Autoxidation of Methyl Linoleate and Methyl Linolenate and Reactions

compounds was studied by several authors (Watt and Otto,. 1947; Gibson, 1948; Fugger, 1951 ... was then extracted with 200 mL of n-heptane and 200 mL ...
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212

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979

Autoxidation of Methyl Linoleate and Methyl Linolenate and Reactions of the Hydroperoxides Formed in n-Heptane Solution Charles F. Hendriks, Pieter M. Heertjes, and Hendrik C. A. van Seek' Laboratory of Chemical Technology, Delft University of Technology, 2628 BL Delft, the Netherlands

Methyl linoleate and methyl linolenate upon autoxidation in n-heptane solution in daylight as well as in the dark are converted into a mixture of hydroperoxides with a cis,trans conjugated diene structure, in which the hydroperoxy group is attached to one of the allylic methylene groups. The initiation reaction is a combination of a thermal and photochemical reaction between the methyl esters and oxygen. The hydroperoxides decompose into the corresponding hydroxy and keto compounds. The stoichiometric and kinetic parameters of these reactions in the absence and presence of cobalt(I1) or cobalt(II1) naphthenate were determined. During the final stage of the autoxidation of the methyl esters fragmentation of the C-C chain and formation of oxo bridges between the methyl ester molecules were observed. Diene polymerization appeared to be inhibited by oxygen.

Introduction During the hardening of films of lacquers and paints, unsaturated components such as polyalkenes and triglycerides of unsaturated carboxylic acids, such as linoleic acid and linolenic acid, are oxidized. These oxidations are catalyzed by Co salts, present in the mixture as metal soaps (siccatives). The mechanism of the autoxidation of the methyl esters of linoleic and linolenic acid as model compounds was studied by several authors (Watt and Otto, 1947; Gibson, 1948; Fugger, 1951; Cannon et al., 1952; Ricciuti et al., 1954; Sephton et al., 1956; Khan, 1959). It was found that aerobic solutions of these esters consumed oxygen with formation of hydroperoxides as the first stable products (Ricciuti et al., 1954; Sephton et al., 1956; Khan, 1959). It is generally accepted that these autoxidations are free-radical chain reactions. Privett (1954, 1959) suggested that the hydroperoxides formed have a cis,trans conjugated structure. During prolonged reaction the hydroperoxides decompose into several fragmentation products such as carboxylic acids (Horvat et al., 1969) and dicarbonyl compounds (Cobb and Day, 1965). Because the information about these autoxidations is very incomplete, the present authors reinvestigated kinetics and product distribution of these reactions and also the influence of cobalt naphthenate on the processes. The solvent used, n-heptane, is not oxidized under the reaction conditions chosen. The results of the investigations are presented in this paper. Experimental Section Materials. n-Heptane was purified by distillation; the fraction used had a boiling point of 98.0-98.5 "C. Methyl linoleate (cis-9,cis-12-octadecadiene carboxylic acid methyl ester) and methyl linolenate (cis-9,cis-12,cis-15-octadecatriene carboxylic acid methyl ester) were of AR quality as received. Their absorption spectra had a peak at 232 nm and a tail absorption of which the intensity decreased with increasing wavelengths. Cobalt(I1) naphthenate was used as a 7.5% solution in white spirit. Cobalt(II1) naphthenate was prepared in solution by oxidation of cobalt(I1) naphthenate with ozone. The excess of ozone was removed by flushing with nitrogen. The hydroperoxides of methyl linoleate and methyl linolenate were prepared by autoxidation of 100 mL of methyl ester during 16 h at 75 "C. The reaction mixture was then extracted with 200 mL of n-heptane and 200 mL 0019-7890/79/1218-0212$01.00/0

of 4:l methyl alcohol/water, using the method described by Privett (1954). The peroxides were isolated from the methyl alcohol/ water layer by evaporation under reduced pressure. The yield was 20% and the purity as determined by iodometric titration was 80%. Because the hydroperoxides slowly decomposed, they were used directly after preparation. Procedures a n d Analyses. The reactions were carried out in a cylindrical Pyrex vessel (diameter 6 cm, length 15 cm), which was equipped with a thermostat and connected at the bottom to a Pyrex vessel (path length 0.30 cm), placed in a Vitatron UC 200 colorimeter. The reaction vessel, which was transparent only for light with X >320 nm, was irradiated with a 100-W tungsten lamp. The solution absorbed -15 W of the radiation. A glass stirrer was used which had two elliptical blades. The stirring rate was 2000 rpm. The initial concentration of the methyl esters and the corresponding hydroperoxides were varied between 0.05 and 1.0 M, the concentration of cobalt(I1) and cobalt(II1) naphthenate between lo4 and M, and the oxygen pressure between 0.10 and 0.96 atm. The 592 nm, emax concentration of cobalt(II1) naphthenate (A, 112.6 M-' cm-') and of cobalt(I1) naphthenate (Am= 580 nm, emax 21.5 M-' cm-') were measured optically. The determination of hydroperoxide concentrations was carried out by adding an aliquot (2 mL) to an anaerobic solution of 10 mL of acetic acid/chloroform (2:1), after which 2 mL of a saturated solution of potassium iodide was added. After flushing with nitrogen for 15 min, 10 mL of water was added and the liberated iodine was titrated with 0.1 N sodium thiosulfate. The consumption of oxygen was measured volumetrically. Equilibrium oxygen concentrations at different temperatures in n-heptane were determined polaroM;50 "C,7.2 X M; and graphically: 25 "C, 15.1 X 75 oc, 5.0 x 10-3 M. The concentration of the methyl esters and their volatile oxidation products was determined gas chromatographically using a 3-m 10% SE-30 on Chromosorb-W column placed in a Varian Aerograph 1522-1B. Carboxylic acids, produced during the autoxidations, were prior to analyses converted into the corresponding methyl esters with diazomethane. Identification of the reaction products was based on their mass spectra (Varianmat 111GNOM mass spectrometer). The hydroperoxides formed from methyl linoleate and methyl linolenate have a UV absorption maximum a t 0 1979 American Chemical Society

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979

Table I. Identification of Some Hydroxy Stearic Acid Methyl Esters, Formed by Reduction of Hydroperoxides as Autoxidation Products of Methyl Linoleate and Methyl Linolenate

methyl ester used methyl linoleate methyl linolenate

characteristic proposed m / e values of position of hydroxy stearic hydroxy acid methyl substiester formed tuent= 297,187,157 297, 243,101 297, 187, 157 297, 229,115 297, 243,101 297, 285,59

9 13 9 12 13 16

See Results and Discussion.

232.0-232.5 nm. The infrared absorption spectra of the hydroperoxides differed from that of the starting methyl esters in the absence of the band at 10.95 and the presence of bands at 2.75 pm (-OHvibration), 9.55 pm (-OOH vibration), 10.12-10.15 pm and 10.19-10.20 pm (&,trans conjugated diene vibrations). The UV and IR absorption spectra were recorded with a Unicam SP 800 and a Beckman IR-20A spectrophotometer. To identify the hydroperoxides in mixtures of autoxidation products, these compounds were first converted into the corresponding hydroxy stearic acid methyl esters, which were identified by their mass spectra (see Table I). The hydrogenation of the hydroperoxides was carried out a t 1 atm H2, using platinum oxide as catalyst. The products were separated on an 1.5-m alumina column using petroleum ether/chloroform (7:l) as eluent. The products (Table I) were formed in almost equal quantities. By thermal decomposition of the hydroperoxides, formed by autoxidation of methyl linoleate (see above), four compounds were found as first stable products with characteristic m l e values of respectively 293, 279, 153; 293, 279, 101; 308, 277, 151; and 308, 277, 99. By thermal decomposition of the hydroperoxides, formed by autoxidation of methyl linolenate (see above), eight compounds were found as first stable products with characteristic m / e values of respectively 291, 277, 151; 291, 277, 111; 291, 277, 99; 291, 277, 59; 306, 275, 149; 306, 275, 97; 306, 275, 57; 306, 275, 109. The structure of the nonvolatile end product of the autoxidation of the methyl esters was determined (after separation from the volatile products by evaporation under reduced pressure) by element analysis, and by the hydroxyl, acid, iodine, and saponification numbers by using the standard methods described by the American Oil Chemists' Society (1966); see Table 11. It was found by iodometric titration that no peroxy bonds were present in these end products. The infrared spectra of these products showed bands a t 2.82 and 2.95 pm (-OH), 5.82 pm (-COOH) and 9.60-9.80 and 8.10 pm (-COC and =COC). Results and Discussion The kinetics of the chain autoxidation of methyl linoleate and methyl linolenate were determined by measuring

the rate of conversion of the methyl ester and of consumption of oxygen at different initial ester concentrations and different temperatures between 25 and 75 "C. The reaction appeared to be first order in the ester concentration a t these temperatures. At 25 "C the rates were independent of the partial oxygen pressure (p0J at values of pol > 0.20 atm. At higher temperatures the reaction rates were also a function of the partial oxygen pressure; see Table 111. The kinetics of the initiation reaction under irradiation and in the dark were studied by inhibiting the chain propagation with 2,6-di-tert-butyl-4-methylphenol (Hendriks et al., 1977). This compound is a strong inhibitor for the propagation reactions in autoxidations. In the presence of this inhibitor the conversion of the methyl esters was much slower than in the absence of the inhibitor. The reaction rates were found to be independent of the inhibitor concentrations if these concentrations were >0.1 M. For reactions carried out in light a t room temperature, the rates were linear with the methyl ester concentration and almost independent of the oxygen pressure. At higher temperatures the rates of these reactions increased with increasing oxygen pressure. For reactions carried out in the dark the rates were linear with the methyl ester concentration and the oxygen pressure. From these observations several conclusions can be drawn. The strong inhibition of the autoxidation by relatively small amounts of the inhibitor proves that the process is a radical chain reaction. For the photochemical initiation reaction it is probable that the rates are linear with the light absorbance by the methyl esters and independent of the oxygen pressure. Since the light absorbance should (at low absorbances) be approximately linear with the methyl ester concentrations, the rates must be first order in these concentrations. This relationship has indeed been found. We assume that this photochemical initiation is a rapid oxidation of the photo-excited methyl ester by oxygen with generation of the chainpropagating radical (see eq 2). An initiation process proceeding via singlet oxygen, as described by Rawls and van Santen (1970) is improbable, since the energy transfer from the photo-excited methyl ester to oxygen should depend on the oxygen pressure, a fact which has not been observed. Information regarding a possible role of singlet oxygen in the initiation reaction can also not be obtained from the structures of the products of the overall autoxidation because the latter are mainly determined by the radical chain process. Finally, it can be observed that the ratio of the rate constants ( h / k , , see Table 111) of the overall autoxidation and the initiation, if both reactions are carried out at the same temperature and under irradiation, is independent of the oxygen pressure. This indicates that the oxygen pressure only influences the initiation reaction and not the chain length of the autoxidation. The relative conversion of the methyl esters, consumption of oxygen, and production of the hydroperoxides were measured. The results indicated that during the

Table 11. Analysis of Nonvolatile Autoxidation Products products exptl autoxidation of methyl linoleate (

19 H3!

2

%H 9.1

61.1

8.4

)

methyl linolenate (C19H,,02

%C 60.9

formula proposed C,,H,,O, ( M = 374) C,,H,,O, ( M = 372)

213

theoret

no. of groups/mol of product (expt)

%C

%H

OH

CO,CH,

C=C

CO,H

61.0

9.1

0.63

0.98

1.95

0.54

61.3

8.6

0.62

0.97

2.96

0.51

214

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979

Table 111. The Kinetics of the Autoxidation of Methyl Linoleate and Methvl Linolenate

PO,,

T,"C 25 25 50 50 75 75 25 25 50 50 75 75

atm

not irrad IO6 k , , min-'

irrad

lo6 k , , min-'

irrad lo4k , min-'

hydroperoxy irrad k/k,

0.96 0.20 0.78 0.16 0.51 0.10

5.5 5.5 7.3 6.8 48.3 50.0

0.96 0.20 0.78 0.16 0.51 0.10

Methyl Linoleate 0.05 4.7 0.08 0.02 4.7 0.08 0.12 5.1 0.16 0.02 5.0 0.14 0.38 6.1 1.32 0.07 5.7 1.26

1.7 1.7 3.1 2.8 21.6 22.1

initial stage the overall autoxidation can be represented by RiRzCH2 + 0 2 RiR2CHOOH (1) (R1R2CH2= methyl linoleate, methyl linolenate). During the autoxidation remarkable changes were observed in the ultraviolet and infrared absorption spectra of the solutions (see Experimental Section). The observed ultraviolet absorption maximum of 232.0-232.5 nm of the products is characteristic for a conjugated diene, which has one alkyl substituent at both sides (Gillam and Stern, 1957). The changes found in the infrared absorption spectrum are the same as were observed by Tolberg and Wheeler (1958), who studied the isomerization of cis-9,cis-12-methyl linoleate in the presence of iodine to cis-9, trans-11-methyl linoleate. Therefore the conclusion is that on autoxidation of the methyl esters the cis,cis nonconjugated diene structure is changed into a cis,trans conjugated structure of the corresponding hydroperoxide. This conclusion is in agreement with the suggestion published by Privett (1954, 1959). To determine the position of the hydroperoxy group in the hydroperoxides, these products were separated from the solution (see Experimental Section) and hydrogenated to the corresponding hydroxy stearic acid methyl esters. From methyl linoleate and methyl linolenate two, respectively four, different hydroxy stearic acid methyl esters were isolated in ratios of respectively 1:l and 1:l:l:l; see Table I. The positions of the hydroxy substituents were determined from the mass spectra of these compounds. This was possible because fragmentation of the C-C chain occurs preferentially next to the carbon atom to which the hydroxyl oxygen is attached (Pasto and Johnson, 1969). It is now concluded from the UV, IR, and mass spectral data that methyl linoleate is converted by autoxidation into a 1:l mixture of 9-hydroperoxy,l0-trans,l2-cis-linoleic acid methyl ester and 13-hydroperoxy,9-cis,ll-trans-linoleic acid methyl ester. Methyl linolenate is converted on autoxidation into a 1:l:l:l mixture of 9-hydroperoxy,l0-trans,l2-cis,15-cis-linolenic acid methyl ester, acid methyl 12-hydroperoxy, g-cis, 13-truns,l5-cis-linolenic ester, 13-hydroperoxy, 9-cis, ll-trans,l5-cis-linolenicacid methyl ester, and 16-hydroperoxy, 9-cis, 12-cis,14trans-linolenic acid methyl ester. Based on these results it is concluded that in the chain propagating radical the free electron is mainly localized on the 9- and 13-positions for methyl linoleate and on the 9- 12-, 13-, and 16-positions in methyl linolenate. It can therefore be assumed that the propagating alkylperoxy

hydroperoxy methyl linolenicester acid

methyl linoleic ester acid

h,

Methyl Linolenate 0.04 2.0 0.11 0.01 2.0 0.11 0.10 2.6 0.19 0.02 2.5 0.17 0.30 2.9 1.40 0.07 2.6 1.30

-

Table IV. Rate Constants and Activation Energies for the Decomposition of Hydroperoxy Linoleic and Hydroperoxy Linolenic Acid Methyl Ester at 25 C

(M-I

min-')

7.3 (E,= 8 . 6 kcal/mol) 0.7 (E, = 9.5 kcal/mol) 0.20 (E, = 4.5 kcal/mol)

k , (M-l min-')

105h, (min-')

,

8 . 0 (E, = 9.2 kcal/mol) 0.9 (E, = 10.1 kcal/mol) 0.20 (E,= 4.5 kcal/mol)

radical (ROO. in reaction 2) selectively abstracts hydrogen from the 11-CH2group in methyl linoleate and from the 11-and 14-CHzgroup in methyl linolenate, converting the methyl esters into a radical which isomerizes to a cis,trans conjugated structure, reaction 2. Presumably this mechanism is also valid for the initiation reaction, in which oxygen attacks the same methylene groups. During /

\

/ \

/

/

\ /

\

c=c

H

c=c

H H

H CH2

H

/

H

\ / \ H H

H

/

H

/

\

\ / H H

prolonged autoxidation the hydroxy and keto compounds derived from the methyl esters are formed (see mass spectral data in the Experimental Section). The same products were found if the hydroperoxides, isolated from autoxidizing solutions of the methyl esters, were decomposed thermally. It may therefore be concluded that the hydroperoxides are the precursors of the hydroxy and keto compounds. For the measurement of the kinetics of this conversion solutions of hydroperoxides previously prepared by autoxidation of the methyl esters were used. The rate of decomposition of the hydroperoxides was found to be linear with the hydroperoxide concentration. The firstorder rate constants k4 and the activation energies calculated from Arrhenius plots over the temperature range 25-80 "C are given in Table IV. Prolonged autoxidation of the methyl esters also resulted in the formation of further reaction products which can be divided into volatile and nonvolatile products. The main volatile products, identified with GC/MS, were saturated hydrocarbons Cz-C8 (30-38% of the total amount of volatile products), mono-unsaturated hydrocarbens C3-Clo (1-4%), methyl heptanoate (lo%), methyl heptenoate (5-7%), methyl undecenoate (10-12%), propionic acid (5%), hexanoic acid (lo%), 1,7-heptanedicarboxylic acid monomethyl ester (8%), and carbon dioxide (11%). These products are formed by fragmentation of the C-C chain. The nonvolatile products were analyzed after separation from the volatile products (see Experimental Section). The elemental analysis of these products gave a C/H ratio equal to the value of the starting methyl ester and an oxygen content corresponding with the uptake of 2.5 mol of oxygen per mole of methyl ester; see Table 11. From the iodine and saponification numbers of the reaction products it followed that the number of unsaturated bonds and ester groups is the same as for the starting methyl ester. Part of the oxygen content is explained by the presence of the

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 215

CO" + *OH

H+

CO"'

+ HzO

(5)

GC/MS analysis showed furthermore that reaction of cobalt(II1) naphthenate with the hydroperoxides gave the hydroxy and keto linoleic or linolenic acid methyl esters, which were also found as products of the thermal hydroperoxide decomposition (see above), in a ratio of 1; see Experimental Section. As other products of the reaction between cobalt(II1) naphthenate and the hydroperoxides were found cobalt(I1) naphthenate and oxygen. The overall stoichiometry was obtained by measuring the relative conversion and formation of reactants and products

-

-

2R1R2HCOOH + 2Co"' R1R2HCOH + RlR2C0 + O2 + 2Co"

Figure 1. The initial rate of the reaction of hydroperoxy linoleic acid methyl ester with cobalt(I1) ( 0 )or cobalt(II1) naphthenate ( 0 )and of hydroperoxy linolenic acid methyl ester with cobalt(I1) ( X ) or cobalt(II1) naphthenate (A) in n-heptane solution (25 "C).

hydroxyl and carbonyl groups in the compounds. The remainder is probably present in the form of COC and =COC bonds between the methyl ester molecules. Indications for the presence of such bonds were obtained from the infrared absorption spectrum of the end products. It seems remarkable that no polymerization of the double bonds of methyl linoleate and methyl linolenate has taken place. It seems probable that oxygen, which acts as an inhibitor for C=C polymerization reactions, prevents this reaction. Under our conditions the reaction mixtures were continuously saturated with air or oxygen by stirring. Finally, the reactions between the hydroperoxides, isolated from autoxidizing methyl linoleate and methyl linolenate (see Experimental Section) and cobalt(I1) or cobalt(II1) naphthenate were studied. In the presence of cobalt(I1) naphthenate the hydroperoxides were converted into products with the same retention times (GC) and mass spectra as for the hydroxylinoleic and linolenic acid methyl esters which were found as reaction products of the thermal hydroperoxide decomposition (see Experimental Section). As other products of the reaction between the hydroperoxides and cobalt(I1) naphthenate were found cobalt(II1) naphthenate and water. The stoichiometry of the reaction was determined from the relative conversion and formation of reactants and products. The results indicate a conversion as is presented in eq 3. 2C0"

+ RlRpHCOOH

-+ 2H+

2Co"'

H 2 0 + RIRzHCOH (3)

The kinetics of reaction 3 were determined by measuring the initial rates. Under these conditions the reaction of the hydroperoxides with cobalt(II1) naphthenate (see below) can be neglected. The rates were found to be first order in the concentration of both reactants (Figure 1). The values of the rate constants k 2 are given in Table IV. The k z values give a linear Arrhenius plot from which the activation energy is calculated (see Table IV). This reaction is quite similar to the oxidation of cobalt(I1) acetate by peracids (Hendriks et al., 1979) and is therefore explained by an analogous reaction mechanism (4-51, in which reaction 4 should be rate-determining: CO" + R1RZHCOOH

H+

CO"'

+ RIRZCHOH + -OH (4)

+ 2H+ (6)

The kinetics of this reaction were also determined by measuring the initial rates in order to exclude the decomposition of the hydroperoxides by CO" (see above). It was found that the rates are first order in the concentration of both reactants (see Figure 1). The values of the rate constants k3 and the activation energy, calculated with the method of Arrhenius, are given in Table IV. Because similar results were obtained for the decomposition of tert-butyl and cumyl hydroperoxide by cobalt(II1) acetate in acetic acid solution (Hendriks et al., 1979), reaction 6 is explained by an analogous reaction mechanism (7-9) in which reaction 7 should be rate-determining. CO"'

+ R1R2HCOOH

-

2R1RzHC00. 2R1R2HCO.

-+

CO" + RlR2HCOO- + H+

-

2R1R2HCO. + 02

RlRzHCOH

+ RlRzCO

(7) (8) (9)

Conclusions The information in the literature about the autoxidation of methyl linoleate and methyl linolenate mainly concerns the formation and isolation of hydroperoxides and the identification of some of the fragmentation products formed therefrom. This paper supplies new and more detailed information concerning the kinetics and product distribution during the different stages of these autoxidations in n-heptane solution. During the initial stage a mixture of hydroperoxides is formed with a cis, trans conjugated diene structure and a hydroperoxy group attached to one of the a-methylene groups. The initiation reaction is a combination of a thermal and photochemical reaction between the methyl esters and oxygen. The hydroperoxides are thermally decomposed to the corresponding hydroxy and keto compounds. With Co" the hydroperoxides react to form the corresponding hydroxy compounds. In the presence of Co"' the hydroperoxides are converted into a 1:l mixture of the hydroxy and keto compounds. It follows from the kinetic and stoichiometric parameters found for these hydroperoxide decomposition reactions that the mechanism is quite similar to the decomposition of peracids and hydroperoxides in CO" or Co"' containing acetic acid solutions (Hendriks et al., 1979). In the final stage of the autoxidations fragmentation of the C-C chain occurs as well as formation of oxo bridges between the methyl ester molecules. The number of double bonds is not changed during the autoxidation of the methyl esters, which means that no diene polymerization has taken place even though the radicals produced by the autoxidation processes mentioned above possess the ability of initiating polymerization reactions. It is possible

216

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Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979

that oxygen, present in the solutions under the reaction conditions used, strongly inhibits this polymerization. The autoxidation of unsaturated components of a paint film causes a continuous production of chain carrying and peroxy radicals. As a result of these autoxidation reactions, hydroperoxides are formed and oxygen is consumed. When oxygen is exhausted to a sufficient extent, polymerization reactions may occur, which lead to the ultimate hardening of the paint film. The hydroperoxides formed by the autoxidation reactions are decomposed by cobalt(I1) and cobalt(II1) naphthenate, which also results in the formation of free radicals. This hydroperoxide decomposition will therefore be one of the reactions by which cobalt naphthenates accelerate the hardening of the paint film. The other reactions by which cobalt(II1) naphthenate can initiate and accelerate the polymerization reactions will be dealt with in a following paper. Literature Cited

Cobb, W. Y., Day, E. A., J. Am. Oil Chem. SOC.,42, 1110 (1985). Cannon, J. A., Zilch, K. T., Burket, S. C., Dutton, H. J., J. Am. 0llChem. Soc., 29, 447 (1952). Fugger, J., Cannon, J. A., Zilch, K. T., Dutton, H. J., J. Am. OilChem. SOC., 28, 285 (1951). Gibson, G. P., J. Chem. Soc., 2275 (1948). Gilhm, A. E., Stern, E. S., "An Introduction to Electronic Absorption Spectroscopy in Organic Chemistry", Edward Arnold Publishers, London, 1957. Hendriks, Ch. F., van Beek, H. C. A., Heertjes, P. M., Ind. f n g . Chem. Prod. Res. Dev., 16, 270 (1977). Hendriks, Ch. F., van Beek, H. C. A., Heertjes, P. M., Ind. fng. Chem. Rod. Res. D e v . , 18, 38 (1979). Hoffmann, G., J. Am. OilChem. SOC.,38, l(1961). Horvat, R. J., et al, J. Am. Oil Chem. SOC.,48, 94 (1969). Khan, N. A., Can. J. Chem., 37, 1029 (1959). Pasto, D., Johnson, C., "Organic Structure Determination", p 272,PrenticeHall, Englewood Cliffs, N.J., 1969. Privett, 0. S., et al., J. Am. Oil Chem. SOC.,31, 23 (1954). Privett, 0.S.,J. Am. Oil Chem. Soc., 36, 507 (1959). Rawls, H. R., van Santen, P. J., J. Am. 011 Chem. SOC.,47, 121 (1970). Ricciuti, C., Wlllits, C. O., Ogg, C. L., Morris, S. G.. Riemenschneider, R. W., J . Am. OliChem. SOC.,31, 456 (1954). Sephton, H. H., Sutton, D. A., J. Am. Oil Chem. Soc., 33, 263 (1963). Tolberg, W. E., Wheeler, D. H., J. Am. 011 Chem. SOC., 35, 385 (1958). Wan, G. W., Otto, J. B., J. Am. Chem. SOC.,6% 836 (1947).

Received for review April 24, 1978 Accepted March 6, 1979

American Society for Testing Materials, "Paint Materials Specifications and Tests: Naval Stores; Aromatic Hydrocarbons", Part 20, 1966.

The Autoxidation of Methyl Linoleate and Methyl Linolenate in the Presence of Cobalt( 11) and Cobalt( 111) Naphthenate in n-Heptane Solution Charles F. Hendrlks, Pieter M. Heertjes, and Hendrlk C. A. van Beek' Laboratory of Chemical Technology, University of Technology, Delft, The Netherknds

In the autoxidation process of methyl linoleate and methyl linolenate carried out in visible light in solutions to which cobalt(I1)naphthenate had been added, three different stages were observed. In the first stage the autoxidation is inhibited by cobalt(I1)naphthenate. Initially the hydroxy derivatives of the methyl esters and Co"' are formed by a slow reaction. Co"' contributes to the initiation by oxidation of the methyl esters. The combined effect of Co" and Co"' therefore leads to an autocatalytic conversion of CO" into Co"', the rate of which increases strongly with decreasing stirring rates. The second stage starts after 90-95% conversion of Con.The CoIn/Co"concentration ratio then reaches a constant value. In this stage the methyl esters are mainly converted into the corresponding hydroperoxides, while the corresponding hydroxy and keto compounds are formed as secondary products. In the third stage fragmentation products are formed and Co"' is converted into an inactive Co" complex. For the first two stages a reaction scheme is proposed which provides an adequate description of the results obtained. Conclusions are drawn for the processes taking place in drying paint films containing cobalt(I1) naphthenate.

Introduction The thermal and photochemical autoxidation of methyl linoleate and methyl linolenate in n-heptane solutions was described in a separate paper (Hendriks et al., preceding paper in this issue). This system was chosen as a model system for the drying of a paint film. Because the rate of the drying of paint is strongly enhanced by the addition of Co salts, the present authors also studied the effects of cobalt(I1) and cobalt(II1) naphthenate on the autoxidation reactions. It is proposed that the autoxidation of the methyl esters catalyzed by cobalt naphthenates can be explained by the reacting scheme given in reactions 1-11 initiation hu, 0, RlR2CHz RlRzCH + HOO. hi (1) Co"'

-

+ RlR2CHz

Co"

+ R1R2CH+ H+

k5, k6

(2)

0019-7890/79/1218-0216$01.00/0

CO"'

+ RlRzHCOOH

propagation RlRzCH

+

+02

CO" + R1RzHC00.

-

-

RlR2HCOO.

R1RZHCOO. + R1R2CH2 RlRzHCOOH termination Co"' + RIRzCH

H2O

-

H+

+ RlR2CH

+ R,R,HCOH + H+

-+

CO"'

0 1979 American Chemical Society

0 2

k3 (3) (4)

k7

+ RlRzHCOOH RiRZHCOH + RlRzCO +

CO" + RiR2HC00. 2RIRzHC00.

Co"

+ H+

k8

(5)

k9 (6) klo

(7)

k11

(8)