INHIBITED AUTOXIDATION OF SQUALANE
2247
of the ground states of NOz+, NO*, and NOz- are properly accouiited for. As for the planar dimer, some insight into the nature of the central bond and into the reason for the low stability of the molecule has been obtained. An obvious shortcoming of the method lies in the poor quality of the charge distribution, which is particularly exemplified in the 4al orbital of NOz. A refinement which would probably improve this situation would consist in modifying the Hamiltonian matrix elements according to the charges on the atoms and in applying an iterative procedure until self-consistency is reached between the atomic charges and the matrix elemenhas
followed here may be helpful in future a priori calculations. Such calculations on systems of the size considered here are necessarily laborious and expensive; any guidelines are worthwhile.
too far, it is worth pointing out that the approach
Clarke, iG. 45,'4743 , (1966).
Acknowledgments. The calculations have been carried out on the CDC 6600 computer of the Courant Institute of New York University. A grant of computer time from the NYU AEC Computing Center is g r a t e fully acknowledged. We are also thankful to Mr. J. R. Hamann who has written the extended Hiickel program.
Inhibited Autoxidation of Squalane
by James C. W.Chien Research Center, Hercules Incorporated, Wilmington, Delaware 19890 (Received December 87, 1966)
The induction period observed in inhibited autoxidation of cumene corresponds to the scavenging of two radicals by the inhibitor (hindered phenols). The induction periods are several times longer when the substrate is squalane, cyclohexene, dicyclohexyl, or 2,4,6-trimethylnonane. This increase is larger at high concentrations of the initiator and the inhibitor. Addition of chlorobenzene, methyl benzoate, or diphenyl ether reduces the induction period to the normal values. The results are interpreted to mean that the inhibition stoichiometry is not invariant and is dependent upon these experimental variables.
Introduction Several investigators have demonstrated that, in inhibited autoxidations, each inhibitor molecule reacts with two chain-carrying radicals. Thus, the inhibition stoichiometric factor, n, was found to be equal to 2 in the hydroquinone-inhibited autoxidation of benzaldehyde' and of ethyl linoleate. Similar results were obtaineda in cumene oxidation inhibited by 2,6di-t-butyl-4methylphenol.
A value of n = 2 implies that all radicals terminate by reacting with the inhibitor. Under these circumstances, the efficiency of an initiator can be determined (1) T.A. Inglee and H. W. Melville, Proc. Roy. SOC.(London), AZ18, 163, 176 (1963). (2) J. L. Bolland and P. ten Have, Trans. Faraday SOC.,43, 20 (1947). (3) C. E. Boozer, G. 8. Hammond, C. E. Hamilton, and J. N. Sen, J . Am. Chem. SOC.,7 7 , 3233,3238 (1965).
Volume 71, Number 7 June 1967
JAMES C.W.CHIEN
2248
by measuring the induction period. I n order to establish the general validity of this stoichiometry, we have investigated the inhibited autoxidations of several aliphatic substrates. This paper describes those conditions where apparent departures from n = 2 stoichiometry were observed.
10
Results c I
Ifany cumene oxidations initiated by di-t-butyl peroxide (DTBP) or a,a'-bis(isobutyronitri1e) (AIBN) were carried out at 62-110" with chlorobenzene as the solvent. The rates of oxidation and the rate constant ratio obtained are in complete agreement with literature values.3 I n the presence of inhibitor such as 2,6-di-t-butyl-4-methylphenol, the observed induction periods correspond to n = 2, regardless of the initiator used. 'The valus of n is calculated from the equation
n
=
2a[initiator]0[1- eXp(-kdtind)]/[IH]~ (1)
where k d is the rate constant of decomposition of the initiator and a is the fraction of radical in the decomposition which escapes primary cage recombinations. The rate of decomposition of AIBN and the value of n were measured by the rate of nitrogen evolution and the disappearance of iodine added as scavenger. The results obtained from 52.5 to 82.5" are in excellent agreement with the literature value^.^ The rate of decomposition of DTBP in squalane was measured from 100 to 145". The disappearance of DTBP and the formation of products were followed by gas chromstography. The reaction is first order in DTBP concentration to better than 95% completion. The results (Figure 1) are in good agreement with those reported5 in other solvents. The gas-phase decomposition rates, also shown in the figure, are about 30% slower than the others. It is assumed that there is no appreciable cage recombination of t-butoxy radicals and that a = 1 in eq 1. In the gas phase, the decomposition product is predominantly a ~ e t o n e ; the ~ products in aromatic solvents are mixtures of t-butyl alcohol and acetone. In squalane, the only product detected is 1-butyl alcohol; acetone i w ; not formed in measurable quantities at the experimental temperatures. The induction periods in squalane oxidation inhibited by either 2,6-di-t-butyl-4-methylphenol (I), 2,6-di-tbutylphenol (11), or 2,4,6-tri-t-butylphenol (111) are appreciably longer than the corresponding reaction in cumene-chlorobenzene (Table I). The calculated values of n increase with the increase of inhibitor concentration (runs 7-13) and of initiator concentration (runs 2-4). The Journal of Physical Chemistry
M* yb'
1.0
c
x a
.z
0.1 (+-)
x
103, DEGREE-'
Figure 1. Rate constants for the decomposition of DTBP: 0,in squalane; A, in cumene;6 0, in the gas phase.6
The changes in the rate of oxygen uptake a t the end of the induction period in all the experiments are abrupt; the maximum rates6 are given in column 7 of Table I. From this rate and with the known rates of initiation, the rate constant ratio k,/kt'/' is calculated by
kt'/' k, =
(E)''[(-%)m
- $]/[RH]
(2)
where k, is the rate constant for hydrogen abstraction, kt is the rate constant for termination, and Ri is the rate of initiation. The variation of the density of squalane with temperature was measured and introduced in this calculation (see the Experimental Section). Figure 2 is the Arrhenius plot of the results, from which we find E , - l/zEt= 16 kcal mole-'. During the induction period, the rates of oxidation are slow but measurable. These rates were found to be approximately equivalent to the rates of decomposition of the initiator (compare columns 6 and 7 of Table I). Addition of chlorobenzene markedly reduces the induction period. A stoichiometric factor of n = 2 was obtained in mixtures of squalane containing 50% or more of chlorobenzene. Table I1 and Figure 3 give these results, where phenol I was the inhibitor. Chloro(4) G. 9. Hammond, J. N. Sen, and C. E. Boozer, J. Am. Chem. SOC.,77, 3244 (1966). (6) J. H. Raley, F. F. Rust, and W. E. Vaughan, ibid., 70, 1336
(1948). (0)
A. V. Tobolsky, D. J. Metz, and R. B. Mesrobian, ibid.,72,1942
(1960).
INHIBITED AUTOXIDATION OF SQUALANE
2249
Table I : Autoxidation of Squalane
(2); x
-Inhibitor--Run no.
[DTBPI, M
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0.015 0.01 0.024 0.06 0.1 0.01 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.5 0.02 1.0 0.5 1.0 1.0 0.46 0.19 0.47 0.2 0.2
Phenol
Concn x 108, M
None
...
I I I I
3.0 1.2 3.0 3.0
None
...
I I I I I I I
Temp, "C
140 140 140 140 140 125 125 125 125 125 125 125 125 115 110 95 95 85 85 95 125 95 125 125
0.2 0.8 2.7 6.0 9.0 15.0 30.0
None I None
...
I
1.0
None I 11 I1 I11 I11 I11
...
106, M sec-1
1.0
...
0.5 1.0 5.9 1.0 6.4 6.3
' Rate of oxidation during induction period.
1.2 4.8
100 91 75 50 25
(-dOz/dt)m
tind
10-1, sec
17.4 14.2 6.7 5.7 5.0
x
108,
n
gee-1
6.4 5.2 2.5 2.1 1.9
3.2 3.8 4.0 3.0 1.7
[DTBP]= 0.5 M ; [phenol I]
=
x 100, M sec-1
lo-*,
sec
n
...
...
5.2 1.1 2.0 0.86
2.5 3.8 6.6 4.7
...
3.6 4.6 5.2 10.8 9.0 11.0 10.0
... 0.11
34.0
...
...
0.21
0.38
14.0
5.5
0.35
0.35
3.2 3.3
6.4 6.4
Rate of initiation.
(kp/ktl/l) X 103, I.'/z moIe-'/z 8ec -l/z
3.0 3.9 4.8 5.8 6.2
10-3 M ; temperature =
95".
Several other solvents were also found to reduce When squalane was diluted with 25 vol % of diphenyl ether. t-butylbenzene, or methyl benzoate, the corresponding values of n were 2.6, 2.6, and 2.9, respectively. These experiments were carried out at tin&
1.8 4.3
tind
0.21
Table 11: Oxidation of Squalane in Chlorobenzenea
x
x
0.13 0.56 2.4 10.5 12.5 27.0 88.0
benzene also increases the value of k,/kt'/'. Similar effects were observed when the inhibitor was phenol I1 or I11 (Table 111).
Squalane, VOl %
Rib 106, M sec-1
x
(kp/kt'/9 x 103, I.l/a moIe-'/2 BBC -l/a
61 69 91
2.2 3.6 2.7
19
1.0
43 39
0.93 0.87
5.0
1.2
4.0 3.9 1.9 1.9 3.5 33 24
0.24 0.37 0.22 0.22 0.34 0.80 0.24
3.6
...
5.7 8.2 3.5 4.9 2.7 2.6
13.0 21.6 3.4 13.6 2.6 2.5
Maximum rate of oxidation.
Table 111: Effect of Chlorobenzene on Squalane Oxidation --Inhibitor--Concn X 108, [DTBP], Squalane, M vol % Phenol M
0.45 0.46 0.19 0.47
25 20 20 25
I1 I1 111 111
0.97 0.94 6.3 0.93
tind
Temp,
X 10-1,
OC
800
n
95 95 125 95
6.86 6.15 2.2 5.0
2.4 2.3 2.2 2.2
125" in the presence of 0.2 M DTBP and 3 X lo-* M phenol I. Nitrobenzene, under the same conditions, gave an even shorter induction period, corresponding to a value of 0.9 for n. Figure 3 shows an apparent dependence of n and of kp/kt'" upon viscosity. This correlation is, however, found to be not a very important factor. Long induction periods were observed for three low-viscosity substrates-cyclohexene, dicyclohexyl, and 2,4,6-trimethylnonane. The viscosity of cyclohexene is nearly the same as that of chlorobenzene, being 0.797 and 0.707 cp at 30°, respectively. The autoxidation of cyclohexene-chlorobenzene mixture (1:2 volume ratio) Volume 71, Number 7 June 1067
JAMES C. W. CHIEN
2250
duced the values of n to 2.1 and 2.0 for dicyclohexyl and 2,4,&trimethylnonane, respectively.
Discussion of Results
I
0.1
I
I
I
I
2.4
2,s
2.6
2.7
($1
x
I
I
I 2.8
2.9
103, DEGREE-'
Figure 2. Variation of kP/k:/' with temperature for squalane oxidation: 0,uninhibited; a, inhibited.
c
t! v)
s
x
-
Y
0
I
I
I
20
40
60
I
80 SQUALANE, VOLUME' %
I
I
100
Figure 3. Variation of kp/kt'/'(0), viscosity (A), and n (0) with squalane concentration.
initiated by 1.07 X M AIBN in the presence of 5X M phenol I has an induction period of 5.14 X 108 sec. The value of n is 1.8. The induction periods and values of n in neat cyclohexene under otherwise identical conditions are 18.4 X lo8 sec and 6.0, respectively. Similarly, in phenol I-inhibited autoxidations of neat dicyclohexyl and of 2,4,6-trimethylnonane1 the observed induction periods correspond to values of n = 3.2 and 5.2, respectively. The initiator used here was DTBP. Dilution by 30% of chlorobenzene reThe Journal of Physical Chemietry
The most significant observation made here is that the inhibited autoxidations of aliphatic substrates have unusually long induction periods. Various experimental artifacts which could increase the induction period have been examined. Anomalously long induction periods could result from cage recombination of the initiator radicals in a viscous environment. This possibility was mitigated by the direct determinations of the rates of decomposition of the initiators and the attendant cage recombination efficiencies of the initiator radicals in the actual experimental systems. The stoichiometric factors were calculated using these measured quantities. Since only t-butyl alcohol was found in the decomposition of DTBP in squalane, every t-butoxy radical must abstract hydrogen from the substrate. From the known reactivities' of t-butoxy radical toward aliphatic hydrogens, secondary and tertiary alkyl radicals are produced in approximately the same ratio (9:7) leading to nearly equal amounts of secondary and tertiary peroxy radicals. It is unlikely that the peroxy radicals would be found in the same solvent cage because during the time it takes for hydrogen abstraction and reaction with oxygen to take place, the radical pair must have diffused out of the solvent cage and become separated. To the extent that these peroxy radicals are still in close vicinity and that a finite probability exists for their rapid termination, the value of n would be increased and the increase is larger in environment of higher viscosity. To resolve this uncertainty, three aliphatic substrates of low viscosities were studied. Those results indicate that geminal recombinations are generally unimportant. If the initiator was appreciably depleted during the induction period, then the oxidation products could assume control of the initiation process and significantly alter the induction period and the apparent inhibition stoichiometry. I n all the experiments reported here, tind is much shorter than the half-life of the initiator. Even in run 13 of Table I, tind is about one-half of the half-life of DTBP a t this temperature. We have furthermore prepared squalane hydroperoxide and studied its thermal decomposition.8 Squalane hydroperoxide decomposes a t the same rate as DTBP at 100"; the rate is about one-half that of DTBP at 135". It appears that the oxidation product could (7) A. L. Williams, E. A. Oberright, and J. W. Brooks, J . Am. Chem. SOC.,78, 1190 (1956). (8) J. C. W. Chien and H. Jabloner, ibid., in press.
INHIBITED AUTOXIDATION OF SQUALANE
not significantly alter the rate of initiation. Nonhydroperoxidic oxidation products could conceivably affect the rates of propagation and of termination. The products cannot be present in amounts exceeding the amount of oxygen consumed during the induction period. This amount corresponds to of the substrate concentration. The values of kp/fctl'* were found to be the same with or without added inhibitor (Figure 1). It is concluded that oxidation products are also not altering the courses of propagation and termination. The possibility that the oxidation products of phenols are also scavengers of radicals was considered. Stilbenequinones derived from phenol I have been shown to inhibit autoxidations. This possibility can be rejected for the following reasons. Known oxidation chemistry of hindered phenols could not account for values of n larger than 3, yet values as high as 10 have been observed. It is difficult to rationalize the effect of aromatic solvents. Finally, phenol I11 which oxidizes quantitatively to 2,4,6-tri-t-butyl-4-peroxycyc1ohexadienonel0also gives n > 2. Based upon these considerations, we conclude that the long induction periods are real and that the inhibition stoichiometric factors are apparently larger than those reported for other systems. Furthermore, this stoichiometry is sensitive to experimental variables such as initiator concentration, inhibitor concentration, temperature, and solvent. Therefore, inhibited autoxidation cannot be employed indiscriminately to determine rates of initiation. The mechanism responsible for the observed stoichiometry is not immediately evident. The dependence of n on inhibitor concentration implicates the inhibitor in higher than first-order reactions. The dependence on initiator concentration suggests the involvement of the chain-propagating radicals. The effect of chlorobenzene and other aromatic solvents can be interpreted to mean that the species responsible are stabilized thus preventing those reactions which lead to long induction periods. An explanation which appears to be capable of explaining the above data is suggested by the proposa1l1J2 that phenol-inhibited autoxidation involves the reaction
1. + R H + R * +IH (3) where I H is the inhibitor. This reaction as written could not affect the inhibition stoichiometry. However, if a radical such as I - abstracts a neighboring hydrogen or the activated a hydrogen of the peroxy radical, the product of this reaction could be carboxylic acid or a stable cyclic peroxide. The value of
2251
n would be effectively increased. This speculative mechanism may be represented by
+ I H +(ROOeIH) + products (ROOaIH) + I . -2IH ROO.
(4) (5)
where the species in the bracket is the complex postulated by Boozer, et a l a Accordingly, the induction period should vary with the square of the inhibitor concentration. By assuming a reaction similar to eq 5 for the peroxy radical with recovery of the inhibitor, the dependence of n on the initiator concentration may be rationalized. Chlorobenzene and other aromatic solvents probably stabilize the radicals to prevent the postulated hydrogen abstraction reactions.l a There are a t least two objections to the reactions postulated here to rationalize the results. First, hydrogen abstraction by phenoxy radical is known for unhindered phenols; it has not yet been established for hindered phenols. Second, if eq 5 is to be consequential, the a hydrogen of the peroxy radical must be sufficiently activated so that its abstraction can compete with the statistical reaction 3. Because of these difficulties, other experiments are underway to seek a better understanding of the processes involved in the inhibited autoxidations of aliphatic substrates.
Experimental Section Materials. Squalane, chlorobenzene, cumene, cyclohexene, dicyclohexyl, a,a'-azobis(isobutyronitri1e) (AIBN), 2,6-di-t-butyl-4-methylphenol,2,&di-t-butylphenol, and 2,4,6-tri-t-butylphenol were purchased from Eastman Organic Chemical Co. Di-t-butyl peroxide (DTBP) was obtained from Lucidol Co. All the liquid compounds were fractionated at 100 theoretical plates. The middle constant-boiling ( h 0.1") fraction wns collected and stored a t 0" in the dark. Dicyclohexyl and di-t-butyl peroxide were fractionated at reduced pressure. Squalane was purified by stirring with concentrated sulfuric acid; the colored acid was replaced by fresh acid. This procedure was repeated until the acid remained colorless after 24 hr of stirring. After separation from the acid, the squalane was neutralized, washed, dried over anhydrous MgS04, and fractionated as above a t reduced pressure. Infrared analysis detected no olefinic impurities. (9) R. F. Moore and W. A. Waters, J . Chem. Soc., 243 (1954). (10) A. L. Bickel and E. C. Kooyman, ibid., 3211 (1963). (11) L. R. Mahoney and F. C. Ferris, J . Am. Chem. Soc., 85, 2345 (1963). (12) W. G.Lloyd and C. E. Lange, ibid., 86, 1491 (1964). (13) C. Walling and P. Wagner, ibid., 85, 2333 (1963).
Volume 71, Number 7 June 1067
JAMES C. W. CFIIEN
2252
I 6 -
R
5 -
I 2.45
0.70
20
40
60 ao TEMPERATURE;
100
120
140
2.55 (+)
2.65
x
2.75
103, DEGREE-'
OC
Figure 4. Density of squalane as a function of temperature.
Density of Squalane. The density of squalane from 25 to 140' was determined volumetrically. The results are shown in Figure 4. Viscosity of Squalane. The viscosity of squalane as a function of temperature is given in Figure 5. The activation energy for diffusion is 4.6 kcal mole-'. The viscosities of squalane-chlorobenzene mixtures a t 95" have been given in Figure 3. Squalane conM phenol I taining up to 1 M DTBP and 3 X
Figure 5. Viscosity of squalane as a function of temperature.
was found to have the same viscosity as neat squalane at a given temperature. Autoxidation. The procedure and equipment used in these experiments have been described elsewhere. Decomposition of DTBP. DTBP was decomposed in capsules sealed under 1 atm of nitrogen. The amounts of undecomposed DTBP and of products were determined by quantitative gas chromatography eluted at 75". A 12-ft column of Carbowax was used in this work.
