Paichha, R C

Panigrahi, G P

English

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IndianJournalof ChemistryVoI."2?A,March 1988,pp.207-210Kinetics&MechanismofOxidationofSulphanilamidebyPeroxomonophosphoricA idG P PANIGRAHI* & R C PAICHHADepartmentof Chemistry,BerhampurUniversity,Berhampur760007Received18November1986;revised30April 1987;rerevisedandaccepted10August1987Oxidationof sulphaniIamideby peroxomonophosphoricacid (PMPA) in thepH range0-7 is first orderboth in[PMPA]and[sulphaniIamide].Dependencein [H+]is complexandrate-pHprofile showsratemaximumaroundpH1.1.The observeddatahavebeenrationalisedin termsof an electrophilicattackby neutralPMPA speciesnamelyH3POS ontheaminogroupof sulphaniIamide.The title investigationforms a part of a broadprogrammefrom our laboratoryon peroxomono-phosphoricacidoxidationof aminobenzoicacidsl,aminopyridinez,aniline3,sulphanilicacid4etc.MaterialsandMethodsAll the chemicalswereof AR grade.All solu-tionswerepreparedin conductivitywater.PMPAwaspreparedby acidhydrolysisof K4PZOg(ref.5)and its concentrationcheckedfrequentlyby io-dometry.The selfdecompositionwasfoundnegli-giblein thepH regionexamined.The aciditywasadjustedby addingappropriateamountsof perch-loric acid or standardbuffers6•Sulphanilamide(EGA-CHEMIE, W. Germany)was used aftercheckingitspurity.The kineticswasfollowedupto threehalf-livesunderpseudo-firstorderconditionsin thetemper-aturerange35-50°Cby measuringtherateof dis-appearanceof PMPA iodometrically,in aceticac-id-acetatebuffer medium(pH 4-5) containingadrop of ammoniummolybdatesolution7•The rateconstantswerecomputedby theusualmethodandwere found to be reproduciblewithin ±7-10%.Dissociationconstant(Kh) for the conjugateacidwasmeasuredby pH-metrictitrationof hydroch-lorideof sulphanilamide.pH valueswerereprodu-ciblewithin±0.05pH units.ResultsGood agreement(within 10%)was noticedinthefirst orderrateconstantvaluescalculateduptothreehalf-livesfor eachrun and thoseobtainedfrom the slopesof the linearplots of 10g[PMPA]versustime.No changein thevaluesof first orderrate constantswas noticedevenin repeatingex-perimentswith varying[PMPA] but constant[sul-phanilamide]and pH (Table 1). At fixed [PMPA]and[acid]with [sulphanilamide]varying,the k1 va-lues (Table 1) increasedwith increasein [sulpha-nilamide]and kz (= k/ [sulphanilamide])wascon-stantindicatingorderin sulphanilamideto be one.•Table I-Rate Constants for theOxidation of Sulphanilamidein Aqueous Medium by Peroxomonophosphoric.1\Acid (PMPA), ~.Temp.pH103lQ4[PMPA]104kl102k2~·C(±0.02)[SulphaniIamide](moldm-3)(s-I)(dm3molS-I)f(moldm-3)350.852.9032.5732.040.855 11 99687 61 121 7420527.161.12195 7364 0486 5~-.•.1.121 .1059,~401.1 .8 8~45.500207INDIAN J. CHEM., VOL. 27A, MA RCH 1988The PI1otondissociationconstants9suggesthatin thepH region0-2 PMPA existsas H3POS and,.--,...•... (5)whereB =(p)H2NC6H4S02NH2However,the valueof Kh is not availableandhenceit hasbeenmeasuredby pH-metrictitrationof sulphanilamidehydrochloride.The Kh valuecomesout to be - 1x 10- 3. This suggestssub-stantialprotonationof sulphanilamidebelow pH3. As has beenestablishedin our earlierworkl-4oxidationsby PMPA are basicallypolar in natureand nucleophilesare oxidisedwith ease.In thepresentcasetheaminogroupgetsprotonated(Eq.5)and it is theunprotonatedsulphanilamidemole-culewhichundergoesoxidationwithPMPA. How-ever, the protonatedspeciesis not capableofreactingwith PMPA presumablydue to reducednucleophilicityas indicatedby veryslowreactivityat high [H+] (pH <1). The observedrate maxi-mumatpH 1.1canberationalisedasfollows:As pH increases,the equilibriumconcentrationof neutralsulphanilarnideincreasesand it is thisspecieswhich undergoesoxidation.Similarlywithincreasein pH, H3POS concentrationdecreasesfrom a highvalueandH2PO; slowlyaccumulatesand in significantamountafterpH 2. Therefore,initial increasein ratewith increasein pH from 0to 1.1is due to increasein neutralsulphanilamideconcentrationand this species is oxidised byH3POS only becauseof its highernucleophilicity.As no H2POS speciesis availablein significantamountatpH 1.1,onecannotexpectanyreactionbetweensulphanilamideand H2PO; below pH1.1.As pH increasesbeyond1.1,H2PO; slowlystartsaccumulatingandhenceonecanexpectoxi-dation of sulphanilamideby both H3POs andHzPOs·In viewof theexperimentalresultsobtained,thereactionstepsinvolvingoxidationof sulphanila-midemaybevisualisedasshownin Scheme1.KhBH+ +:t B+H+H2POS, former beingpresentin significantcon~centrationover the latter.It is reasonableto ex-pectthatin thepH range0-1, it is H3POS specieswhichis mostimportantandhenceanyreactioninthis regionis due to neutralH3POS' This is con-sistentwith the ideaput forth by SeccoandVen-turinilO who demonstratedhigher reactivityofH3POS over H2PO; in terms of greaterleavinggroupabilityof H3P04 overH2P04. EquilibriumgivenbyEq. (4) isof littlesignificance.Sulphanilamideis alsolikelyto be protonatedinthispH regionandequilibrium(5)maybe import-ant.... (1)Rate=kr[Sulphanilamideh[PMPA]... (3)... (4)wherek1 is thesecondorderrateconstantandthestibscrip~T denotestotal concentration.The ratemaximuQIat pH 1.1 mayarisefrom contributionof proto* dissociationequilibriafor both theoxi-dantandisubstrate.The PMPA is knownto under-godissocfationasshownbyEqs (2-4) (ref.9).H3POS ~ H2PO;+H+ (K1=8xlO-2at2SOC)... (2)Discussif:)OBased!on thekineticresults,theratelaw canbeexpresse~byEq. (1)atconstantaciditySincetherateswerenot influencedby addedsalts,no atfempt was made to maintain the ionicstren~ constantduring the rate measurement.The p'ot of kobs versuSpH (variedfrom 0 to 7) at45 an~50°C,showedratemaximumaroundpH1.1 in4icatingparticipationof differentspeciesofPMPAi resultingfrom dissociationas well as in-volvenientof protonationequilibriaof sulphanila-mide.Ad<ij.tionof acrylamidedid not affectthekinet-ic resu~tsor produceany turbidityindicatingthatno ract;.calintermediateis involvedin the oxida-tionof fulphanilamidebyPMPA.Und r t e condition[sulphanilamide]~[PMPA],onemqlof PMPA wasfoundto consumeonemolof sulp~anilamide.However,theproductof oxida-tion c~ld not be isolated.It may be concludedhatsu,phonamidederivativeof h droxylamineisformedII since the NHz group is convertedintoNHOH\ whenheterolyticoxidationoccurs.Furthertherea9tionmixtureswerealmostcolourless.For-mationpfcondensationproductslike azoor azoxyderivati~eswould haveresultedin colouredmix-tures.This is at variancewith the observationsofSrivastafaetal.8who identifiedthecorrespondingazoxyd~rivativein theoxidationof sulphanilamidewithpe~xodisulphateion.Plotslof log k2 (pH =1.1±0.02)versusliT (Tvaried iP the range 308 to 323K) were linear.The emtgyof activationEa andentropyof activa-tion~st werecalculatedfromthelinearplotsandthe valqeswere found to be 34.1kJmol-1 and-213Jl<-lmol-l respectivelyin the pH range2.0to 5.~.208I· l' II I I , I .1~'11fIl HilI<PANIGRAHI &PAICHHA: OXIDATION OF SULPHANILAMIDE BY PEROXOMONOPHOSPHORIC ACIDScheme1Thesestepsleadtoratelaw(6)Equilibrium concentrationcan be expressedbythe followingrelationshipsin termsof their totalconcentrations,sincewhole of PMPA in the pHrari'gestudiedwould be presentas H3POS andH2PO; only.FromEq. (5),weget,BattagliaandEdwards9 and Kh valuesdeterminedby us were used to evaluatethe left hand sideterm.In factsucha plot waslinearwitha positiveslopebut almostnegligibleintercept.As the rateexpression(Eq. 10) suggeststhe slopeand inter-ceptshouldbe a measureof k1 (oxidationbroughtby H3POS) and k2 (oxidationby H2PO;) respect-ively. However,zero interceptsignifiesthat thesecondterm in Eq. (10) i.e. k2 Kl Kh is equaltozero.SinceKl andKh arefinite,it impliesk2 shallbe eitherzero or negligiblysmall.Hencethecon-tributionto the rateof oxidationby H2POS spe-ciesis notverysignificant.It canthereforebe con-cludedthatoxidationof sulphanilamideis only af-fectedby H3POS but not by any of the chargedPMPA species.In the sameway it can be con-cludedthatprotonatedsulphanilamide,is also in-ert towardsthe oxidantspecies.ConsequentlythereactionstepwhichshouldrightfullyrepresentheoxidationprocessisandFromEqs 2and3,This mechanismof the reactionin essencewouldbe a polar one involvingelectrophilicattackonthe lone pair of nucleophilicaminogroup in therate-limitingstepforminga bimolecular~2 trans-itionstate(A).... (8)... (7)... (9)(A)SubstitutingEqs 7-9, in therateexpression(6)wegetEq. (10)d[PMPA);r klKh[H+]+k2KhKl [Blr{PMPAJ-r... (10)Equation(10)issimilartoEq. (1)wherek' = klKh[H+]+k2KhKl(Kh+[H+])(Kl +[H+])Validity of the rateexpression(Eq. 10)conse-quentlyof theoxidationstepsrepresentedby Eq.(5) andthe last two stepsin Scheme1 would re-quirethatplot of k' (Kh+[H+]) (K1 +[H+])versus[H+]shouldbe linear with a positiveslope andpositive intercept.To test this, Kl reportedbyThis is supportedby the observedkineticdataandlargenegativeentropyof activationwhich in-dicatesa large structuredunit with much de-creasedrotationalandvibrationaldegreesof free-domll.The mechanismpostulatedhere is differentfrom that of the oxidationof sulphanilamidebyS20~- proposedby Srivastavaand othersRwhichessentiallyinvolveda radicalprocess.Thesework-ersobservedthattherateincreasedupto a pH of5.34 only beyondwhich the rate remainedcon-stantandnosharpmaximumwasreported.AcknowledgementOne of us (RCP) wishesto thank the UGC,New Delhi,for grantof a teacherfellowshipunderFacultyImprovementProgramme.209INDIAN 1.CHEM., VOL. 27A, MARCH 1988Referehces1 Panitrahi G P & PandaA K, Bull chemSoc (Japan),54(l981) 1554.2 Mah~patroS N, PandaA K & PanigrahiG P, Bull chemSpc(Japan),54(1981)2507.3 Pani~ahiG P, PandaA K & MahapatroS N, J org Chem,4~(1981)4000.4 PaniWahiG P & NayakR N, ReactKin CatalLeft (Hung),2.(1982)283.5 CrutqMield M M, Peroxodiphosphoricacid in peroxideriJpctionmechanisms,Edited by J aEdwards(Intersci-e~ce,NewYork) 1961,p.41.210'I i i 1,1 ' I I' "."'"6 Lange N A, Handbookof chemistry,(McGraw-Hill, NewYork) 1962,p 791.7 KapoorS &GuptaY K, J chemSoc(Dalton),(1976)473.8 SrivastavaS P, MittaJ A K & GuptaV K, IndianJ Chem,20A (1981)806.9 BattagliaC J & EdwardsJ 0, InorgChem,4 (1965)552.10 SeccoF & VenturiniM, J chemSoc(Dalton),(1977)634.11 Moore W J, Physical chemistry,(Orient Longmans,NewDelhi)1969,298..•..

Kinetics &Mechanism of Oxidation of Sulphanilamide by Peroxomonophosphoric Acid

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Kinetics &Mechanism of Oxidation of Sulphanilamide by Peroxomonophosphoric Acid

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