1520
E. J. Y. SCOTT AND A. W. CHESTER
Kinetics of the Cobalt-Catalyzed Autoxidation of Toluene in Acetic Acid. The Role of Cobalt by E. J. Y. Scott* and A. W. Chester Mobil Research and Development Corporation, Central Research Division, Princeton, New Jersey 08640 (Received October 6, 1971) Publication costs assisted bu the Mobil Research and Development Corporation
The cobalt(II1) acetate-catalyzed autoxidation of toluene in acetic acid was studied at 87" by determining hydrocarbon consumption and cobalt(II1) concentration as a function of time. At steady-state concentrations of Co(II1) and Co(I1) acetates, the reaction is first order with respect to toluene concentration. A twoterm rate law was found to hold
toluene. At 87", the values of the rate constants are k , = 1.0 x M'/z sec-1 and kb = sec-l. The rate law is consistent with a two-path mechanism, derived from a Co(II1) monomer-dimer equilibrium. The term half-order in Co(II1) is consistent with oxidation of RH to its radical cation by the very active cobalt(II1) monomer; the term second order in cobalt(II1) arises from the oxidation of RH by the predominant cobalt(II1) dimer. where RH
=
3.9 X
The homogeneous catalytic autoxidation of hydrocarbons by transition metal salts has considerable theoretical and practical interest. For example, this area of chemistry is the basis for important industrial processes such as the manufacture of terephthalic acid from p-xylene. Although the cobalt-catalyzed autoxidation of toluene to benzoic acid may be regarded as a prototype for the catalytic autoxidation of alkyl aromatics to aromatic carboxy acids, the kinetics of this system are not yet understood. Morimoto and Ogata2 observed the simple rate law (1) for the autoxidation of toluene catalyzed by Co(II1) acetate in acetic acid a t 90".
-d'RH1 - k[Co(III)][RH] dt
Recent work done in this laboratory3s4and reported in the literature516led us to doubt this simple formulation. We therefore sought to find a more general rate law for the cobalt-catalyzed autoxidation of toluene.
Experimental Section Toluene and cobalt acetate were Baker reagent grade, chlorobenzene was obtained from Matheson Coleman and Bell, and oxygen was obtained from Airco. Toluene, chlorobenzene, and stock solutions in glacial acetic acid were tested for peroxide iodometrically prior to use. No effect on reaction rate was noticed when toluene was first passed through a column of alumina and silica. Co(II1) acetate was prepared by passing ozone through an acetic acid solution of Co(I1) acetate tetrahydrate. The solution was evaporated at 35-40" The Journal of Physical Chemistry, Vol. 76, No. 11, 197d
at a pressure of 4 mm on a rotary evaporator. The solid, obtained by further drying in a vacuum desic, all its cobalt as cobalt(II1). cator over P z O ~contained The cobalt (111) acetate-catalyzed autoxidation o f toluene in acetic acid was studied at 87' by determining hydrocarbon consumption and cobalt(II1) concentration as a function of time. The reactor was a jacketed 100-ml round bottomed flask, fitted with condenser, stirrer, and thermometer. Trichloroethylene was refluxed in the outer jacket to maintain a temperature of 87". Oxygen was admitted into the reactor through a sensitive pressure regulator set at 1.01 atm, a Wet Test meter, and a drying tower. Zero time was considered to be the time at which the temperature of the reactant solution attained 86". During the heating period the thermal decomposition of cobaltic acetate decreased its concentration to almost half the initial value (see Figure 1). The initial volume of reactant solution was 80 ml and l-ml samples were pipetted from the reactor at appropriate intervals. Toluene and chlorobenzene (internal standard) were analyzed on a 6-ft column of Polypak 2 at 200" using an F and h!t 810 chromatograph. Co(II1) acetate was measured spectroscopically by absorption of a filtered W. F. Brill, Ind. Eng. Chem., 5 2 , 837 (1960). T. Morimoto and Y. Ogata, J . Chem. Soc. B, 62, 1353 (1967). A. W. Chester, to be published. E. J. Y. Scott, J . Phys. Chem., 74, 1174 (1970). (5) K. Sakota, Y . Kamiya, and N.Ohta, Can. J . Chem., 47, 387 (1969). (6) E. Koubeck and J. 0. Edwards, J . Inorg. Nucl. Chem., 25, 1401 (1963). (1) (2) (3) (4)
KINETICSOF
0
THE
COBALT-CATALYZED AUTOXIDATION OF TOLUENE
0 5 M TOLUENE 0 5 M CHLOROBENZENE 0.25M Co(Il1) ACETATE 1.01 atm OXYGEN 88'C.
0
0.4
kobp=5.68x10-6SEC:'
0.7
0
0.3
0.2
Figure 1. First-order plot for the Co(II1) acetate-catalyzed autoxidation of toluene in acetic acid at 88'.
sample a t 625 mp on a Unicam SP800D spectrophotometer.
Results The cobalt (111) acetate-catalyzed autoxidation of toluene in acetic acid was studied at 87" by determining hydrocarbon consumption and cobalt (111) concentration as a function of time. The change of log [toluene] and log [Co(III)] with time is shown in Figure 1. After the concentrations of Co(1II) and Co(I1) acetates reach steady-state values, the logarithm of the toluene concentration decreases linearly with time. The computed first-order rate constants (kobsd) at 87" are shown in Table I. Comparison of the last two runs indicates that the reaction is approximately first order with respect to toluene concentration (see also Figure 4). These data may not be used directly to establish order with respect to Co(II1) acetate until it is known to what extent Co(I1) acetate affects the system. The effect of 0.1 M Co(I1) acetate tetrahydrate on the Co(II1) acetate-catalyzed autoxidation of toluene
Table I: First-Order Rate Constants for the Cobalt-Catalyzed Autoxidation of Toluene in Acetic Acid at 870a 1O6kobsd,
[RHIO
[Co(III)laa
[CO(II)l,,
8ec-1
0.5
0.0179
0.5 0.5 0.5 1.0
0.053
0.0446 0.0720 0.1035 0.1315 0.138
0.455 1.47 2.93 5.68 4.74
0.084 0.1186 0.112
[RH], = initial molar concentration of toluene. [Co(III)].,, [Co(II)lS. = steady-state molar Concentration of Co(I11) acetate and of Co(I1) acetate. [Co], = [Co(III)],, [CO(II)],~.
+
1521
0.5M TOLUENE 0,5M CHLOROBENZENE 0 l 2 5 M Co(III1 ACETATE 1.01~ l m OXYGEN . BB'C
I
a
.IM Co( 111 ACETATE ADDED
0
CONTROL ( N O CO(I1))
I -
'\
1.
.
1 100
zoo
300
T I M E , MIN.
Figure 2. Inhibition effect of Co(I1) acetate on Co(II1) acetate-catalyzed autoxidation of toluene in acetic acid at 88'
in acetic acid at 88" is shown in Figure 2. The Co(II1) acetate concentration is higher in the presence of Co(I1) acetate. However, no enhancement of rate is noticed for about 100 min. This "induction period" indicates that the reaction is retarded by Co(I1) acetate. The effect of adding Co(I1) acetate tetrahydrate after 100 min to the Co(II1) acetate-catalyzed autoxidation of toluene is shown in Figure 3.7 The rate of autoxidation of toluene is sharply reduced and subsequently increases as Co(I1) acetate is converted to Co(II1) acetate. To determine whether the retarding effect could be ascribed to acetate ion or water, 0.1 M potassium acetate and 0.4 M water were added after 406 and 481 min, respectively. Water produced no change in rate while the retarding effect of potassium acetate was significant but small. The main retardation was therefore due to Co(I1) acetate. An initial attempt was made to relate the observed rate constants (kobsd) to simple powers of the corresponding cobaltic acetate steady-state concentrations, i e . , l/t, 1, and 2. Each of these orders had been proposed at various times by various workers for the reactions of cobaltic acetate with aromatic compounds in acetic acid s ~ l u t i o n . ~ ~The ~ ~ observed ~,* rate constants in the present work were found t o be nonlinearly dependent on [Co(III)]"' and [Co(III)] but linearly dependent on [CO(III)]~.I n view of the observed inhibition by Co(1I) acetate the relations between kobsd and [CO(III)]"[CO(II)]-~,where m = 1, or 2, (7) This experiment was suggested by E. I. Heiba. (8) E. I. Heiba, R. M. Dessau, and W. J. Koehl, Jr., J. Amer. Chem. Soc., 91, 6830 (1969).
The Journal of Physical Chemistry, Vol. 76, No. 11, 1972
1522
E. J. Y. SCOTT AND A. W. CHESTER
.5
0 X
0. I M Co(II) PiCETATE ADDED CONTROL (NO C o ( 1 I ) )
O,SM l D L U E N E O.5M CHOLOROBENZENE 0.125M C o l I I I ) ACETATE 1.01 atm, OXYGEN .4 D.IM Co(II)~OAc)14Hp0
ADDED
z T z .3
3
L
.I2
.2
I
z v 08
z4
c 0
&L I
06
Co'Ul) ADDED
0
100
I
200
I
300 400 TI ME, min.
I
I
500
600
L
04
0
i
I
3
I 4
I
6
M%
+
+
[CO(II)1
kb[Co(III) ]'[RH]
, 103
I
5
Figure 4 'Test of rate expression -d[RH]/dt = k,[Co(III)] '/'[RH] / [Co(II)] ka[Co(III)]2[RH].
were also checked. Other negative orders of Co(II), such as - ' / z and -2, seemed intrinsically unreasonable and had in fact not been previously proposed. Except for simple second-order dependence, no linear relationship was found. I n this instance the slope was computed to be 4.0 X sec-' and the intercept 3.1 X 10-6 sec-'. The positive intercept indicated that a second term in the rate expression was required to account for an increased reaction rate at low Co(1II) acetate concentrations. Of the three plots of f k o b s d - 4.0 x ~ o - ~ [ c o ( I I I ) ] ~as ) functions of [Co(III)]m[Co(II)]~ where rn = '/2, 1, or 2 and n = 0 or -1, only the case where rn = '/2, n = -1 gave an approximately linear plot with a positive slope. The rate expression9 most consistent with the data is therefore dt
I
2
[CO(ir)] [ ~ ~ ( I I IX) ]
Figure 3. Addition of Co(I1) acetate tetrahydrate to the Co(II1) acetate-catalyzed autoxidation of toluene in acetic acid a t 87".
--dIRH] = ka[Co(I1I)]'/'[RH]
I
I
(2)
The observed first-order rate constant with respect to R H is given by
in Figure 4. The rate constants, obtained from the M1" sec-' and slope and intercept, are ka = 1 X k b = 3.9 X &f-2sec-l at 87". Note that the point for [toluenelo = 1.0M in Figure 4 lies close to the line, confirming the first-order dependence on toluene. This rate expression is not necessarily the only one consistent with the data but is the one which is most consistent with the accumulated information of all research on this and closely related systems.
Discussion The reaction of Co(II1) acetate with alkyl aromatic hydrocarbons has been studied in the absence of oxygen. Heiba, Dessau, and KoehP and Sakota, Kamiya, and Ohta5 have independently postulated that the ratedetermining step is an electron transfer from the aromatic hydrocarbon to Co(II1). Heiba, et al., studied the reaction with a competitive technique; in some related cases they were able to identify the epr spectrum of the intermediate radical cation.* Sakota, et a1.,6 studied the Co(II1) oxidation of toluene and its derivatives kinetically and found the following rate law
- d[Co(III)] - k[RH][Co(III)I2
which may be rearranged to
A plot according to (3) for [toluenelo = 0.5 M is shown The Journal of Phgsical Chemistry, Vol. 76, No. 11, 197.9
(4) dt [CO(II)1 which is consistent with the following reaction scheme (9) Co(II1) concentration is the experimental total Co(II1) conoentration and comprises Co(II1) in both monomeric and dimeric forms: [Co(III)] = [monomer] 2[dimer].
+
KINETICSOF
THE
+ RH. + Co(II1) +R + + Co(I1) + H+
Co(II1) RH. +
COBALT-CATALYZED AUTOXIDATION OF TOLUENE
+ RH
Co(I1)
+
(5)
1523
followed by the irreversible, rate-determining loss of a proton to give the benzyl radical R
(6)
If the rate-limiting step is the same in the catalytic autoxidation of alkyl aromatics, a similar inverse dependence on Co(I1) acetate concentration would be expected. Morimoto and Ogata2 did not look for or observe such a dependence (Eq 1). The present results also differ from those of Morimoto and Ogata in that no induction period was observed. Measurement of oxygen absorption and Co(II1) acetate and toluene concentrations indicated that no induction period occurs when the reaction is initiated by Co(II1) acetate. These results are consistent with those of other workers who did not observe an induction period when the autoxidation of p-xylene1° or ptoluic acidl0S1l is similarly initiated. It is possible that Morimoto and Ogata's results were modified by the presence of a retarder. Rlorimoto and Ogata also observed that the steady-state Co(II1) acetate concentration mas ,57% of the total cobalt present. I n Table I, Co(II1) acetate concentration varies from 29% [Co], for [Co], = 0.0625 M to 47% [Co], for [CO]O= 0.25 M. This variation is consistent with an independent observation in the absence of oxygen that the rate of Co(II1) acetate decomposition decreases with increasing Co (111)acetate concentration.
M
+ RH
Ki
Co(I1)
RH.+-%R.
+ RH.
+
+ H+
(10) (11)
The involvement of Co(I1) on the right side of equilibrium 10 results in a retardation of the monomer oxidation path by Co(I1). The products, R + and Re from (9) and (11) react further in non-rate-determining steps with the eventual formation of the aromatic acid. These steps have been adequately discussed by Morimot0 and Ogata.2 Equations 9 and 11 predict that the addition of acetate ion should accelerate the reaction rate whereas the experimental data indicate a slight reduction in rate. However, the addition of acetate ion also was independently shown to accelerate the decomposition of cobaltic acetate,a consistent with the reduction of cobaltic acetate concentration after the addition of potassium acetate (Figure 3). These compensating effects could not be separated. Combining (S), (9), (lo), and (11) results in the rate law for the two paths
The incorporation of equilibrium 7 and the relationship derived from it results in rate law (2) with
Mechanism Rate law (2) is consistent with a two-path mechanism in which a cobalt(II1) acetate dimer is the oxidant in one path and a cobalt(II1) acetate monomer the oxidant in the other. It has previously been proposed that cobalt(II1) acetate exists predominantly as a equilibrium dimer in s o l ~ t i o nand ~ ~ a~monomer-dimer ~'~ has been invoked to explain a variety of kinetic results. 3-6 The monomer-dimer equilibrium may be written as
RH. +
KD
D z 2 M
(7)
where D and M represent dimeric and monomeric Co(II1) acetate. The oxidation of R H by D probably involves the reversible formation of a complex f D,RH) followed by the irreversible, rate-determining reaction of the complex with a further D molecule. Kz
+ R H (D,RH] JD,RH) + D -%R + + H + + 2Co(III) + 2Co(II) D
I t is of interest, however, that the oxidation of R H by D is not retarded by Co(I1). An alternate mechanism may be written in which only M is involved in oxidizing RH, e.g., if equilibrium 10 is followed by reaction 11 and
(8)
+ M "e Co(I1) + R + + H +
(14)
in competition. Such a mechanism, however, results in a rate law in which Co(I1) retardation occurs in both terms, a conclusion not in accord with the experimental results . The nature of the electron transfer in the two paths bears some discussion. I n the case of the monomer (M), the electron transfer undoubtedly occurs via an outer-sphere process. It is unlikely that alkyl aromatics would directly coordinate to Co(II1) in acetic acid, a relatively polar solvent, prior to the electron transfer. The outer-sphere oxidation of alkyl aro-
(9)
The overall reaction is second order with respect to D; moreover, Co(I1) acetate does not retard the reaction. The oxidation of R H by M involves a reversible electron transfer with formation of the radical cation R H . +,
(10) P.S. Landis, unpublished work. (11) Y.Kamiya, M. Ohm, and N. Ohta, Kogyo Kagalcu Zasshi, 71, 999 (1968). (12) S. S. Lande and J. K. Kochi, J. Amer. Chem. SOC.,90, 5196 (1968); S. S. Lande, C. D. Falk, and J. K. Kochi, J.Inorg. Nucl. Chem., 33, 4101 (1971). The JOUTW~ of Physical Chemistry, Vol. 76, No. 11, 1972
1524
R. P. WIDMANAND J. ROBERT HUBER
matics by a heteropolyion containing cobalt(II1) has already been dem0n~trated.l~The oxidation by M can occur by conduction through the carboxyl group of a monodentate acetate ligand, with orbitals of the uncoordinated oxygen overlapping with aromatic r orbitals. A similar mechanism can be invoked where alkyl aromatics are more rapidly oxidized by cobalt(111) acetate in the presence of chloride,s in which case the monomeric hexachlorocobaltate(II1) ion is formed. l4 The overlap of chloride orbitals with aromatic r orbitals undoubtedly facilitates electron transfer. The oxidation by the dimer (D) is less easily explained, as is the nature of the intermediate “complex” (D,RH). Dimeric cobalt(II1) acetate is thought to contain two bridging hydroxo groups and chelating acetate ligands3v6 although Lande and Kochi have proposed that bridging acetate is involved.12 The electron transfer may occur via the bridging hydroxo groups; the need for two dimers during the electron transfer may be a requirement for more favorable energetics. The proposed mechanism is consistent with the rate law derived from experiment and emphasizes the importance of the monomer-dimer equilibrium in solutions of cobalt(II1) acetate. The mechanism
specifically accounts for the observed retardation by Co(II), a factor that was neither observed nor explained by Morimoto and Ogata.’ Both the monomer and dimer forms of cobalt(II1) acetate are therefore active oxidants in the cobalt-catalyzed autoxidation of aromatics in acetic acid. Under the reaction conditions in this study, both terms contribute equally to the observed rate. However, the monomer is a much more reactive species since it is present to only a very small extent. The contribution of the monomer to the overall rate of oxidation would be increased by decreasing the Co(II1) concentration. The contribution of the second term may be increased by increasing the Co(II1) concentration since the second term is second order whereas the first term is half order with respect to Co(II1).
Acknowledgment. The authors are indebted to Dr. P. S. Landis and Dr. E. I. Heiba for helpful discussions. The original oxidation apparatus was designed by Dr. P. s. Landis. (13) A. W. Chester, J . Org. Chem., 35, 1797 (1970). (14) A. W. Chester, E. I. Heiba, R. iM. Dessau, and W. J. Koehl, Inorg. Nucl. Chem. Letters, 5, 277 (1969).
Temperature Effects in the Intersystem Crossing Process of Anthracene
by R. P. Widman and J. Robert Huber* Photochemistry and Spectroscopy Laboratory, Department of Chemistry, Northeastern University, Boston, Massachusetts 0.2116 (Received December 1971) Publication costs borne completely by The Journal of Physical Chemistry
The temperature effect in the intersystem crossing process (ISC) of anthracene in a polymethylmethacrylate matrix has been investigated between room temperature and 77°K. The temperature dependence of the ISC rate constant ( k T ) can‘be expressed in terms of an Arrhenius-type function ~ T ( T=) TO A exp(-EE/RT) with E = 200 f.30 cm-1 and A = (3 =k 0.6) x 108 sec-1. On the basis of recent theoretical results by Henry and Siebrand [J.Chem. Phys., 54, 1072 (1971)], ISC mechanisms consistent with this temperature behavior with T3 slightly above S1, involving a purely electronic are discussed. The ISC route S1(1B2,),-+ T3(3Bau), spin-orbit coupling (Hnm(l))appears to be the most favorable mechanism.
+
The detailed mechanism of intersystem crossing (ISC) of planar polynuclear aromatic hydrocarbons continues to receive much attention from both the experimental and theoretical points of Anthracene, whose intersystem crossing efficiency shows strong dependences On both and temperature’ sents a particularly interesting case. The Variation of the Isc ratc with temperature has been interpreted The Journal of Physical Chemistry, Vol. 76, No. 11, 197.2
in terms of temperature-dependent and temperatureindependent processes. For this compound the second triplet state T2(3B1,)lies energetically very close to the (1) J. B. Birks, “Photophysics of Aromatic Molecules,” WileyInterscience, New York, N. Y . ,1970. (2) W. S. Veeman and H. J. Van der Waals, Mot. Phys., 18, 63 (1970); R. Englman and J. Jortner, ibid., 18, 145 (1970). (3) B. R. Henry and W. Siebrand, J . Chem. Phys., 54, 1072 (1971).
