1940 J.C.S. DaltonKinetics of Reactions of Catechol (o-Dihydroxybenzene) and Quinol(p-Dihydroxybenzene) with Oxidizing Metal Ions present in Excess : TheReactions with Iron(iii), Vanadium(v), and Thallium(1ii) in Aqueous Per-chloric Acid SolutionBy Ezio Pelizzetti, Edoardo Mentasti, Edmondo Pramauro, and Guido Saini," lstituto di Chimica Analitica,The reactions of catechol with excesses of Fe'II, Vv, and TI"' ions and of quinol with an excess of V' have beenstudied by means of a stopped-flow technique in aqueous acid solution (HCIO,) at 25.0 "C. Kinetic dependenceson the reagent concentrations, different from those obtained in the presence of excess of the dihydroxybenzene,have been found for the reactions with Fe"I and Vv ions, while the same kinetic dependence has been found for thereaction with TI'".Universiti di Torino, ItalyProposed schemes of reaction are discussed.THE kinetics and mechanism of oxidation of phenols ~ i t hmetal ions has been investigated mainly by working withthe phenols present in excess.In previous papers thekinetics of oxidation of excess of catechol (o-dihydroxy-benzene, H,cat) with aqueous FelI1,l Vv,2 and TlIIIions were investigated. Further experiments with anexcess of FeIII or Vv ions showed a different kineticdependence on reactant and hydrogen-ion concentrations.This prompted a kinetic study of the reaction of catecholwith the above oxidants (as well as with T P ) presentin large excess. It was also of interest to investigateranges wcre: [FeIII] 7 1.0 x 10-2-3.0 x [H,cat] =2.0 x lo-,, [H'] = 0.10-0.40 (HClO,), and I = 1 .0 0 ~(NaC104), for the reaction FeJII + catechol; [Vv] =1.0 x 10-3-1-0 x [H,cat] = 1.0 x [H+] =0.50-1-00 (HClO,), and I = 1-00iu (NaClO,), for thereaction V" + catechol; [VV] = 2-5 x 10-3-7.0 x lo+,[H,quin] = 2.0 x [H -1 = 0.30-5-00 (HClO,), and I =5 . 0 0 ~ (NaCIO,), for the reaction Vv -+ quinol; and [TlIIIj =1.0 x 10-3--3-0 x [H+] =1-00-2-00 (HClO,), and I = 2.00~1 (NaClO,), for tlicreaction TIIT1 + catechol. Measureriients were carried outat 25.0 & 0.1 "C.[H,cat] = 1.0 x lo-,,Fe3*Laq 1Hca< fast bFe2'(aq)+ o-qno +Fe2'( aq)4SCHEME 1 PI is the formation constant of the complex [Fe(Hcat)]'+, I<, the hydrolysis constant of Fe3+, and KH theaciddissociation constant of [Fe(Hcat)]2+the reaction between quinol (9-dihydroxybenzene,H2quin) and an excess of Vv ions; this reaction, inexcess of quinol, was previously- studied by Wells andK ~ r i t s y n .~EXPERIMENTALThe reagents and procedure were as described previ-o u ~ l y . ~ - ~ In the present experiments, the oxidationproduct of catechol ( H p t ) was o-quinone (o-qno), asshown by its absorption spectrum. The reaction betweenquinol (H,quin) and Vv ions was followed at 370 nm; thereaction product was p-quinone (p-qno) , Concentration1 (a) E. Mentasti and E. Pellizzetti, J.C.S. Dalton, 1973, 2605;(b) E. Mentasti, E. Pelizzetti, and G. Saini, ibid., p. 2609.E. Pelizzetti, E. Mentasti, and G. Saini, Gaizetta, in thepress.E. Pelizzetti, E.Mentasti, and G. Saini, J.C.S. Dalton, 1974,721.RESULTSFeTTr + CntechoZ.-The increase in absorbance at 390 nm(due to o-qno) was first order with respect to [H,cat] (thereagent not in excess). Observed rate constants kohs a tvarious [H+] and [FeTrl] values are given in Table 1.The rate of reaction depends on [FeT1'I]lz, where +z is anumber between 1 and 2, which in turn is a function of[H']. By recalling tlie previously suggested schenie ofreaction,lb in excess of catechol, and taking into accountthe fact that, when FeTT1 ions are present in large excess,the reverse reduction duc to FeII can be neglected, Scheme(1) can be proposed. T o account has been taken of dis-sociated species of catechol because of its low dissociationconstant (pK, ca.10) .5 -41~0 tlie complex [Fe(H2cat)l3+(aq)4 C. 17. WellsandL. V. liuritsyn, J . Chem. SOC. ( A ) , 1970, 1372.5 I,. G. Sillen and A. E. Martell, ' Stability Constants of Metal-ion Complexes,' Special Fubl. No. 17, The Chemical Society,London, 19641974 1941can eitlici- react with another I7eIII ion giving the finalproducts of reaction (FcT1 and o-qno) or decompose intoFeT1 antl Hcat'; Hcat' is rapidly oxidized to o-qno byanother FcTTI ion (as shown previously).TABLE 1\-alucs o f thc obscrvccl rate constant k o L s (0) for tlieositiation of catecliol with FeTI1 ions (25.0 "C, I = 1.00,H,catj, = 2-0 x t O - 4 3 ~ )[HClO,]/ar9-10 0.12 0.14 0.17 0-20 0.25 0.400-042 0.029 0-024 0.018 0.014 0.0078 0.00550.063 0.043 0.039 0.029 0.021 0-015 0.00800.079 0.055 0.045 0-032 0.026 0.016 0.00920.12 0.059 0.050 0.049 0.038 0.028 0-0130.11 0.098 0.064 0-052 0.035 0.0160.1 1 0.055 0.037 0.0180.1 4 0.075 0-048 0-024The kinetic expression wliicli can be deduced, assumingthat reactions of hydrolysis and coiiiplcs formation are fastcompared with the electron transfer and that FeIII, in the[H+] range investigated, is iiiaiiily present as Fe3+, is asi n equation (1).Here I<cq is the equilibrium constant ofthe rcactioii Fe3-:- + H,cat + [Fe(cat)]'(aq) $- JH'(4-3 ;.; 1 0 niol 1 l),ln and ml is given by equation (2).C I ~ := /<2(311<l~,€3 '.:,--' +- (k2'Pl.K13 -; k3Keq)[H']-2 +k,'Kll<eq[H'=-3 (2)Sincc tlic data of a previous paper,ln where formatioil oftlie complc~s [Fe(cat)]+(aq) was investigated, showed thatat /-H+] = 0 .1 0 ~ there was no evidence of forination of aprotonatecl i Fe(Hcat)lS' (aq) species, i t follows that theacid-dissociation constant of tlie comples [Fe(Hcat)l2+-(aq) should be >0.1 niol l-l, and p1 < 2-6 x lo2 1 mol-l.'I..lius tlie term ( plIc1 + ~,,lT,H+]-l)[Felll?[H"]-l in the mostunfavoursble case assumes the value of ca. 0-1 and will bencglected i n the following treatment. Equations (1) and(2) can then be siiiiplified to give (3). Figure 1 shows plots,k0~,/[Fe1I1] = hlPII<l[H']-l + m1[Ferrrjaccording to cquation (3), a t various [H']. From theintercepts ilie \-due of tlie constant KIPIK,, which has thesame meaning as the constant k,' given previously,1b waswyaluated as 0.20 s-l (cf.0.19 lb), from which h,P, = 1.2 x102 1 mol-1 s-l. Furthermore, from gradients of the plotsin Figure I , tlie dependence on [H"]-2 was assessed aiicl avalue of 2.2 sI t is to be noted that, in this [H'] range, the climerspecies (Fe--O-Fe)*'- is present in less than ly(, of [Fe111]T.6v7The participation of this species in tlie redox reaction canbe excluded since the rate of formation of the dimer 8 islow compared with that of tlie redox reaction; in fact,under the prcsent experimental conditions, the kinetictreatment used ant1 the suggested rca.ction scheme shouldnot be applicable.Vv 4- CtrfeclroZ.-TIie absorbance of the solution at505 nm increased very rapidly due to formation of a[Vv(H,cat)l' (as) comples whose stability constant isK.M. Milburn, J . Aweit. Clzcwz. SOC., 1957, 79, 537.J < . M. Milburn, J . Anger. Chenz. SOC., 1955, 77, 1362; D. D.H. Wentlt aiid H. Strehlov-, 2. Elekbroclzena., 1060, 34, 131.(3)calculated for (h2'(31K12,+ k3.K-q).Perrin, J . C1i.r-n.z. SOC., 1959, 1710.ca. 4 x lo2 1 111ol-~; the absorbance then decreased withfirst-order kinetics with respect to H2cat (the reagent not inexcess). Values of the observed rate constant kobs' atdifferent experimental conditions are given in Table 2.- E 'r / //' MI/-/300 'I 21021Fe"11 / MFIGURE 1 Plots of h,,,,/[FeIII] against [FeIII] for the reaction ofFeIII with catechol at [H,cat], = 2.0 x I = 1-00h1,25-0 O C , and [HCIO,] as follows: ( a ) , 0.10; (b), 0.12; (G), 0.14;( d ) , 0.17; antl ( e ) , 0.20rrTABLE 2Values of the observed rate constant 1O2K0b,' (s-l) for theoxiclatioii of catechol with V" ions (25.0 OC, I = 1.00,[H,cat], = 1.0 :.: l W 4 ~ )[HC104]/~7-- 1 103[vv]/~ 0-50 t ) .C i O 0.70 0-85 1-001.0 1.3 1 4 2.1 2.32.0 4-0 4.2 6.0 7.43.0 7.6 8.3 13 144.0 6.1 10 11 17 195.0 9-5 17 18 23 267.0 15 26 29 38 4410.0 24 39 45 55 63The order with respect to cvv]T does not correspond to awhole number (first order was found when Vv was not inexcess). A mechanism consistent with the experimentalresults, taking into account the scheme suggested forcatechol in excess, is shown in Scheme (2). The corre-sponding kinetic expression which can be deduced, assum-ing that reactions of protonation and coniplex formationare fast compared with electron transfer, is in equation(4), where ay is given by ( 5 ) .Since K,[H+] < 1 (ref. 2) anda2 = k, + (h,'K2 + h5K3)[Hi] + h,'K2K,[H"I2 (5)K3[Hi] Q 1, equation (6) is obtained. According toequation (6), by plotting [VV]T/hobi as a function of 1/[v"]Ta straight line is obtained with intercept l/a2 and gradientl/a2P2. Such a plot, corresponding to [H'] = 1 - 0 0 ~ , i1942 J.C.S. Daltonshown in Figure 2. The average value obtained for p2from the experimental data was (3.7 f 0-7) x lo2 1 mol-l,in agreement with that obtained directly from spectro-photometric data (ca. 4 x lo2 1 mol-1).2 a2 Values showedThe decay in absorbance is first order in [H,quin] (thereagent not in excess) and values of the pseudo-first-orderrate constant hob;‘ are given in Table 3.It is to be notedthat Kobs” is proportional to [VVIn, where I I is a numberSCHEME 2 K , = [~(OH),2t]/[V(OH),+jiH+j, 1C3 = [V(H,Cat)’+]/[V(H,cat)+][H+], and Pr and P3 are formationconstants of the intermediate complexesa dependence on [H’], indicating that, in the [H+] rangeinvestigated, paths (4’) and/or ( 5 ) are preferred. Accord-ingly (h4’K, + k5K,) has been estimated to be 74 l2 m o P s-l.between 1 and 2. By applying to this system the consider-ations previously suggested, Scheme (3) can be proposed.The corresponding kinetic expression which can be deducedVv - Qui$zoZ.-Even in excess of VT, as previously with the same previous assumptions is as in equation (7)reported for reaction in the presence of excess of quin01,~an initial rise in absorbance followed by a decay was1t-0 5 101f21V “I -’ / M-’FIGURE 2Vvionswithcatecholat [H,cat], = 1.0 x1 = 1 .0 0 ~ and 25-0 “CPlot of [VV]T/k&e’ against l/[VV] for the reaction of[HClO,] = 1.00,TABLE 3Values of the pseudo-first-order rate constant ko&” (0) forthe oxidation of quinol by V” ions (25.0 OC, 1 = 5.00,[H,quin] = 2.0 x 10-4~~)[ HClO,] /M1 0 3 [ f T v ] ~ / M ‘ 0.30 0.50 1-00 2-00 3.00 5‘00’1.5 3.4 6.3 6.2 10 16 262.0 4.5 7-1 8.6 14 23 372.5 5.7 9.0 12 17 30 463-0 7.1 11 15 23 40 603.5 8.4 14 18 26 55 814.0 10 16 22 33 58 965.0 13 21 28 46 80 1387-0 20 30 42 69 118 207observed a t 370 nm.intermediate complexes.This can be ascribed to formation ofwhere K,P5 = @,IT5.-4ssuniing that [V(OH),+] = [VVlT(K4[Hf] < 1 (refs. 2 and 4)}, equation (8) is then obtained.whereandy = K6 -+ k,’K2[Hf] (9)a3 = K , + (k,’K4 + KSK5)[H7-] + hs’K4K,[H’I2 (10)Figure 3 shows plots of the experimental data accordingto equation (8), calculated by using values of P4 (5.5 1 mol-1) //FIGURE 3 Plot of left-hand side of equation (8) against [Vv],for the reaction of Vv ions and quinol a t I = 5 . 0 0 ~ , 25.0 “C,and [HClO,] as follows: ( a ) , 5.00; ( b ) , 3.00; ( c ) , 2.00; ( d ) , 1.00;(e), 0.50; and (f), 0 . 3 0 ~and K, (3.2 1 rnol--l) reported by Wells and Kuritsyn.4Figure 4 shows the dependence of a3/[H’] on [H+]; it ca1974be seen that path (7) does not contribute significantly.From the intercept (k,’K, + k,KJ = (5.6 f 2.2) x lo4l2 m o P s-l and k,’K, = (9.0 & 3.6) x lo3 l2 m o P s-l canbe obtained.The paths (7’) and (8) are kinetically indistin-guishable. Intercepts of the plots in Figure 3, plotted as aVO‘ ‘(aq) + p - qnofast [V (0 H Id ( aq I I V02’(aq) t Hquin’H* It K5/ig‘V02*(aq) +- Hquin’showed a linear dependence on [H+]-l. The above observ-ations are the same as those found previously with excessof catech01.~Therefore the mechanism suggested for the reaction inthe presence of excess of catechol can be assumed to applyI ifast/[V(OHld*(aqlV 0 2 * ( a q ) + p-qnoSCHEME 3 = [I-(OH),*+]/[V(OH),+][H+], K , = [Vv(H,q~in)~+]/[VV(H,quin)+][H+], and p4 and p5 are formationconstants of the complexes [VV(H,quin)]+(aq) and [V(H,q~in)]~+(aq)functiun of [H’], ga\-e k 6 = (3.6 +- is:) x lo2 and A,‘ =( 1 .1 & 0.4) 1O2 s-l [cf. (3.3 = 0.0) x lo2 and (1-3 f0.3) s l o 2 5 - l respecti\-ely *I!-4-2 4[H’] /MFIGURE 4 Plot of a3/[H+] against [H+] for the reactionbetween Vv ions and quinol at I = 5 . 0 0 ~ and 25-0 “CTllI1 + CntechoZ.-Yo spectrophotometric evidence ofcomplex formation was found. Kinetic runs were moni-tored a t 390 nm. The absorbance increased with time andthe kinetics were first order in [H,cat]. Pseudo-first-orderrate constants kObS”’ are given in Table 4. The reactionwas also first order in [Tlr1IIT. The expression kO’”(l +K,[H+]-I), where KO”’ is the second-order rate constantG. Biedermann, REC.Trav. chinz., 1956, 75, 716; T. E.Rogers and G. M. Waind. Trans. Favadav SOC.. 1961. 5’9. 1360.also when TlIII ions are in excess. From the presentexperiments, K , was found to be (1.3 & 0-1) x lo5 1 mol-l s-l,TABLE 4ITalues of the obserx-ed rate constant Kok,;” (s-l) for theoxidation of catechol with TllIr ions (25-0 “C, I = 2-00,[H,cat], = 1.0 :.: 10-41r)~HC~OJM1 O3[TPI1J/~r 1 *00 1-50 2-061.0 8.7 6.4 4.91.5 14 9.2 7.69.0 18 12 102.5 23 15 123.0 30 19 15in agreement with the value given previ~usly.~value, as previously, was assumed for K,.3r 9,(The sameT I 3 (aq ) - H2cat Flm( H2cat I] 2 * (aq) & k TL*(aq)+ o-qno/ [TiOH]”(aq)DISCUSSIOKThe experimental results suggest that in the oxidationof catechol with FeIII ions in excess, the rate-determin-ing step is mainly oxidation of intermediate complexes,in equilibrium with the reactants, by means of FeIII;there are two alternative paths, kinetically indistinguish-able, i.e.the reaction of the ion [FeOHI2+(aq) with[Fe(Hcat)12+(aq) and/or Fe3+(aq) with [Fe(cat)] +(as).Of the few papers on the oxidation of organic compoundsbv FeIII ions, a first-order dependence on the reactant1944 J.C.S. Daltonwas observed with acetoin lo (the experiments werecarried out with a large excess of organic substrate). I tis noteworthy that in the oxidation of quinol with Fe1I1ions in excessll (where no evidence of intermediatecomplexes was found), no term in [I;e1I1I2 was found.However, experimental data on the oxidation of sulphiteion (in presence of an excess of metal ion) are in agree-ment with an equation which includes a term pro-portional to [Fe111]2.12When the oxidation of catechol is carried out withVv ions in excess, there are two alternative paths forthe rate-determining step, i.e.reaction of [V(OH),I2+(aq)ions with [VV(H2cat)]+(aq) and/or [V(OH),]+(aq) with[Vv(H2cat)l2+(aq). Similar beliaviour is proposed forthe reaction of quinol with an excess of Vv ions. Asecond-order dependence on Vv has already been foundin other oxidation reactions (forma1deliyde,l3 chloral,14nialonic acid,15 and arsenious acid lG). This kineticbehaviour was explainecl by assuming formation ofintermediate complexes such as (I).r H 1(1)10 J . I<. Thomas, G.Trudel, arid S . Bywatcr, J . Phys. Ckciiz.,l1 J. H. Baxendale, H. I<. Hardy, aiirl I,. I-I. Sutcliffe, Tvaizs.12 D. G. Karraker, J . Phys. CAem., 1963, 67, 871.1960, 64, 51.Favaday Soc., 1951, 47, 963.Kinetic dependences on the reactants in the oxidationof catechol with TIrr1 ions do not change in the presenceor absence of an excess of the oxidant.The occurrence of these different inechanisins doesnot seem to depend simply on the number of electronstransferred. However, it is relevant that the redoxreaction occurs through interaction of the oxidizingmetal ion and the organic substrate, with the possibilityof intermediate complex formation, tlie stability ofwhich depends both on the cation and the organicmolecule. In particular, for one-electron oxidants, it issuggested that, when formation of a stable complexdoes not occur (Le. for FeTJr and quinol), the kineticbehaviour does not change in the presence or absence ofan excess of the oxidant. On the other hand, whenintermediate-complex formation occurs (i.e. with l'eIi1 4-catechol, 17" + catechol, ancl I T I T + quinol) two com-petitive paths are expected, one being direct decom-position of the complex witli formation of tlie productsof reaction, the other being further reaction of thecomplex witli another oxidizing ion. Additional workon a number of redox reactions is required in order toclarify this point.W e thank the Consiglio Xazionnle tlelle I<iccrclie (12onit~)for support.[3/3390 ~L'('cPIL?('LZ, 21st X O U ~ Y J L ~ ~ ~ , 197:;~'1. J . licriip arid JY. -4. Waters, Proc. l i o ~ f . Soc., I963, A2'74,480.'I. T. I<eiiip in ' Comprehensive: Chcinical Kinetics,' eds.C . H. B&iford and c' F. H. Tipper, El,cvicr, ,\nisterdam, 1972,1.5 T. J . Iicinp and JV. -4. W a t e r s , J . Chrm. Soc , 1964, 1610.l6 31. Robtclskjr ancl Al. Glasner, J . Awter. Chem. Soc., 1!343, 64,vol. 7 , p. 379.1462