Char, P Narasimha

Rao, T Navaneeth

Sethuram, B

Sondu, S

English

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IndianJournalofChemistryVol.27A,March 1988,pp.211-213KineticsofCo-oxidationofEthyleneGlycol& Propan-2-01byAcid BromateP NARASIMHA CHAR, SSONDU, B SETHURAM &T NAVANEETH RAO·DepartmentofChemistry,OsmaniaUniversity,Hyderabad500007Received24April 1987;revised22 July 1987;accepted10August1987Acid bromateoxidationof a mixtureof ethyleneglycolandpropan-2-o1proceedsmuchfasterthanthatof theeitherof the two substratestakenalone.The productsof oxidationare acetonand formic acid. The order in[BrOi] is unity and in [ethyleneglycol]and (propan-2-o1]it is fractional.The rateincreaseswith increasein [H+]and percentageof aceticacid in the reactionmedium.The amountof acetoneformedfrom propan-2-o1duringaparticulartimeperiod in thepresenceof ethyleneglycolis nearlytwicethatformedin its absence.A mechanisminvolvingformationof a termolecularcomplexinvolvingethyleneglycol,propan-2-o1andbromateanddecomposi-tion of this complexin the rate-determiningstepis proposed.The natureof thecomplexformedhasnot beenas-certained.Recentlywe have reportedthe co-oxidationof1,2-glycolsandaliphaticestersby periodatein ba-sic medium1.Preliminarystudiesin thepresentin-vestigationhave shownthat the rate constantofacid bromateoxidationof a mixtureof ethylene-glycol (EG) and propan-2-01is almosttwicethatof the sumof the rateconstantsof .theindividualsubstratesunder identicalconditions.This indi-catesthat co-oxidationis takingplace.Detailedkineticinvestigationof acidbromatecoordinationof EG +propan-2-01is thereforecalledfor in or-dertoarriveatasuitablemechanism.MaterialsandMethodsPotassiumbromate,sulphuricacidandmercuricaccetatewereof AR grade(BDH) andwereusedas such.Acetic acid (AR, BDH), ethyleneglycol(EG) andpropan-2-ol(AR) sampleswerepurifiedbeforeuse.All theexperimentswerecarriedoutin aqueousmediumunderpseudo-firstorderconditionsusingten-foldexcessof substratesoverthe oxidantandthe kineticswerefollowediodometrically.Mercu-ric acetatewas used as a scavengerfor Br-formedas oneof theproducts.The rateconstantswerecalculatedbytheleastsquaresmethod.Acetoneandformicacidwereidentifiedas thereactionproductsby theircharacteristictests.Oneof the oxidationproducts,viz. acetonewas esti-matedcolorimetricallyby measuringthe absorb-anceof thecolouredcomplexformedby theaddi-tion of salicylaldehydeand KOH solution andcomparingit with the standardBeer's law plot.The resultsindicatedthat 1mol of acetonewasformedfor everymol of bromateconsumedin thepresenceofEG in accordancewithEq. (1).3Br~+3CH3-CH-CH3 2CHz-CHz H+I + I IOH OH OH3CH3-CO-CH3+4HCOOH +3B; +5HzO ••••••••(1)ResultsandDiscussionThekineticresultsaresummarisedbelow:(i) Under theconditions[substrate]~[BlOi]theorderin [BrOi] wasunity(Fig.1A).(ii) The ordersin [EG] and [propan-2-.o1]werebothfractional(FigsIB andle).(iii) The rate increasedwith increasein [H+]and the order in [H+]was two, indicatingthe in-volvementof two protonsin this process(Table1).(iv)The rateincreasedwith increasein percen-tageof aceticacidin thereactionmediumandtheplot of logkobsversusliD waslinearwith a posi-tiveslope(Fig. ID) indicatingthatprobablya pos-itiveion anda dipoleare involvedin the rate-de-terminingstep.Secondorderdependenceof rateon [H+]indi-catesthattwoprotonsareinvolvedin thereactionand the probabilityis the protonationof BlOi(seeEq.2).BrOi +2H+ ~ BlOt +H20 ...(2)This showsthatBlOt is thereactivespecies.Sucha reactivespecieshasalsobeenproposedby earli-erworkers2in thebromateoxidation.(v)Linear plots of 11kobs versus 11[EG] and1i[propan-2-01]with definiteinterceptson Y-axesindicatestheinvolvementof thesubstratesin com-plexwithBr (V).(vi)The first order rate constant(kobs x 104S-1) underthe conditions[BrOi = 5.00 x 10-3211t .•'.,_.__.IV16.628.366.5__ h ••• ._ •••• __ • II__ __•........ 11.942.153.35[H2C-CH2 J+b b CH3•••••. /' ••. J IIIHO/Br,~ :?-l- CH3H H1C2)Compl,..H29-fH20, +...0BrHO.•....'OH- 2 HCHO +HOBr + CH3 COCH3+ H20+ H+.Iow1.794.197.670.5940.7881.11[W]moldm-3c"",p'",+H~C-CH2 + 8,=0I I IIHO OH 0Table1-EffectofVarying[H+]onRateConstantsofBr(V)OxidationofEthyleneGlycol+Propan-2-o1[BrOi] =5.500x 103moldm-3; [ethyleneglycol]=2.50x 10-2moldm-3 [propan-2-01]=2.50x 10-2 moldm-3; [Hg(OAcU=0.100moldm- 3;temp=318Kkoh, (10-4) [W]S-I moldm-3- +HCHO + HOBr - HCOOH + Br + H ••-- ••------- --.- vSCHEME Ito decideat thisstage.No inducedpolymerisationof acrylonitrilewasobservedwhenit wasaddedtothe reactionsystems.The experimentalobserva-tion thatthe rateconstantof acidbromateoxida-tion of a mixtureof two substratesis twicethatoftheindividualsubstratesunderidenticalconditionssuggeststhatco-oxidationis takingplace.In orderto seewhetherthe rate enhancementdue to co-oxidationresultsin the formationof more of thereactionproductsfor a certaintime interval,wehave estimatedone of the products,i.e. acetonecolorimetrically.The results indicatethat underthe conditions[BrOi] = 5.00x 10-3moldm-3,[H2S04] = 1.00moldm-3, [Hg(OAc)2]0.10moldm-3, temp = 313K, [EG]0.25moldm-3 and [propan-2-ol]0.24moldm-3 the amountsof acetoneformedinpresenceand absenceof EG is 0.414mmol and0.218mmol (for 20ml of reaction mixture in30min) respectively,thus providing additionalsupportfor the occurrenceof co-oxidationof thetwo substrates.Basedon the aboveexperimentalobservationsthe probablemechanismis showninScheme1.'·2,·00·8'" +; 'IN 1.40·40·50·60·7INDIAN J. CHEM., VOL. 27A, MARCH 198825 30 -- Timt(min) (A),·0 1·2_2+log(tthyltnt glyCOI](BlI-a 1·2-2+logUsoProponol](Cl10 15 200-4 0,6 0-80-4 0·6 0·8~ ~D •o ,• <'" 'o 0~ ~t t0·8 6·00·4\Fig. l--(A) Plot of log(a/a- x)versustime([Bra)] t=5.00x 1O-3;moldm-3; [propan-2-01]=2.50 x 10-2mold~- 3; [ethyleneglycol]=2.5010- 2 moldm- 3; [H2S04]=1.QOmoldm"3[mercuricacetate]=0.100moldm- 3;, temp=313K)I(B) Plf>tof logkobs versuslog[ethyleneglycol](lEG]=2.00x 10-2 to5.00X 10-2 moldm-3; [propan-2-01]=5.00x '0-2 moldm-3; otherconditionssameasin (A)).i(C) Plot if logkobsversuslog[propan-2-01]([Propan-2-01]=2.00xl ·2 to 3.00X 10-2 moldm-3; [EG]=5.00x 10-2moldm-3).iI (D) Plot ofIogkobsversus1/Dmoldmt3, [propan-2-0Ij= 2.50x 10-2moldm-3,[EG] do 2.50 x 10-2 moldm-3, [H2S04]1.00mof dm- 3, [mercuric acetate]0.100m~ldm-3 are 7.29, 7.31,7.25 and 7.33 at[NaCIOdi = 0.05, 0.10,0.15 and 0.20moldm-3respecti~elyindicatingthatNaCI04 has no effectonthertfction rate.In t~ oxidationof 1,2-glycolsby acidbromatethefo ationof a complexbetweenthesubstrateand oxi ant is well established.However,no suchcomplex!ihasbeenobservedbetweenbromateandpropan-~-oP.The fact thatthereis no interactionbetween!propan-2-01and EG as such and thefractionaJorder dependenceof rate on [EG] and[propan-f-ol]indicatethatboth the substratesareinvolvedlintheformationof somesortof complexwith theioxidant.The kineticevidencefor suchacomplexi. comesfrom the plot of 1/kobs versus1/[subst$te]whichgivesclearintercepton 1/kohsaxis.The exactnatureof the complexis difficult212I i II 1 I' 1,-11I!IlI Hill'NARASIMHA CHAR etat.: Co.-o.XIDATIo.N o.F ETH¥LENE GLYCo.L & PRo.PAN-2-o.L BY ACID BRo.MATEScheme1leadstoratelaw(3)-2'3031109[B'~------- •.~ob.-HRatelaw(3)envisagesfirstorderdependencein[Br(V)] and fractionalorder dependencein both[EG] and [propan-2-ol]in keepingwith the ob-servedresults.It alsoexplainsthe increasein ratewith increasein [H+].The reciprocalof the Eq.(3)aftersimplificationleadstoEq. (4)whichI [I+!il[H+J2] [I ] [I ]-- --- --+1 -----+1~ob.- k~I [tfO]2 ~ [EG] ~J [P,oPon-2-ol].•.......•.•...(4)predictslinearplotswith sameinterceptsfor (i) 1/kobsversusl/[EG] at fixed[propan-2-ol]and[H+]and(ii) 1/kobs versus1/[propan-2-ol]at fixed[EG]and [H+]. Such plots have been realisedin thepresentwork.The Kl and K2 valuescalculatedfrom theoxi-dationof EG alonewith Br(V) werefound to be0.24 and 22.9 respectively.The K2 valuescalcu-latedfrom the slopeand interceptof the plot of1/kobs versusl/[EG] at higher[propan-2-01](Eq.4) is foundto be 22.6whichis in excellentagree-mentwith thevalue(22.9)obtainedfor the indi-vidual oxidationof EG indicatingthe validityofthe rate law and hencethe proposedmechanism.The K3 valuecalculatedin a similarfashionwas5.00.Substitutionof thesevaluesin Eq. (3) givesthekvalueas3.29x 10-2 moldm-3s-1•References1 SwarnalakshmiN, UmaV, SethuramB & NavaneethRaoT, IndianJ Chern,23A (1984) 646.2 Vijayalakshmi& SundaramE V, Indian J Chern, 20A(1981)300.3 NatarajanR & VenkatasubramanianN, TetrahedronLet!,57(1969)5021;30(1970)785.213

Kinetics of Co-oxidation of Ethylene Glycol & Propan-2-ol by Acid Bromate

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Kinetics of Co-oxidation of Ethylene Glycol & Propan-2-ol by Acid Bromate

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