Inhibitors in Cracked Gasoline 11. Correlation of‘ Inhibiting Action and Oxidation-Reduction Potential C. D. LOWRY, JR.,GUSTAVE ~ L O F FJ. , C. hfonmm, AND C. G. DRYER Universal Oil Products Company, Riverside, Ill. OUREU aiid Dufraisae (17) have reviewed their own extensive work on antioxidants. Earlier experiments in this field were reported by Seyewetz and Sisley ( 2 0 . Progress to date in aut o x i d a t i o n has been summa-
A correlatiora is established betueen the efectiveness gasoline inhittitors as measured by
accelerated oxidution lest and their critical oxidation Potentiah. The best inhibitors tested have potentials between 0.600 and 0.800 volt; fair inhibitors are in the range 0.800 to 1.043 volts. ?,,ilh the lathr figare have practically no inhibiting action under the conditions Of test. ffydroquinone and cerlain ethers show less i7hibiting action in the bomb kat than their potentials would lead one to th,is is attributed to their tendency to undergo direct oxidation.
compoun&
rized Thebypower Milasof( 2 4preventing ). action of free oxygen on sensitive niaterial is possessed largely by s u b s t a n c e s which are themselves readily oxidized. This was by and Dufraisse (17) and pointed out e a r l i e r b y M i t t r a a n d Dhar (16) who stated, “it seems possible that the pheuommon of negative catalysis is possible only nhen the catalyst is liable to be oxidized.” In contradiction, Milas (14) reports inhibition of oxidation of anethole by benzoqninone and anthraquinone, which, be st,ates, “are far from being easily oxidized.” Of import in this connection is inhihition hy l,%naphthoquinone, which was reported by Mat.ill (13) and in Part I of this paper (7). Fieser (9) su~gcststhat tlie mechanisni of inliibition by this substance is addition of water to form a trili3.droxynaphthalene, an oxidizable subst,ance wlriclr is the actual inhibitor. Such an addition was shown by Fieser and Peters (IO) to be the first step in the decomposition of 1,2-naplithoquinone, which is urLst.able in solution. During the period in which it protects a substance from oxidation, an inhihitor is slowly oxidized, and its oxidation is an induced react,ion. This %-asclearly shown by Alyea and Backstrom ( I ) in a study of the inhibition by alcohols of rodiiini sulfite oxidalion, in whir11 tbry nnea~riredt!!e rates
of o x i d a t i o n of tlie alcoliols used as inhibitors. Dhar (6‘)
z:r:
~ ~ ~ h ~ tive is actually oxidized.” Monreu and Dufraisse (171, while assuming oxidation of the
::’’I
~ ~ ~ ~ ~ tion and write, “One recovers the three bodies (oxidizable substances, inhibitor, and oxygen) in t.heir original state.” Their mechanism, however, has been severely criticized by Milas (14) and Perrin ($0). The chain theory of reaction meclianism is useful in explaining inliibitor action. The hypothesis of so-called chains of reacting molecules was crystallized in 1924 by Christiansen ( 3 ) . He stated that in a bimolecular reaction, unless it is immeasurably fast., only a fraction of the pairs of molecules which collide are able to react. Those reacting are activated molecules w!iose energy exceeds a certain value. Just after reaction, “the molecules of the reaction products possess an available energy great.ly in exccss of t,be mean energy a t the temperature considered. Now, these very ‘liot’ molecules have sufficient energy to activate molecules of the rcactants a t the first encounter, and when these react, the resultants in their turn again are able to act as activators, and so on. Consequently, it is possible that the occurrence of one elementary reaction mill give rise t,o a whole series of such reactions.” In negative catalysis, “foreign molecules * * * a r e able to take up the energy from the [hot’ molecules of the reaction products.” In a later publication Chrisdiansen (4) declared that the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
autoxidations of organic aldehydes investigated by hloureu and co-workers “are strongly inhibited by very small amounts of oxidizable substances, as, e. g., hydroquinone, and must consequently, according to our theory, be chain reactions.” By similar reasoning the inhibition of gum formation in gasoline may involve a chain mechanism. Backstrom ( 2 ) has also stated, “The role of the inhibitor, in thermal as well as in photochemical oxidation, consists in the breaking of reaction chains.” Similarly. Alyea and Backstrom ( 1 ) in their study of negative catalysis of sulfite oxidation by alcohols postulated “that the alcohol is oxidized in breaking the reaction chain, and that this induced oxidation is completely incapable of carrying on the reaction chains.” To account for tlie ability of an inhibitor to prevent the continuance of reaction chains, Milas (2 ?) has recently proposed a mechanism based on current studies of electron relationships. He considers as a more general property of inhibitors than their oxidizability, the possession of loosely bound molecular valence electrons by which. on collision with a reactive “dative” peroxide (assumed as the first product of an autoxidation), all the excess energy of the latter is completely absorbed, initiation of new reaction chains prevented, and rate of autoxidation greatly reduced. “The active molecules of the inhibitor can then be oxidized either by the organic peroxides or by free oxygen, or can combine with the active autoxidant molecules forming unstable complexes which may decompose to yield the original inhibitor molecules.” The chain mechanism has not been universally accepted. Moureu and Dufraisse (19) and Dhar (6) have raised objections to its employment as a means of explaining inhibitor action. The present authors do not wish to imply, in this summary of literature statements which are related to their work, that they believe all inhibitions of autoxidation to have the same characteristics. Substances found to have antioxidant properties in some degree are so varied-for example, Milas (14) cites inhibition by sulfuric acjd, ferric chloride, iodine, and trinitrotoluene--that it is scarcely to be expected that one set of generalizations will cover all. OXIDATION-REDUCTION POTENTIAL The statements of prior investigators indicate that an inhibitor is usually an oxidizable substance, that it undergoes oxidation as an integral part of its action in checking autoxidation, and that its oxidation in some way stops the propagation of reaction chains. Assuming this to be true, a relationship might be expected between the ease with which a compound is oxidized and its inhibiting effectiveness. The more easily an inhibitor is oxidized, the more quickly might it break a chain of reacting molecules, and, since these reactions appear autocatalytic, the more effective it should be, provided it is not so sensitive as to be destroyed by side reactions under the conditions of use. Moureu and Dufraisse (18) were apparently inclined to such an idea when they stated: “The substances (antioxidants) should be sufficiently oxidizable but not too much so. One is thus led to suppose that for each autoxidizable body, situated under definite conditions, there should correspond an optimum degree of oxidiaability to be possessed by the catalyst for use with it.” I n the case of a compound which is a member of an oxidation-reduction system in which both reductant and oxidant are stable, the normal oxidation-reduction potential is a measure of the tendency to be oxidized or reduced. For example, since hydroquinone and quinone are stable compounds, the normal oxidation-reduction potential of the hydroquinone-quinone system serves as a measure of the
80 5
ease of oxidation of hydroquinone. But for components of unstable oxidation-reduction systems, usual methods of oxidation-reduction potential determinatipn are not applicable. Most of the effective inhibitors for gasoline are members of such unstable systems, and their normal oxidation-reduction potentials have not been determined. Their tendencies to be oxidized can be measured, however, by their critical oxidation potentials. The concept of “critical” oxidation potential was introduced by Fieser (8) to make possible quantitative measurement of the oxidizability of reductants of oxidation-reduction systems in which the oxidants are unstable. Critical oxidation potential is defined by Fieser as the potential a t which the rate of oxidation of the reductant of an oxidation-reduction system becomes so small as to be just detectable. It is determined by measuring the potential of a solution of a stable oxidation-reduction system, usually an inorganic one, in which “a certain small amount of the substance being tested (0.5 per cent) is oxidized in five minutes when equivalent amounts of the sample and of the oxidizing agent are employed.” In some systems both the normal and critical potentials can be determined, and the critical oxidation potential is found to be about 0.136 volt below the normal oxidation-reduction potential where one hydrogen atom is removed by oxidation, as in the case of a monohydric phenol, and 0.068 volt below the normal potential in a compound of the hydroquinone type, where two hydrogen atoms are removed. Differences and similarities in structure have been found by Fieser to have consistent effects on critical oxidation potentials. From his study of seventy phenolic and amino compounds, he has arranged a table of the Effect on the Critical Oxidation Potential of Ortho or Para Substituents. This allows calculations of the potentials of many compounds without direct measurement, by their relationship to compounds whose potentials have been determined. The potentials in the present work designated “calculated” were found by use of this table. The potential of 4-chloroZaminophenol, for example, was estimated by adding to the critical potential of o-aminophenol, which is 0.673 volt, the value given by Fieser for the effect of introducing chlorine, 0.005 volt. Subtracting the effect of a methyl group, -0.052 volt, from the potential of p-aminophenol, 0.673 volt, gave 0.621 volt for the potential of 2-amino-5-hydroxytoluene. A substituent in the meta position is reported by him to have but 16 to 18 per cent of the influence on the potential that is exerted by the same group placed ortho or para. CORRELATION OB POTENTIAL AND INHIBITING ACTION The authors have found that the critical oxidation potentials of inhibiting substances can be correlated with their value as inhibitors in gasoline, as determined by the accelerated oxidation test. The compounds with which this correlation is demonstrated were selected to cover a wide potential range. They were tested in equimolecular concentrations, using 0.00005 mole of inhibitor per 100 grams of gasoline. Where necessary, because of sparing solubility in gasoline, the inhibitor was dissolved in 5 cc. of an organic solvent before addition to 200 cc. of gasoline. As earlier work had shown that a solvent sometimes affected the induction period, with each inhibitor, tests were made, where possible, using more than one solvent, and, if the results did not agree, the longest induction period found was used in the tables. The gasoline was a Pennsylvania cracked product which had been treated with fuller’s earth in the vapor phase, and, until used, was kept under nitrogen t o prevent deterioration. It was the
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INDUSTRIAL AND ENGINEERING CHEMIS'rRY
gasoline employed in comparing - inhibitors in Part I of this series ( 7 ) . Its properties were as follows: Color, ' haybolt G u m mg./100 cc.: Cobper dish 4 i r jet Octane number Induction period (oxygen bomb), min.
30 163 0 73 45
A. 8 . T. Id. 100-CC. DISTILLATION
c.
O
F
33
92
62 134 192 205
144 274 378 40 1
The tests were made in bombs designed in this laboratory, which have been described ( 7 ) . As in earlier work, the gasoline samples in glass bottles were exposed to 100 pounds per square inch (7 kg. per sq. cm.) oxygen pressure a t 100" C.
Vol. 25, No. 7
compounds over their meta isomers found when comparine: resorcinol and catechol, or the cresols and xylenols, has been pointed out as a structural regularity. It is now seen to be linked with the effect of ortho or para substitution on potential. Similar to the ortho-meta relationship is the superiority of a- over &naphthol, which is related to the lower potential of the former compound. The &naphthol induction period does not fall, however, exactly on the curve in Figure 1. -
TABLE
I
I. COMPOUNDS CONTAINIXG HYDROXYL GROUPS (0 00005 mole per 100 grams gasoline)
CRITICALOXIDATION POTENTIAL^ VGh Pennsylvania gasoline alone p-Sitrophenol 1:433 o-Nitrophenol 1,433 m-Nitrophenol 1.147 Phenol 1,089 m-Cresol 1,080 o-Cresol 1.040 n-Cresol 1.038 2-Hydroxy-1 4-dimethylbenzene 1.03Sb 4-Hydroxy-l:2-dimethylbenzene 1.036b p-Hydroxydiphenyl 1,036 ,%Naphthol 1.017 2-Hydroxy-l,3-dimethylbenzene 0.8958 4-Hydroxy-1,3-dimethylbenzene 0.895 Phloroglucinol 0.799 m-Kaphthol 0.797 Catechol 0.742 1,5-Dihydroxynaphthalene 0.673 Hydroquinone 0.631 0.609 Pyrogallol a D a t a from Fieser except calculated values. b Calculated.
INDUCTION
PERIOD Min. 45 45 60 60 60 90 120 165 160 150 150 255 255 255 75 1920 1890 1380 135 1440
If a substituted group increases the potential of a hydroxy compound, the meta derivative has the lowest potential and is no longer inferior to its ortho and para isomers. This is the case in the nitrophenols. While in the concentrations used in Table I their inhibiting properties were almost negligible, in an experiment in which the concentrations mere increased twenty times (0.001 mole per 100 grams of gasoline, with benzene added t o effect solution) the following induction periods were obtained: p-Nitrophenol o-Nitrophenol m-Nitrophenol
Mznuies 105 165 150
The meta compound is nearly as effective as the ortho and distinctly superior to the para derivative. I Table I contains two figures n-hich require explanation. 400 .?OO .&a0 ,.ooo /./OO /.zoo The induction period of phloroglucinol is much shorter than C R / r / C A L O X / D A r / O N POT€HT/A$ V O L T S might be predicted from its potential. Phloroglucinol, FIGURE1. CORRELATION BETWEEN INDUCTIOX P E R I O D however, besides having all its hydroxyl groups in the unAND CRITICAL OXIDATION POTEXTI.4L FOR COMPOUNDS COIVTAINING HYDROXYL ASD AMINOGROUPS favorable meta relationship, is a sensitive compound which may rearrange or break down during the test. Moureu following the procedure of Hunn, Fischer, and Blackwood and Dufraisse (18) explain the weak inhibiting action of (11). The time from the beginning of heating until rapid phloroglucinol in their tests as perhaps due to its existence absorption of oxygen began, minus a correction of 15 minutes in the ketone form, which would not be expected to possess for the time during which the bomb was heating, was recorded inhibiting power. Hydroquinone is another exception, as it gives a short as the induction period. The correlation between induction period and critical induction period, although it has a low potential, and storage oxidation potential is best shown among related compounds. tests have shown it to be an effective inhibitor. Alyea and Table I gives data for a number of compounds containing Backstrom ( 1 ) also obtained irregular results in quantitative work on the inhibiting action of hydroquinone. They hydroxyl groups. These data, together with those for compounds containing believed that the inhibiting action of this substance was amino groups, are shown graphically in Figure 1. The disturbed by its direct oxidation. It is probable that in names of compounds whose values do not fall close to the the bomb test, hydroquinone is autoxidized and for this reason gives a short induction period. This direct oxidation curve are given. As the potential decreases, the induction period lengthens, would be more pronounced a t the elevated temperature used shows a sharp peak between 0.800 and 0.700 volt, and drops in the accelerated test, where hydroquinone gives poor slightly a t still lower potentials. The variations of induction results, than in storage a t room temperature, where it is period with structure, which were emphasized in Part I (Y), effective. That this explanation is probably correct is are paralleled by relationships between induction period and indicated by results of oxidizing 5 per cent solution in alcohol potential. The superiority of ortho- or para-substituted of hydroquinone and of catechol, which behaves normally in
July, 1933
INDUSTRIAL AND ENGINEERING
the bomb test, under the usual conditions (100 pounds per square inch, or 7 kg. per sq. cm., initial pressure a t 100" C.). The oxygen pressure in the bombs dropped as follows: DROPI N O X Y Q E N PRESSURE Hydroquinone Catechol Pounds/souare inch iko./so. . _ em.). 0 ( 0 ) 5 (0.35) 7 (0.49) 9 (0.63) 10 (0.70) 11 (0.77) 13 (0.91) 13 (0.91)
Alcohol alone was not oxidized under these conditions. The results indicate autoxidation of the hydroquinone a t a much more rapid rate than catechol. While it is not possible from this experiment a t high concentration to predict the rate of hydroquinone autoxidation a t the low concentration used in testing its inhibiting effectiveness, the relative rapidity of its oxidation supports the conclusion that direct oxidation is ' the cause of the ineffectiveness of hydroquinone in the oxygen bomb test. The critical oxidation potential is perhaps a! better index of the value of hydroquinone as an inhibitor than the bomb test. The rate of autoxidation of an inhibitor cannot be predicted from potential measurements, although compounds of low potential probably have a greater tendency to autoxidation than those of high potential. A careful distinction should be made between direct or autoxidation of the inhibitor, which impairs its utility, and induced oxidation of an inhibitor, probably brought about by the first products of gasoline oxidation, vhich appears to be essential to its protective action. I n Table I1 correlation of potential and induction period is shown for a number of compounds containing amino groups.
111.
A M I S 0 AND
HYDROXY ETHERS
CRITICAL OXIPATION POTESTIhL'
Volts Pennsylvania gasoline alone p-Anisidine Guaiacol Eugenol Pyrogallol dimethyl ether Isoeugenol
0:892 0.868 0.831 0.760 0.757
INDVCTION PERIOD .%f
