionation of the ester through a C - C bond clea-vage to yield formaldehyde. This product undergoes oxidation with a faster rate than glycolic acid en-ding up with formic acid, inert towards further oxidation. An independent study of oxidation of for-maldehyde is also made and results are correlated 'fith the observations of the later part of the reac-tion. Related activation parameters are evaluated and discussed. An attempt is also made to comparethe similar oxidations of other substrates and oxidants.
Glycolic acid (GA) being a bifunctional com-pound, its oxidation can proceed by many possi-hie routes. Rocek and co-workers' in the GA oxi-dation by chromic acid, demonstrated that Cr(VI)acted as a three electron oxidant. In a similarstudy with permanganate, Sengupta et al? noticedthat the oxidation involves the rupture of C - Cbond without proceeding through usual steps viz.,alcohol - carbonyl compound -- carboxylic acid, inwhich Mn(IV) was the active species rather thanMn(VII). A kinetic isotope effect of 5.8 was ob-served by Banerji" in the pyridinium chloroch-romate-GA reaction. The reaction rate increasedby substituting alkyl group in HOCH2 - COOHand hydride ion transfer was assumed in the ratedetermining step. In the study of oxidation bymanganese(III) sulphate, Khamrui et at: summa-rised that the mechanism involved rapid and re-versible formation of Mn(III)-GA complex fol-lowed by its decomposition to yield a transientfree radical. Similar mechanism was also for-warded by Mehta and coworkers" with aquo-man-ganese(III). On the otherhand, a transient 1:1 cyc-lic complexation between GA and Mn(III) pyro-phosphate, with the stability constant of 44.0 moldm - 3, which was followed by a C - C bond clea-vage in the rate determining step to yield pro-ducts was proposed by Rao and Gandhi". The ox-idative kinetics of the same substrate by Ce(IV)
was studied by a number of workers. Grover andGupta", made a comparative kinetic study of sub-stituted glycolic acids by eerie ions in sulphuricacid medium. Catalytic activity by manganese(II)was also noticed in these investigations. In a simi-lar study with Ce(IV) as an oxidant, Dayal et al.R
and later Prasad and Choudary? postulated C - Cbond cleavage as the rate determining step. Am-zad et al.1O and Calvaruso et al." studied the oxi-dation of a-hydroxy acids including GA byCe(IV) in perchloric acid medium and establishedan innersphere mechanism involving a thermody-namically more stable and kinetically more reac-tive complex. In the Cu(II)-catalysed oxidation ofGA by vanadium(V) in sulphuric acid medium,Rao et al.'? suggested prior formation of a com-plex between copper(II) and GA, which got oxi-dised by V(V) to give an aldehyde as an end pro-duct. A similar study was also undertaken byChoudhari and Prasad':'. Cobalt(III) perchlorate'v "and cobalt(III) sulphate'? were extensively usedfor the oxidation of different types of organic sub-strates including GA. A general pattern in allthese oxidations included the formation of a shortlived intermediate complex which subsequentlydisproportionated in the rate determining electrontransfer step.
Similar studies using acid bromate as an oxi-dant are lacking and hence the present work. In
REDDY et al.: OXIDATION OF GLYCOLIC ACID BY ACID BROMATE 409
this study it is observed that the product is alsogetting oxidised by acid bromate, hence two dis-tinct reactions, a slow reaction followed by a fas-ter one. An attempt is also made to explain thecomplex and/or competitive reactions.
Materials and MethodsAll the reagents employed were of AnalaR
grade or high purity. All standard solutions weremade in conductivity water and/or in glacial ace-tic acid following the standard procedures. Glyco-lic acid was standardised by cerimetry".
Under subdued light and nitrogen atmosphere,the solutions were thermostated for an hour atthe desired temperature. The reactions were in-itiated by the addition of bromate to the tempera-ture equilibrated solutions of substrate and otherreagents. The progress of the reaction was moni-tored by estimating unreacted bromate iodometri-cally to a starch end-point. The plots of log[brom-ate 11 versus time exhibited bi-variancy with respectto depletion of [bromate 1. All reactions proceededin two distinct steps, with an identical characteris-tic of an initial slow reaction followed by a fasterone. The plots of log[bromate 11 against time, com-praised of two distinct straight lines, (with corr.coeff. 0.995 in each case) having two differentslopes corresponding to slow and faster stages ofthe reaction. Hence the pseudo-first order rateconstants (kG and kr) corresponding to slow andfast stages of the reaction, with respect to brom-ate, were computed from the slopes of two linearparts of the plots of log[bromate 11 versus time.Second order rate constants (k2, dm ' mol- I s - 1 )
were calculated by dividing kl (s - 1) with [substr-ate] Triplicate kinetic runs showed that kineticdata was reproducible within ± 5% and meanvalues were reported.
A Schimadzu multipurpose recording doublebeam spectrophotometer model MPS-5000equipped with a temperature controller was usedfor absorption studies.
StoichiometryIn the presence of adequate acid, mercuric acet-
ate, and excess of bromate over the [substrate],the stoichiometry established is 3:2, amounting tofour electron oxidation of the substrate.
The reaction mixture was allowed to stand for60 h at the same conditions as in the kinetic de-termination to enable the completion of the reac-tion. At the end, reaction mixture was separatedwith diethyl ether, washed with water and concen-trated. Formic acid was the product of oxidation
as identified by its qualitative tests" and quanti-tatively estimated with chromotropic acid after re-ducing it with Mg powder. The optical densitywas measured at 570 nm and the concentrationof formic acid was calculated using e' = 0.57 dm 'ug : 1 (ref. 20).
ResultsThc molecular bromine is expected to be gen-
erated by an autocatalytic bromate-bromide (re-duction product of bromate) reaction. As Br 2 isalso a powerful oxidising agent, its interferencewith the reaction under study was eliminated bythe addition of a bromo-complexing agent likemercury(II) acetate, which forms unionisablebromocomplexes ". Multi-fold mercuric acetateconcentrations did not affect the rate constants,revealed that it acted as only a bromide ion sca-venger without perturbing the kinetic data of theBr(V)-GA reaction.
Preliminary investigations in which [bromate 1was monitored, taking excess of mineral acid andsubstrate revealed that the depletion of bromateoccurred in two distinct steps; a slow step fol-lowed by a rapid one, indicating that the reactionfollowed a complicated kinetics, possibly involv-ing competitive and 'Consecutive reactions.
Dependence of rate on the initial bromate concen-tration
Kinetic data were collected for initial [bromate 1in the range 2.0 to 20.0 X 10-4 mol dm-3. All theexperiments exhibited an identical characteristic,an initial slow reaction followed by a faster one.The transition to rapid stage was almost inde-pendent of the [bromate]; The plots of loglbrom-ate It versus time, for different initial concentr-ations, had two distinct straight lines for the slowand faster steps of the reaction with correlationcoefficients of 0.996 and 0.994 respectively. Thefairly constant, kG (s -1) values for slow step andkr(s -I) values for fast step confirms that slow andfast depletions of bromate followed first orderkinetics (Fig. 1). The oxidation of formaldehydestudied independently revealed that the rate wasindependent of initial [bromate 1. The slow deple-tion of bromate was solely attributed to the oxida-tion of glycolic acid and rapid one may be due tointerference of more reactive intermediate/pro-duct (to be visualised in due course to be for-maldehyde) or both.
Dependence of rate on [glycolic acid]kG values increased linearly with increase in in-
itial [GA], indicating first order in [GA] (Table 1),
410 INDIAN J CHEM, SEe. A, MAY 1996
Table I-Effect of varying [reactants] and solvent composition on the rate of acid bromate oxidation of glycolic acid at 313 K
104 x [Br(V)] 102x[GAj [H2S04] HOAc-H20 104 X rate constant (S-I)
(e) 16 x 10-4 mol dm"?(GAj=O.Ol mol dm="; HOAc-HP=30-70% (v/v)[Hg(OAch]-O.Ol mol dm-J; temp = 313 K
whereas k, values increased slightly and reached amaximum. The oxidation of formaldehyde (inqe-pendent study) also exhibited a fractioilal orderdependence on initial [HCHO].
Effect of[ acid] on the rate of reactionThe effect of sulphuric acid on the rate of reac-
tion was studied at".constant ionic strength. Fromthe plots of log kG versus loglacid], it was ob-served a second order dependence on [acid] inthe range of 0.5 to 2.00 mol dm ", kG and kr va-lues increased with increase in [acid] and the dif-ference in their values was narrowed on increas-ing [acid] due to less acceleration of the later partof the reaction involving the oxidation of the pro-duct. The same is in agreement with the observa-tion of the oxidation of formaldehyde wherein,acid dependence is less than unity in the range(0.1 to 1.0 mol dm-3) studied, as its rates of oxi-dation are very high at higher [acid] and cannotbe measured in the present setup.
Effect of dielectric constant of the mediumThe increase in dielectric constant (e) of the
medium by varying the composition of acetic acidand water, decreased the rate constant (Table 1)
REDDY et al.: OXIDATION OF GLYCOUC ACID BY ACID BROMATE 411
and the plot of log k (kG or kf) versus liE was lin-ear with a positive slope.
Added salts like Na2S04 and NaHS04 had noeffect on the rate of reaction.
The rate of reaction was neither accelerated inthe presence of benzoyl peroxide nor retarded inthe presence of acrylonitrile, ruling out the pos-sibility of free radicals in the reaction.
Effect of temperatureGA- Br(V) and HCHO- Br(V) reactions were
studied at different temperatures ranging from308 to 328 K in sulphuric acid' and evaluated ac-tivation parameters are recorded in Table 2.
DiscussionIn acid solutions bromate exists in the protonat-
ed and unprotonated forms. Amis et alP pro-posed Bf02 as the oxidising species in bromateoxidations of iodide. Ambar and Guttrnan-' aswell Wright and Bartorr'" suggeste2 that in moder-ately strong acid solutions H2X03 is the existingform of halate; in the case of bromate it is
+H2Br03• However, Reddy et al. observed thatboth, HBr01 and H~B}03' are the species exist-ing in acid medium with protonation constants ofKp)=0.529 drn ' mol-) and Kp2=0.21 drn" mol"?respectively/Y" .
The rate acceleration with an increase in [acid]and' the study of dielectric constant effect on therate of the reaction, mitigate the chances of parti-cipation of BrO:3 as the reactive species. The sec-ond order dependence of rate on [acid] in therange studied also confirms that H2Bfo3 is thereactive species.
In an aqueous solutions of GA, 12.16% of it ispresent in the dissociated form when [GAl = 0.01mol dm - 3 as its Pk, = 3.85. In the presence ofmolar sulphuric acid the GA dissociation is com-pletely suppressed and only 1.48 x 10 --6 moldm - 3 is present in the dissociated form, hencenegligible. Therefore, it is more appropriate to as-sume undissociated glycolic acid as the reactivespecies.
Inspite of considerable variations in the struc-ture, glycolic and lactic acids are oxidised at thesame rate (1.43 x 10-2 and 1.42 x 10-2 dm 'mol-) s - )). The structural insensitivity observedin the hydroxy acids oxidation by Br(V) rules outthe possibility of bromate ester decomposition be-ing the rate determining step. This step involvesthe cleavage of stronger C - H bond and is boundto be influenced by structural changes in hydroxyacids, as was seen in chromic acid oxidatiorr'" orin the oxidation with bromine.". The absence ofkinetic isotope effect (kH/ ko = 1) also confirmsthat C - H bond is not cleaved in the rate deter-mining step. The relative ease of oxidation of gly-colic acid shows that, the oxidative decarboxyla-tion is energetically more favourable and a-hydrogen is not involved in oxidation at all. Theassumption of bromate ester formation being theslow step could only explain the observed insensi-tivity to structural influences. Inorganic esterifica-tion reactions of this type are known to be not in-fluenced by structural variations in alcohol moie-ty29.30.
Acid catalysed reactions are expected to be fas-ter in D20 as D~O is a stronger acid than H;O.However, labile hydrogens like those present in
Table 2-Activation parameters of acid bromate oxidation of glycolic acid and formaldehyde>
Slow step of GA-oxidation102 x kG (mol "! dmJ S-I)
Fast step of Ga-oxidation102 x k,(mol-I dm3 S-I)
Oxidation of formaldehyde104 x k,(s-I)
(al'The average error in the values of 6H', 6S' and 6G' (at 313 K) are: ±2 kJ mol-., ±5 J mor ' K-I and ±3 kJ mor ' re-spectively.(b'Value in the parenthesis corresponds to the oxidation offormaldehyde
412 INDIAN J CHEM, SEe. A, MAY 1996
hydroxyl or carboxyl groups undergo rapid ex-change in D20. If 0 - H bond is cleaved either inrate determining step or prior to it, 0 - H/O - Disotope effect comes into play. This cancels therate enhancing effect of D20 and may result in aninverse isotope effect. In the present reaction, aninverse solvent isotope effect (kH,O/ ko,o = 1.8± 0.01 at 303 K) observed indicating that - OHgroup and/or - COOH group is/are involved inthe rate determining step.
Based on the aforesaid reasons, the possiblemechanism for the reaction is outlined as in (Eqs1 and 2)
~ + k 7 +H-~-OH + HOBr02H \ .10:) H-~-O- Br02H-H~
COOH COOH (, )
HI + fasf +
H"~JO- Br02H -- HCHO+c02+ H8r02+H
O'lC~_H(2 )
The bromate ester formed in Eq. (1) may alsoreact with proximate carboxylic acid group toform a cyclic ester (Eq. 3), which may subse-quently decompose to products.
An analogy for the above cyclic ester could bethe cyclic chromic ester or a Pb(N) ester, whichvicinol diols are capable of forming ester withchromic acid and Pb(N) acetate respectively ". Inthese cases the formation of monoester is morelikely to be the slow step, followed by its rapiddecomposition.
In the above mechanistic picture (Eq, 1), theformation of bromate ester is due to the nucleo-philic attack by the +alcoholic hydroxyl at the bro-mine atom of H2Br03• But an alternate mode ofattack (Eqs 4 and 5) by carboxyl group cannot beruled out. .
Such an ester has been conceived in the reac-tion between bromine and oxalic acid-? Eqs (6and 7) and acid bromate oxidation of oxalicacid:".
O~_/OH 0,>- OH'1; 8 slow "'L;~
I + r2= IAC.•.•.. C
o~ OH O~ '08.
+ H8r (6 )
O~C/OHI ~ 2 CO2+H8r
0~C'08r(7 )
The other alternative mechanism (Eqs 8, 9) thatcan also satisfy the kinetic results is the formationof glyoxylic acid as the intermediate.
(8 )
(9)
Glyoxylic acid (OHC ~ COOH} and oxalic acid(HOOC - COOH), which have the 'same degreeof oxidation are oxidised all the way to carbon di-oxide and water and could not be any intermedi-ates.
Formaldehyde is oxidised to formic acid whichis inert towards further oxidation. Under the con-ditions [Br(V)] ~ [GA], the formation of HCHOand HCOOH are identified and estimated quanti-tatively. H~nce the formation of glyoxylic acid asintermediate and its further oxidation to carbond-ioxide is ruled out under the present experimentalconditions, ([Br(V)] < [GA]).
Hence it is confirmed that the glycolic acid isoxidised to HCHO (further to formic acid)through Eqs (1) to (5).
The rate equation in consonance with Eqs (1)to (5) is given by Eq. (10).
k = d[Br(V)]/ dt = k K K [H+]2[GA]G [Br(V)] X' pi p2.
... (10)
The esterification constant k; is evaluated as1.43 x 10-1 from the slopes of the plots of kGversus [H +F or kG versus [GA] which are in goodagreement with each other.
The mechanism proposed here is further sup-ported by the study of energy of activation (Ea). IfE; value is about 50 kJ mol- 1 the oxidation ofhydroxy acids/alcohols to carbonyl compoundstakes place in normal way in which H of C - H
REDDY et af.: OXIDATION OF GLYCOUC ACID BY ACID BROMATE 413
bond is attacked, as per the observations of Cha-terji and Antony " in the oxidation of alcohol toaldehydes by chromic acid. Whereas Price andKnell'" observed E, values to be 92.94 and 93.37kJ mol- 1 in glycol cleavage by chromic acid andperiodic acid respectively, in which oxidation istaking place when H of 0 - H is attacked result-ing the rupture of C - C bond. The energy of acti-vation, of about 90 kJ mol- 1, suggests C - Cbond fission=-'? via reversible complex formationbetween hydroxy acid and Br(V) species. In thepresent study of acid bromate oxidation of glycol-ic acid, energy of activation (76.63 kJ mol-I) is inbetween 50 and 90 kJ mor ! suggesting that theoxidation to carbonyl compounds is taking placemostly by the cleavage as shown in Eqs (1) to (5)and also in the normal way to some extent. Butthe product analysis confirms that the oxidation istaking place only by fission of C - C bond as theoxidative product is only HCOOH obtained byfurther oxidation of HCHO.
There is a formal analogy between the mechan-ism proposed herein, and chromate ester mechan-ism for the oxidation of alcohols by Cr(Vl)oxide'". Chromic acid oxidations are known to beexhibit a pronounced kinetic isotopic effect, (kH/kD = 7) for the oxidation of isopropyl alcohol, anda negative p has consistently been obtained forthe oxidations of several substituted alcohols.These observations establish unequivocally thatdecomposition of the chromic ester constitutesthe slow step. But, the oxidation of alcohols byBr(V) is characterised by the absence of substitu-ent effect and kinetic isotopic effect'".
Other way of demonstrating that in the presentsystem the decomposition of the ester is a fastone is to study the oxidation of other hydroxy ac-ids like benzilic acid. According to the proposedmechanism, it is predicted that the rates of oxida-tion of benzilic and mandelic acids should be thesame. The same is realised from their oxidations.
Fast stage of the reactionIt is observed that in most of the reactions, the
product is also oxidised along with the substrate,when the product has tendency to undergo theoxidation. However, its contribution to the totalrate is not appreciable as [product] is in negligibleproportion than [substrate].
On the other hand when producns) has greatertendency to react with oxidant, through its highcomplexing activity, its contribution to the ratewill be appreciable. Some times it superceeds theoxidation of substrate, ultimately the preferentialoxidation of product alone takes place leaving
back the substrate unreacted, consuming availableoxidant, though its concentration is also very lowin comparison with the substrate.
In the present study, the later part of the kineticdata pertains to the oxidation of product, for-maldehyde.
To establish this fact, the oxidation of formalde-hyde is also studied independently. From the kin-etic data available (Table 3), the mechanism of ox-idation of formaldehyde is shown as below.
Ky kHCHO +Br(V) ? [complex] ~ products
slow
... (11)
In consistance with Eq. (11) the rate law is mani-fested by the following equation at low concentra-tion of acid (0.1 mol dm":'), treating HBr03 asthe reactive species.
kF= -d[Br(V)]/dt[Br(V)h
~= 1+Kp1[H+] +~k, kdKyKpdH+][HCHO] kd
... (13)
Table 3-Effect of varying [reactants] and solvent composi-tion on the rate of acid bromate oxidation of formaldehyde at
From the plot of 11kF versus 1I[HCHO], whichis linear the values of kd and Ky are evaluatedfrom the intercept and slope as 5 x 10- 4 (s- I) and8.75 x 103 (dm' mol-I) respectively. Similarly itcan be shown that the complexation constant ofH2Br03 and HCHO as the reactive species athigher acid concentration is also of the same or-der. Though formaldehyde involves in rapid com-plexation with HBr03 as -and when formed, itscontribution to the rate is not appreciable in thebeginning as decomposition of its complex is aslow step (kd = 5 x 10- 4 S - I). But it will be ac-countable when HCHO reaches an optimum con-centration where its rate of oxidation is greaterthan glycolic acid.
The transition from slow stage to a fast stageoccurs after completion of 40% of the reaction asvisualised from the plots of log(a - x) versus time(Fig. 1). At inflection point the [HCHO] can beestimated as equivalent to the [bromate] con-sumed is 4 x 10-4 mol dm -3.
From the knowledge of Ky and kd values, andthe rate constant (kf) of the later part of the reac-tion, the .[HCHO] can be evaluated theoreticallyfrom Eq. (12). The [HCHO] evaluated as4.5 x 10-4 mol dm-3, is in good agreement withthe observed data. Hence the later part of the oxi-dation is attributed to be the preferential oxida-tion of formaldehyde over glycolic acid on ac-count of its high complexing ability, (Ky> kx) withBr(V), though, its concentration is very low.
Further evidence is also obtained from thestudy of oxidation of formaldehyde, with low con-centration of Br(V) i.e., of the order of 5 x 10-4
mol dm - 3 which is the concentration expected tobe "in the reaction mixture under the kinetic con-ditions of present study in the presence and abs-ence of glycolic acid. Both reactions have pro-ceeded with the same rate till the completion offormaldehyde. In the presence of glycolic acid, theoxidation of glycolic acid is observed only afterthe complete oxidation of formaldehyde and aplot completely inversed to the oxidation of gly-colic acid alone is obtained i.e., fast depletion fol-lowed by the slow depletion of bromate (Fig. 2).The rate constant kr= 2.8 x 10- 4 S - 1 andkF= 1.43x 10-4 S-1 obtained from this plot arealso in good agreement with the present study ofoxidation of GA alone.
The substrate effect reveals the first order de-pendence on [GA], from the slow stage of thereaction and zero order dependence from the la-ter part of the reaction. In the fast stage of thereaction, the rate is manifested by the unreacted
temp.=313K[B]= Bromate oxidation of formaldehyde[Br(V)]o=O.OO1 mol dm='; [HCHO] = 5 x 10-4 mol
HOAc-HP=30-70% (v/v); [H2S04] = 1.0 mol
[Hg(OAch]= 0.01 mol dm -3; temp = 313 K.
[Br(V)]. in the reaction mixture and [HCHO] ob-tained in the present study of oxidation of GA.
At inflection point unreacted [Br(V)] is 6 x 10-4
mol dm - 3. The theoretical rate constant can becalculated with maximum possible concentrationof Br(V)- HCHO complex is 6 x 10-4. mol dm-3
obtained by 1:1 complexation of Br(V) with for-maldehyde and from the knowledge of kd and Kyas 3.2 x 10- 4 S - 1. It is in good agreement with therate constants (kF) observed at higher [S]. Thisalso supports the assumption that the first (slow)stage of the reaction pertains to the oxidation ofGA alone and second (fast) stage of the reactionis attributed solely to the preferential oxidation offormaldehyde. .
Moreover the activation parameters related tothe later part of the glycolic acid oxidation are al-so in good agreement with the oxidation of for-maldehyde with in experimental errors supportsthe assumption (Table 2).
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REDDY et al.: OXIDATION OF GLYCOUC ACID BY ACID BROMATE 415
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