2979
Oxidation of Quinols by Hexachloroiridate(1V)
Kinetics and Mechanism of Oxidation of Quinols by Hexachloroiridate(IV) in Aqueous Acidic Perchlorate Media Ezio Pelizzetti,' Edoardo Mentasti, and Claudlo Balocchi lstituto di Chimica Analitica dell'Universita, 10 725 Torino, lfaly (Received March 16, 1976; Revised Manuscript Received September 15, 1976) Publication costs assisted by the Consiglio Narion.de delle Ricerche, Rome, ltaly
The kinetics and mechanism of oxidation of a series of quinols with hexachloroiridate(1V) have been investigated by means of a stopped-flow technique. The reaction rate showed first-order dependence on both reactants and a small effect of acidity and temperature was assessed. The data are in agreement with the Marcus theory, and a reorganizational parameter, A, of 26 kcal mol-l is derived; this value supports that the rate-determining step is a simple electron transfer. The intrinsic and extrinsic parameters for the oxidation of this class of reversible redox organic systems have been estimated.
Introduction Relationships between free energies of activation and the corresponding free energies of reaction have provided useful information in the elucidation of redox reaction mechanisms.' A successful quantitative treatment has been achieved but confined almost entirely to reactions involving metal-ion complexes. In this laboratory the possibility of extension of similar relationships to organic redox systems in aqueous solution, when reacting with oxidizing aquometal ions (Mn(III), Co( HI), Fe(III), Tl(III), V(V)),zhas been investigated. Owing to the interest of such organic systems, the reactions of a series of substituted quinols with hexachloroiridate(1V) have been investigated. In such systems, the interactions between the reactive centers are probably small (outer-sphere mechanism) so that examination of the collected data can suggest useful criteria to distinguish between different possible mechanisms, such as electron-transfer or hydrogen atom transfer. Experimental Section Reagents. Sodium hexachloroiridate(1V) was supplied by Merck and the spectrum of fresh solutions agreed with literature data.3 The quinols (K & K or Merck) were purified, when necessary, by recrystallization and the solutions were prepared daily. The following quinols have been investigated: benzene-1,4-cliol ( I ) , 2-methylbenzene-1,4-diol (2), 2-chlorobenzene-1,4-diol (31, 2,5-dihydroxybenzoic acid (4), 2,5dihydroxybenzenesulphonic acid ( 5 ) , 2,5-dihydroxy-1,4disulphonic acid (6), 2,3-dicyanobenzene-l,4-diol(7). Procedure. The reactions were followed with a DurrumGibson stopped-flow spectrophotometer a t X 487 nm (qr(l") 4070 M-' cm-1).3Kinetic runs were performed with [Ir(IV)] = 1.0 X M and excess organic substrate in the range 1.0-20 X M. Measurements were carried out a t [HC104] = 1.00 M, w = 1.0 M, and at different temperatures. Other measurements were performed a t [HC104] = 0.50 M (w = 1.0 M with LiC104 addition) and the observed rate constants showed very slight differences. A series of kinetic runs was also carried out in the presence of sodium hexachloroiridate(II1) in concentration of up to 12 times the initial concentration of Ir(1V); no kinetic effect was observed, thus the effect of any reverse reaction effect was neglected.
The rate constants were evaluated with a weighted leastsquares method (based on the deviation of the single points of each run) and the other kinetic parameters were derived by assigning the weights on the basis of standard deviations. The formal reduction potentials, Eo, of the couples quinone I quino1,for the different derivatives, were evaluated with a Metrohm E388 potentiometer, equipped with a saturated KC1-calomel electrode (saturated NaCl bridge) and a platinum electrode. A solution of quinol derivative, a t [HC104] = 1.00 M, I.L = 1.0 M, and 25.0 "C, was partially oxidized (25,50, 75%) by addition of thallium(II1) perchlorate (which rapidly reacts with 1:l stoichiometry, giving the corresponding quinone),4 and the formal potentials (compared with quinol value, 0.699 V)5 were estimated from the emf readings. Results Potentiometric Data. The following reduction formal potentials were determined (literature data are reported in parentheses, when available ) : 5 2,0.644 (0.645); 3,0.712 (0.712); 4,0.769; 5,0.787; 6,0.851; 7,0.910 (0.971) V. Stoichiometry. By means of spectrophotometric measurements with Ir(1V) in excess, the following overall equation has been derived 2Ir(IV)
+ H2Q
+
2Ir(III)
+ Q + 2Hf
(1)
where H2Q represents the quinol and Q the corresponding quinone. The values of the potentials for the couples Ir(1V) IIr(II1) (0.957 V, in 1M acid, HC104 and HzS04; 22 and QI H2Q show that all the reactions go to completion. Kinetic Data. Plots of In (A, - A,), where At and A, represent the absorbance at time t and at equilibrium, against time, were found linear for a t least two half-lives. The observed rate constants also showed linear dependence on the concentration of the organic substrates. Thus -d[Ir(IV)]ldt = ho[Ir(IV)][H2Q]
(2)
The ho values, concerning the different substrates, permit an estimation of the values of the specific rate constants collected in Table I, together with the activation parameters (obtained from kinetic measurements at 10.0 and 25.0 "C). The Journal of Physical Chemistry, Vol. BO, No. 27, 1976
2980
E. Pelizzetti, E. Mentasti, and C. Baiocchi
TABLE I: Kinetic Parameters for the Reactions of Quinols with IrCl& at 25.0 "C, [HClOJ = 1.00 M, 1.1 = 1.0 M
7.3 x 104 2.5 X los 1.8 X lo4 2.25 x 103
4.4 4.3 4.9
5
1.9 x 10'
6 7
27
2.2 4.7 4.7
1
2 3 4
32
5.5
-21.4 -18.4 -22.5 -24.8 -36.3 -36.5 -35.7
4.15 2.6 4.55 6.3 6.8 8.9 10.55
8.35 7.6
9.2 10.4 10.35 13.05 12.95
8.7 7.9 9.0
4.2
4.6 5.25 4.45 3.7 3.55 2.8 2.3
5.0
4.65 4.1 3.55 4.15 2.4
10.0~
10.35 11.7 12.85
M-I s-l; the error is f3-5%. b kcal mol-I. The error is f0.7-1.2 kcal mol-I. cal mol-' deg-1; the error is f2.4-4.0 cal mol-' deg-l. Calculated by assumingKl(sq)= 10 M for the unsubstituted quinol. f Calculated from eq 7 with X = 26 kcal molw1. ( A G o ) g (I
=
(AG*i)expt-
(AG*-s)expt.
Discussion
Scheme I
Since Ir(1V) is a well-known one-electron oxidant, the present noncomplementary oxidation takes place through two successive one-electron steps, as follows Ir(1V)
k3 + H2Q e Ir(II1) t SQ k -3
Ir(IV) t SQ
(3)
2Ir(II1) + Q
(4)
where SQ represents the semiquinone radical, irrespective of its protonated form (the protons are omitted). If the steadystate condition is applied to the semiquinone radical, the following equation is obtained d [Ir(IV)] - 2k 3h 4[Ir (IV)]2[H~Q1 dt k-?[Ir(III)] t k4[Ir(IV)]
(5)
The observed first-order plots of In ( A t - A m )vs. time, and the absence of Ir(II1) effect, suggest that k&r(IV)] >> h-3[Ir(III)],hence -d [ Ir (IV)]ldt = 2k 3 [ Ir (IV)][HzQ]
(6)
Thus, ho = 2k j . Comparison of the kinetic constants and of the activation parameters with those involving displacement of a chloride ion in the coordination sphere of the hexachloroiridate(1V) anion supports the conclusion that the first oxidation step follows an outer-sphere mechanism. A similar mechanism has been found to occur in the oxidation of phenol,6acyclohexanone,61band in the oxidation of organolead compoundsGC by means of the same oxidizing agent. When this mechanism is operating, a relation between the rates of reaction and the overall free energies involved is expected. A theoretical model, which relates these quantities, has been developed by Marcusl and the approximate equations (neglecting the small work terms to bring reactants and products together in the transition state) are AG*12 = X l z ( 1 + AG"12/X12)2/4
for IAG"l2l 6 X1z
AG*lp = 0
for AGO12
< -A12
(74 (7b)
AG*12 =
for AGO12 >, X I 2
(7c)
AGO12
where h = 2 exp(-AG*12/RT), 2 being the collision frequency in solution (lo1' M-I s-l); A12 is defined as 2(AG*ll+ AG*22), (where I G * l l and I G * L 2 refer to the self-exchange reactions of the reagents) and is approximately equal to 4AG*o (that is the value of AG*12 a t AGO12 = 0 ) . These simple relations, derived originally for weakly overlapping electron transfers, have found a wide applicability also The Journal of Physical Chemistry, Vol. 80, No. 27, 1976
for atom or proton transfers and for strong overlapping electron transfer^.^ Besides, Marcus and Sutin have also recently extended these conclusions to reactions involving large negative activation entropy variations, as in the present experimentse8Equations 7b and 7c apply to reactions in solution when most of the reorganization comes from the bonds being broken and formed, rather than from all the other coordinates. Inspection of eq 7a shows that an approximately linear relationship between AG*12 and AGO12 can be observed with slope 0.5(1 t AG012/2X12),which reduces to 0.50 if (AG012/2X12)
