C. J. PEDERSEN Jackson Laboratory,
E. I. du Pont de Nemours and
Co., Inc., Wilmington, Del.
Mechanism of Antioxidant Action in Gasoline
T H E first commercial gasoline antioxidants were developed about 1930 (73), and derivatives of p-phenylenediamine, p-aminophenol, and phenol are used today to protect billions of gallons of cracked gasoline during storage. Although there has been much speculation, no general theory has yet been advanced which explains how antioxidants terminate autoxidative chain reactions in gasoline. This article reviews the different hypotheses on the mechanism of antioxidant action in the light of experimental evidence obtained with antioxidants in cracked gasoline under a certain set of conditions and shows that antioxidants of different types must function in different ways. Autoxidation of Gasoline The autoxidation of gasoline is a chain reaction invoIving peroxy and hydrocarbon free radicaIs. T h e initial stage of the oxidation is characterized by a n induction period during which the hydrocarbons absorb very little oxygen and only small amounts of oxidation products are formed. .-Since the usefgl life
of gasoline lies fully within the induction period, only the reactions occurring during this period need be considered in discussing how antioxidants delay the deterioration of gasoline. T h e relevant reactions are.
+R.Initiator + Product (endothermic)
(1 )
(exothermic)
(2)
(endothermic)
(3)
Interaction of these substances among themselves and with antioxidant
(4)
R-H
-
-.f
+ R-0-0. R-0-0. + R-H R. + R-0-0-H R.
0 2
-+
where R-H is the autoxidizable hydrocarbon and R.is the corresponding free radical. Reactions 2 and 3 tend to be selfperpetuating and are the propagating steps of autoxidation. Antioxidants function by reacting with the chaincarrying free radicals. T h e peroxy radicals, R-0-O., probably predominate in this termination step, since hydrocarbon free radicals react very readily with molecular oxygen. Test Method T h e efficiencies of the antioxidants were determined in thermally cracked I-
gasoline by the oxygen bomb induction period method (ASTM D-525-49), at 100’ C. and about 7 atmospheres of oxygen pressure. They are expressed as molar efficiencies relative to N,N’di(sec-butyl)-p-phenylenediamine,which is arbitrarily classified as 1 0 0 ~ oefficient. The derivatives of $-aminopheno1 and $-phenylenediamine were tested a t concentrations between 5 X 10-6 and 2.5 X lop4 mole per liter of gasoline (approximately 0.002 to 0.005% by weight). The precision of this test method is estimated to be 10%. I t is assumed that antioxidants are added to the gasoline before it has been autoxidized to any extent, and that it is not contaminated by pro-oxidant metal catalysts. Mechanism of Antioxidant Action Autoxidation was not considered a chain reaction until about 1923 when Backstrom began to apply the chain mechanism to the autoxidation of benzaldehyde ( 2 ) . Hence, theories published before 1923, including those of Moureu and Dufraisse (76, 77), are only of historical interest. VOL. 48, NO. 10
OCTOBER 1956
188 1
A generally accepted theory of antioxidant action of aromatic amines is based on the removal by the peroxy radical of a labile hydrogen atom of the amino group (6). If the removal of the hydrogen is critically involved in chain termination, the replacement of the N-hydrogen in the antioxidant molecule by deuterium should decrease the rate of chain termination and thereby lower the efficiency of the antioxidant. For example, Westheimer and Nicolaides have reported that the Gate of chromic acid oxidation of isopropanol which involves the initial removal of the hydrogen on the 2-carbon atom, should be about six times faster than that of pure 2-deutero-2-propanol (23). The effects of deuterated and nondeuterated antioxidants on the induction period of gasoline are compared in Table I. I n order to minimize the possibility of deuterium-hydrogen exchange, the gasoline used for these tests was freed from compounds containing OH, SH, and N H groups by treatment with alumina gel. The induction period of the gasoline thus treated and containing no added antioxidant was 23 minutes. The differences in effect are within the limits of experimental error, and it is clear that the efficiency is not lowered by deuteration. The equality of these two antioxidants was confirmed by their effects on the autoxidation of waterfree terpinolene a t room temperature. I n this case hydrogen abstraction cahnot be the rate-controlling step in chain termination. This is in accord with Boozer and Hammond, who found that the effects of diphenylamine and N-D-diphenylamine on the induction period of azonitrile-initiated autoxidation of cumene are identical (6). Hence, the chain-carrying capacity of R-0-0. is not destroyed by being converted to a hydroperoxide according to the following equation : R-0-0.
1 + :N-H+
I
I + :Pi.
R-0-0-H
(5)
but probably by being changed into a peroxy anion by abstracting an electron from the amino group: R-0-0.
-
I + :N-H+
Table 1.
I
R-0-0:-
+ (*i!-H)+ I
(6)
If Reaction 6 is irreversible, a chain is terminated without involving in any way the detachment of hydrogen from the amino group. Whether R-O-0:subsequently acquires a proton and becomes a hydroperoxide, or whether the ion pair on the right-hand side of the ]I 882
equation persists until it reacts, for example, with another R-0-0.: cannot be decided from the available data. Boozer and Hammond favor the formation of an antioxidant-R-0-0. complex which reacts with another R-0-O., each antioxidant molecule thus terminating two chains (6). Boozer, Hammond, Hamilton, and Sen have published the results of recent work on the inhibitory effects of phenols and amines on the autoxidation of cumene and tetralin dissolved in benzene or chlorobenzene and initiated by azobisisobutyronitrile (7, 14). They have arrived a t a unified theory of the mechanism of antioxidant action which involves the antioxidant-R-0-0. complex and the stoichiometry mentioned above, and correlates the efficiencies of all antioxidants according to Hammett’s equation. Their conclusions, therefore, are contrary to some of the conclusions presented in this article. N, 2V’,-\’’-tetrame thy1 p phenylenediamine is 3oy0 as efficient as N,-V’di(sec-butyl)-p-phenylenediamine. Since the former compound has no labile hydrogen on the amino group, its effectiveness must be dependent upon its ability to donate an electron to the peroxy radical. The abstraction of an electron by R-0-0. will be the easier the higher the electron density of the amino group, and hence the efficiencies of p-phenylenediamine derivatives must be directly related to the electron density of the arylamino groups or to their basicity. This is shown by Table I1 where the efficiencies of symmetrically AV,S’-disubstituted p-phenylenediamines are compared. Substitution by these hydrocarbon groups increases the efficiency of p phenylenediamine. The electron-releasing amino group and dimethylamino group in the hydrocarbon substituents enhance the efficiency still further, while the reverse is true for the electron-attracting nitrile group. Therefore, the efficiencies of p-phenylenediamine derivatives are related to the electrondensity of the arylamino groups.
Antioxidant Concn., Molar
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
Deuterated and Nondeuterated Antioxidants
-
Increase in Induction Period of Gasoline, M i n .
H 0.000066 0.000132 0.000264
The study of other p-phenylenediamine derivatives, however, compels the further conclusion that the effectiveness of these compounds is dependent not only on their basicity but also on the steric situation prevailing. I n Table I11 are shown the efficiencies of symmetrically N,?lr’-disubstituted p-phenylenediamines in which the substituents differ from one another chiefly in regard to steric hindrance. I n the second and fourth compounds of Table 111 the carbon atom attached to the nitrogen is a member of the cyclohexyl ring and is a tertiary carbon atom. The sterically hindered derivatives are obviously more effective than the less hindered ones. These differences cannot be due mainly to the electron-releasing tendencies of the substituents which are about the same, as shown in Table IV by the similarity of the p K , values for the four different monobutylamines ( 7 ) . T h e p K a values of the monobutylamines are about the same. The p K , values are a measure of basicity or of proton affinity, the proton being little affected sterically. O n the other hand, reactions involving much bulkier substances than the proton are greatly affected by steric hindrance, as shown by the relative rates of aminolysis of methyl acetate by the butylamines. It also has been found that p-phenylenediamine derivatives in which the substituent on one nitrogen atom is different from that on the other have efficiencies about equal to those of the symmetrically disubstituted derivatives having the less effective substituent. For example, the efficiency of AT,A!-’di(sec- butyl) -p phenylenediamine is 10070, that of ,V,,N‘-diisobutyl-p-phenplenediamine is 40%, and that of ,!’-isobutyl- 1V’- (sec- butyl)-p-phenylenediamine is 37y0. All this suggests that the nonhindered compounds are less effective antioxidants because they participate much more readily in some undesirable reactions than the more hindered ones. Although no satisfactory experimental evidence exists, two reactions appear possible in this connection-reaction
115 226 396
?
? 114 250 369
A D D I T I V E S IN FUELS
Table II. Efficiencies of N,N’-Disubstituted p-Phenylenediamines
R in R - N H ~- N H - R
Molar Efficiency,
% 25
H CH3 CHs-CHz-CH-
I
100
CHI
with molecular oxygen and reaction with the hydroperoxide, R-0-0-H, formed slowly during the induction period. The former reaction is less likely to be the cause of the difference because the oxygen molecule is a relatively small one and it tends to react like a free radical. If one or both of these reactions do occur, the antioxidants are destroyed without stopping chain reactions and their efficiencies are lowered to the extent that they participate in these reactions. The more hindered derivatives react much more slowly with
The effects of the substituents on the antioxidant efficiency of p-aminophenol are less widespread than for p-phenylenediamine. This suggests that something common to these compounds has become dominant, probably the hydroxyl group. However, 0-alkylated p-aminophenols are at most extremely feeble antioxidants, hence N-substituted p-aminophenols must terminate chains by reacting with R-0-0. a t the hydroxyl group. The ineffectiveness of the ethers points to the following termination reaction:
137
R-0-0.
CH3
I CH3-CI
96
CHa
I I
220
CH3 CHI
I I
NC-CHz-C-
51
CHa CH3 NC-C-
I I
31
CHz
Table Ill. Efficiencies of N,N’-Disubstituted p-Phenylenediamines Molar El%-
caency,
% 38
CHa-CHz-CHz-CHz-
0
+ R-0-0-H
+ R~-NH---/~-o
-
(4)
CHa
HaN-CHz-C-
+R ~ - N H - ~ - O H
oxygen and hydroperoxide and conserve themselves for useful work, but their effectiveness would be diminished if the hindrance applied also to free radicals. There is evidence, however, that free radical reactions are less influenced sterically than reactions between molecules. For example, an ethylenic compound which is sterically hindered to such an extent that it is unreactive toward molecular reagents can be reactive enough towards radical reagents (27). It also has been found that the blue 2,4,6-tri(tert-butyl) phenoxy free radical in benzene reacts readily with the yellow N, N-diphenyl- N’-picrylhydrazine to give the colorless phenol and the purple N, N-diphenyl-N’-picrylhydrazyl free radical. These substances react with each other in spite of strong steric hindrance. The arguments presented receive support from the following facts: Although the efficiencies of N-(sec-butyl)-N’-(tertbutyl)-p-phenylenediamine and N, N’di(tert-butyl)-$-phenylenediamine a r e about equal to that of N,N’-(se+butyl)-kphenylenediamine, it has been reported that the first two compounds are more resistant to air-oxidation than the third
rather than electron donation, since a n electron should be as readily abstracted from an ether oxygen as from a hydroxyl oxygen. The fate of the p-aminophenyl free radical is not yet known but it probably involves chemical rearrangement which does not initiate, either directly or indirectly, an oxidative chain reaction. The effectiveness of phenol antioxidants also is dependent upon the presence of the free hydroxyl group, and their ethers and esters are without activity. Rosenwald, Hoatson, and Chenicek have shown that the efficiency of phenol is greatly improved by alkylation in the 2-, 4-, and 6-positions (20). For optimum results, all three positions must be substituted and one tert-butyl group must be in an ortho-position and a methyl in the para-position. At
(4,22).
R in R - - N H ~ - - O H
The efficiencies of some N-substituted p-aminophenols are given in Table V.
Table V.
Efficiencies of N-Substituted p-Aminophenols Molar E ficiency, I
%
H
94
CH~--CH~--CH~-CH~--
127
CHa-CHz-CHz-CHz-
83
O - C H B -
29
CHs CHa-CH-CHzI
Table IV. 107
CHa
I
CHs-CH-CHr
40
CHa
I CHa-CHs-CH-
100
Properties of Monobutylamines
R in R-NHz
pK,
H n-Butyl
9.27 10.61 10.42 10.56 10.45
Isobutyl sec-Butyl tertButy1
Relative Rate 01 Aminolysis of Methyl Acetate 1.56 12.4 5.1 0.27
Immeasurably slow
82
CHs CHa-CHz-CH- I
80
(CHs)zN-CHz-CHr
109 65
Q-CHZCHs NC-C-
i
68
I
CHa
VOL. 48, NO. 10
OCTOBER 1956
1883
and the phenol terminates a chain by Reaction 8, but if Reaction 10 occurs, the phenol acts merely as a chaintransfer agent and the over-all rate of autoxidation is not decreased. If IV abstracts a hydrogen from the phenol, the antioxidant is very shortly depleted. Thus, beneficial substituents must increase the electron density of the oxygen atom and prevent undesirable side reactions. All the termination reactions discussed previously involve peroxy radicals, but the antioxidant efficiencies of two derivatives of iv,A”-diphenyl-p-phenylenediamine shown in Table VI suggest the
best these products are about a third as effective as N,N‘-di(sec-butyl)-p-phenylenediamine. Ziegler and coworkers demonstrated that R-0-0. can abstract a hydrogen atom from a phenolic hydroxyl group. In the presence of pyrogallol, hexaphenylethane is oxidized by molecular oxygen to triphenylmethyl hydroperoxide rather than to bis(triphenylmethy1) peroxide which is formed in the absence of pyrogallol (24, 25). According to recent publications on the oxidation of phenols (3, 5, 8-72, 75, 78), the ring-substituted phenols can participate in the following reactions:
Ri R
