Kuriacose, J. C.

Pillai, G. Chithambarathanu

Rajaram, J.

English

NOPR

Indianl Journal of ChemistryVol. lilA, July 1977,pp. 608-611Kinetics & Mechanismof Oxidationof Methyl Ethyl Ketoneby Vanadium(V)in Dilute Sulphuric AcidG. CHITI-IAMBARATHANU PILLAI, J. RAJ ARAM & J. C. KURIACOSEDepartmentof Chemistry,Indian Institute of Technology,Madras 600036Received 25 September 1976;accepted17 January 1977Theoxidationofmethylethylketone(MEK)byV(V)in thepresenceofsulphuricor perchloricacid,is firstordereachin MEK, V(V)andtheacid,thereactionbeingslowerin theformeracid.NaCIO.haspracticallyno effecton therate,NaHSO.increasestherateandNa2SO.decreasesthe rate. The proposedmechanisminvolvesa freeradicalintern1ediate,whosepresencehasbeenestablishedby inducedpolymerizationof acrylonitrile.Fig. 1- First orderplots with respectto V(V)alwaysin excess.Whenthe logarithmof the rateconstantsare plottedagainstlog [MER], a linearplot with a slopeof unity is obtained(Fig. 2A),indicatingfirst order in MER. This kinetics isobservedwhen[MER] is varied from 0·8 to 1·5Mand [V(V)] from 1-5xlO-~ to 5·0xlO-2M. Withhigher and lower concentrations,there is somedeviation. The rate increaseswith increase.in[H2S04]. Due to the difficultiesin maintainingthe ionic strengthconstantin H2S04, HCl04 wasusedinsteadof H2S04 and NaCI04 was addedtokeeptheionicstrengthconstant.A plot of logkJ vslog [HCI04J gavea slopeof 0·99showingfirst orderdependencein acid (Fig. 2B).Effect of addedsalts~ Changein ionic strengthby the additionof NaCI04 hasnegligibleeffectonthe rateof the reaction(Table1). Sodiumbisul-T ~ANADIUM(V) has been investigatedas anV I oxidizing agent for many organic and in-i organicsubstratesl-5.Theoxidationof methyleth~ ketone(MER) by alkalinehexacyanofcrrate-(III~ ion~,chloramine-T7and Cr(VI)8 has beeninvettigated,but no detailedkimtic studyhas sofar Ibeenmade of its oxidation by V(V). Theresuitsof a detailedstudyof theoxidationof MERby IV(V) are presentedhere.Ma~eriaISandMethodsM~R was distilled and its purity checkedbyGL. Ammoniummetavanadate(AR, Hungary)diss lved in dil. H2S04 wasusedfor the prepara-tionof V(V) solu ions. Therateof thereactionwasfOll~ed by quenchingan aliquotof the reactionmix urein a measuredexcessof ferrousammonium-sulp ate and back titrating the' unreactedFe2+agai st standard dichromate using N-phenyl-ant ranilic acid as the indicator. All the experi-mens Wt~r2 condu tedat 300±0.10 unl"ssotherwisestattd.St,pichiometryand product analysis-. The stoi-chi~etry of thereactionwasdeterminedby keepingreaq:ionmixturescontaining xcessof V(V) untilthe~reactionwas completeand then determiningthe unreactedV(V). The stoichiometryis foundto e 2 g ions of V(V) to onemoleof the ketone.The presenceof formicacidafterdistillationof thereac/tionmixturewas indicatedby GLC. Carbondio~idewas not one of the products.Re~uItsandDiscussion~ing a 20-100fold excessof MER in comparisonwitl V(V), the reactionwas foundto be pseudo-firs order in V(V) (Fig. 1). There is very littlevar~'1tion in the pseudofirst-orderrate constantscalc lated for the different sets of experiments,indi ating that first-orderrate constantsare in-depndentof the initial [V(V)].Theorderin MER wasfoundto beoneby keeping~hejinitial V(V) concentratio?constantand cha?g-Il1g! the ketone concentration,the latter bel11g6081I,1.71.2=~"'-'§+.. lO1.40.7 •1.992.93.3DoB•4 7L 0.20 100200TIME IN MINUTESTIME IN MINUTES100 200 300 I,()()[MEKJ=1,5M[H2SOJ=2MTEMP.=30·C400I I I n'llIllPILLAI et al.: OXIDATION OF METHYL ETHYL KETONE BY V(V)TABLE 1- EFFECT OF ADDiTION OF NaCIO, ON THEREACTION RATEoo1·0o1'991·0001.0 _'"0,80·23·760'9990.30'81-62'261'0000'70'33-630'999k1X 103 Correl.min-1coeff.1'330'9752-43, 984·818,90913' 70' 920 2560'61'22·650,9990,60,43-520'9990,41·83·230,9990,50,53,340'9990·22·43·610,9990,30·73·040,9990.1o1.099- 080.5~...1->+ 0.5..•[Acid] H2SO,M k1X 103Correl.min-1coeff.1·21'620,992·12 2783,03 891·00095-514 89,755 76'0 9(a) [V(V)]=2,05xl0-2M; [MEK]=l'OM; [H.SOJ=2·OM;temp.=30°TABLE 2 - EFFECT OF ADDiTION OF SALTS UNDERCONSTANT IONIC STRENGTHD.4~0.2?I ••-oj 0 .I' §1-0 +1Zljl.. ....__- --'------'- - - --.1-04•• .o:z -LOGo •.•••.•.. ,¥ .0.1 0.2 0.3-LOG 0HzoFig. 3- Plots of rateconstantsversusacidity (Ho) functionsand activity of water (aH,o){[V(V)]=1-42X 10-2M; [MEK]=lM; temp.=30°}HeIO,TABLE 3 - COMPARISON OF THE RATES OF THE REACTIONAT THE SAME CONCENTRATION OF HCIO, AND H2SO,(b) [V(V)]=2·01x10-2M; [MEK]=1'OM; [H2SO,]=2·0M;temp.=30°[Na2SO,]. M 0[NaCIO,], M 3·0k1X 103,min-1 4·28Correl. coeff. 0·997[NaHSO.], M 0[NaCIO,], M 1·0k1X 103•min-1 2·92Correl. coeff. 0·999§0.4 +..,o0-21.01.413o [V(vn=4.75x102M[HzS04]=2MTEMP.:30'c1+LOG ljiC104]1.1 1.21.00.9• (V(vij =5.E8X.oZM1.1t- • [MEK]=IMl'EMP.=30C1.0o.e1.20.9I •r gas~0.70.6.5V .00.10.2 0.3 0.4LOG [MEK)Fig. 2 - (A) First orderplotswith respectto MEK. (B) Firstorderdependencein acidphate when addedunder constantionic strength(NaCl04wasaddedto keeptheionic strengthcon-stant)showedincreasein the ratewhe:easNa2S04is foundto decreasethe rate (Table2).Comparisonof ratesin HClO4 and H2S04 - In1M acid, the rate is similar in both acids. Athigherconc~ntrations,thereactionis fasterin HCl04indicatingthat in HCl04 a moreactiveformof theoxidizingagentis present(Table3). Similarresultshavebeenobtainedby Mehrotra9in the oxidationof 2-hydroxy-2-methylpropanoicacidby V(V).Acid catalysis- The fact that the rate increaseswith increasein HCl04 at constantionic strengthclearlyshowsthat the reactionis acid-catalysed.The plotsof log k1vs -Ho werereasonablylinearin the caseof H2S04 and HCl04. The valuesof-Ho are thosegiven by Paul and LongH• Theslopesof the linearplotsaremuchlessthanunitybeing0·5 and0·6 in H2S04 andHCl04 respectively(Fig.3). Theseresultsarecomparableto theresultsreportedby Mehrotrafor the oxidationof citricacid by V(V)12 ..Littler andWaters3haveshownthata dependenceof log k1 on Ho, the Hammettacidity function,ratherthan a dependenceon [HaO+]canbe takenasanindicationfortheformationof a cyclictransi-tion state. In the presentcase,thereis a depen-[MEK]=1·0M; [V(V)]=1·19x 10-2M; [H2SO,] =2'OM;temp. 30°; ionic strength was not kept constant[NaCIO,]. Mk.X 103min-1CorrelationcoefI.o2·820,9950·22'550,9970,42·600,9970,72·530,9990·852·750,9971·02·360,995609INDIAN J. CHEM., VOL. 15A, JULY 1977actuallya productof a rateconstantandtwoequili-briumconstants.Reactive V (V) species- The differencein theratesbetweenthe reactionsfor the sameacidcon-centrationsof H2S04 and HCl04 indicates thepresenceof two differentactivespeciesin theseacids.ThoughV(V) is amphotericit existsas a cationin solutionsof aciditygreaterthan 0·05M10. Themarkedcolour of the H2S04 solutionsof V(V)suggeststhat one HS04 ion may be incorporatedin theactivespecies2asshownin Eqs. (1)and (2).VO;+H20+H+~V(OHW (1)V(OHg++HS04~V(OH)3HSOt (2)Sinc~theratesaredifferentin HCl04 andH2S04and increasewith increasein H2S04, the reactivespccis maycontainthesulphateorbisulphategroup.The sulphatecomplexis not a probableactivespeciessince addition of sulphateion decreasesthe rate. So V(OHhHSO~ can be the reactiveV(V) speciesin H2S04 medium. In HCl04 no suchcomplexformationis probablesinceNaCl04, whenadded,causesIY'gligibheffector decreasesthe rateslightly and henceV(OHW may be the activeoxidantin HCl04.Mechanism - In oxidationreactionsketonescanreact either directly or through the enol form.Oxidation ratesfasterthan the rateof enolizationhavebeenobservedwith cobaltic14,15,ceric15,16andmanganicsalts15as oxidants, indicatingthat theketonesreactdirectlv. All of theseoxidantsunder-go one-electronred~lctionand the reactiontakesplacethrougha freeradicalmechanism.Oxidationinvolvingenolintermediateis proposedfor thallic,mercuricand permanganatesalts17and for manga-nic pyrophosphate18. In all these cases, theformationof the enolis the rate determiningstepand the reactionsarc thereforezero order in theoxidantwith the ratesof oxidationsand enoliza-tions beingequal. An unambiguousindicationoftheformin whichtheketonereactscanbeobtainedonly whentherateof oxidationis greaterthantherate of enolization.Bromination of ketone isconsideredto take place throughan enol inter-mediateS.In thepresentsystemtherateof enoliza-tion measuredby brominationis muchgreaterthanthe rate of oxidation. Under identical experi-mentalconditions,the brominationis over within2 min whereasoxidation takes around400 minfor completion.Sincethe enolizationis very rapid, the ketonecould be reactingthrough an enol intermediateunderthe experimentalconditions. The enol-V(V)complexmay be subsequentlydisintegratingtogive the products. However,a direct attack ofV(V) specieson the ketoneand the subsequentdecompositionof the ketone-V(V)complexcannotbe ignored. In the absenceof zeroorderkineticswith respectto theoxidant,it is immaterialwhichform of the substrateis consideredas takingpartin the reaction.One may visualizethe reactionas taking placethroughthe formationof an intermediatecomplexbetweenthe ketoneand the V(V) speciesin anUl.0:!N$2in12.01.0!Y(vil=4.15x162M~50J=2MTEMP.dO·Cl/[\1EK]0·6 C·Br----To 08.41.81.21.21.82.46.66006'''b 4.8•••~ 1,.2.J'3.63.0[ME!!jMIFig.4-IPlot of l/[MEK] versuskabs and[MEK] versuskabsIdence0 log k1 on Ho and hencethe formationofa compex of the type (I).maybe envisaged.Thenegativ valueof the grossentropyof activationmaybeconsideredto supportthis idea.Acco ing to BunneP3,the plots of log (k+Ho)againstlog aH20 aregenerallylinearandthe slopesdefine parameterw. A w value of -2'5 to 0indicats non-participationof waterin the transi-tion st e,a valuebEtween+1,2and+3'3indicatesthat w ter actsas a nucleophileanda value>3·3indicats thatwateractsasa protontransferagent.In the presentcase,the plots of -(log k1+Ho)vs -10 aH20 werenotlinearin theentireacidrangeused. p to 3M HCl04 the slopeof the Bunnetplot w s nearly 10 indicatingthat watermay beacting~s a proton transferagent. At [HCl04J>3M, he w valuesobtainedfromthe linearplotswere3· for H2S04 and 3·8 for HCl04 (Fig. 3).Suchb rderlinevaluesdo not p rmita clear-cutassignmentof a role for watEr.Inter1ttediates- Inducedpolymerizationof acrylo-nitrile Iduring the reaction betweenV(V) andMEK t'nd the absenceof polymerizationof themonomr by V(V) or MEK indicatedthe formation0:[ a fr e radicalintermediate.The V spectrumof thereactionmixtureshoweda verytsmall optical exaltationaround 240 nmindicati 15 som:~complexformation. A plot ofl/kobs v l/[MEK] at constantV(V) waslinearwitha smal intercepton the Y-axis, offeringkineticsupportfor complexformation(Fig. 4).TherJ odynarnic parameters - The ordersin V(V),MEK d HCl04 are independentof temperaturein the angeof 30°to 50°. Th·eps'.~udofirst-orderrateco stant (kab,) is calculatedby conductingthen~actioat temperaturesof 30°,35a, 40°,45° and50°. F omtheslopeof theplot of log kabs against1/1' th energyof activationis foundto be 16·6i: 0·3 aI/mole. The valuesof ~Gt and ~st are21·5kc l/moleand-16'5 e.u.respectively.kabs is610IPILLAI et aT.: OXIDATION OF METHYL ETHyL KETONE BY V(V)equilibriumstep followedby its rapid transforma-tion to givetheproducts(Scheme1)K,VO~+H20+H+~V(OH);+ ...(3a)K(VO++H20+H2S04~V(OH)aHSOr ...(3b)It is assumedthat V(OH);+and V(OH)aHSOrare the respectiveactivespeci~sin perchloricandsulphuricacids12respectively.Rate= ka[complexJ= kaK2[MEKJ[V(OH)aHSO:n= kaK2K~ [MEK][VO;J[H2S04J ...(7)Substitutingfor [V(V)h and[VO;J,Rat'"=~2K~[MEKJ[H2S04J[V(VUT_ (8)~ 1+Ki[H2S04J+K2Ki[MEKJ[H2S04J ...At constant [V(V)h,k kaK2K~[MEKJ[H2S04J ( )obs =1+K~[H2S04J+K2KaMEKJ[H2S0J 9_~=_1_{ } +__1 }+l (10)kObS [MEKJ RaK2K1[H2S04J kaK2 kaA plot of thereciprocalof observedrateconstant(kobs) against1/(MEK) waslinearwith a slope=321and an intercept=7·32.If K~ is very smallcomparedto kaK21 1 1- +k K =k K K'[H SO J •.• (11)a2 a212 41 1 1 1-k =-- ----- +- ...(12)obs [MEKJ kaK2K1[H2S04J kaFromtheslopeandinterceptvalues,kaandK2K~are calculatedto be equalto 0·137and 1·14x10-2respectively,the [H2S04Jbeing2M.References1. WATERS, W. A., Rev.chim.Buc., VII (1962),1371.2. MEHROTRA, R. N., J. chern.Soc., (B) (1968),1563.3. WATERS, W. A. & LITTLER, J.S., Oxidationin organicchemistry,Part A, editedby K. B. Wiberg (AcademicPress, New York), 1965,204-39.4. LITTLER, J. S. & WATERS, W. A.,J. chern.Soc.(1959),4046.5. JONES, J. R. & WATERS, W. A., J. chern.Soc.(1962),2068.6. SINGH, V. N., SINGH, H. S. & SAXENA, B. B. L., J. Am.chern.Soc.,91 (1969).2643.7. MUSHRAN, S. P., SANEHI, R. & AGARWAL, M. C., Z.Naturforsch,27b (1972),1161.8. TANDON, S. K., BANERJEE, K. K. & BAKORE, G. V.,Indian J. Chern.(1971),677.9. MEHROTRA, R. N., J. chern.Soc. (B) (1968),642.10. LITTLER, J. S. & WATERS, W. A., J. chern.Soc. (1959),1299.11. LONG, F. A. & PAUL, M. A., Chern.Rev.,57 (1957),935.12. MEHROTRA, R. N., J. chern.Soc., (B) (1968),1564.13. BUNNETT, J. F., J. Am. chern.Soc.,83 (1961),4956.14. HOARE, D. G. & WATERS, W. A., J. chern.Soc.,(1972),971.15. LITTLER, J. S., J. chern.Soc., (1962),832.16. VENKATAKR1SHNAN, S. & SANTHAPPA, M., Z. physik.Chern.(Frankfurt).16 (1958).73.17. LITTLER, J. S., J. chern.Soc. (1962),827.18. DRUMMOND, A. Y. & WATERS, W. A., J. chern.Soc.,(1955),497.Sincein the denominatorof expression(9), com-paredto (1)K{ andK2K~ arefoundto besufficientlysmall, to be neglected.kobs=kaK2K~[MEKJ[H2S04J ... (13)and at constantconcentrationof the acid,kobs =k~K2K~[MEKJ=k'[MEKJA plot of kobs vs [MEKJ shouldbe linearpassingthroughthe origin (Fig. 4).kavalue(0·137)calculatedfromthe valueof theinterceptof theplot of l/kobs vs l/[MEKJ{(Eq. 10)when[H2SO4J is kept constantat 2M} agreeswellwith thatcalculatedfromtheplot of kobs vs [MEK](Eq. 13,ka=0·143).By usingthe slopeandinter-ceptof thesetwo plots,K~valueis calculatedandis foundto be equalto 1·28X 10-2whichis seentobe negligiblecomparedto the calculatedvalueofkaK2=0'125, supporting the line of argumentproposed.... (4)... (5)... (6)fastRadicalintermediate+V(V)-+products+V(VI)Scheme1K,V(OH)aHSO~+MEK~ Complexk.Complex--+ Radicalintermediate+V(VI)slow611

Kinetics & Mechanism of Oxidation of Methyl Ethyl Ketone by Vanadium(V) in Dilute Sulphuric Acid

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Kinetics & Mechanism of Oxidation of Methyl Ethyl Ketone by Vanadium(V) in Dilute Sulphuric Acid

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