38
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
Reactions of Some Peracids and Hydroperoxides with Cobalt( 11) and Cobalt(II1) Acetate in Acetic Acid Solution Charles F. Hendriks, Hendrlk C. A. van Beek,' and Pieter M. Heertjes Laboratory of Chemical Technology, University of Technology, Delft, The Netherlands
The kinetics and reaction products were determined of the oxidation of Co" by several peracids, the catalytic decomposition of these peracids by Co"', the reduction of Co"' by h drogen peroxide, tert-butyl and cumyl hydroperoxide and the catalytic decompositiqn of these peroxides by Go! The results were compared with those of the thermal decomposition of these peroxides. From the information obtained some conclusions were derived concerning the decomposition reactions of oxy and peroxy radicals in acetic acid in the presence and absence of Co" and Cot" acetate.
Introduction Liquid phase autoxidations of hydrocarbons, aldehydes, and ketones are radical chain reactions, during which peroxides are formed as intermediate products. It is well known that these reactions can be catalyzed by complexes of transition metals, in particular by cobalt complexes. Decomposition of peroxides by the metal complexes into chain carrying radicals is presumably one of the reactions which are responsible for the catalytic effect. Several investigations dealing with the reactions between peroxides and cobalt complexes have led to scattered and incomplete information. I t has often been found that the decomposition of a peracid ultimately leads to the formation of the corresponding acid and oxygen or to the corresponding acid and oxidation products of the solvent. There is no definite evidence concerning the mechanisms of the formation of these different products. Bawn and Jolley (1956), Koubek and Edwards (19631, and Boga et al. (1973) all found different results for the kinetics and product composition of the catalytic decomposition of peracids in the presence of Co"' acetate in different solvents. The investigations of the decomposition of tert-butyl hydroperoxide in the presence of Co" and Co"' acetate (Dean and Skirrow, 1958; Richardson, 1965; Onuma et al., 1967) showed that tert-butyl alcohol and acetone are the main reaction products. However, the stoichiometry and kinetics of this reaction as reported by these authors differ largely. Therefore we reinvestigated these reactions and studied both the thermal decomposition and the decomposition in the presence of Co" and Co"' acetate of a number of peracids, hydrogen peroxide, and tert-butyl and cumyl hydroperoxide in acetic acid solution. Experimental Section Procedures. The reactions were carried out in a Pyrex glass vessel, which was connected at the bottom to a Pyrex glass cell (path length 0.30 cm) placed in a Vitatron UC 200 colorimeter. The solutions, with a total volume of 100-150 mL, were stirred a t a rate of 2000 rpm. Materials. Acetic acid was purified by distillation (bp 118-120 "C a t 1 atm, water content 0.1 M). Co" acetate was of AR quality as received. CO'~'acetate was prepared by oxidation of Co" acetate with a stoichiometric amount of peracetic acid (see below). Hydrogen peroxide was used as a 30% solution in water, tert-butyl hydroperoxide as a 70% solution in tert-butyl alcohol, and cumyl hydroperoxide as a 70% solution in cumene. Peracetic acid was prepared in a n-propyl acetate solution (300 mL) by adding 50% hydrogen peroxide (1.5 mol) to acetic acid (1.95 mol) in the presence of a sulfonic 0019-7890/79/1218-0038$01.00/0
acid resin ( 5 g) and Victawet 35B (Na5R5(P30&, with R = 2-ethylhexyl, 0.2 g) (Phillips and Starcher, 1958). Perbenzoic acid was prepared by reaction of dibenzoyl peroxide (0.21 mol) with sodium methoxide (0.20 mol) in a 2:l chloroform/methanol mixture (300 mL); after extraction with water and benzene, crystalline perbenzoic acid was obtained by removal of the solvent under reduced nitrogen pressure (yield 80%, mp 41 'C). Substituted perbenzoic acids were prepared by adding 50% hydrogen peroxide (1.0 mol) to a mixture of the corresponding benzoic acid (0.3 mol) and methanesulfonic acid (1.5 mol) (Silbert et al., 1962). After addition of ammonium sulfate, the mixture was extracted with benzene. The peracids were isolated by evaporation of the latter under reduced nitrogen pressure (yield (50-65%). Analysis. Co"' was determined optically by measuring the absorbance a t 652 nm ( 6 160 M-' cm-l), where the absorbance of Co" is negligible. Peroxides were determined by iodometric titration. The production of oxygen was determined volumetrically. Measurement of the amount of carbon dioxide produced was performed by absorption into a solution of barium hydroxide and determination of the precipitated barium carbonate by weight. Water was determined by the Karl Fischer titration method. Determination of the other reaction products was performed gas chromatographically on a 3 m 10% SE-30 on Chromosorb W column by using a Varian Aerograph 1522-1B. Carboxylic acids were prior to analysis converted into the corresponding methyl esters by diazomethane. Identification of the reaction products was based on their mass spectra, recorded by using a Varianmat 111 GNOM mass spectrometer. The formation of peroxy radicals during the decomposition of the peracids and hydroperoxides was confirmed by addition of 2,6-ditert-butyl-4-methylphenol to the solution as a radical scavenger. After complete conversion of the peroxide the reaction products were separated chromatographically on a silica column (Horswill and Ingold, 1966) and isolated by evaporation of the eluent (trichloroethylene). In each case the infrared spectrum of the main product showed a twin absorption band at 6.0 pm, characteristic for quinoide compounds and a band at 11.4 pm, caused by a peroxy bond. The results of the other analysis methods are listed in Table I. Results Reactions with Peracids. All peracids investigated decompose slowly in acetic acid solution following firstorder kinetics. The rate constants of the decomposition were determined and the reaction products obtained were 0 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
39
Table I
peracid/hydroperoxide usid
a
%C
%H
ratio of protons (NMR) aromatic:vinyl: methy1:tert-butyl
4-methylperbenzoic acid
found theor.
74.8 74.6
8.3 8.1
4:2:6:19 4:2:6:18
perbenzoic acid
found theor.
73.9 74.2
8.2 7.9
5:2:3:19 5:2:3:18
4-chloroperbenzoic acid
found theor.
67.4 67.6
6.8 6.9
4:2:3:17 4:2:3:18
hydrogen peroxide
found theor.
71.0 71.4
9.8 9.5
2:3:18 2:3:18
terf-butyl hydroperoxide
found theor.
73.9 74.0
10.3 10.4
2:3:26 2:3:27
cumyl hydroperoxide
found theor.
77.9 77.8
9.0 9.2
structure proposed 2,6-di-tert-butyl-4-methyl4( 4-methylbenzoylperoxy)-2,5-cyclohexadienone 2,6-di-tert-butyl-4-methyl4-benzoylperoxy-2,5cyclohexadienone 2,6-di-fert-butyl-4-methyl4 4 4-chlorobenzoylperoxy)-2,5-cyclohexadienone 2,6-di-tert-butyl-4-methyl4-hydroperoxy-2,5-cyclohexadienone 2,6-di-tert-butyl-4-methyl4-tert-butylperoxy-2,5cyclohexadienone 2,6-di-tert-butyl-4-methyl4-cumylperoxy-2,5-cyclohexadienone
5 : 2 :3 : 1 7 :6O 5:2:3:18:6
Isopropyl protons.
Table 11. The Thermal and Catalytical Decomposition of Some Peracids (0.5-0.1 M ) a t 25 "C in Acetic Acid (0.1 M Water). A = Thermal Decomposition ; B = Reaction with Co" Acetate,b C = Reaction with Co"' Acetate A: k .1O-' (mini') C :k (M' min-') RC0,H
peracid (RCOJ3) perbenzoic acid peracetic acid 2,4-dichloroperbenzoic acid 2,6-dichloroperbenzoic acid
A B C A B C A B C A
9.1 19.0 3.0 31.8 7.4 17.6 6.8
B 3,4-dichloroperbenzoic acid 2-chloroperbenzoic acid 4-chloroperbenzoic acid 4-bromoperbenzoic acid 4-methylperbenzoic acid 4-isopropylperbenzoic acid cyclohexanepercarboxylic acid
C A B C A B C A B C A B C
A B C A B C A B C
10.7 6.6 16.4 7.1 14.0 6.5 18.4 7.2 19.2 10.2 32.4 10.1 28.4 8.1 23.1
0.89 1.00 1.00 a a a
0.88 1.00 0.99 0.88 1.01 1.01 0.88 1.01 0.99 0.88 1.00 1.00 0.88 0.88 1.01 0.91 0.99 0.99 0.90 0.98 0.98 0.91 0.99 1.00 0.89 0.99 1.02
products (mol/mol of peracid decomposed) CH,CO,-
0,
H,O
CH,
CO,
0.28
0.57 0.95
0.60
0.62
0.73 0.98 0.75 0.55 0.99
0.75
0.69
0.74 0.63
0.72 0.53
0.48 0.11 0.14 0.31 0.48 0.27 0.48 0.28
0.60 0.97
0.47 0.26
0.59 0.98
0.47 0.27
0.55 1.02
0.48 0.28
0.57 1.01
0.47 0.32
0.52 1.02
0.51 0.30 0.51 0.28 0.50
a Accurate determination impossible because of choice of solvent.
identified. The results are summarized in Table 11. The rates of decomposition of perbenzoic acid were measured a t different temperatures. The rate constants obtained gave a linear Arrhenius plot, from which an activation energy of 27.2 kcal/mol was calculated. Peracids oxidize CO" acetate instantaneously with formation of the corresponding carboxylic acid and water. The stoichiometry of the reaction was determined by measuring the relative changes in the concentrations of the
0.59 0.96
Co"'
RH
0.10 2.00 2.01 0.09 1.97
0.03 0.62
0.62
0.14 2.01
0.03 0.62
0.56
0.12 2.02
0.02 0.62
0.61
0.10 1.99
0.01 0.63
0.60
0.11
2.00 0.02 0.63
0.10
0.64 2.00
0.02 0.55
0.57
0.07 1.99
0.55 0.96
0.57
0.56 1.01
0.61
0.59
0.10 2.01
0.57
0.08 2.02
0.01 Concentration range 10.' to
M.
reactants and the products (see Table 11) and can be represented by 2Co"
-
+ RCOBH
2H+
2Co"'
+ RCOZH+ HzO
(1)
With Co"' acetate a slower decomposition reaction of the peracids was observed. The corresponding acid and oxygen were found as main reaction products, with the exception of the case of peracetic acid, where methyl
40
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
0
0
20
10
30
40
50
t Irnl"1
Figure 1. Decomposition of 0.5-0.1 M peracetic acid and perbenzoic M Corn acetate acid in acetic acid solution in the presence of lo4 to at 25 OC: 0,peracetic acid, [HzO] = 0.1 M; 0 , perbenzoic acid, [H20] < 0.01 M; X , perbenzoic acid, [H,O] = 0.1 M; 0 , perbenzoic acid [HzO] = 2.0 M.
acetate, water, and carbon dioxide were the main products. For the other peracids investigated these compounds were found as byproducts. The complete product distributions are given in Table 11. Optical measurements showed that within experimental accuracy (0.05%), the Co"' acetate concentration remained equal to the initial concentration. The reaction rate of the catalyzed peracid decomposition is linear with the peracid and Co"' acetate concentrations, as is illustrated by Figure 1 for the decomposition of perbenzoic acid and peracetic acid. This kinetic relation was found over the whole concentration range used for the peracids mentioned (I 1 M) and Co"' acetate to M). It is also demonstrated in Figure 1 that an increase of the water content of the solvent causes a decrease of the rate constant of the catalytic decomposition of perbenzoic acid. It will be discussed in a separate paper that this is correlated to an exchange of water and acetic acid ligands in the Co"' complex. Rate constants, measured over the temperature range 25-55 "C for the catalytic decomposition of perbenzoic acid and peracetic acid in acetic acid solution (0.1 M water), gave linear Arrhenius plots, from which activation energies of 17.0 f 0.5 and 22.1 f 0.5 kcal/mol, respectively, were calculated. Indications for the formation of acylperoxy radicals during the decomposition of the peracids were obtained upon addition of 2,6-di-tert-butyl-4-methylphenol to the solution as a radical sqavenger. The reaction product which was identified (see Experimental Section) is formed by oxidation of the phenol by an acylperoxy radical, followed by addition of a second acylperoxy radical on the 4-position. Reactions with Hydroperoxides. Hydrogen peroxide, tert-butyl hydroperoxide, and cumyl hydroperoxide slowly decomposed in acetic acid solutions following first-order kinetics. The rate constants and the reaction products found are given in Table 111. In the presence of Co"' acetate the hydroperoxides mentioned above were oxidized to products, which are also given in Table 111. Determination of the relative consumption of the reactants and formation of the products gave eq 2 as the overall reaction for the oxidation of hydrogen peroxide by Co"' acetate and eq 3 for the oxidation of tert-butyl and cumyl hydroperoxide. 2Co"' + H202 2Co" + 2H+ + O2 (2)
-
2Co"'
CHBCOzH
+ 2RC(CH3)200H 2Co" + 2H+ + RC(CH3),0H + RCOCH3 + CH3C02CH3
(R = CH3, C,jH5) (3)
-
[ROOHJ~ [CO
tot],
Figure 2. Kinetics of the reduction of Co"' acetate to lo-' M) by some hydroperoxides (lo-,to lo-' M) in acetic acid solution (0.1 M water) at 25 "C: 0 , hydrogen peroxide; X , tert-butyl hydroperoxide; 0,cumyl hydroperoxide.
I
1
I
-3 5
-3 0
1 -2 5
I -2 0
I
'
Figure 3. Pseudo-first-order rate constants of the decomposition of some peroxides as a function of the CO" concentration in acetic acid (0.1 M water) ( a = slope): 0,hydrogen peroxide at 25 "C, [Co"] < M, a = 2.02; 0 , tert-butyl M; a = 0.98; [Co"] > 1.5 X M,a = 1.00; [Co"] > lo-* hydroperoxide at 60 "C, [Co"] < 5 X M, a = M, 01 = 2.00; X, cumyl hydroperoxide at 60 "C, [Co"] < 1.01; [Co"] > 2 X lo-' M,a = 2.01.
Measurement of the initial rates (the first 3-4 min), using high Co"' concentrations gave the kinetics of the reactions 2 and 3. At longer reaction times and realtively low Co"' concentrations, the Co"-catalyzed decomposition of the hydroperoxides (see below) also occurs at an appreciable rate. The rates of the oxidation by Co"' acetate were found to be linear with the concentration of both reactants, as is represented in Figure 2. The rate constants are given in Table 111. In separate experiments the catalytic influence of Co" acetate on the decomposition of the hydroperoxides was studied. During this reaction no Corn acetate was formed. The reaction products and the stoichiometry of the reaction follow from the data presented in Table 111. The reaction rates were found to be linear with the hydroperoxide concentrations. Figure 3 shows the relation between the logarithms of the pseudo-first-order reaction
41
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18,No. 1, 1979
Table 111. The Thermal and Catalytical Decomposition of Hydrogen Peroxide, tert-Butyl Hydroperoxide, and Cumyl Hydroperoxideb in Acetic Acid at 25 " C (0.1 M water). A = Thermal Decomposition, B = Reaction with Con Acetate,C C = Reaction with Co"' AcetateC ~~
~~
rate constants B A (M-' temp, 1 0 S k , min-') "C (min-I) k, 25 1.7 10.3
peroxide hydrogen peroxide tert-butyl hydroperoxide
60
cumyl hydroperoxide
60
0.21
0.24
0.34
0.32
(M-'min'') k3
2.0 X
lo4
C (M-' min-') k 1510
93.3
850
690
35.4
a Accurate determination impossible because of choice of solvent. range t o lo-' M.
rate constants and the Co" concentrations used. From this graph the conclusion can be drawn that the reaction order for the Co" concentration increases from 1 and relatively low concentrations to 2 at higher concentrations. The complete rate equation for the thermal and catalytic decomposition of these hydroperoxides can therefore be given by --d[ROOH]/dt = { h , + ~,[CO"]+ ~,[CO"]~}[ROOH] (4) The values of the rate constants obtained are given in Table 111. Upon addition of 2,6-di-tert-butyl-4-methylphenol to the solution the formation of peroxy addition products was found (see Experimental Section). This confirms the formation of peroxy radicals during the decomposition of the hydroperoxides. With dibenzoyl and diacetyl peroxide no reaction was observed with Co" or Co"' acetate in acetic acid solution. Discussion Reactions w i t h Peracids. The slow thermal decomposition of peracids has been studied by Tokumatu et al. (1962) in alcoholic solvents. They concluded from kinetic experiments that homolytic dissociation of the peracids, e.g., the fission of the 0-0 bond, was the rate-determining step. The present results show that for the decomposition in acetic acid the same conclusion can be drawn. The following reaction scheme (reactions 5-10) rationalizes the main product distribution found. RCOBH RCOp. + HO. (5) -+
HO.
+ RCOBH
-
-
HzO
+ RC03. +
2RC03. 2RC02. 0 2 RCOp. + CHBCOpH ---* RCOzH + CH,COZ* CH3C02. CHy + COZ CH3. + CH,CO,* CH3COZCH3
--
(6)
(7) (8)
(9)
(10) The H abstraction from acetic acid by acyloxy radicals (8) and by phenyl radicals (12) is well known (Traylor et al., 1969; Bridger and Russell, 1963, Pryor and Guard, 1964). Data obtained by Anbar and Neta (1967) show that H abstraction by HO- from acetic acid, used as solvent, is
reaction products (mol/mol of peroxide decomposed) CoIIacetate water Co acetate tert-butyl alcohol acetone di-tert-butyl peroxide methyl acetate water Co acetate cumyl alcohol ace tophenone di-cumyl peroxide methyl acetate water oxygen
A
B
1.01 0.48
0.99 0.48
Q
Q
0.41 0.01 0.47 0.50 0.24
0.48 0.02 0.48 0.51 0.26
0.46 0.50 0.03 0.51 0.41 0.21
0.47 0.50 0.02 0.51 0.48 0.23
-
-
Concentration range 0.5-0.1 M.
C 2.03 0.95 1.01 a 0.47 0.02 0.46
-
0.44 1.01 0.41 0.46
0.03 0.41
-
0.45
Concentration
much faster than the diffusion-controlled disproportionation of the hydroxyl radicals. To account for the oxygen evolved in the present reaction, however, we must assume that, instead of H abstraction from the solvent, H abstraction from the peracid takes place (61, followed by the disproportionation of the acylperoxy radicals formed ( 7 ) in the manner as described by Howard (1971) for alkylperoxy radicals. The methyl acetate formation can be explained by the combined decarboxylation (9) and recombination (10) of acetyloxy radicals. The hydrocarbons found in minor quantities must be formed by the following side reactions RCOy R* + Cop (11)
-
Re + CHBCOzH
RH + CH3COp. (12) The oxidation of Con acetate by peracids has been reported by several authors (Bawn and Jolley, 1956; Koubek and Edwards, 1963; Boga et al., 1973),but no information has as yet been given concerning the stoichiometry and the mechanism of the reaction. In analogy to the well known reactions of Fe" and Ti"' with hydrogen peroxide (Dixon and Norman, 1963) we consider this reaction as a reductive dissociation of the 0-0 bond, resulting in the formation of a carboxylate anion and a hydroxyl radical (13). Formation of acyloxy radicals (together with hydroxyl anions) can be excluded, because no decarboxylation products formed from these radicals have been found among the products. Based on the overall stoichiometry of the reaction it follows that the hydroxyl radicals formed rapidly oxidize Co" to Co"' (14). CO" + RCO,H
H+
---*
CO"'
-
+
CO" + *OH
H+
+ *OH+ RCOpH
(13)
CO"' + HzO (14) For the Co"'-catalyzed decomposition of peracids Bawn and Jolley (1956) proposed an electron transfer from Co"' to the peracid as rate-determining step. Boga et al. (1973) proposed the formation of a Co"'-peracid complex on the basis of their kinetic results. The present results, consisting of the kinetics and the stoichiometry of the reaction, the trapping of acylperoxy radicals in the reacting solutions
42
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
by means of a radical scavenger, and the influence of substituents in the perbenzoic acid derivatives investigated on the reaction rates (an increase in rate with decreasing Hammett (r values of the substituents) support a ratedetermining step involving Con' and the peracid. Although the results do not lead to a conclusion as to the exact nature of the process, it can apparently be rationalized as a decomposition of the peracids in a Co"'-peracid complex into acylperoxy radicals and Co" (15). The acylperoxy radicals escaping from the complex can then disproportionate according to (7). The acyloxy radicals formed in this process are able to reoxidize Co" to Co"', thereby producing the corresponding acids. CO"'
+ RCOBH s [Cor1'*-RC03H]s CO" + RCOy + Hf
(15)
The rates of the overall reactions should be determined by the complex concentration and therefore be approximately linear with the concentrations of the reactants. From the product distributions also some interesting conclusions can be drawn. The CoI"-catalyzed decomposition of the aromatic peracids differs from the thermal decomposition in that no methyl acetate and C 0 2 are formed. This indicates that the reduction of the acyloxy radicals formed in (7) by Co" is faster than their reaction with the solvent (8). For peracetic acid the same product distributions are formed for the catalyzed and the thermal decomposition, and in both cases mainly methyl acetate and COS are formed. It follows therefore that the reduction of acetyloxy radicals formed in (7) by CO" is slower than their decarboxylation (9), followed by the recombination (10). The Co" formed in (15) should then be reoxidized to Cor" by the peracetic acid present in the solution. Reactions with Hydroperoxides. The thermal decomposition of hydroperoxides is well known (Emanuel et al., 1967). For the decomposition in acetic acid, however, no quantitative data on the kinetics and the stoichiometry of the reaction are known. The characteristic results of the present investigations are that the decompositions follow first-order kinetics and that from tert-butyl and cumyl hydroperoxide for each mole of oxygen 2 moles of the corresponding alcohol and ketone and of methyl acetate are formed. The latter results can be explained by the following reaction scheme, which starts with the homolytic rupture of the 0-0 bond. RC(CH3)z OZH RC(CH3)20. + HO. (16) HO.
--
+ RC(CHJ202H 2RC(CH3)20y
2RC(CH3)20.
Hf
H2O
+ RC(CH3)202- (17)
2RC(CH3)20. + 02
RC(CH3)20++ RC(CH3120H
(19)
+ CH3+ CH3C02CH3+ H+
(20)
RC(CH3)20+-* RCOCH3 CH3+
(18)
-
+ CHBC02H
(21)
For the hydroperoxides investigated, this reaction scheme (eq 16-19) is in agreement with the experimental results. Dialkyl peroxides, probably formed by dimerization of the alkoxy radicals, were obtained in small quantities. The decomposition of hydrogen peroxide in acetic acid can be explained by a similar process, consisting of a homolytic cleavage with formation of hydroxyl radicals, H abstraction by these radicals from hydrogen peroxide, and disproportionation of the resulting hydroperoxy radicals. That no H abstraction from the solvent by the hydroxyl radicals takes place follows from the fact that no methyl acetate was found in the reaction products.
The mechanism of the oxidation of hydrogen peroxide by Co"' acetate in acetic acid solution apparently is the same as has been proposed by Baxendale and Wells (1958) for this reaction in water, e.g.
+ H2O2 CO"' + HOO.
CO"'
+
CO" + HOO.
-
+ H+ CO" + 02 + H+
(22) (23)
in which ( 2 2 ) has to be the rate-determining step. The oxidation reaction ( 2 3 ) is analogous to the oxidation of hydroperoxy radicals by Fe"I as proposed by Norman and Lindsay-Smith (1965). For the oxidation of tert-butyl and cumyl hydroperoxide by Co"' acetate no stoichiometry has as yet been reported. Chuev et al. (1969) and Sapunov et al. (1974) found that these reactions were first order in the Co"I and hydroperoxide concentrations in acetic acid/chlorobenzene mixtures. We have now found analogous kinetic rate equations. The stoichiometry of the reactions indicates the following mechanism CO"'
+ RC(CHJ2OOH
+
CO" + RC(CH3)200*+ H+ (24)
2RC(CH3)200*----* 2RC(CH3)20*+ 02
(R = CsH5, CH3) ( 2 5 )
followed by the disproportionation reaction (19) and reactions 20 and 21. Reaction 25 has also been suggested by Howard (1971) for the decomposition of alkylperoxy radicals in benzene. The Co" acetate catalyzed decomposition of hydroperoxides has been studied by Onuma et ai. (1967), Dean and Skirrow (1958), and Richardson (1965). Rates have been found with a reaction order for Co" acetate varying from 0 (thermal decomposition) to 1'/? and for the hydroperoxides varying from 1 to 11/2.The product distribution depended upon the catalyst concentration. The present results indicate that in acetic acid the ratio of the amounts of the ketone, the alcohol, and methyl acetate formed as products was 1:l:l in all cases. We therefore may conclude that in acetic acid Co" catalyzes the decomposition of the hydroperoxides with formation of the same products as in their thermal decomposition. The stoichiometry of this reaction can therefore be explained by reactions 16-21. From the kinetic data obtained, no conclusions can, as yet, be drawn regarding the role of CO" in this process. A simple rationalization of the influence of Co" acetate would be that the rate-determining steps in the catalytic decomposition of the hydroperoxides are the formation of respectively 1:l or 2:l Co"/ hydroperoxide complexes, which decompose with formation of the corresponding alkyloxy and hydroxyl radicals. Conclusions Based on the results in this paper we are now able to present the stoichiometry and the kinetics of the thermal decomposition and the decomposition in the presence of Co'I and Co"' acetate of a number of peracids, hydrogen peroxide, tert-butyl and cumyl hydroperoxide. For these reactions mechanisms have been proposed, which start with the homolytic rupture of the 0-0 bond in the peracids and hydroperoxides. From these mechanisms the following conclusions were drawn regarding the properties of the radicals, which occur as intermediates in the processes. The aromatic acyloxy radicals, when formed in acetic acid solution apparently oxidize the solvent, with formation of acetyloxy radicals. The known facile decarboxylation
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
of the latter leads to the observed formation of methyl acetate and COP Hydroxyl radicals, which presumably are formed upon decomposition of the peracids and hydroperoxides together with the acyloxy radicals, apparently react faster with the peracids or hydroperoxides than with the solvent. The resulting peroxy radicals should disproportionate with formation of oxy radicals and oxygen. tert-Alkyloxy radicals appear to disproportionate with formation of the corresponding ketones and alcohols. In acetic acid this process is accompanied by the formation of methyl acetate. From the results it follows also that the oxy radicals are stronger oxidizing agents than the acylperoxy radicals. Both types of radicals must have a higher oxidation potential than Co"'. Hydroperoxy radicals are oxidized by Co"'. Literature Cited Anbar, M., Neta, P., Int. J . Appl. Radiat. Isot., 18, 493 (1967). Bawn, C. E. H., Jolley, J. E., Proc. R. SOC.London, Ser. A , 237, 297 (1956). Baxendale, J. H., Wells, C. F., Trans. Faraday Soc., 53, 800 (1957). Boga, E., Kiricsi, I., De&. F.. Marti, F., Acta Chim. Acad. Sci. Hung., 78. 89 (1973).
43
Bridger, R. F., Russell, G. A., J . Am. Chem. Soc., 85, 3754 (1963). Chuev. I. I., Shuskunov, V. A., Shechennikova. M. K., Abakumov, Kinet. Catal. (USSR), IO, 75 (1969). Dean, M. H., Skirrow, G., Trans. Faraday Soc., 54, 649 (1958). Dixon, W. T., Norman, R. 0. C., J . Chem. Soc., 3119 (1963). Emanuel, N. M., Denisov, E. T., Maizus. Z. K., "Liquid Phase Oxidation of Hydrocarbons", Plenum Press, New York, N.Y., 1967. Horswill, E. C., Ingold, K . U., Can. J . Chem., 44, 269 (1966). Koubek, E.. Edwards, J. O., J . Inorg. Nucl. Chem., 25, 1401 (1963). Norman, R. 0. C., Lindsay-Smh. J. R., in "Oxidases and R e h t d Redox Systems", Vol. 1, p 131 ff, H. S. Mason, T. E. King, and M. Morrison, Ed., Wiley. New York, N.Y., 1965. Onuma, K., Wada, K., Yamashita, J., Hashimoto, H., Bull. Chem. SOC.Jpn., 40, 2900 (1967). Phillips, B., Starcher, P. S.,J . Org. Chem., 23, 1823 (1958). Pryor, W. A., Guard, H., J . Am. Chem. Soc., 88, 1150 (1964). Richardson, W. H., J . Am. Chem. Soc., 87, 247, 1096 (1965). Sapunov, V. A., Selyutina, E. F., Lebedev, N. N., Kinet. Cafal. (USSR), 15, 315 (1974). Silbert. L. S.,Siegel, E., Swern, D., J . Org. Chem., 27, 1336 (1962). Tokumaru, K., Simamura, O., Fukuyama, Bull. C b m . Soc. Jpn.. 35, 1673 (1962). Traylor, T. G.. Sieber, A., Kiefer, H., Clinton, W., Intra-Sci. Chem. Rep., 3, 289 (1969). Walling, C., Padwa, A., J . Am. Chem. Soc., 85, 1593 (1963).
Received f o r review January 9, 1978 Accepted November 14, 1978
The Structure of Cobalt(I1) Acetate and Cobalt(II1) Acetate in Acetic Acid Solution Charles F. Hendriks, Hendrik C. A. van Beek," and Pieter M. Heertjes Laboratory of Chemical Technology, University of Technology, Delft, The Netherlands
Optical and ion migration experiments were carried out with acetic acid solutions of Co" and Co"' acetate. The results indicate a mononuclear structure with six ligands. The largest amount of these complexes is uncharged. I t is proposed that in anhydrous acetic acid Co" acetate can be represented a s Co" (OAc-), (HOAc), and Co"' acetate as Co"' (OAc-), (HOAc),. Addition of water, (ring substituted) benzaldehyde, benzoic acid, or phenol results in exchange reactions with acetic acid ligands.
Introduction Co"' compounds are used as catalysts for the liquid phase autoxidation of aldehydes and hydrocarbons and for the drying of paints (Sheldon and Kochi, 1973). In acetic acid solutions the catalyst is usually prepared by electrochemical or peracid oxidation of Co'I acetate to Co"' acetate. The ligand structure of the Co acetates in solution is still not elucidated. The literature supplies uncertain information, which is mainly caused by the fact that the metal complexes were analyzed after isolation from the solutions (Sharp and White, 1952; Peschanski and Wormser, 1962; Koubek and Edwards, 1963; Lande et al., 1971; Zidkowski e t al., 1973). The different isolation procedures which were used by the authors presumably caused ligand exchange reactions of the original Co complexes in the solution. The present investigation deals with the analysis of the structure of Con and Con' acetate in acetic acid solution in the absence and presence of compounds such as aldehydes and their oxidation products. The information provided by the results obtained therefore strictly concerns the structure of the Co" and Co"' complexes in acetic acid solutions. Experimental Section Materials. Acetic acid was purified by distillation (bp 118-120 "C at 1atm) and dried by boiling with acetic acid 0019-7890/79/1218-0043$01.00/0
anhydride. Co" acetate was used as tetrahydrate and was of AR quality as received. Anhydrous Co" acetate was prepared from the tetrahydrate by heating at 110 "C for 2 h. CoTnacetate was prepared by oxidation of Con acetate with a stoichiometric amount of peracetic acid. The aldehydes, phenols, carboxylic acids, and hydrocarbons used were purified by distillation or crystallization. Procedures. Optical measurements of the Con and Corn concentrations were carried out with a Zeiss PMQ I1 spectrophotometer. Magnetic susceptibilities were measured by the Gouy method (Selwood, 1961). Ion migration experiments were carried out in a W-shaped vessel, having three compartments isolated from each other by a sintered glass disk. The solution under investigation was placed in the central compartment, the solvent in the two outer compartments. The migration of the pink Co" complexes and of the green Co"' complexes was measured visually. The radii of the complexes were calculated from the stationary migration rates with the aid of the equation Q V / d = 67rwRu (1) with Q = charge of the complex, V / d = electrical field strength, p = viscosity of the solvent (acetic acid 1.3 X kg s-l m-l; water 0.9 X kg s-l m-l), R = radius of the complex, and Y = migration rate. Determination of water concentration was performed with the aid of the Karl Fisher titration method. Al0 1979 American Chemical Society
