ANTIOXIDANT INHIBITION BY SUBSTITUTED P H E N O L S Antioxidant E$icacy as a Function of Structure and Reactivity W . G. L L O Y D , ' R . G. Z I M M E R M A N , A N D A. J . D I E T Z L E R The Daw Chemical Ca., Midland, Mich.
Induction periods conferred by a number of substituted phenols at 0 . 0 0 2 M have been measured under standard conditions in cumene a t 126.5' C. and in a-methylstyrene at 81 .O' C. Antioxidant efficacies in both systems show good correlations with the polarographic half-wave oxidation potentials of the phenols and also correlate well with the sums of electrophilic substituent constants, even for several trisubstituted phenols. The emergence of a well-defined optimum reactivity in the cumene inhibition data is interpreted in terms of a dual competition of the chain-transfer inhibition reaction with both substrate propagation and direct antioxidant-oxygen reactions.
HE effects of temperature and phenolic inhibitor concentraT t i o n upon the induction periods of cumene were explored in a previous paper (26). The results of a study of substituent effects upon the antioxidant efficacies of some simple phenols are now reported here. Most simple autoxidations have been found to follow the course of Reactions 1 to 4, with Reaction 3 the rate-determining propagation step. Because of the long kinetic chain length of most autoxidations, a great deal of oxidative degradation can occur as the result of a small amount of adventitious initiation. The ideal chain-transfer antioxidant reacts readily with the chain-carrying organoperoxy radical, yielding products substantially incapable of continuing the oxidation chain (Reaction 5 ) . Usually the resonance-stabilized radical formed by Reaction 5 stops a second peroxy radical, as indicated by Reaction 6 (2, 5, 6 ) . The crucial competition is that between Reactions 3 and 5 : For effective inhibition the latter must dominate, even though the antioxidant concentration will frequently be only 1OW2 to 10-6 that of the substrate material to be protected. A good antioxidant, therefore, must be highly susceptible to hydrogen abstraction by an organoperoxy radical.
(Initiation)
Re
+
nR.
(1)
ROO.
(2)
-F
0 2 +
+ RH ROOH + R . ROO + ROO. inert R O O . + AH A . + inert ROO. + A. inert
ROO.
+
+
-F
-+
(3) (4)
(5) (6)
As susceptibility to Reaction 5 is increased indefinitely, however, another problem emerges-namely, that of direct reaction between antioxidant and oxygen. The analogy with Reaction 5 is evident if molecular oxygen is represented as a diradical : *OO.
+ AH+
HOO.
+ A.
(7)
At best Reaction 7 is a side reaction wasting antioxidant. At the worst, HOO. radicals escaping the solvent cage are likely 1
326
Present address, The Lummus Co., Newark, N. J. I & E C PRODUCT RESEARCH AND DEVELOPMENT
to enter into further homolytic reactions, making of Reaction 7 another oxidation-initiation step. I n any particular system, therefore, the desired antioxidant must be so highly susceptible to attack by peroxy radicals that it can successfully compete for these radicals against the much more abundant substrate material, and yet it must not be susceptible to the analogous direct oxidation by molecular oxygen. For effective inhibition, Reaction 5 must dominate both Reaction 3 and Reaction 7. Evaluation of chain transfer antioxidants under nonbranching conditions-e.g., with chemically initiated systems a t low temperatures-typically shows a plateau of efficacy. Experiments under such conditions are useful for studies of initiation rates and inhibitor stoichiometry, but are not likely to afford sensitive discrimination among fairly good-to-excellent antioxidants. I n degenerately branching systems, however, induction periods are responsive not only to the qualitative dominance of us over wg but also to the chain length of the inhibited autoxidation-Le., to the kb/k3 ratio. I n these systems, therefore, measurements of induction period provide no plateaus, but instead show a continuum of varying induction periods among fairly good-to-excellent antioxidants. Furthermore, under conditions such that the most reactive antioxidants are also susceptible to Reaction 7, a specific optimum antioxidant reactivity may be found. Several studies have provided useful guidelines to antioxidant reactivity. Bolland and ten Have found that electrondonating substituents enhance the antioxidant efficacy of simple phenols (3, 4 ) , and a similar effect has been found with substituted catechols (30). The rate of Reaction 5 with l-tetraalkylperoxy radicals has been correlated with the calculated energy of the highest occupied molecular orbital for a series of substituted phenols ( g ) , and a good correlation has been reported between gasoline induction periods and the critical oxidation potentials of a group of phenolic inhibitors (37). Several studies have shown that antioxidant efficacies of phenols show a fairly good Hammett p-u correlation (74, 75, 24), which is further improved when steric factors are held constant or corrected for. The reaction rates of phenols with diphenylpicrylhydrazyl-also follow apother radicals-e.g., proximately a p-u relationship (79). Recent studies by Howard and Ingold have indicated better correlations for
several systems with u + (electrophilic substituent) constants (20-22, 24). The present study seeks to illuminate the three-cornered relationship among molecular structure, chemical reactivity, and antioxidant efficacy for a group of simple phenols. Specifically an accessible parameter of reactivity of a candidate phenol is sought that will c o r r e l a t e a t least under model oxidation conditions-with the actual antioxidant efficacy of the phenol. A second objective is the correlation of this reactivity parameter with the structure of the phenol. Experimental
Phenols used were purified by recrystallization to constant melting point, or by refractionation under nitrogen a t reduced pressure until a constant refractive index was attained. Cumene and a-methylstyrene were center cuts of the best available commercial grades, fractionated under prepurified nitrogen and stored under argon in light-shielded glass vessels. For cumene oxidations, stock solutions were prepared containing exactly 0.0200 mole per liter (1M) of each phenol (0.0100M for the bisphenols), and immediately before each oxidation run, the appropriate phenolic solution was diluted volumetrically with freshly withdrawn cumene to provide operating phenol concentrations of 0.00200M (0.OOlOOM for the bisphenols). For oxidations with a-methylstyrene the phenols were weighed directly into freshly withdrawn 100-ml. portions of hydrocarbon immediately before each run. Oxidations were carried out with a standard gasoline oxidation apparatus (7), modified to permit operation a t temperatures above 100' C. (25, 26). The oxidation procedure and its precision have been noted (26). In addition to those oxidations carried out to measure induction period, a series of extra cumene oxidations was carried out for 18.0 hours a t 126.5' C., which period fell entirely within the induction periods of the best 14 phenols tested. These 18-hour oxidates were then examined for oxidation products formed during the induction periods. Aliquots of the 18-hour oxidates were purged with argon and assayed for total oxygen by neutron activation analysis. (The oxygen present owing to the phenolic inhibitor is negligibly small, less than O.OlyL,.) Oiher aliquots were examined by gas chromatograph, on an 8-foot X I/d-inch column of diethylene glycol succinate 20y0 and phosphoric acid 2yo on Celite a t 150' C. The major products of cumene oxidation under these conditions-methanol, acetophenone, cumyl alcohol, and a-methylstyrene-were determined with the aid of calibration curves from pure standards. Cumyl alcohol readily undergoes partial dehydration under these conditions of analysis ( 7 3 ) ; the analytical results in Table I are corrected for this systematic error. Cumene Oxidation Products
Cumene autoxidation proceeds via formation of cumyl hydroperoxide [Reactions 1 to 4, R = C,jH&(CH3)2.], which itself is subject to a slow first-order and a faster radical-induced decomposition (70, 29), leading to acetophenone and cumyl
alcohol. The alcohol is likely to undergo in situ dehydration to a-methylstyrene (73, 77), which in turn is known to be highly susceptible to autoxidation, yielding principally 2methyl-2-phenyloxirane ("a-methylstyrene oxide"), acetophenone, and a polymeric peroxide (78, 27). These expected products were found in nearly all of the inhibited autoxidation mixtures (Table I). For the first two sets of analyses in this table, all of the products found were formed during the induction periods; the latter sets show increasing contributions from postinduction-period oxidation. Acetophenone predominates over cumyl alcohol in the better inhibited systems, but the increasing prominence of the alcohol with decreasing inhibition suggests it to be the major product of radical-induced hydroperoxide homolysis, as it is in the straight preparative autoxidation of cumene ( 7 7). aMethylstyrene is found most abundantly in weakly inhibited systems. This is reasonable if it is principally formed by dehydration of cumyl alcohol (77), which itself is scarce in the better-inhibited systems; with uninhibited systems, on the other hand, a-methylstyrene is so highly susceptible to autoxidative polymerization (78, 27) that its concentration is held to a low level. 2-Methyl-2-phenyloxirane is a measure of the extent of polymerization-depolymerization of a-methylstyrene (27), and hence is abundant only in weakly inhibited and uninhibited systems. A comparison of total products with total oxygen gives a crude material balance. Since methanol (detected but not assayed quantitatively) is the stoichiometric coproduct of acetophenone, and water is the coproduct of a-methylstyrene, the total oxygen content for the well-inhibited systems is accounted for, within experimental error, by these nonpolymeric products. With progressively poorer inhibition, however, the total oxygen content becomes increasingly greater than that accounted for by the sum of nonpolymeric products. This, together with the presence of the oxirane, suggests that significant amounts of poly (a-methylstyrene peroxide) are formed under noninhibited conditions. Induction Periods and Half-Wave Potentials
The induction periods obtained with 0.00200M concentrations of simple phenols, in cumene a t 126.5' C. and in amethylstyrene a t 81.0' C., are listed in Table 11. Also shown are the polarographic oxidation half-wave potentials for each phenol, measured a t 25' C. in anhydrous methanol using a fresh-surfaced graphite electrode. Bisphenols are often used to advantage where low volatility is desired. Several bisphenols have also been examined, a t 0.00100M concentrationLe., a t the same phenolic functional group concentration as that of the monohydric phenols. Results of these runs, with the corresponding half-wave potentials, are shown in Table 111.
Products from Inhibited Autoxidation of Curnene at 126.5' C.a 2-Methyl-2Total Total Antioxidants Acetophenone Cumyl Alcohol a-Methylstyrene phenyloxirane Products Oxygen 3 bestb 0,0025 vi
Ar-0-H
.. .. .. ..
-0-0-R
0.6
e
c;t,
[
Ar-0-H
..
5 0
r!
:0-0-R
I=
.. ..
Ar-0. H-0-0-R
.. ..
.’
I
-N
W
Such a transition state has been predicted from theoretical considerations (72).
0.4
Conclusions
A
0.2
-I
- 0.5
+ 0,5
+I
Figure 2. Polarographic oxidation half-wave potentials of 16 phenols as a function of the sum of electrophilic substituent constants 0 Monosubstituted phenols H Disubstituted phenols A Trisubstituted phenols
Commercially important problems of antioxidant stabilization have traditionally been solved by brute force-i.e., by massive antioxidant evaluation studies which must be repeated for each new problem. The foregoing relationships suggest other modes of attack upon these problems. Actual antioxidant efficacies have been correlated with various electrochemical measurements of the inherent oxidizibility of the phenolic inhibitors. Stillson and Sawyer have used Conant and Pratt’s “critical oxidation potential” (8, 37). Meier and Mebes have correlated the control of linseed oil drying with oxidation-reduction potentials (28). The present study has shown consistent correlations for two model systems VOL. 5
NO. 4
DECEMBER 1 9 6 6
329
with polarographic oxidation half-wave potentials. Other more elegant measurements of the ease of electron abstraction, such as ionization potential which for phenols is known to correlate with u+ values (34,are also available. For any specific substrate and condition of oxidative exposure there probably exists an optimum value range for this reactivity parameter. This optimum may possibly be located by a direct fundamental measurement of substrate reactivity; but it will always be empirically accessible by a single set of induction period measurements using a selected group of antioxidants of appropriately spaced reactivities. Given the apparently general correlation of phenolic antioxidant efficacy with the sum of electrophilic substituent constants, it is possible to predict antioxidant efficacy with fair confidence from chemical structure. This in turn suggests that for many systems massive inhibitor screening may profitably be replaced by the purposive tailoring of antioxidants for optimum performance under the desired conditions. Nomenclature
AH A. El/z
= = = k3,k5,etc. =
M
= =
n
RH R. ROO
= =
-
= =
S.C.E.
=
ti P
= =
U U+
~ 3 w6, , etc.
= =
phenolic antioxidant antioxidant-derived radical polarographic oxidation half-wave potential rate constants for indicated reactions gram moles per liter initiation efficiency factor hydrocarbon hydrocarbon radical organoperoxy radical saturated calomel electrode (reference potential) induction period Hammett reaction constant Hammett substituent constant electrophilic substituent constant rates of indicated reactions
Ac knowledgment
Polarographic potentials were measured by R. Bidwell and total oxygen by neutron activation analysis was determined by 0. U. Anders. We are indebted to H. G. Scholten and T. Alfrey, Jr., for valuable discussions.
Literature Cited
(1) Am. SOC. Testing Materials, “Standards,” ASTM Test D
525-55,Part 7, pp. 263ff, 1958. (2) Bickel, A. F., Kooyman, E. C., J . Chem. SOC.1953, p. 3211. (3) Bolland, J. L., ten Have, P., Discussions Faraday Sac. 2, 252 (1947). (4) Bolland, J. L.,ten Have, P., Trans. Faraday Sac. 43,201 (1947). (5) Boozer, C.E., Hammond, G. S., Hamilton, C. E., Sen, J. N., J . Am. Chem. Sac. 77, 3233 (1955). (6) Campbell, T.W., Coppinger, G. M., Zbid., 74, 1469 (1952). (7) Charton, M., J . Org. Chem. 28, 3121 (1963). (8) Conant, J. B., Pratt, M. F., Chem. Revs. 3, l(1926). (9) Davies, D.S.,Goldsmith, H. L., Gupta, A. K., Lester, G. R., J . Chem. Sac. 1956, p. 4926. (10) Fordham, J. W. L., Williams, H. L., Can. J . Res. 27B, 943 ( 1 949). (11) Fortuin, J. P., Waterman, H. I., Chem. Eng. Sci. 3, spec. suuul.. 60 (1954). (12)’Pudno,T.,Rde, T., Eyring,H., J . Phys. Chem. 63,1940(1959). (13) Gray, P., Pearson, M. J., J . Chem. SOG. 1964,p. 5725. (14) Hammond, G. S., Boozer, C. E., Hamilton, C. E.,. Sen,. J. N., J . Am. Chem. Sac. 77,-3238(1955). . (15)Hedenburg, J. F., Znd. Eng. Chem. Fundamentals 2, 265 (1963). (16) Hedenburg, J. F., Freiser, H., Anal. Chem. 25, 1355 (1953). (17) Helden, R.,van, Bickel, A. F., Kooyman, E. C., Rec. Trau. Chim. 80, 1257 (1961). (18) Hock, H., Siebert, M., Chem. Ber. 87, 546 (1954). (19) Hogg, J. S.,Lohmann, D. H., Russell, K. E., Can. J . Chem. 39, 1588 (1961). (20) Howard, J. A., Ingold, K. U., Zbid., 41, 1744 (1963). (21) Zbid., p. 2800. (22) Zbid., 43, 2724 (1965). (23) Ingold, K.U.,J. Phys. Chem. 64, 1636 (1960). (24) Ingold, K. U.,Zbid. 41, 2816(1963). (25) Lloyd, W. G., J . Polymer Sci. Al, 2551 (1963). (26) Lloyd, W. G.,Zimmerman, R. G., IND.END.CHEM.PROD. RES.DEVELOP. 4, 180 (1965). (27) Mayo, F. R., Miller, A. A., J . Am. Chem. Sac. 80, 2480 (1958). (28) Meier, K., Mebes, K., Farbeu. Lack 58,215 (1952). (29) Melville, H. W.,Richards, S., J . Chem. SOC.1954, p. 944. (30) Sethi, S. C., Aggarwal, J. S., Subba Rao, B. C., Indian J. Chem. 1,435(1963). (31) Stillson, G. H., Sawyer, D. W., cited in ( 75). (32) Stock, L. M., Brown, H. C., “Advances in Physical Organic Chemistry,” V. Gold, ed., pp. 35ff, Academic Press, New York, 1963. (33) Suatoni, J. C., Snyder, R. E., Clark, R. O., Anal. Chm. 33, 1894 (1961). (34) Tait, J. M. S., Shannon, T. W., Harrison, A. G., J. Am. Chem. Sod. 84, 4 (1962). RECEIVEDfor review April 21, 1966 ACCEPTED October 12, 1966
TELOMERIZATION OF ETHYLENE WITH TRIMETHYL BORATE W . T. H O U S E , S. D . S U M E R F O R D , l A . H . N E A L , A N D W . J . P O R T E R ’ Esso Research Laboratories, Baton Rouge, La.
(5) first coined the term “telomin 1942, a considerable amount of research has been done in this field. An extensive review of the literature was made in 1958 by Fox and Field ( 4 ) . We report here the results of our studies on the telomerization of ethylene with trimethyl borate using di-tert-butyl peroxide as initiator. INCE Hanford and Joyce
S erization”
Experimental
The reactor used was a 1-gallon stirred autoclave of Monel construction. A nominal charge of 2 liters of trimethyl 1 2
330
Present address, 2017 Hollydale Ave., Baton Rouge, La. Present address, Esso Chemical Co., Inc., New York, N. Y l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
borate (TMB) was used. The T M B was heated to reaction temperature, and its partial pressure was noted. Ethylene was pressured into the reactor to the desired ethylene partial pressure. A solution of di-tert-butyl peroxide in T M B was then pumped continuously into the reactor at a predetermined rate throughout a given run. At the end of a run, the total liquid product was withdrawn, and most of the unreacted T M B was removed by distillation. Methanol was then added to convert the higher borate esters to higher alcohols and trimethyl borate. The distillation was then continued to yield a bottoms product essentially free of methanol and TMB. The average molecular weight of the product was determined by freezing point depression, and the C, H, and 0 content was determined by direct analysis. The product was further characterized by infrared, gas chromatography, and chemical analysis for functional groups.
