Singh, H. S.

Singh, J. M.

Singh, V. P.

Srivastava, P. N.

English

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-- ---------- ..~Indian Journal of ChemistryVol. 15A,June 1977,pp. 517-519Kinetics & Mechanism'of Alkaline FerricyanideOxidationof2-Methylpentane-2,4-diol& Pentane-I,5-diolH. S. SINGH, V. P. SINGH, J. M. SINGH & P. N. SRIVASTAVADepartmentof Chemistry,University of Allahabad, Allahabad 211002Received 13 February 1976;accepted27 September 1976Oxidationof 2-ml'thylpentane-2,4-dioland pentane-I,5-diolby hexacyanoferrate(III)inaqueousalkalinemediumhasbeenstudied.The reactionis first orderwith respecto hexa-cyanoferrate(III).The first orderkineticshas beenobservedwith respectto bothOH- anddiol at low concentrations;thefirst ordertendstowardszero-orderat higherconcentrations.A probablereactionmechanismhasbeenproposed.OXIDATIONOF 2-METH,YLPENTANE-2,4-DIOLTABLE 1- RATE CONSTANTSAT VARYING CONCENTRATIONSOF HEXACYANOFERRATE(III){(I) [NaOH] =15 x 10-IM; [2-methyl-pentane-2,4-diol]=O'IM; temp.=35°}{(2)[NaOH] =4 x 10-2M; [pentane-1,5-diol]=0'1M}dc[K2Fe(CN)6] X 103M -lit x 105 hl'>;- loa_ , _ (mm"I)Initial Consumed moleslitre-Imin"Iglutaricacidas the final oxidationproductin thecaseof pentane-l,S-diol.ResultsandDiscussionPseudo-firstorderrate constants(kl) have beenobtainedfrom the relation(1) and the valuesaregivenin Table1. Thesevaluesarefairly constantfor the oxidationof 2-methylpentane-2,4-diolbutdecreaseon increasingthe [hexacyanoferrate(III))in thecaseof pentane-l,S-diol.Suchananomalousbehaviourin the. case of pentane-l,S-diolmightk1=- (~~)I[hexacyanoferrate(III)] ...(1)bedueto theglutaricacidformedduringthecourseof reaction. The complicatedfeatureof the re-action was observedwith varying [OH"] and1·931'361'061-101·091·017·826·4.55·342·371·661-362·004·004·506·608·202·100·801-601-502·103·204'200·390·650·852·804-806·800·430·911'371·822·893·86OXIDATIONOF PENTANE-1,5-DIOL0·500·801·003·005·007·000,501·001·502'003·004·00THE earlier studieson alkaline hexacyano-ferrate(III) oxidationof organic compoundsshowedthat the anion derived from theorganic substratesare frequently oxidized byhexacyanoferrate(III)via an electron transferprocessl•Recently,Singhandcoworkers2,3suggestedthat the oxidation of aldehydesand ketonesalso takesplacevia an electrontransferprocess.Whereasin one of the studiesin which the oxi-dation of iodide ion was carriedout with hexa-cyanoferrate(III), formation of an intermediatecomplexwas assumed'.Similarly another typeof bridgingwas found in the reactionof Co(II)ethylenediaminet traacetatewith hexacyanoferrate(III)5, in the first reversibleequilibrium.Thecomplexwas then assumedto disproportionateslowly to productsin the subsequentstep. Thisdualityof viewsregardingthemechanismof alkalinehexacyanoferrate(III)oxidation of organic com-poundsled us to study the kineticsof oxidationand 2-methyl-2,4-pentane-and pentane-l,S-diols.A probablereactionmechanismhasbeenproposedto explainthe experimentalresults.MaterialsandMethodsThe solutionsof 2-methylpentane-2,4-diolandpentane-l,S-diol (Fluka), potassium hexacyano-ferrate(III), potassium chloride and sodiumhydroxide (AR, BDH grade)were preparedindoublydistilledwater.After equilibratingthe solutionsof the reactants[hexacyanoferrate(III),NaOH and diols] at thedesiredtemperature,thesewere mixed and theamountof hexacyanoferrate(II)producedat diffe-rent time intervals was estimatedby titratingagainsta standardsolutionof ceric(IV) sulphateusingferroinas an indicator.Ceric(IV) sulphate being an oxidizing agentcould also oxidizediols but it was observedthatthe estimationof hexacyanoferrate(II)ion byceric(IV)sulphatewasnot affectedin the presenceof diols.Product analysis- 2-Methyllacticacid, aceticacid and oxalic acid have beenidentifiedas theoxidationproductsof 2-methyl-pentane-2,4-dioland517.INDIAN J. CHEM., VOL. 1SA, JUNE 1977... (el... (d)-d[Fe(CN)~-]Vi =---dt--_2~&J~~dK+] [S]COHo][Fe(CN)~-h(k+k_l) {1+Kl [K+]}+klKKl [K+][S][OH-]... (3)Eq. (3) is apparentlyconsistentwith the observedkinetics. It shows the first-orderkinetics withrespect to hexacyanoferrate(III)ion and alsoexplainsthe observedcomplexkinetic featureofthe reactionratewith respecto [diol] and COHo].Now consideringtheviewsof previousworkers7,sthe inequalityKl[K+]~l would manifestand therate law (3) reducesto Eq. (4).v. =-d[Fe(CN)~-]= 2kk;K[SJ[OH'][FeyJT (4)• dt (k+k_l)+k_K[S] [OH-] ...At verylow COHo]and[diol]theinequality (k+k_l)~ kl K[S] [OH-] will hold good and under thisconditiontheratelaw (4)reducesto Eq. (5).Scheme1whereC representsa complexbetweendiol anionand KFe(CN)~-. Similarstepsmight be proposedfor 2-methylpentane-2,4-diol.The formationof the alkoxideion of the diol(stepa) is supportedby theobservationsof Stewartand coworkers6who have studiedthe kineticsofpermanganateoxidation of alcoholsin alkalinemediumandsuggestedthatthealkoxideion formedby the ionizationof alcoholsis oneof the reactingspecies. It has alreadybeenreportedby severalworkers7,sthatthestep(b)of tbeproposedreactionschemewill be mainly towardsright.Now consideringthe steadystateconditionandthe total hexacyanoferrate(III)as [Fe(CN)i-h=Fe(CN)~-+KFe(CN)~-+ Complex the rate lawin terms of decreasing[hexacyanoferrate(III)ion]wouldbekComplex (C)-+HO.CH2.(CH2)..CH20·+KFe(CN)rHO.CH2.(CH2)..CH2.O'+KFe(CN)~'h2--+HO.CH2.(CH2l3.CHO +KFe(CNli-fastKHOCH2.(CH2h-CH20H+OH-~HO,CH2'(CH2hCH2,O-+H20 ...(a)KlK+ +Fe(CN)i-",KFe(CN):- ...(b)hlHO.CH2.(CH2la,CH2.O+KFe(CN)~-"'C(;mplex(C) ... (e)h_lwhereS representsthe [diols]and k, is the specificrate constant. The values of third order rateconstant (k,) have been calculatedby dividingd[Fe(CN)3'J- --dt~with hexacyanoferrate(III),OH- anddiol concentrationsand thesevalueswere foundto be 1·66X 10-3mole,2litre2 sec-l for 2-methyl,pentane-2,4-dioland 7·83X 10-2 mole-2 litre2 sec-lfor pentane-l,S-diolat 30°.Mechanism- Keeping these experimentalfactsin view a probablereactionmechanismis shownin Scheme1.-~(A) [K3 fe(CN)6]= 2.00 X to M,(NaOH]=o.60M, ,t/=2.ooM(11) [1<3Fe(CN)~:::2.00X 103M,[No OH]:=0.05 M, P = 1.00MA6.010.0I 6.010.0l'c20 4.0' 6-0 .9.0 10.0A0.5' 1.0 15 2.0 B[SUBST~ATE]xlOMFill. 1- Plot of (-dc/dt) versus[substrate]at 35°(Al [I:, Fe (CN)6]~ 2.00x 163M,[2-melhyl-2.;4-perrlane diolJ= 0·4 M,P=2.·00M(B) [k,Fe(CN),] =2..00X IO'M,[l,,-penfane diol]" O.2.M, p= 1.0M~18.0~'"o';16.020.0 40.0 60.0 80.0 100.0 A2.0 4.0 6.0 8.0 10.0 B[OliJ X 102MFig. 2- Plot of (-dcfdt) versus[OH-] at 35°[diol . The reactionrate is directlyproportionalto t e COHo]and [diols]at lower concentrationsbut ends towardszero-orderat higher concen-trati ns (Figs. 1 and 2).Th s, consideringthe abovefacts the generalrate (2) may be suggestedfor the oxidationofthesediols at lower concentrationsof alkali anddiols_ d[~~;N)~']=k, [S][OH-][Fe(CN):-] ...(2)518SINGH et al.: OXIDATION OP 2-METHYLPENTANE-2,4-DIOL & PENtANE-1,5-D10LFig. 3 - Plot of (-dc/dW1 versus[substrate]-1(A) [K3Fe(CN)~=2.00)(103M,[2-rne/hyl -2,4-pentane diol]=O.4M,P=2.ooMlB) [K3Fe (C/"JJ=2.00X fO~M,~ A I C',~-penfanediol]=O.2M, J.I =;;1.0M10·0Varia-tionk +k_lkkl K(from Figs. 3and 4)2-METHYLPENTANE-2,4-DIOL11·90 16·006·25[OH-]13-703 28 33(DiolPENTANE-1,5-DIOL0·310·289·26~]0·3338 3k +k_lM1K(from Figs. 1and 2)TABLE 2 - RATE CONSTANTS AT VERY Low CONCENTRA-TIONS OF HYDROXYL ION AND DIOLStwo differentsetsof conditions,viz. varying[diol]and COHo]are quite closeto eachother,.therebysubstantiatingthe validity of rate law (5).At higherCOHo]and [diol] the valuesof kIK[S][OH-]will bequitelargeand,thus,theretardingtrenddueto theOH- anddiolswouldbe obtainedand the previousinequalitywill not be valid and,therefore,at higherCOHo]and [diol] the rate ex-pression(4) can be rewrittenas1 k+k_l 1 (11; =2kk1K[S][OH-][Fe(CN):-] +?J.m~rrl\.Tl3-1 •••6)This Eq. (6)predictsthat a plot of 11rateagainst1/[S] or 1/[OH-] shouldbe linearwith a positiveinterceptat y-axis. Such a trend is shown inFigs. 3 and4. From the interceptandslope,thevaluesof k and k+k_l/kkIK havebeencalculatedrespectivelyandthevaluesaregivenin Table2..Here againthe valuesof k+k_llkklK, obtainedundertwo setsof conditions,viz. varying [diol]and COHo]are in good agreement.This alsoconfirmsthe validity of rate law (4). Hence aproposedreaction schemeappearsto be quiteprobable.References1. WILSON, 1. R, Rev.Pure appl. Chem.,16(1966), 103.2. SINGH, V. N., GANGWAR, M. C., SAXENA, B. B. L. & SINGH,M. P., Can. J. Chem.,47 (1969), 105.3. SINGH; V. N., SINGH, M. P. & SAXENA, B. B. L., IndianJ. Chem.,8 (1970), 529.4. (a) INDELLI, A. & GUARADDI, G. C., J. chem.Soc., (1964),36.(b) MAFlD, Y. A. & HOWLETT, K. E., J. chem.Soc. (A),(1968), 674.(e) HALPERM, J. LEGASE, R. J. & LUMSY, R, J. Am.chem.Soc.,85 (1963), 680.5. ADAMSON, A. W. & GONICK, E., Inorg. Chem.,2 (1963),129.6. (a) STEWART, R, J. Am. chem.Soc.,79 (1957), 2057.(b) STEWART, R. & VANDERLINDEN, Discuss. FaradaySoc., (1960), 211.7. JAMES, J- C. & MONK, C. B., T1'ans~Faraday Soc., 46(1950), 1041.8. EASTON, W. A., GEORGE, P. & HANANIA, G. 1. H., J.phys. Chem.,71 (1967), 2016.Bo4.0 /j.O 8.0 10.0 ,.8.0 12.0 16.I) 20.0 &. [olif'M(A) [1<3Fe (CN)6]= 2.00 x 10-3M,[NaOH] =0.60 M, 1-/=2.ooM(B) [K3Fe(cN)J=2.00Xf03M,[NaG"'] =0.0,M, p=1.00M8.010.0E~••l~_x'~:-ll~4.0~..!..,Tc:_x 10.0'•....•~8.0 4·0-'i' 6.0..•..4.0 2.02-00.0Fig. 4 - Plot of (-dc/dt)-1 versus [OH-]-1'i"0:E:e<t1012.06.0.=-d[Fe(CNn-]= 2kkIK[S][OH-][Feyh (5)V., k k ...t + -1The rate law (5) clearly accountsfor the first-orderkineticswith respecto hexacyanoferrate(III),hydroxylion and dials at very low concentrationsof hydroxylion and diols. Thus, the rate law (5)obtained at low concentrationclearly supportsthereactionmechanism.The valuesof k+k_I/kklKwerecalculatedfromthelinearplotsin Figs.1and2(i.e.at verylow concentrationof diol andhydroxylion)andthevaluesaregivenin Table2. A perusalof Table2 indicatesthat the valuesobtainedfrom519

Kinetics & Mechanism of Alkaline Ferricyanide Oxidation of 2-Methylpentane-2,4-diol & Pentane-1,5-diol

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Kinetics & Mechanism of Alkaline Ferricyanide Oxidation of 2-Methylpentane-2,4-diol & Pentane-1,5-diol

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