Bakore, G. V.

Shanker, R.

Sharma, S. L.

English

NOPR

Indian Journal of ChemistryVol. 14A, February 1976, pp. 82-84Kinetics of V (V) Oxidation of Esters of Mandelic AcidS. L. SHARMA, R. SHANKER & G. V. BAKOREDepartment of Chemistry, University of Udaipur, Udaipur 313001Received 20 January 1975; accepted 1 A priX 1975The kinetics of V(V) oxidation of esters of mandelic acid (PhCHOHC02R, R = Me, Et,Pr», Pr+, Bun, Bus But and -CH2Ph) is first order in [V(V)]and [substrate] and second order in[H+]. A kinetic isotope effect (kHjkD = 2·6)in the oxidation of ethyl mandelate (a-H) and (a-·D) isobserved. Oxidation products rule out the possibilities of rate limiting or concomitant hydro-lysis. On the basis of product analyses and the absence of vinyl polymerization V(V)seems tobehave as a two electron oxidant. No definite order in the reactivity is observed when the ratesof oxidation of the esters at different temperatures are compared and the order does not conformto the isokinetic model.INits reactionsV(V), like Cr(VI), can act bothas one and two etoectron oxidant.'. WhileJones et aJ.2 maintain that oxidation of cx.-hy-deoxy acids by (V) proceeds through one electronoxidant involving C-C bond cleavage in the ratedetermining step. Bakore and Shanker", in thereaction of glycollic acid with V(V) suggest thatV(V) acts as a two electron oxidant involving C-Hbond rupture in the slow step, similar to theoxidation of cc-hydroxy acids by Cr(VI)4. Since noinformation is available on the reaction of estersof e-hydroxy acids with V(V), oxidation of estersof mandelic acid was undertaken. While this workin progress, a kinetic study of the oxidation of ethylmandelate by chromic acid- has been published andit is of interest to compare the reactivities of theseoxidants.Materials and MethodsV(V) solution in perchloric acid was preparedfrom ammonium metavanadate (Riedel). Aceticacid (E. Merck) was purified by refluxing over Cr03and subsequent fractional distillation. Esters ofmandelic acid [PhCHOHC02R, R=Me, Et, Prn,Bun, BuS, Bu' and CHzPh] were prepared byFischer's method" and their purity checked. Man-delic acid (-cx.a) was prepared by the method of Kempand Waters? (98% deuterated ; PMR analysis).Reactions were carried out in acetic acid-watermixture at 30° ± 0·02° unless otherwise specifiedand followed colorimetrically using Hilger patternBiochern absorptiometer by measuring the absor-bance of V(V) at 680 nrn. The cell was kept in ametal block through which water from the thermo-stat was circulated. Perchloric acid (60%, AR,Baker) was used as a source of hydrogen ions. Ionicstrength was kept constant or varied by addingNaCl04 (AR, Baker).Product AnalysisProduct study was made under kinetic conditions.Oxidized mixture was completely neutralized by82sodium bicarbonate and then extracted with ether(3 to 4 times). Aqueous layer gave no precipitatewith 2,4-DNP indicating the absence of benzoyl-formic acid(s). Ethereal layer when treated with2,4-DNP gave hydrazones which were identified bym.p. and mixed m.p. determination. Keto estersformed were also identified by elemental analysisand IR spectroscopy,For quantitative analyses, control and blankexperiments were run along with the test experi-ments. The results showed that 90 ± 5·0% of themandelate esters were converted to the correspondingketo esters (d. Table 1).No induced polymerization of vinyl cyanide wasobserved in all the cases indicating the absence offree radicals.Results and DiscussionAll the esters of mandelic acid, PhCHOHG02R(where R=Me, Et, Prn, Pri, Bun, BuS, Bui and -CH2Ph) behave alike. The data presented in this paper is,therefore, typical of all and represents the reactionof ethyl mandelate with V(V). Pseudo-first orderrate constants do not change with varying initial[V(V)J. The order of reaction with respect to sub-strate is one and with respect to hydrogen ions comesout to be two (Table 2). The extent of variationof these parameters are limited by the solubilitiesof the esters in acetic acid-water and their rates.TABLE 1 - ESTIMATION OF KETO ESTERSEster Wt (g) of 2,4-DNP of ReactionPhCHOHC02R keto esters (%)----------_.Expected Founcl*1. R = Et 0·25 0·2310 92·42. = Pri 0·30 0·2702 90·03. = Bun 0·25 0·2260 90·4*Average of three determinations after correcting for thecontrol experiments.SHARMA et al.: V(V) OXIDATION OF ESTERS OF MANDELIC ACIDTABLE 2 - EFFECT OF VARYING [SUBSTRATE] AND [H+] ON THE REACTION RATE{[V(V)]= 0'042M; solvent: 60% AcOH (v/v); [.10 = 4M; temp.= 30°C}kl X 104 kl/[Ester] X 10' [H+]X M k2 X 102sec! litre mole'? see'? litre mole'? sec-1[H+] = 0'27M [Ester] = 0'021M25'3 120 1'16 5·5029'0 121 1·60 9·1033·3 123 2·06 16·838'9 121 2·50 23·042'7 122 2·95 30·948·3 121 3·20 38·0Average: (121±2)[Ester] xM0·210·240·270'320·350·40k2/[H of] X 102litre" mole=s sec-14'083·503·963·683·563·71Average: (3'71 ±0'3)Effect of added NaClOQ, - Addition of NaCl04increases the rate constants considerably. Thevalues of rate constant (k2) are 4·6, 9'4, 11·6 and21·5 X 102 litre mole! see+ at [NaCl04J = 2'05, 2·85,3·25 and 4'OSM respectively. The increase in ratemay be attributed to the decrease in activity ofwater molecules thereby decreasing the microscopicdielectric constant. However, dependence of rateon NaCl04 is almost second order. This unusuallystrong effect cannot be solely attributed to themedium effect alone and may be due to specific effectof one of the' ions, viz. Na".Kinetic isotope effect - A kinetic isotope effect(kH/kD) = 2,6 is observed in the oxidation of ethylmandelate (o:.-H) and (o:.-D) and the values of k2 are128 and 48·8 X 104 litre mole+ sec-1 respectively forethyl mandelate and deuterated ethyl mandelate.A second order dependence of rate on [H+] canbe explained by postulating V(OHh(H20W as theVOi+3H20+2H+~V(OHMH20W ... (1)active oxidant (Eq. 1). Existence of quinque-coordi-nated V(V) species has been demonstrated by Mishraand Symons".A primary kinetic isotope effect of 2·6 suggest thatC-H bond is affected in the transition state. Acomparison of the magnitude of isotope effects inthe oxidation of cyclohexanoi'P (= 3,6) and mandelicacid? (= 2,0) by V(V) points to some similaritiesin these reactions. However, on the basis of productanalysis* and the absence of radical mechanism V (V),in its reaction with esters of mandelic acid seemsto act as a two electron oxidant like Cr(VI) assuggested by Bakore and Shanker" and Pickeringand McAuley9. The mechanism of oxidation ofthese esters can be expressed as shown in Chart 1.There is neither kinetic nor spectrophotometricevidence for the existence of intermediate (I) andan outer-sphere bimolecular interaction cannot beruled out. However, in redox reactions involvingsulphydryl group (-SH) viz. 2-mercaptosuccinicacid9 and 2-mercaptoisobutyric acid (unpublished*It is interesting to compare the products obtained inthe oxidation of ethyl mandelate by V(V), Cr(VI) andMn(III) under acid conditions. While 90% of the ketoester is obtained in case of Cr(VI) (d. Jha and Bakore6)and V(V), benzaldehyde is the main product with Mn(III)- a typical one equivalent oxidant. [R. K. Malkani andK. S. Suresh (unpublished data)]. This, all the moresupports the proposal that V(V) in the present studyresembles Cr(VI) and behaves as a two electron oxidant.O OHH 3+~ 2"v -OH/ '- 2HO OH 2Chart 1work) and V(V), a transient intermediate complexformation has been characterized spectrophoto-metrically using stopped flow technique. The rateof ligand substitution is much faster than the rate ofelectron transfer. Paucity of relevant data doesnot permit one to conclude whether the reactionis inner sphere or outer sphere ..Absence of benzaldehyde(s) in the ethereal extractand benzoylformic acid(s) in, the aqueous layerclearly rules out the possibility of a rate limitingor concomitant hydrolysis in the oxidation of theseesters by V(V) (see also Jha and Bakore").For comparison the rates of oxidation of l-phenyl-ethanol and mandelic acid were determined. Underexperimental conditions mandelic acid reacts toofast to be measured by conventional techniques.I-Phenylethaaol, however, reacts at a rate slowerthan those of the esters (Table 3). Although theorder of reactivity COOH > COOR > CH3 is inaccord with the -1 effect of the substituents, therate of oxidation of mandelic acid is much faster thancan be accounted for on the basis of -l effects alone.The formation of cyclic ester anhydride (II), as1183INDIAN J. CHEM., VOL. 14A, FEBRUARY 1976TABLE 3 - EFFECT OF VARYINGTEMPERATUREON THE REACTIONRATE([Ester] = 0·021M; [V(V)] = 0'042M; [H+] = 2'05M; solvent: 35% AcOH (v/v); ••.= 2'2M}PhCHOHC02R k2 X102 litre mole'? see"! L\Et L\StR= (kcal mole"! (e.u.)18° 25° 30° 35° 40° 45°Me 6'8 10'& 15'7 20'0 27'6 36'9 11·5 • -25l<:t 6·6 9'7 17'1 21-4 26·6 35'9 11'8 -24Pr» 4'7 7'5 11·0 14·6 19·0 25·7 11·4 -26Pro 3'9 6·4 8·1 14·1 17·4 24·2 12·3 -22Bun 2·1 3·8 4·5 7·4 11·9 12·6 14·1 -17Bu, 6·0 10'7 15'7 23·8 31·4 44·5 13·7 -18Bus 6·1 8'5 10·9 14·3 16·2 22·3 8·0 -37-CH2Ph 7·9 10·5 11'9 15·2 8·0 -38I-Phenyl ethanol* 0·4 0·55 0'7 1-4 11'8 -35*{[H+] = 2'8M; [Alcohol] = 0·12M; [V(V)] = 0'021M; ••.= 3·0M}suggested by Jha and Bakore-, may account for theenhanced rate of oxidation of mandelic acid by V(V).A comparison of rates at different temperatures(Table 3) do not conform to any definite order ofreactivity. The random behaviour of the substi-tuents point to the concept of isokinetic model. Aplot of I1Ht versus 115t is linear with isokinetictemperature, ~ = 300°K. Since ~ is within experi-mental range, an inversion in order of reactivity isexpected with the change in temperature through ~.However, this method of testing the isokineticmodel has been criticized!''. The model is betterunderstood if it is formulated as a family ofArrhenius lines with a common point of intersec-t.ion-". Thus a plot of log k at Tl versus log k at T2should be linearly dependent according to thefollowing Eq. (2). -log kT~ = a+b log kTl ... (2)The parameters a and b are dependent on thechoice of Tl and T2• When this criterion is appliedto the present data (Table 3), no linearity is observedwith the choice of Tl and T2. It, therefore, appearsthat the variation of rates amongst the esters84themselves can be attributed to the operation ofsolvent and mild steric effe cts which have a mutuallycompensating effJe,cton the changes in enthalpy andentropy, resulting in the linear plot of I1Ht versus115t.References1. HIGGINSON,W. C. E. & MARSHALL,J. W., J. chem, Soc.,(1957), 447.2. JONES, J. R., WATERS,W. A. & LITTLER, J. S., J. chem,Soc., (1961), 630.3. BAKORE, G. V. & SHANKER, R., Can. J. Chem., 44(1966), 1717.4. BAKORE, G. V. & NARAIN, S., J. chem, Soc., (1963),3419.5. JHA, D. S. & BAKORE,G. V., J. chem, Soc. (B), (1971),1166.6. FISCHER, E. & SPENCER, A. S., Ber, dt. chem. Ges.,(1895), 3255.7. KEMP, T. J. & WATERS, W. A., J. chem, Soc., (1964),1192. . •8. MISHRA,H. C. & SYMONS,M. C. R., J. chem, Soc., (1962),4404.9. PICKERING, W. F. & McAULEY, A., J. chem. Soc.,Dalton, (1968), 1173.10. WOLD, S. & EXNER, 0., Chim. Scr., 3 (1973), 5.11. LITTLER, J. S. & WATERS,W. A., J. chem, Soc., (1959).4046.

Kinetics of V(V) Oxidation of Esters of Mandelic Acid

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Kinetics of V(V) Oxidation of Esters of Mandelic Acid

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