Mehrotra, Raj N.

Sankhla, Prem Singh

English

NOPR

Indian Journal of ChemistryVol. 14A, September 1976, pp, 663-666Kinetics & Mechanism of Oxidation of Butane-r.j-diolby Ce(IV) Sulphate in Aqueous Sulphuric Acid Solution:Evidence for Ce(IV) DimerPREM SINGH SANKHLA & RAJ N. MEHROTRA·Department of Chemistry, University of Jodhpur, Jodhpur 342001Received 3 January 1976; accepted 31 January 1976The kinetics of oxidation of butane-l,3-diol by Ce(IV) in dil. H2SO, indicates a fractional orderdependence in the [diol'] consistent with the complex formation between Ce(IV)and diol prior toits rate limiting disproportionation. The linear dependence of the observed first order rateconstant (k) on the inverse [Ce(IV)] indicates the presence of dimeric Ce(IV) and its participationin the reaction mechanism. The data also indicate that the association constant of the dimericCe(IV) is appreciable and its significance cannot be minimized in view of the dependence ofthe rate on the initial [Ce(IV)]. The diol is oxidized to crotonaldehyde via an intennediate freeradical. The mechanism is discussed and the activation parameters at a given [Ce(IV)] arereported.THE kinetics of oxidation of diols, with theexception of glycols by Ce(lV) sulphatehas been reported earlier by USI-3. Theseoxidations exhibited a first-order dependence in[diol]; the plot of observed first-order rate constantkl versus [diol] was linear and passed through theorigin in each case. These oxidations were con-sidered partly to proceed through Ce(lV)-diol ccm-plex formation and partly through the bimolecularmechanism. The present paper reports the necessarykinetics evidence for the forrnaticn of the inter-mediate complex and a different dependence, unlikethe one reported in the oxidation of pentane-Lfi-diol'',of kl on [Ce(lV)]. The differing dependence ofkl on [Ce(lV)] is considered as a possible potentialtool in distinguishing the nature of the intermediatecomplex formed in the oxidation of pentane-IS>and butane-l,3-diols.Materials and MethodsThe cerium(lV)-sulphate (Fisher) was used assuch. The stock Ce(IV) solution was preparedin Basynth's analar sulphuric acid. The acid strengthin Ce(lV) solution was determined as describedear lierl-3.The stock solution of butane-1,3-diol (Fluka,purum grade) was prepared by direct weighingof the sample. Other chemicals used were of eitherBDH or Sarabhai-Merck grade.The reaction was followed over two half-lives(80%) and the usual first-order plots we~e lin~arthroughout the range covered. The reaction nux-tures were prepared so as to have [diol] in muchexcess over that of [Ce(lV)]. The slope valuesof the plots, wherever needed, were calculated bythe least squares method. Other experimentaldetails were similar to those reported earlier':". Therate constants, reported here, are the average values•Author to whom all correspondence should be addressed.of two to three runs and agreed within 3% withthe individual values obtained from identical runs.Results and DiscussionAlthough the first-order plots were linear at all[Ce(lV)], the first-order rate constant kl with respectto Ce(lV) was not found to be independent of initial[Ce(IV)]. It rather decreased with the increasein [Ce(lV)] (Table 1).There was no appreciable change in kl at different[H2S04] (Table 2), except for a very small retarda-tion and acceleration in ki in solutions <1·sMand > 1·sM sulphuric acid respectively.The kl measured at different [H+] but constantionic strength did not increase proportionatelywith the increase in [dial] (TabJe 3). The ionicstrength was kept constant by using mixtures ofTABLE 1 - EFFECT OF VARYING [Ce(IV)] ON kl{[Butane-1,3-diol] = 0·3M; [R.SO.] = 1·5M; temp.= 40°C}102[Ce(IV)]M0·250·501·0106k, 109[Ce(IV)] 105k1sec< M see'<3·90 2·0 1·952·78 2·5 1-902·22TABLE 2 - DEPENDENCE OF THE RATE CONSTANT (kl) ONVARYING INITIAL [H2SO.]([Ce(IV)] = 0·OO5M; [Butane-l,3-diol] = 0'3M;temp. = 40°C]}[H.SOJ 106k1 [H.SOJ 106111M see'? M sec-l0·3 3·3 2·5 3·11·0 2·9 3·0 3·2··1-5 2·8 3·5 3'32·0 3·0 4'0 ss663INDIAN J. CHEM .• VOL. 14A. SEPTEMBER 1976TABLE 3 - DEPENDENCE OF RATE CONSTANT (kl) ON[BUTANE-l.3-DIOL] AT VARYING [H+]([Ce(IV)] = 0'005M; [HS04] = 1·5M; temp.= 40°C}[Butane-l.3-diol]M0·100·150·300·450·751'00[ff+]=0'8M l'OM 1·2M 1·5M120·7 0·8 0·9 1·21-1 1·2 1·3 1·71·8 2·2 2·4 2'8 '.x-H 2·7 3·0 3·& A3·6 4·6 4-6·0 7·0 '0[HSO'] 105kl [HSO,] kl X 10'M see"! M sec<0·& 4'6 1·2 3·41'0 3·8 1·5 2-8 00TABLE 4 - EFFECT OF VARYING [HSO,] ONkl AT CONSTANT [H+]{[Ce(IV)] = 0'005M; [Butane-1.3-diol] = 0'3M;temp. = 400qTABLE 5 - EFFECT OF IONIC STRENGTH ONOBSERVED RATE CONSTANT (kl){[Ce(IV)] = 0'005M; [Butane-1.3-diol] = 0'3M; [H+J=l'SM[HtSO.] = 1'5M; temp.= 40°C}[NaCIO.] io», [NaCIO.] io»,M see'? M see"?0·00·12·82·60·30·52'42-3TABLE 6 - EFFECT OF TEMPERATURE ONRATE CONSTANT (kl) AND ACTIVATION PARAMETERS([Ce(IV)] = 0'005M; [Butane-l,3-diolJ = 0·45M;[H.SO.] = 1'5M}Temperature(0C)4050Temperature(0C)556023·941·33·7514·3~Et = 24·6 ± 1·6 kcal mole>, ~St = -4 ± 5 cal deg ?mole'<.sulphuric acid and sodium hydrogen sulphate. Theplots between l/kl versus I/[diolJ were linear (Fig. 1).The- intercepts on the rate axis were independentof [H+J.The rate decreased with the increase in [HS04J(Table 4) at constant [H+]which was kept constantby using mixtures of sulphuric and perchloric acidsin such a way that the overall [H2S04] was always>[HCI04]. It ensured the continuity of the Ce(IV)species in this mixture. The rate also decreasedwith the increase in ionic strength (Table 5), whichwas varied using sodium perchlorate. The values ofrate constant (k1) and the activation parameters aregiven in Table 6.The kinetic evidence for the existence of theCe(IV)-diol complex was indicated by Michaelis-6645 10 15[OIOLf'Fig. 1- The plot between l/kl and [butane-Lj-diolj'< at40° with [Ce(IV)] = 0'005M; [HS04] = 1'5M; and [H+]= (A) l'5M; (B) l'2M; (C) 1M; (D) O'&M~or-----------------------------~3·0i-s0 0" 0·2 0'3 0'4IO-l[C~(IV1)'Fig. 2-The plot of observed rate constant kl versus[Ce(IV))"lMenten plots (Fig. 1). The assumption of thecomplex formation in the oxidation of propane-l,3-and butane-Ls-diolst-t is thus justified and thedeviation from Michaelis-Menten plot in thesecases could be ascribed to the smaller or negligiblevalue of the formation constant for Ce(IV)-diolcomplexes.The data in Table 1 gives a linear plot betweenkl and [Ce(IV)]-l with a positive intercept on therate axis (Fig. 2). >This correlation between kland [Ce(IV)J is different from the one observedin the oxidation of pentane-I.s-diols where it wasnoted that the plot of l/kl versus [Ce(IV)] was.linear. This difference in the dependence of theSANKHLA & MEHROTRA: OXIDATION OF BUTANE-l,3-DlOL BY Ce(IV) SULPHATErate on [Ce(IV)] in the oxidation of butane-l 3- andpentane-lSvdiols could be ascribe~ to the re;ctivityor lTIactlV1t~ of some Ce(IV) species present in dil.H2S04 solutions. We believes this species is dimericCe(IV).Our efforts to establish the existence of dimericCe(IV) by spectrophotometric measurements remaininconclusive in the sense that the equation cone-lating the optical density with the various molarextinction coefficients corresponding to variousCe(IV) species and their concentrations is not easyto resolve. However, it should be noted that thereis a shift in the maximum of absorption towardsshorter wavelength with increasing [H2S04) (ref. 4).We have ourselves noted some kind of shift in ).maxwith increasing [Ce(IV)] at a given [sulphuricacidl.Considering the well established existence ofdimeric Ce(IV) in non-complexing perchlorate& andcomplexing nitrate medias and a trimeric Ce(IV)species in acetic acid", we conclude that there shouldbe no valid objection to such an assumption insulphate medium, especially when the same is "erymuch indicated and supported by the kinetics ofthe present reaction and oxidation of other diolssuch as propane-Ls-, butane-Ls-, hexane-Lo- andhexane-2,5-diols8. It may be added that the depen-dence of the rate on [Ce(IV)) in the oxidation ofpropane-1,3-, butane-1,4- and hexane-2,S-diols issimilar to one observed in the present case, whereasin the oxidation of hexane-lti-diol it is similar tothat observed in the oxidation of pentane-I,S-dio13•The differing dependence of kl on (Ce(IV)] couldpossibly be used as a potential tool in visualizingthe nature of the intermediate complex formedbetween Ce(IV) and the diol. A chelate complexis indicated in the oxidation of butane-1,3-diolif one assumes the abilitv of the reactive Ce(IV)species including dimeric Ce(IV) in successfullybridging the gap between the two hydroxyl groupsof butane-1,3-diol. An acyclic complex formationin the oxidation of pentane-1,S-diol is expectedbecause of the inability of Ce(IV) species includingdimeric species in bridging the increased gap betweenthe two hydroxyl groups of the diol.These conclusions based on experimental obser-vations are however not in agreement with thoseof Young and Trahanovsky? who believe in theformation of the acyclic complex between Ce(1V)and the diol. However, further support for ourconclusions comes from a recently completed studyon the oxidation of these diols by Ce(IV) in per-chlorate medium-s. The conclusions are in agree-ment with those of Hintz and Johnsonll who favourthe formation of a chelate complex. Also Dukeet al.l2-14 have reported the chelate formation betweenCe(IV) and organic compounds having more thanone hydroxyl groups.I t is interesting to note that the formation constantfor dimeric Ce(1V) from monomeric Ce(1V) specieshas an appreciable value in nitrate" (17 ±2 at 30°),perchlorate= (16·2 at 2S0) and 1·62x 105 for theformation constant of trimeric Ce(IV) from mono-meric Ce(IV) in acetic acid medium'. Thus itwould not be erroneous to assume that in the presentmedium also the equilibrium constant for the equi-librium between monomeric Ce(IV) and dimericCe(IV) species has an appreciable value.Since it is not easy to identify which of the mono-meric Ce(IV) species present in dilute sulphuric acidwluti~ns15 fo:m the dimeric Ce(IV) species, thefoJ]CW1.rg.baSIC. n:~chani~m is proposed in view ofthe principal kinetic results obtained in the presentstudy.KCe(IV)+Ce(IV)~lCe(lV)]210",Ce(IV)+diC'l~C( n.plextm)loa[CE(IV)J2+dic'] ~ Cr rr.plextd)10",C(mplex(m)-~Ce\III)+R'+H'"lotiC(mplex(d)-:-~Ce(lV)-Ce(IIl)+R·+H+Cc(IV)+R-->Cc(III)+P+H'"fastwhere the pre duct P is crotonaldehyde, R is thefree radical CHa·CHOH-CH2·CHOH, the formationof whi~h. is indicated by the polymerization ofacrylonitrile added to the reaction mixture. Therewas no polymerization when the monomer wasadded to Ce(IV) solution in the absence of the diol.The formaticn cf the dimer Ce(IV)-Ce(III) is againrased. on the existence d such dimeric species inthe nitrate" and perchlorate mediums. Blausteinar.d Glyde16a have Examined the' data of Sherillct. al.l6 ar.d have concluded that Ce(UI)-Ce(IV)dimer s are present in perchloric acid medium.The rr act ic r s (1)-(6) give the follcwing rate Eq, (7)-d[Ce(IV)]dt_ 2(k",K",+kaKdK[Ce(IV)])[Ce(IV)][diol]- (1 +K",[diol])+K[Ce(IV)J(1+Kd[diol]) ... (7)New if it is assumed, as a measure of approxi-mat icn , Hat K[Ce(IV)](l +Kd[diol]) > (I4-K", [diol))became of the appreciable value of K as statedearlier and Kd is expected to l::e >K"" the Eq. (7)is reduced to Eq. (8).-d[Ce(IV)) _ k _ 2(k",K",+kdKJ([Ce(IV)J)[diol]dt[Ce(IV)] - 1- K[Ce(IV)](1+Kd[diol])••• (8}At a given [dioi] He Eq. (8) cculd be expressedby Eq. (9)_ 2k",K",[diol] 2kaKa[diol]k1- K(l+Ka[diol])[Ce(IV)] + (I+Kd[diol]) •.. (9)Thus Eq. (9) is consistent with the linear plot(Fig. 2) between kl and [Ce(IV)]-l. Again, at afixed [Ce(IV)), the Eq. (8) could be rearranged togive the Eq , (10) which is consistent with the linearplots between 1/kl and Ij[diol] (Fig. 1).1 K[Ce(IV)]~= 2{k",K",+kaKJ([Ce(IV)}[diol]KKd[Ce(IV))+ 2{k",K",+kaKJ([Ce(IV))} ... (10)It is thus seen that within the assumed approxi-mations, the proposed mechanism and the rateequations derived therefrom are consistent with••. (1)•.. (2)•.. (3)•.. (4)•.. (5)... (6)INDIAN J. CHEM., VOL. 14A, SEPTEMBER 1976the observed results. The existence of dimericCe(IV) species in sulphuric acid solutions seemslikely' and requires a careful reinvestigation of theequilibria between various Ce(IV) species in thisacid medium. It should be pointed out that withoutthe assumption of dimeric Ce(IV) and its formationof a chelate complex with butane-lB-diol, it isdifficult to explain the differing dependence of therate on the initial [Ce(IV)] in the oxidation ofperrtane-I ,5- and butane-l,3-diols.Stoichiometry - The reaction product is repeatedlyextracted with ether from the reaction mixtureshaving different [diol] and a known [Ce(IV)J. Theextracted product (80%) formed 2,4-DNP deri-vative, m.p. 187-88°, identical with the 2,4-DNPderivativet? of crotonaldehyde. In a separate experi-ment the absence of HCHO in the extracted productis established with chromotropic acidIS.AcknowledgementAuthors are thankful to the CSIR, New Delhifor the financial assistance -to the project and forthe award of a junior research fellowship to one ofthem (P.S.S.). Grateful thanks are due to Prof.R.C.Kapoor for the keen interest and encouragement.References1. SANKHLA, P. S. & MEHROTRA,R. N., Indian]. Chem., 10(1972), 1077. .6662. SANKHLA, P. S. & MEHROTRA, R. N., J. inorg, nucl,Chem., 35 (1973), 891.3. SANKHLA.P. S. & MEHROTRA.R. N .• Indian J. Chem.,13 (1975). 904. .4. BUGAENKO,L. T. & KUAN-LIN. HUANG. Russ. J. inorg.Chem .• 8 (1963). 1299.5. (a) HEIDT. L. J. & SMITH. M. E .• J. Am. chem, Soc., 70(1948). 2476; (b) HARDWICK. T. J. & ROBERTSON.E .•Can. J. Chem .• 29 (1951). 818; (c) KING. E. L. &PANDOW, M. L., J. Am. chem, Soc .• 74 (1952). 1966;(d) ARDON. M., J. chem, Soc., (1956), 104.6. (a) BLAUSTEIN.B. D. & GRYDER.J. W .•J. Am. chem. Soc.,79 (1957). 540; (b) DORFMAN.M.K. & GRYDER. J. W.,Inorg. Chem .• 1 (1962). 799.7. FORD. P. C. & WIBERG. K. B .• Inorg, Chem .• 7 (1968),369.8. MEHROTRA.R. N., (unpublished work).9. YOUNG, L. B. & TRAHANOVSKY.\V. S.• J. Am. chem,Soc .• 91 (1969), 5060.10. PRAKASH. A., MEHROTRA,R. N. & KAPOOR. R. C. (tobe communicated).11. HINTZ. H. L. & JOHNSON.D. C.• J. org, Chern., 32 (1967),556.12. DUKE, F. R. & FORIST. A. A.• J. Am. chem, Soc., 71(1949). 2790.13. DUKE, F. R. & BREMER. R. 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Kinetics & Mechanism of Oxidation of Butane-I,3-diol by Ce(IV) Sulphate in Aqueous Sulphuric Acid Solution: Evidence for Ce(IV) Dimer

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Kinetics & Mechanism of Oxidation of Butane-I,3-diol by Ce(IV) Sulphate in Aqueous Sulphuric Acid Solution: Evidence for Ce(IV) Dimer

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