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Kinetics of the redox reaction between thallium(III) and 1-hydroxy-2-methoxybenzene in aqueous perchlorate media
J. inorg, nuc£ Chem., 1975, Vol. 37, pp. 537-540. Pergamon Press. Printed in Great Britain
KINETICS OF THE REDOX REACTION BETWEEN THALLIUM(Ill) AND 1-HYDROXY-2-METHOXYBENZENE IN AQUEOUS PERCHLORATE MEDIA EDOARDO MENTASTI, EZIO PELIZZETTI, EDMONDO PRAMAURO and GIANFRANCO GIRAUDI Istituto di Chimica Analitica dell'Universit~ di Torino, Torino, Italy
(Received 6 March 1974) Abstract--The reaction of TI(III) with 1-hydroxy-2-methoxybenzene has been studied in aqueous perchloric acid, in the range 0.10 ~<[HC10,] ~<2.00 M, at I = 2.00 M, at 15.0, 25.0 and 32.0°C, with the stopped-flow technique. The reaction stoicheiometry has been found to be consistent with: Tl(III) + G + H:O ~ TI(I) + o -quinone + CH3OH + 2H ÷. The overall rate has been found to be first order with respect to both reagents, either in the presence of an excess of oxidant or of the organic substrate. TI(I), even in excess, showed no significant effect on the rate. The acidity dependence of the overall reaction rate has been explained in terms of two reaction paths; these paths have been discussed in terms of alternative mechanisms, involving differently protonated species, and the results compared to previous findings on the reaction of catechol with oxidizing metal ions. INTRODUCTION THE KINETICS of oxidation of 1,2-dihydroxybenzene (catechol) with different oxidizing reagents, such as Fe(III)[la], V(V)[Ib], and TI(III)[2], has been previously investigated in this laboratory. It has been shown that, when the oxidation is performed with Fe(III) or V(V), the kinetic dependence on reagents is different according to which of the reagents (organic substrate or oxidizing agent) is in excess, unlike in the case of oxidation with TI(III) where the kinetic dependence did not change, irrespective of the reagent in excess [3]. In the former case, the different kinetic dependence was explained in terms of the formation of intermediate complexes. When the oxidizing agent is in defect, there is a rate-determining decomposition of the complexes giving the semiquinone which undergoes a further fast oxidation with the oxidant, whereas, when the oxidizing agent is in excess, its direct reaction with the complexes becomes kinetically relevant. On the contrary, in the case of TI(III), the absence of such a behaviour did not allow to point out whether the reaction could advance through an outer-sphere or an inner-sphere mechanism. In the latter case the participation of excess oxidant is not needed for the reaction progress, as TI(III) is a two-electron oxidant. In order to achieve additional data on the kinetic aspects of the above redox reactions, it seemed of interest to investigate the effect of the substitution of one of the hydroxy groups of catechol with a methoxy group on its oxidation by means of TI(III). EXPERIMENTAL
Reagents Sodium perchlorate and perchloric acid (analytical grade chemicals) were used for bringing the solutions to the desired
ionic strength and acidity. All the solutions were brought to ionic strength I=2.00M, and the investigated acidity range was 0.10 ~<[HCIO4]~<2.00 M. l-hydroxy-2-methoxybenzene (Guaiachol, referred to as G) solutions were prepared daily by dissolution in water of weighted amounts of the analytical grade compound. TI(III) solutions were obtained by dissolution of T1203 in aqueous perchloric acid and TI(III) content was determined by Edta titrations (1-(2-pyridilazo)2-naphthol as indicator). TI(I) perchlorate solutions were obtained by dissolution of weighted amounts of T12CO3 in aqueous perchloric acid. Double-distilled water was used for all the solutions.
Procedure The kinetic runs were performed with a stopped flow Durrum-Gibson spectrophotometer at 390nm (wavelength of maximum absorption of the oxidation product, the other reactants giving no significant absorption in the investigated concentration ranges), and the transmittance variations with time were photographed on the storage screen of a 564 Textronix oscilloscope. Absorption spectra at equilibrium were recorded by means of an EPS-3T Hitachi-Perkin-Elmer spectrophotometer. The equilibrium constants for the first and second T] 3+ hydrolysis, K~ and K2 respectively, have been obtained, for the present experimental conditions, from literature data[4], as previously described [2]. The dependence of Ks with temperature is not known so that it has been assumed to be invariant in the temperature range investigated (15 +32°); this assumption does not give rough approximations owing to the fact that terms containing K2 do not appreciably modify the experimental data treatment (see Eqns 4 and 5). RESULTS AND DISCUSSION Stoicheiometry The spectrum of the final reaction product showed a maximum absorption at 390 nm, and did not differ from that of o-quinone (qno), as expected. For evaluating the stoicheiometry, known amounts of TI(III) were mixed 537
538
E. MENTASTIet al.
with an excess of guaiachol and the concentration of qno formed was determined spectrophotometrically at 390 nm (eq,o = 1.46 x 103 1 mol-l cm -1, [la]) where no interference has been found either from unreacted guaiachol or from TI(I). In the investigated acidity range and at 25.0°C it was found that A[TI(III)]/A[qno] = 1'02-+0'03. Therefore the overall reaction can be represented by: TI(III) + G + H20 ~ TI(I) + qno + CH3OH + 2H ÷.
T13":
'
K~TH + TIOH~
~T ,
K251'H+
k 1 \'
' (TI(III)-G)2+
1~
TI(0Hh+.. ~
~2 T I + + qno + CH,0H
Z
) (TI(III)-G)~
Table 1. Pseudo-first-order rate constants kob, (see -1) for TI(III)guaiachol reaction, in various experimental conditions. ( I = 2.00 M, [TI(III)] = 2.0 × 10-" M)
Kinetics of reaction The kinetic runs performed with at least a five-fold excess of guaiachol by respect to TI(III) showed first-order dependence in respect of the reagent in deficit (see Fig. 1). The corresponding pseudo-first-order rate constants, kobs,are collected in Table 1 together with the other experimental data. The dependence of kobson [G]r (the stoicheiometric concentration of guaiachol) indicates
103 x [G]~[HC10,](mol 1-') (moll -J ) 0.10 0.20 0.30 0.50 0.70 1.00 1.50 2.00
1.0
T = 15.0°C 1.3 1.15
1.2
1.5
2.0
1.7
1.8
1.65
2.0 3.0 4.0 5.0
2.6 3.8 4.95 5.8 T = 25.0°C 1.3" 1.3' 2 . 0 5 2.5* 3.0* 3.75 3.5 4.05 5.2 4.5 5.3
2.4 3.15 3.85 4.8
2.35 3.6 4.9 5.8
2.1 3.5 4.4 5.25
1.3' 2.75* 4.5 4.85
1.45' 2.75* 3.9* 5.55* 6.5* 8.05*
0.5 1.0 1.5 2.0 2.5 3.0 4.0 5.0
I0
4 i
08
1.0 1.5 2.0 3.0 4.0 5.0
06
,
' (TI(III)-G)'÷~
I
I
200
Time,
I
I
1.9 3.1 4.1
6.0 7.35 7.6 7.5 10 10.5
7.6 11
1.25' 2.4* 3.6* 4.7 6.25 7.2 9.3 11
8.2 10.5
T = 32.0°C 3.9 4.6 6.15 6.8 7.95 8.85 11 12 14 16 18
1.1
4.2 6.4 7.75 10.5 15 20
1.0' 2.2* 3.85* 4.75 6.9 9.0 11.5
4.05 5.9 7.5 11 15.5 17.5
*For these runs: [TI(III)] = 1.0 x 10-4 M.
400
msec
2o-
+I'
Fig. 1. Plots of -log (A® - A , ) as a function of time for some typical runs. [HCIO4]= 1.00 M, [TI(III)] = 1.0 × 10-4 M, T = 25.0°C, I=2.00M, [G]T: A, 3.0x 10-3; B, 2.0x 10-3; C, 1.0×10-3; D, 0.5 x 10-3 M.
first order dependence also on the organic substrate (see Fig. 2) at constant acidity. Other experiments, performed with [HC10,] = 1.00 M, and TI(III) in excess, showed the same first order dependence both on the organic substrate and on the metal ion. The second order rate constant was in good agreement with that calculated from experiments with TI(III) in deficit. Further experiments carried out with an excess of TI(I) (this reagent was added to guaiachol before mixing with the TI(III) solution) showed that this addition does not affect the reaction rate, thus indicating that the overall reverse reaction can be ruled out. In order to explain the experimental data, the following reactions, in which account is taken of the participation of hydroxylated species of TI(III), were considered:
q-
IO
j '
I
I O
20 IO 3 x[G]r,
i
40 mole
t -j
Fig. 2. Plots of kob, (sec-') against [G]r, in 1.00 M perchloric acid at different temperatures: T: O, 15.0°C; ©, 25'0°C; +, 32'0°(]. I=2.00M.
Kinetics of the redox reaction between thallium(III) and l-hydroxy-2-methoxybenzene With the assumption that the protolytic equilibria corresponding to vertical arrows in the scheme are fast as compared to the electron transfer steps, the following kinetic equation can be derived: d[TI"P]T k' + k"K~h + k'"K~K:h 2 [G]r • [TI"]T dt
(4)
1 + Klh + KIK2h 2 k r
ko~,~= ko[G]T =
6O
--
o
(6)
20
, 0
I 1.0
2.5
I 2.0
TI3÷ does not become a reactive species, as is true for guaiachol, whereas an inner-sphere mechanism could explain such differences on the basis of the different co-ordination capability of the two organic substrates with respect to TI(III). An alternative explanation of the different behaviour of guaiachol and catechol towards TI(III) on the basis of an outer-sphere intermediate might attribute it to the basicity of the methoxy group of the former compound compared to the hydroxy group of the latter. In the former case the first reaction step is an interaction between TI(III) and guaiachol with outersphere complex formation; in this fast step proton transfer is possible from a water molecule co-ordinated to the metal ion, to the ethereal oxygen, so that, if the reaction partners were T13÷ and G, the proton transfer would lead to the formation of TIOH 2÷ and GH ÷. Such a proton transfer catalyzed mechanism through an "inner hydrolysis" was advanced by Eigen for Fe(III) complexes formation [9]. However, some preliminary measurements performed on the oxidation on 1,4-dihydroxybenzene (quinol) and its monomethylether by TI(III)[10], have shown kinetic behaviour similar to that of guaiachol (the overall redox reaction proceeds through two distinguishable paths corresponding to the first and the second term of Eqn (6)). Taking into account that an acid catalyzed mechanism involving protonation of the substrate appears to be very unlikely in the case of unetherified dihydroxybenzenes, it
0.10 0.20 0.30 0.50 0.70 1.00 1.50 2.00
(15.0°C) (25.0°C) 2.1 (32.0°C)
I
[H÷], m o l e t - l Fig. 3. Plots of k~ as a function of [H÷] at different temperatures. I = 2.00M.
Table 2. Second order rate constants ko (1 mol-' sec-') for Tl(III)-Guaiachol reaction, at various acidities and temperatures (I = 2.00 M)
103xko
25"0°C
(5)
The left-hand side of Eqn (6) plotted as a function of h -~ gave a linear relation with significant intercepts, at the temperatures investigated (see Fig. 3). This suggests that two contributions, the first linearly dependent and the second independent of [H ÷] in Eqn (6), are responsible for the overall reaction progress: the former contribution concerns path 1, whereas the latter corresponds to path 2 of the above scheme. These paths are assumed in the scheme to be reactions which give the final products through intermediate species formation; these species (as pointed out in the introduction) could be either outer-sphere complexes (such as ion-dipole associations) or inner sphere complexes. No spectrophotometric or kinetic evidence has been reached confirming this latter possibility, but TI(III) has been previously found to give redox reactions through intermediate inner-sphere complexes (for example with cyclohexanol [5], formic acid [6], oxalic acid [7]). Thus path 1 could be ascribed either to the reaction between T13÷and guaiachol, with an outer-sphere mechanism, or through a (TI(III)-G)3÷ species formation; and similarly for path 2 two undistinguishable mechanisms can be proposed. These findings can be compared with the behaviour of TI(III) in the redox reactions with catechol[2] and its 3-methyl, 4-methyl and 4-chloro derivatives [8]. These reactions proceed by a single path corresponding to path 2 or the present scheme. Comparing with the present experiments, an outer-sphere mechanism does not explain why, in the case of catechol,
[HCIO4](molel-'):
/ "O'C/
L
where: K~ = [T1OH÷÷][H+]/[TI3÷], K2 = [TI(OH)2+][H÷]/[T1OH2+], h = [H÷] -~, and ko is the second order rate constant. Equations (4) and (5) are in agreement with the observed first order dependence on both reactants, and Table 2 collects the computed values of the second order rate constant ko. In order to explain the observed dependence of ko on h, Eqn (5) can be rewritten as follows: k~ = ko(h '+ K, + KIK2h) = k ' h - ' + k"K1 + k"'KiK2h.
32
0
40
+ k IrKib + K m K1K2h 2 rG 1 -1--~K , h + ~ K ~ h 2 t Jr
--
539
1.2 2.6 3.8
1.0~ 1.2 1.1 2'65 2.65 2.65 2.4 4.05 3.9 3.7
2.4
540
E. MENTASTIet al. Table 3. Specific rate constants and activation parameters for TI(III) oxidations (T = 25.0°C, I = 2.00M) Compound 1-hydroxy-2-methoxybenzene 1-hydroxy-2-methoxybenzene 1,2-dihydroxybenzene
Rate const. (1 mole-' sec-1)
AH~ AS~ kJ mole-1 J K-' mole ' Reference
k' 2.5 -+0.3 x 103 +46-+5 k" 4.0 -+0.4 x 10 3 +50 -+5 k" 1.4-+0.1× 105 +46-+5
seems reasonable to prefer the mechanism via an inner-sphere intermediate. However it is worth mentioning that, unlike the previously investigated oxidations of dihydroxybenzenes with one electron oxidizing agents[l,3] (where the inner-sphere mechanism was supported by the different kinetic dependence on the oxidizing agent whether in deficit or in excess, and in some instances by direct spectrophotometric evidence), no other direct or indirect evidence of the suggested mechanism can be given for the present reaction. Moreover, in the case of Tl(III), the above findings do not bnng additional information about the mechanism, due to the fact that Tl(III) is a two-electron oxidant: the reaction order should not change (when the reagent in excess is changed) either with an outer-sphere or with an inner-sphere mechanism (no additional oxidant is necessary for the reaction progress). Table 3 collects k' and k" values together with the corresponding activation parameters as well as the ones previously found for the reaction between TI(III) and catechol. The kinetic parameters have been evaluated by taking from the literature the data concerning the first hydrolysis of TI3+. According to these data the corresponding AH is rather small (ca. 4 kJ mol-l). However other authors have raised doubts about the reliability of this value on account of the higher AH (20 +40 kJ mo1-1) for other transition metal ions such as In(III), U(IV)[ll]. If this hypothesis should be true, the AH:~ values, listed in Table 3 would be corrected so that the corresponding reactions should occur with very low enthalpies of activation. Some kinetic investigations on redox reactions of TI(III) pointed out that TI(III) can react in two oneelectron successive steps, with TI(II) formation as intermediate[5,12]. In the present case a two step
-16-+ 15 +4 -+15 +26-+ 15
This work This work [2]
mechanism which could involve TI(II) and the guaiachol semiquinone formation, cannot be ruled out. With this hypothesis the second electron exchange should then be very fast without diffusion of the intermediate species, as both TI(II) and the semiquinone are highly reactive; thus the two steps would occur within the "solvent cage"; such a mechanism is kinetically equivalent and undistinguishable from a single two-electron exchange step. REFERENCES 1. (a) E. Mentasti, E. Pelizzetti and G. Saini, J. chem. Soc. Dalton 2609, (1973); (b) E. Pelizzetti, E. Mentasti and G. Saini, Gazzetta, in press. 2. E. Pelizzetti, E. Mentasti and G. Saini, J. chem. Soc. Dalton. 721, (1974). 3. E. Pelizzetti, E. Mentasti and G. Saini,J. chem. Soc. Dalton. In press. 4. G. Biedermann,Arkiv Kemi, 5,441 (1953);Rec. tray. chim. 75, 716, (1956);G. Biedermann and L. G. Sill~n,Arkiv Kemi 10, 103, (1956); T. E. Rogers and G. M. Waind, Trans. Faraday Soc. 57, 1360, (1961). 5. J. S. Littler, J. chem. Soc. 2190, (1%2). 6. H. N. Halvorson and J. Halpern, J. Am. chem. Soc. 78, 5562 (1956);J. Halpern and S. M. Taylor, Discuss. Faraday Soc. 29, 1741, (1960). 7. L. B. Monsted, O. Monsted and G. Nord, Trans. Faraday Soc. 66, 936, (1970). 8. E. Mentasti, E. Pramauro, M. E. Carlotti and E. Pelizzetti, Atti Accad. Sci. Torino., In press. 9. M. Eigen, Advances in the Chemistry of the Co-ordination Compounds (Edited by S. Kirshner), p. 371. Macmillan,New York N.Y. (1961). 10. E. Pramauro and C. Baiocchi, Atti Accad. Sci. Torino. In press. 11. N. A. Daugherty, J. Am. chem. Soc. 87, 5026, (1965); A. C. Harkness and J. Halpern, J. Am. chem. Soc. 81, 3526,(1959). 12. C. E. Burchill and G. G. Hickling, Can. J. Chem. 48, 2466, (1970).
KINETICS OF THE REDOX REACTION BETWEEN THALLIUM(Ill) AND 1-HYDROXY-2-METHOXYBENZENE IN AQUEOUS PERCHLORATE MEDIA EDOARDO MENTASTI, EZIO PELIZZETTI, EDMONDO PRAMAURO and GIANFRANCO GIRAUDI Istituto di Chimica Analitica dell'Universit~ di Torino, Torino, Italy
(Received 6 March 1974) Abstract--The reaction of TI(III) with 1-hydroxy-2-methoxybenzene has been studied in aqueous perchloric acid, in the range 0.10 ~<[HC10,] ~<2.00 M, at I = 2.00 M, at 15.0, 25.0 and 32.0°C, with the stopped-flow technique. The reaction stoicheiometry has been found to be consistent with: Tl(III) + G + H:O ~ TI(I) + o -quinone + CH3OH + 2H ÷. The overall rate has been found to be first order with respect to both reagents, either in the presence of an excess of oxidant or of the organic substrate. TI(I), even in excess, showed no significant effect on the rate. The acidity dependence of the overall reaction rate has been explained in terms of two reaction paths; these paths have been discussed in terms of alternative mechanisms, involving differently protonated species, and the results compared to previous findings on the reaction of catechol with oxidizing metal ions. INTRODUCTION THE KINETICS of oxidation of 1,2-dihydroxybenzene (catechol) with different oxidizing reagents, such as Fe(III)[la], V(V)[Ib], and TI(III)[2], has been previously investigated in this laboratory. It has been shown that, when the oxidation is performed with Fe(III) or V(V), the kinetic dependence on reagents is different according to which of the reagents (organic substrate or oxidizing agent) is in excess, unlike in the case of oxidation with TI(III) where the kinetic dependence did not change, irrespective of the reagent in excess [3]. In the former case, the different kinetic dependence was explained in terms of the formation of intermediate complexes. When the oxidizing agent is in defect, there is a rate-determining decomposition of the complexes giving the semiquinone which undergoes a further fast oxidation with the oxidant, whereas, when the oxidizing agent is in excess, its direct reaction with the complexes becomes kinetically relevant. On the contrary, in the case of TI(III), the absence of such a behaviour did not allow to point out whether the reaction could advance through an outer-sphere or an inner-sphere mechanism. In the latter case the participation of excess oxidant is not needed for the reaction progress, as TI(III) is a two-electron oxidant. In order to achieve additional data on the kinetic aspects of the above redox reactions, it seemed of interest to investigate the effect of the substitution of one of the hydroxy groups of catechol with a methoxy group on its oxidation by means of TI(III). EXPERIMENTAL
Reagents Sodium perchlorate and perchloric acid (analytical grade chemicals) were used for bringing the solutions to the desired
ionic strength and acidity. All the solutions were brought to ionic strength I=2.00M, and the investigated acidity range was 0.10 ~<[HCIO4]~<2.00 M. l-hydroxy-2-methoxybenzene (Guaiachol, referred to as G) solutions were prepared daily by dissolution in water of weighted amounts of the analytical grade compound. TI(III) solutions were obtained by dissolution of T1203 in aqueous perchloric acid and TI(III) content was determined by Edta titrations (1-(2-pyridilazo)2-naphthol as indicator). TI(I) perchlorate solutions were obtained by dissolution of weighted amounts of T12CO3 in aqueous perchloric acid. Double-distilled water was used for all the solutions.
Procedure The kinetic runs were performed with a stopped flow Durrum-Gibson spectrophotometer at 390nm (wavelength of maximum absorption of the oxidation product, the other reactants giving no significant absorption in the investigated concentration ranges), and the transmittance variations with time were photographed on the storage screen of a 564 Textronix oscilloscope. Absorption spectra at equilibrium were recorded by means of an EPS-3T Hitachi-Perkin-Elmer spectrophotometer. The equilibrium constants for the first and second T] 3+ hydrolysis, K~ and K2 respectively, have been obtained, for the present experimental conditions, from literature data[4], as previously described [2]. The dependence of Ks with temperature is not known so that it has been assumed to be invariant in the temperature range investigated (15 +32°); this assumption does not give rough approximations owing to the fact that terms containing K2 do not appreciably modify the experimental data treatment (see Eqns 4 and 5). RESULTS AND DISCUSSION Stoicheiometry The spectrum of the final reaction product showed a maximum absorption at 390 nm, and did not differ from that of o-quinone (qno), as expected. For evaluating the stoicheiometry, known amounts of TI(III) were mixed 537
538
E. MENTASTIet al.
with an excess of guaiachol and the concentration of qno formed was determined spectrophotometrically at 390 nm (eq,o = 1.46 x 103 1 mol-l cm -1, [la]) where no interference has been found either from unreacted guaiachol or from TI(I). In the investigated acidity range and at 25.0°C it was found that A[TI(III)]/A[qno] = 1'02-+0'03. Therefore the overall reaction can be represented by: TI(III) + G + H20 ~ TI(I) + qno + CH3OH + 2H ÷.
T13":
'
K~TH + TIOH~
~T ,
K251'H+
k 1 \'
' (TI(III)-G)2+
1~
TI(0Hh+.. ~
~2 T I + + qno + CH,0H
Z
) (TI(III)-G)~
Table 1. Pseudo-first-order rate constants kob, (see -1) for TI(III)guaiachol reaction, in various experimental conditions. ( I = 2.00 M, [TI(III)] = 2.0 × 10-" M)
Kinetics of reaction The kinetic runs performed with at least a five-fold excess of guaiachol by respect to TI(III) showed first-order dependence in respect of the reagent in deficit (see Fig. 1). The corresponding pseudo-first-order rate constants, kobs,are collected in Table 1 together with the other experimental data. The dependence of kobson [G]r (the stoicheiometric concentration of guaiachol) indicates
103 x [G]~[HC10,](mol 1-') (moll -J ) 0.10 0.20 0.30 0.50 0.70 1.00 1.50 2.00
1.0
T = 15.0°C 1.3 1.15
1.2
1.5
2.0
1.7
1.8
1.65
2.0 3.0 4.0 5.0
2.6 3.8 4.95 5.8 T = 25.0°C 1.3" 1.3' 2 . 0 5 2.5* 3.0* 3.75 3.5 4.05 5.2 4.5 5.3
2.4 3.15 3.85 4.8
2.35 3.6 4.9 5.8
2.1 3.5 4.4 5.25
1.3' 2.75* 4.5 4.85
1.45' 2.75* 3.9* 5.55* 6.5* 8.05*
0.5 1.0 1.5 2.0 2.5 3.0 4.0 5.0
I0
4 i
08
1.0 1.5 2.0 3.0 4.0 5.0
06
,
' (TI(III)-G)'÷~
I
I
200
Time,
I
I
1.9 3.1 4.1
6.0 7.35 7.6 7.5 10 10.5
7.6 11
1.25' 2.4* 3.6* 4.7 6.25 7.2 9.3 11
8.2 10.5
T = 32.0°C 3.9 4.6 6.15 6.8 7.95 8.85 11 12 14 16 18
1.1
4.2 6.4 7.75 10.5 15 20
1.0' 2.2* 3.85* 4.75 6.9 9.0 11.5
4.05 5.9 7.5 11 15.5 17.5
*For these runs: [TI(III)] = 1.0 x 10-4 M.
400
msec
2o-
+I'
Fig. 1. Plots of -log (A® - A , ) as a function of time for some typical runs. [HCIO4]= 1.00 M, [TI(III)] = 1.0 × 10-4 M, T = 25.0°C, I=2.00M, [G]T: A, 3.0x 10-3; B, 2.0x 10-3; C, 1.0×10-3; D, 0.5 x 10-3 M.
first order dependence also on the organic substrate (see Fig. 2) at constant acidity. Other experiments, performed with [HC10,] = 1.00 M, and TI(III) in excess, showed the same first order dependence both on the organic substrate and on the metal ion. The second order rate constant was in good agreement with that calculated from experiments with TI(III) in deficit. Further experiments carried out with an excess of TI(I) (this reagent was added to guaiachol before mixing with the TI(III) solution) showed that this addition does not affect the reaction rate, thus indicating that the overall reverse reaction can be ruled out. In order to explain the experimental data, the following reactions, in which account is taken of the participation of hydroxylated species of TI(III), were considered:
q-
IO
j '
I
I O
20 IO 3 x[G]r,
i
40 mole
t -j
Fig. 2. Plots of kob, (sec-') against [G]r, in 1.00 M perchloric acid at different temperatures: T: O, 15.0°C; ©, 25'0°C; +, 32'0°(]. I=2.00M.
Kinetics of the redox reaction between thallium(III) and l-hydroxy-2-methoxybenzene With the assumption that the protolytic equilibria corresponding to vertical arrows in the scheme are fast as compared to the electron transfer steps, the following kinetic equation can be derived: d[TI"P]T k' + k"K~h + k'"K~K:h 2 [G]r • [TI"]T dt
(4)
1 + Klh + KIK2h 2 k r
ko~,~= ko[G]T =
6O
--
o
(6)
20
, 0
I 1.0
2.5
I 2.0
TI3÷ does not become a reactive species, as is true for guaiachol, whereas an inner-sphere mechanism could explain such differences on the basis of the different co-ordination capability of the two organic substrates with respect to TI(III). An alternative explanation of the different behaviour of guaiachol and catechol towards TI(III) on the basis of an outer-sphere intermediate might attribute it to the basicity of the methoxy group of the former compound compared to the hydroxy group of the latter. In the former case the first reaction step is an interaction between TI(III) and guaiachol with outersphere complex formation; in this fast step proton transfer is possible from a water molecule co-ordinated to the metal ion, to the ethereal oxygen, so that, if the reaction partners were T13÷ and G, the proton transfer would lead to the formation of TIOH 2÷ and GH ÷. Such a proton transfer catalyzed mechanism through an "inner hydrolysis" was advanced by Eigen for Fe(III) complexes formation [9]. However, some preliminary measurements performed on the oxidation on 1,4-dihydroxybenzene (quinol) and its monomethylether by TI(III)[10], have shown kinetic behaviour similar to that of guaiachol (the overall redox reaction proceeds through two distinguishable paths corresponding to the first and the second term of Eqn (6)). Taking into account that an acid catalyzed mechanism involving protonation of the substrate appears to be very unlikely in the case of unetherified dihydroxybenzenes, it
0.10 0.20 0.30 0.50 0.70 1.00 1.50 2.00
(15.0°C) (25.0°C) 2.1 (32.0°C)
I
[H÷], m o l e t - l Fig. 3. Plots of k~ as a function of [H÷] at different temperatures. I = 2.00M.
Table 2. Second order rate constants ko (1 mol-' sec-') for Tl(III)-Guaiachol reaction, at various acidities and temperatures (I = 2.00 M)
103xko
25"0°C
(5)
The left-hand side of Eqn (6) plotted as a function of h -~ gave a linear relation with significant intercepts, at the temperatures investigated (see Fig. 3). This suggests that two contributions, the first linearly dependent and the second independent of [H ÷] in Eqn (6), are responsible for the overall reaction progress: the former contribution concerns path 1, whereas the latter corresponds to path 2 of the above scheme. These paths are assumed in the scheme to be reactions which give the final products through intermediate species formation; these species (as pointed out in the introduction) could be either outer-sphere complexes (such as ion-dipole associations) or inner sphere complexes. No spectrophotometric or kinetic evidence has been reached confirming this latter possibility, but TI(III) has been previously found to give redox reactions through intermediate inner-sphere complexes (for example with cyclohexanol [5], formic acid [6], oxalic acid [7]). Thus path 1 could be ascribed either to the reaction between T13÷and guaiachol, with an outer-sphere mechanism, or through a (TI(III)-G)3÷ species formation; and similarly for path 2 two undistinguishable mechanisms can be proposed. These findings can be compared with the behaviour of TI(III) in the redox reactions with catechol[2] and its 3-methyl, 4-methyl and 4-chloro derivatives [8]. These reactions proceed by a single path corresponding to path 2 or the present scheme. Comparing with the present experiments, an outer-sphere mechanism does not explain why, in the case of catechol,
[HCIO4](molel-'):
/ "O'C/
L
where: K~ = [T1OH÷÷][H+]/[TI3÷], K2 = [TI(OH)2+][H÷]/[T1OH2+], h = [H÷] -~, and ko is the second order rate constant. Equations (4) and (5) are in agreement with the observed first order dependence on both reactants, and Table 2 collects the computed values of the second order rate constant ko. In order to explain the observed dependence of ko on h, Eqn (5) can be rewritten as follows: k~ = ko(h '+ K, + KIK2h) = k ' h - ' + k"K1 + k"'KiK2h.
32
0
40
+ k IrKib + K m K1K2h 2 rG 1 -1--~K , h + ~ K ~ h 2 t Jr
--
539
1.2 2.6 3.8
1.0~ 1.2 1.1 2'65 2.65 2.65 2.4 4.05 3.9 3.7
2.4
540
E. MENTASTIet al. Table 3. Specific rate constants and activation parameters for TI(III) oxidations (T = 25.0°C, I = 2.00M) Compound 1-hydroxy-2-methoxybenzene 1-hydroxy-2-methoxybenzene 1,2-dihydroxybenzene
Rate const. (1 mole-' sec-1)
AH~ AS~ kJ mole-1 J K-' mole ' Reference
k' 2.5 -+0.3 x 103 +46-+5 k" 4.0 -+0.4 x 10 3 +50 -+5 k" 1.4-+0.1× 105 +46-+5
seems reasonable to prefer the mechanism via an inner-sphere intermediate. However it is worth mentioning that, unlike the previously investigated oxidations of dihydroxybenzenes with one electron oxidizing agents[l,3] (where the inner-sphere mechanism was supported by the different kinetic dependence on the oxidizing agent whether in deficit or in excess, and in some instances by direct spectrophotometric evidence), no other direct or indirect evidence of the suggested mechanism can be given for the present reaction. Moreover, in the case of Tl(III), the above findings do not bnng additional information about the mechanism, due to the fact that Tl(III) is a two-electron oxidant: the reaction order should not change (when the reagent in excess is changed) either with an outer-sphere or with an inner-sphere mechanism (no additional oxidant is necessary for the reaction progress). Table 3 collects k' and k" values together with the corresponding activation parameters as well as the ones previously found for the reaction between TI(III) and catechol. The kinetic parameters have been evaluated by taking from the literature the data concerning the first hydrolysis of TI3+. According to these data the corresponding AH is rather small (ca. 4 kJ mol-l). However other authors have raised doubts about the reliability of this value on account of the higher AH (20 +40 kJ mo1-1) for other transition metal ions such as In(III), U(IV)[ll]. If this hypothesis should be true, the AH:~ values, listed in Table 3 would be corrected so that the corresponding reactions should occur with very low enthalpies of activation. Some kinetic investigations on redox reactions of TI(III) pointed out that TI(III) can react in two oneelectron successive steps, with TI(II) formation as intermediate[5,12]. In the present case a two step
-16-+ 15 +4 -+15 +26-+ 15
This work This work [2]
mechanism which could involve TI(II) and the guaiachol semiquinone formation, cannot be ruled out. With this hypothesis the second electron exchange should then be very fast without diffusion of the intermediate species, as both TI(II) and the semiquinone are highly reactive; thus the two steps would occur within the "solvent cage"; such a mechanism is kinetically equivalent and undistinguishable from a single two-electron exchange step. REFERENCES 1. (a) E. Mentasti, E. Pelizzetti and G. Saini, J. chem. Soc. Dalton 2609, (1973); (b) E. Pelizzetti, E. Mentasti and G. Saini, Gazzetta, in press. 2. E. Pelizzetti, E. Mentasti and G. Saini, J. chem. Soc. Dalton. 721, (1974). 3. E. Pelizzetti, E. Mentasti and G. Saini,J. chem. Soc. Dalton. In press. 4. G. Biedermann,Arkiv Kemi, 5,441 (1953);Rec. tray. chim. 75, 716, (1956);G. Biedermann and L. G. Sill~n,Arkiv Kemi 10, 103, (1956); T. E. Rogers and G. M. Waind, Trans. Faraday Soc. 57, 1360, (1961). 5. J. S. Littler, J. chem. Soc. 2190, (1%2). 6. H. N. Halvorson and J. Halpern, J. Am. chem. Soc. 78, 5562 (1956);J. Halpern and S. M. Taylor, Discuss. Faraday Soc. 29, 1741, (1960). 7. L. B. Monsted, O. Monsted and G. Nord, Trans. Faraday Soc. 66, 936, (1970). 8. E. Mentasti, E. Pramauro, M. E. Carlotti and E. Pelizzetti, Atti Accad. Sci. Torino., In press. 9. M. Eigen, Advances in the Chemistry of the Co-ordination Compounds (Edited by S. Kirshner), p. 371. Macmillan,New York N.Y. (1961). 10. E. Pramauro and C. Baiocchi, Atti Accad. Sci. Torino. In press. 11. N. A. Daugherty, J. Am. chem. Soc. 87, 5026, (1965); A. C. Harkness and J. Halpern, J. Am. chem. Soc. 81, 3526,(1959). 12. C. E. Burchill and G. G. Hickling, Can. J. Chem. 48, 2466, (1970).