6802 These are the fragment ions formed from the unrearby F I should not be significantly influenced by external ranged cyclohexene-3,3,6,6-d4 ion, assuming that the electric fields at times > 2 X lo-" sec. We, therefore, mechanisms in Schemes I and I1 are valid for the reconclude that with cyclohexene and cyclohexeneactions induced by EI. The increased ion currents 3,3,6,6-d1 the reactions induced by low-energy (nomimay, therefore, indicate that, as might be e ~ p e c t e d , ~ * nally ~ ~ ~ 12 eV) E1 are probably essentially similar in nature on raising the electron energy the rates of the fragmentato the reactions occurring at 10-11-10-5 sec following tion reactions are enhanced to a greater degree than the FI. Major differences are likely to be limited to reacrates of the allylic rearrangements effecting H-D rantion rates being somewhat more rapid following El domization. due to greater internal excitation energies.
Conclusion The same reactions of cylohexene appear to be induced by FI at 10-11-10-5 sec and by nominally 12-eV El, although the amount of internal excitation energy is somewhat greater i n the case of EI. The same H-D rearrangements appear to be induced in cyclohexene3,3,6,6-d4 by both E1 and FI. The reactions induced
Acknowledgments. We wish to thank Mr. F. C. Walls for running the El mass spectra on the DuPont (C.E.C.) 21-492 and Dr. D. Wilson for performing the nmr experiment to confirm the identity of the deuterated molecule. We are indebted to the National Aeronautics and Space Administration for financial support (Grant No. N G L 05-003-003).
Gas- and Liquid-Phase Oxidations of n-Butane Theodore Mill,* Is. Frank Mayo,'* Harold Richardson, Katherine Irwin,'& and David L. AllaraIb Conlribution f r o m the Stanford Research Institute, Menlo Park, California and Bell Laboratories, Murray Hill, New Jersey 07974. Received January 14, 1972
94025,
Abstract: The rates and products of t-BuzOz-initiatedoxidations of n-butane have been studied in the gas and liquid phase at 100 and 125". After allowance for concentration changes, effects of phase change at 125" are surprisingly small. In neat liquid butane at 125", yields of hydroperoxide are as high as 77 at low rates of initiation. As the rate of initiation increases and as the concentration of butane decreases, yields of sec-butyl alcohol and methyl ethyl ketone (chain termination products) and of ethanol and acetaldehyde (cleavage products of sec-BuO . radicals) increase, and kinetic chain lengths decrease. With 0.044 M butane in the gas phase, less than 0.5 molecule of butane is consumed per initiating t-BuO. radical, and oxidation of methyl radicals (from t-BuO. radicals) accounts for much of the oxygen consumed. The methyl and ethyl (peroxy) radicals from cleavage of t-BuO. and sec-BuO. radicals complicate determinations of some important ratios of rate constant, but we estimate that 50-70z of the interactions of sec-Bu02. radicals are nonterminating (in either phase). The ratio of cleavage to propagation for sec-BuO. radicals measured in separate experiments is about eight times as large as for t-BuO. radicals in the liquid phase and significantly larger than estimated from oxidation experiments. From the ratio of sec-BuOH to n-BuOH formed, we calculate that the relative reactivities of the secondary and primary C-H bonds for attack by peroxy radicals at 100" is about 45 : 1.
E
arlier fundamental studies of autoxidations of hydrocarbons have been concerned with liquid-phase oxidations below loo", gas-phase oxidations about 250", and reactions of alkyl radicals with oxygen in the gas phase at 25". In a previous investigation of the transitions between these three regions, we studied the rates and products of oxidation of isobutane2 between 50 and 155". We now have extended that investigation to n-butane in order to determine the effects of temperature, concentration, and phase change on the rates and products of oxidation of n-butane, to establish the most important elementary steps, and to compare the oxidations of n-butane and isobutane.
Experimental Section Materials. Oxygen (Matheson) and n-butane (Phillips) were research grade gases. Di-tert-butyl peroxide (Lucidol) was distilled (1) (a) Stanford Research Institute; (b) Bell Laboratories. (2) D. L. Allara, T . Mill, D. G. Hendry, and F. R. Mayo, Adaan. Chem. Ser., No. 76, 40 (1968).
Journal of the American Chemical Society
under vacuum; no tert-butyl alcohol or acetone was detected by glpc. We prepared sec-BulOn in 98% purity by the procedure of Mosher, et aL3 Procedures. The equipment, techniques, and procedures used for this work were similar to those used previously for isobutane2 except for the analyses of neat n-butane. When benzene was used as a solvent, it was first weighed into the reaction flask. n-Butane, measured in a larger calibrated vessel and introduced into this flask through a capillary side arm and stopcock, was frozen, and a known amount of oxygen was forced into the flask with a Toepler pump. Initiator from a tared microliter syringe was then injected through another capillary side arm fitted with a serum cap. After the flask was charged, both capillary arms were sealed off, and the flask was immersed in a constant temperature bath. During liquid-phase runs, the vessel was shaken with a Burrell wrist-action shaker. Oxidation was stopped by quickly cooling the flask to room temperature. Analyses. Analyses on noncondensable gases were carried out by the same procedures used in isobutane experiments,2using a Cu-CuO combustion furnace coupled to a gas buret. Mixtures of CO and H2 from sec-BuzOz experiments were analyzed by mass (3) F. Welch, H. R. Williams, and H. S . Mosher, J . Amer. Chem. SOC.,77, 5 5 1 (1955).
94:19 1 September 20, 1972
6803 Table I. Rates of Oxidation (Ro)of n-Butane in the Liquid Phase at 100 and 125" _
_
_
_
_
_
_
--Reaction Run no.
Temp, "C
23 102 6
100
15
100
25 110 111 93 89 75 91 52 57 82 104 106 31
100 100 100 125 125 125 125 125 125 125 125 125 125 125
100 100
100
_
~
~
~
~
~
~
conditionsReaction Liquid time, vol, min ml
-Initial t-BUz02 X
15
1544 1508 1439 1462 4339 284 1124 1452 1230 960 960 935 907 489 201 201 178 178
lo3
1.87 2.44 41.8 63.2 65.9 38.4 2.25 0.914 0.899 0.897 0.915 0.927 0.933 0.902 1.94 1.93 2.91 33.2
20.12 17 8.9 4.3 7.78 7.94 12.1 11.5 10.70 8.02 9.00 9.55 18.3 19.35 18.95 8.9 7.4
concentrations, MBenzene 0 0 0 0 0 0 0
9.34 9.27 8.85 7.59 4.76 1.27 0.015 b b 0 0
n-BuH
8.12 8.12 8.12 8.12 8.12 8.12 8.12 0.19 0.386 0.717 1.69 3.89 6.23 7.04 7.04 7.04 7.04 7.04
Reactants --consumed, mM-t-Bu20~4 0 2
0.118 0.150
2.5 3.8 11.7 0.768 0.167 0.647 0.582 0.499 0.509 0.507 0.500
0.306 0.330 0.329 0.429 5.01
4.38 4.72 20.9 26.0 84.6 6.56 4.34 3.73 2.86 4.10 8.78 15.2 20.6 11.4 7.70 8.02 6.4 37.2
~~~
Rates -M/min X loKRo Ri"
2.83 3.13 14.5 17.8 19.5 23.4 3.84 2.57 2.32 4.27 8.48 16.26 22.71 23.37 38.3 39.8 36 204
0.158 0.199 3.41 5.16 5.38 3.12 0.18 0.891 0.948 1.04 1.06 1.08 1.10 1.12 2.99 2.99 4.6 50.6
4 Ri = rate of initiation = 2A[t-BuzOz]/time, calculated from kd = 8.47X min at 125"[extrapolated from data of L. Batt and S . W. Benson, J. Chem. Phys., 30, 895 (1962)J;k d = 4.08X min at 100" (in toluene solution, ref 10). Solutions containing approximately 0.1% benzene because initiator was added as a benzene solution.
Table II. Products of Oxidations of Neat +Butane in the Liquid Phase at 100 or 125"0 -Run Temp, "C A02
At-BuzOzb
co coz
6
15
100 20.9 2.5
100 26.0 3.8
25 100
84.6 11.7
110 100
6.56 0.77 0.72 a. For x = 0.1 and a = 0.30.6, k j / k 6 = 3.5-0.80. The best values appear to be 0 < x < 0.1, and 1 < k j / k 6< 2 at both 100 and 125", 0.3 < a < 0.5 at 100" and0.3 < a < 0.4 at 125". These values of k j / k 6are in fair agreement with our extrapolation of data on the interactions of tertiary and secondary alkylperoxy obtained at lower temperatures. Recent experiments by two groups of workers'O~'' have shown that for the equilibrium between two tertiary alkylperoxy radicals and a tetraoxide molecule (corresponding t o Ksecin eq 5 and 6), AH" and A S o are nearly the same for a wide variety of alkyl groups. It seems reasonable then that, for simple alkylperoxy radicals, Ksec Ktert. From a simple steady-state treatment of reactions 5 and 6 , k,,, = k6KSeC and k,,,, = keKSeC where the rate constants k,,, are defined as
-
[Zsec-BuO.]
/ [Lrec-BuO2 .Ioage
+
0 2
% via eq 6
sec-BuOH
(10) J. E. Bennett, D. M. Brown, and B. Mile, Trans. Furaduy Soc., 3803 (1969).
I
\ SI
( 1964).
66, 386 (1970). (11) J. A. Howard, K. Adamic, and K.
I
I
+ AcEt + 02
(12) R. Hiatt and S. Szilagyi, ibid.,48, 615 (1970). (13) R. Hiatt, T. Mill, K. C. Irwin, and J. K. Castleman, J . Org. Chem., 33, 1428 (1968).
Mill, et al.
Gas- and Liquid-Phase Oxidations of n-Butane
6806
from a concurrent homolytic scission of peroxide;12 a second is possible formation of some sec-BuOH and AcEt by cage combination of sec-BuO * radicals, the amount of which can be estimated from the excess of AcEt over H,; the third complication is conversion of some AcH t o CO, presumably via the sequence t-BuO.
+ AcH
--f
t-BuOH
+ CH3Co
+ CO + i-BuH +CH, + t-Bu. CHI. + CH3CH0 -+-CHa + CH3C0 C H 3 c o ---t CH3.
CH3.
Thus, the modified form of the equation used to calculate k,/kd is 3 7% CAGE 2
I
I
Three experiments with degassed solutions of 0.0940.099 M sec-BuzOzin i-BuH were run at 100" for 30-68 hr. Product analyses indicated the presence of both CO and H2 in noncondensable gases. Neither ethane nor methane, the presumed products from reaction of At 30" and below, rate and isotope effect studieslO%ll Me. or E t . radicals with i-BuH, were found by our indicate that sec-R02. radicals terminate mostly or entirely by a fast, cyclic process corresponding to kcyc procedures. More AcEt was found in runs 115 and 117 than expected from Hz analyses. Results are sumabove with an average value at 30" of k,,., = lo6 l./(mol marized in Table 111. sec) and with an activation energy of 1.9 kcal/mol (estimated for the self-termination of secondary heptylperoxy radical).'O Rate parameters for k,,,, may be Table 111. Abstraction-Cleavage Ratio for see-BuO at 100" estimated on the basis that they must be similar to those in 7.46 M Isobutane for 2t-R02. radicals for which cage recombination is Expt 115 116 117 the only path leading to termination. At 30" rate 29.7 68 65.4 Time, hr constants for termination for t-ROz. radicals range 98.5 94.2 98.0 [sec-Bu~O~lo, mM from 1.4 X l o 3to 7.3 X lo3( M sec)-l for tertiary butyla[sec-Bu~O~], mM 14.7 29.2 32.7 peroxy and 2-methyl-2-pentylperoxy radicals, respec0.8 3.1 co tively. with corresponding energies of activation of 2@ 1.1 8.7 and 9.3 kcal/mol. Taking an average value of 4.5 Hz 8.5 17.5 10.5 AcH X lO3(Msec)-'andu = 0.13gives3.6 X 104(Msec)-'for 15.2 19.5 32.1 XC-BuOH the total rate of interactions of z-RO,. radicals equal to AcEt 4.1 9.4 14.2 kcage for sec-R02.radicals at 30". 6P 91 97 Mass balance, Zb Combining the values above, kcage(300)/kcyc(30") = 0.18 0.11c 0.23 k,/kd, I./mol (eq 10) (3.6 X 104)/(l.0 X lo6) = 0.036 and from the definition "Approximate value, sample lost. b(AcEt + sec-BuOH + of the constants, k j ( 3 0 0 ) / k 6 ( = 3 00.036 c) and E5 - E , = AcH + C0)/(2Asec-BuZO2). Not corrected for CO formation. 9.0 - 1.9 = 7.1 kcal/mol. Extrapolating k j / k 6using = 0.33 and an Arrhenius plot (Figure 2), k5(1000)/k6(1000) k j ( 1 2 ~ 0 ) , / k 6= ( 10.60. 2 5 0 ) These values are in good agreeValues of ka/kd calculated from eq 10 are shown in ment with the lower value of k j / k 6= 0.7, calculated from Table 111. Run 115 gave too little CO and H2 t o make our data. reliable corrections on sec-BuOH and AcH but did give Abstraction-Cleavage Ratio for sec-BuO . Radicals in an excellent material balance. Since the mass specn-Butane. To avoid the problems associated with trometry sample for run 116 was lost, excess AcEt was analysis for AcH in n-butane, we have carried out several assumed to be zeIo. Run 117 gave a useful total experiments with sec-BuzO2 to measure the ratio of product analysis, although the 91 material balance was abstraction (reaction 7) to cleavage (reaction 8) of secpoorer than for run 115. If we neglect run 116, and BuO. radicals at 100" in isobutane. The value of average the other two, (k,/kd)z-BuH = 0.20 i 0.03 AIw1. k,ikd in n-BuH was then estimated from the value in To be useful in interpreting the butane oxidation exi-BuH using data for t-BuO. radicals in these two solperiments, we converted the value of (k,Ikd)Z-BUH to the vents:'j (ka!kd)t-BUo' = 1.5(1-BuH); (kR/kd)L-BUo' = corresponding value in n-butane by using the average 0.72(n-BuH). relative reactivity of n-butane and isobutane toward From reactions 7 and 8 k,/kd = [sec-BuOH]/[AcH]. t-BuO. and t-AmO. radicals at 100" : kan-Bull/ k a t - B u H = 0.48 for t-BuO. ; kan-BuH = k a l - B u H = 0.66 [RH]. However, we must modify this equation to take into account several complications in the decomposition for t-AmO.. If the same relative reactivities apply for of sec-Bu202. One is the formation of AcEt and Hz for sec-BuO . radicals is sec-BuO . radicals, (ka/kd)n-BUH 0.20 X 0.57 = 0.1 1 M-l. Comparison of sec-BuO. and (14) J. A. Howard and I
