J. P h p . Chem. 1980, 8 4 , 1309-1314
phase. Therefore, the comparison must be restricted to those products which are formed in both phases and to the solution chemistry studies where secondary processes are observed (in these cases the agreement between the gasphase and the solution data is obtained), as not only the order of the dealkylation rates shows, but also the small but negligible production of ethylbenzene from isopropyl benzene confirms. Finally, the absence of transalkylation products and the possible formation of the correspondent alkenesz0might suggest that in the gaseous phase the dealkylation processes take place monomolecularly. In order to confirm such a hypothesis, further experiments will be carried out.
References and Notes V. Gold and F. L. Tye, J. Chem. SOC , 2172 (1952). C. Reid, J. Am. Chem. SOC., 76, 3264 (1954). D, M. Brouwer, E. L. Mackor, and C. Mac Lean in "Carbonium Ions", Vnl. 2, G. A. Olah and P. V. R. Schleyer, Eds., Wiley-Interscience, New York, 1970. R. 0. C. Norman and R. Taylor In "Electrophilic Substitution in Benzenoid Compounds", Elsevier, Amsterdam, 1965. H. C. Brown and C. R. Smoot, J. Am. Chem. Soc., 78,2176 (1956). G. A. Oiah, R. H. Schlosberg, R. D. Porter, Y. K. Mo, D. P. Kelly, and 0. D. Mateescu, J. Am. Chem. Soc.,94,2034 (1972), and references therein. G A. Olah and Y. K. Mo, J . Org. Chem., 38, 3221 (1973). E. M. Ernett and J. W. Larsen, J. Am. Chem. SOC., 91, 1438 (1969). H. Cerfontain, A. W. Kaandorp, and F. L. J. Sixma, Red. Trav. Chim. fays-Bas, 82, 565 (1963).
(21) (22) (23) (24) (25) (26) (27) (28)
1309
A. W. Kaandorp, H. Cerfontain, and F. L. J. Sixma, Red. Trav. Chim. Pays-Bas, 82, 113 (1963). R. Taylor in "Comprehensive Chemlcal Kinetics", C. H. Banford and C. F. H. Tipper, Ed., Elsevier, Amsterdam, 1972, p 323. V. Aqulbntl, A. GhrdlniGuidoni, and G. 0. Volpi, Trans. Faraday Soc., 3282 (1968). M. S. B. Munsonand F. H. Feu, J. Am. Chem. Soc.,89, 1047 (1967). G. Perez, J. fhys. Chem., 82, 506 (1978). P. Ausloos and G. Lias, J . Chem. Phys., 40 3599 (1964). P. Ausloos, Prog. React. Kinet., 5, 113 (1969). G. Perez, J. Chem. fhys., 80, 2983 (1976). The substitution of deuterium for helium In these runs approaches the energy-bansfec processes of these systems to the arenedeutetium mixtures. J. Weiss and W. Bernstein, Radiat. Res., 6, 603 (1957). !It was assumed G,,+ = GHs+= 2.75. In order to study the fate of the side chain fractions, reactbn mkxtures were analyzed by gas chromatography on a 2-m sillca-gel column at 50 O C , and traces of the correspondent alkenes were detected. A swcific inadation run of a D,-DdliioDroovlbenzene svstem showed the'formationof propgeneas 6i-i most siriiicant produ6t elutedi u h r such conditions. M. T. Bowers and P. R. Kemper, J. Am. Chem. SOC.,93, 5352 (1971). F. C. Fehsenfeld, W. Lindinger, and D. L. Albritton, J. Chem. Phys., 63, 443 (1975). R. Yamdagni and P. Kebarle, J. Am. Chem. Soc., 98, 1320 (1976). F. Cacace, R. Cipollini, and 0. Occhiucci, J . Chem. Soc., Perkin Trans. 2, 84 (1972). K. Tanaka, G. I. Mackay, and D. K. Bohme, Can. J. Chem., 56, 193 (1978). F. Hatch and B. Munson, J. fhys. Chem., 82, 2362 (1978). G. Perez, Radiochem. Radioanal. Letf., 20, 383 (1975). F. Cacace, R. Cipollini, P. Gacomello, and E. Possagno, Gazz, Chim. Ita/., 104, 977 (1974).
Reactions of Isobutane and the ferf-Butyl Radical with Atomic and Molecular Oxygen Nobuaki Washlda The National Institute for Environmental Studies, P. 0. Yatabe, Tsukuba, Ibaraki, 300-21 Japan
and Kyle D. Bayes" Department of Chemistry, University of Callfornla, Los Angeltw, California 90024 (Received December 26, 1979) Publication costs assisted by the National Science Foundation
The reaction of oxygen atoms with isobutane in a fast-flow system has been followed with a photoionization mass spectrometer. The rate constant for the initial attack of O(3P)on isobutane is (1.0 f 0.2) X cm3 molecule-l s-l, in agreement with the prediction of Herron and Huie. Direct observation of the tert-butyl radicals formed from 0 + (CH3),CD demonstrates that most of the initial hydrogen abstraction occurs from the tertiary carbon. The subsequent reaction of the tert-butyl radicals with atomic oxygen forms mostly isobutene (80%) with acetone (20%) as an alternate channel. Competition experiments show that the tert-butyl radicals react (2.68 f 0.36) X times more slowly with O2than with O(3P). In the presence of an excess of O2the same two products are observed but in a different proportion-isobutene (27%)and acetone (73%). This is interpreted to result from the initial formation of the tert-butyl peroxy radical, which then reacts with O(3P)to form acetone + (CH3+ 02) and isobutene + (OH + 02). The behavior of the CH3signal supports this interpretation. When a previously determined absolute rate constant is used together with the results of competition experiments, cm3molecule-' the rate constant for the reaction O(3P)+ (CH3)3C products is calculated to be (8.7 f 1.9) X
-
S-1.
Introduction Herron and Huie studied the rate of attack of groundstate oxygen atoms on various alkanes.l They showed that, to a good approximation, the overall rate can be expressed as a sum of the rates of attack on the individual hydrogen atoms. Although they did not study isobutane (reaction l),they predicted a value for ,Ezl (1.1X cm3molecule-l W3P)t- (CH3)3CH products (1) s-l a t 300 K) and noted that this disagreed with the one
-
0022-3654/80/2084-1309$01 .OO/O
experimental value by a factor of 17. Since their predictions for other alkanes agreed with experimental rate constants to within a factor of two, the literature value for hl is suspect. Using the generalized rate constants of Herron and Huie one can predict that at room temperature 96% of the hydrogen abstraction from isobutane by O(3P)will occur a t the 2-position, to form the tert-butyl radical (t-Bu), while only 4% of the abstraction will involve the primary hydrogens. The available experimental evidence supports
E)1980 American Chemical Society
1310
The Journal of Physical Chemistry, Vol. 84, No. 11, 1980
TABLE I: Ionization Potentials of Various Molecules and the Resonance Lamps Used to Detect Them m/e
molecule
IP. eV
1 5 CH, 9.84 56 isobutene 9.23 57 tert-butyl 6.93 58 acetone 9.71 58 isobutane 10.5 59 isobutane-d 10.5
lamp (window)
resonance lines. eV
Kr(MgF,) Kr(CaF,) Xe(sapphire) Kr(CaF,) Ar(LiF) Ar(LiF)
10.03, 10.64 10.03 8.44 10.03 11.62, 11.83 11.62, 11.83
this prediction; Blacet, Hall, and Leighton2 observed 2nitro-2-methylpropane, as expected for t-Bu + NOz,as the major product from the photolysis of isobutane-NOz mixtures. Recently photoionization mass spectrometers have been used to detect free radicals in oxidation reactions at low pressure^.^-^ Absolute rate constants can be determined by observing the rate of approach of the radicals to their steady-state con~entration.~,~,~ Ratios of rate constants can be measured by observing the effect of additives on the steady-state radical c o n ~ e n t r a t i o n . ~The J ~ ~free radicals that have been studied include m e t h ~ l formyl,' , ~ ? ~ acetyl,1° and several radicals formed in the acetylene-oxygen atom reaction.5J1 In the present report, the reaction of oxygen atoms with isobutane has been used to generate the tert-butyl radical, and the reactions of this radical with atomic and molecular oxygen have been studied.
Experimental Section The experimental apparatus has been described previ0us1y.~A cylindrical fast-flow reactor with a movable inlet was coupled to a photoionization mass spectrometer. Oxygen atoms were usually generated by flowing a mixture of O2 in He through a microwave discharge. For comparison, some experiments used the titration of nitrogen atoms by NO to produce oxygen atoms. The methods used to determine the oxygen-atom concentrations and the precautions taken were as described previo~sly.~ Molecular oxygen (Liquid Carbonic, 99.999 %) was added directly to the gas flow, using the increase in reading of a capacitance manometer (MKS Baratron 77H-3) as a measure of the partial pressure of 02. These pressure increments were converted to concentrations by using a separate calibration of the flow as a function of pressure. For comparison some runs used a replacement procedure, decreasing the pressure of N2 as the O2was added so that the total pressure remained constant. Both methods of O2 addition gave the same kinetic parameters. The isobutane (Phillips Petroleum Co., 99.99%) and deuterated isobutane (Merck Laboratory Chemicals, (CHJ3CD) were added as mixtures (0.7% in He) through the movable inlet. Isobutene (Phillips Petroleum Co., 99.6%) and acetone were used directly to calibrate the mass spectrometer. The photoionization lamps used, all powered by microwave discharges (2450 MHz, 10-40 W), are listed in Table I along with the ionization potentials of the various molecules that were detected. All measurements were done at room temperature, 24 f 4 "C. Results Butyl radicals were easily detected when oxygen atoms were mixed with isobutane. These could be either tertbutyl radicals, formed by abstraction of the 2-hydrogen from isobutane (reaction la), or isobutyl radicals, formed (CH3)BCH + 0 (CH,),C + OH (la) (CH3)SCH + 0 CH2(CH3)2CH + OH (lb) by hydrogen abstraction from a methyl group (reaction lb). +
-+
Washida and Bayes
TABLE 11: Isotopic Abundances Observed for Isobutane and the tert-Butyl Radicalu
-
molecule isobutane, (CH,),CH isobutane, (CH,),CD tert-butyl (from 0 (CH,),CH) tert-butyl (from 0 + (CH,),CD)
+
m/e for M I(M)
I(M t 1)
100 100 100 100
4.6 i. 0.2 4.2 i. 0.2 4.6 i. 1.7 16.7 + 2.4
58 59 57 57
Measurements on isobutane used a total pressure of 4.9 torr and a partial pressure of isobutane of 0.18 mtorr. The tert-butyl radicals were formed by reacting 0.28 mtorr oxygen atoms with 1.0 mtorr of isobutane at 3.2-ms reaction time and a total pressure of 4.5 torr. The quoted errors on the intensities represent one standard deviation, assuming Poisson statistics.
In order to distinguish between these two abstraction reactions, isotopic measurements using normal isobutane and the monodeuterated isobutane, (CH3)3CD,were made. When the Ar resonance lamp was used, the normal isobutane showed a parent peak at mass 58, with a mass 59 peak of 4.6%, as expected for C4H10 having the natural abundance of 13C (1.11%) and D (0.015%). Similarly the monodeuterated isobutane gave a parent peak at 59 with again the expected intensity at mass 60 (see Table 11). The deuterated isobutane contained less than 5% C4H10, but there was a significant impurity of monodeuterated butene; the intensity of the mass 57 peak was approximately 20% of the mass 59 peak. This impurity should not interfere with the isotope experiment. Switching to the Xe resonance lamp so that only tertbutyl or isobutyl radicals would be photoionized and adding oxygen atoms to the normal isobutane gave the expected result, a parent peak at mass 57 with the heavy isotope contribution at mass 58. When this experiment was repeated with the monodeuterated isobutane, mass 57 was again the dominant peak, demonstrating that it is predominantly the tertiary D atom that is abstracted by O(3P). However, the mass 58 peak was significantly larger than that expected from the 13C contribution (last line, Table 11). Subtracting the 13Ccontribution from the mass 58 peak gives a C4H8Dsignal of 12.2 f 2.4% compared to the C4H9signal. I t is concluded that reaction l a is the dominant but not exclusive abstraction reaction. Two stable products, acetone and isobutene, were observed during the 0 + isobutane reaction. The time dependences of these products for two different oxygen-atom concentrations are shown in Figure 1. On the time scale shown, acetone rises linearly with time while isobutene approaches a constant concentration at long time. Figure 1 shows that the concentration of both compounds correlate with the product [Olt, that is with the time integral of the rate of attack of O(3P) on isobutane12 (the t-Bu radical reaches its steady state very quickly on this time scale, i.e., for [ O ] t 1 3 X lo9 atoms s (see below)). The time dependence of the isobutene concentration can be understood as the result of the formation of a product that can react further with oxygen atoms. At long times the isobutene approaches its steady-state concentration, [C4H8],,. The mathematical treatment of the approach to the steady state is the same as that developed for radicals,6 namely, eq I where kz is the rate constant for the removal (1) [C4&] = [C&ISs(1 - eXP(-kz[O]t)) of isobutene by O(3P) according to reaction 2. When the C4H8+ 0 products (2) data shown in Figure 1are fitted to this expression, a value for k2 of (2.0 f 0.3) X 10-l1cm3molecule-l s-l is calculated, which agrees well with the accepted value.13
-
--Ird
The Journal of Physical Chemistty, Vol. 84, No. 11, 1980 1311
Reactions of Isobutane and the tert-Butyl Radical 2c
I
I
I
/
I
6 I?
-
'0 Q)
n
c
e a
8 IC
em \
iij
This experiment was done with a large excess of oxygen atoms, 14.1 mtorr partial pressure, to assure the above steady-state conditions and to give a significant decay of isobutane. Combining the pseudo-first-order decay rate with the absolute oxygen-atom concentration gave a value for kl of (1.0 f 0.2)X cm3 molecule-l s-l, in good agreement with the prediction of Herron and Huie.l Many attempts were made to use the approach to the steady-state method to determine the absolute rate constant for oxygen atoms reacting with t-Bu. The analysis proved to be more complex than that used previously because at the low oxygen-atom concentrations necessary to see the approach, the hydroxyl radicals formed in reactions 1and 3 were not consumed primarily by reaction 4, but instead they could react with isobutane to form 0 + OH ---* 02 H (4) additional t-Bu radicals by reaction 5. It is possible to (5) OH + C4H10 t-Bu + H2O find an analytical expression for the case where two interconverting radicals, e.g., OH and t-Bu, are approaching their steady states simultaneously and then fit the data to this expression by using literature values for k4 arid kg. Although this procedure demonstrated that the value of k3 was large (>2 X 10-locm3molecule-l s-l), the different sets of data did not give consistent values for kB. Part of the difficulty was due to the necessity of inserting a wall loss rate for OH radicals, but other problems, not fully understood, were present. In view of these inconsistencies the approach to steady-state method was abandoned. When molecular oxygen is added, a competition is set up between O(3P) and O2 for the t-Bu radicals. If one assumes that the formation rate of t-Bu is not altered14 by the addition of 02,then the steady-state t-Bu concentration should follow a Stern-Volmer type law
+
E
+
C
Figure 1. Time dependence of the stable products acetone (triangles) and isobutene (circles). The conditions for the open symbols: [O], = 0.90 rntorr; [C,H,,] = 0.86 mtorr; total pressure 3.99 torr. Conditions for the filled symbols: [O],= 1.78 mtorr; [C4H1,] = 0.86 mtorr; total pressure, 4.17 torr. The method used to calculate the average oxygen-atom concentratlon, [a], is given in ref 12. The isobutene signals corrected for subsequent reaction, [C4HBIT,are indicated by square symbols.
Equation X has been fitted to the data for isobutene and is shown as a solid Bine in Figure 1. It is also possible to correct the observed isobutene signals for subsequent reactions by multiplying each observed isobutene value by k2[O]t(l - e ~ p ( - k ~ [ O ] tin )-~ order to give the total amount of isobutene formed, [C&]T; these corrected points are shown as squares in Figure 1,and they are well represented by the dashed straight line. The ratio of the slope of the dashed straight line to that for acetone is 5.5. The relative sensitivity of the photoionization mass spectrometer for NO, isobutene, and acetone was measured as 1:6.82:3.85 by using a Kr lamp (CaF2). Thus the ratio of the rate of formation of these two products was isobutene:acetone = 3:l for these particular conditions. As will be shown below, this ratio is close to 4:l when only O(3P), and no 02,is present. This ratio is most logically assigned to the branching ratio for reaction 3. The tert-butoxy radical 0 + t-BU ---* CH2=C(CH3)2 OH (34
+
0 + t-Bu --c (CH,),CO'
CH3COCH3 + CH,
(3b) formed in reaction 3b would have approximately 90 kcal/rnol excess energy and should rapidly decompose to acetone and a methyl radical. Thus oxygen atoms attack the weakly bound methyl hydrogens four times as rapidly as they attack the radical carbon. The overall rate constant for reaction 1was determined by following the steady-state concentration of isobutene as a measure of the first-order decay of isobutane in an excess of oxygen atoms. The decay of isobutane itself could not be monitored directly with the Ar lamp because of interference from the product acetone, which is also at mass 58. When both t-Bu and isobutene have attained their steady-state concentrations, then -+
[C4H,Ias = ki[C4Hiol/k3 (11) and as the isobutane concentration decays, so will [C4H8],.
where the subscript zero indicates the condition with no O2 present and k6 is the effective second-order rate constant for the reaction of t-Bu with O2 (eq 6). Results of t-Bu O2 products (6) an O2addition experiment are presented in Figure 2. The contact time for these experiments was kept short, approximately 2 ms, so that the buildup of products such as isobutene, hydrogen atoms, etc. was kept to a minimum. For these measurements it was necessary to take into account the O2surviving from the discharge, so that a specific value for [t-Bu]owas not measured directly. Instead [tBu1-l was plotted against [02]/[0] and a least-square fit then provided [t-BuIo-l as the intercept. Then when this extrapolated value for [t-Bu], was used, the Stern-Vdmer plot in Figure 2 was generated. There is no evidence for a pressure dependence of the O2 effect over the range of 1.8-5.7 torr. Also it is clearly the ratio [02]/[0] that controls the t-Bu steady-state concentration, as expected for a simple competition. The conditions used and the results of several competitions are collected in Table 111. The weighted average of the slopes of the Stern-Volmer lines is (2.68 f 0.36) X which is the best estimate of
+
k6/k3.
-
When large amounts of O2 were added, the same two stable products were observed, namely acetone and isobutene, but in different ratios than when just oxygen atoms were present. The sum of acetone and isobutene (when corrected for subsequent reaction) remained constant within the experimental error as the 02/0ratio was
1312
The Journal of Physical Chemistty, Vol. 84, No. 11, 1980
Flgure 2. Stern-Valmer plot for the competition between O2and O(3P)
for the tert-butyl radicals. The conditions: [OjO= 4.30 mtorr; [O] = 3.77 mtorr; [C4H,0] = 2.71 rntorr; reaction time, 2.43 ms; total pressure, 1.75 torr.
1.75 3.78 5.74
2.71 2.79 4.19
4.30 8.50 7.04
3.77 7.61 6.03
2.43 2.00 1.87 wt av:
Figure 3. Behavior of the products acetone and isobutene as the [O,]/[O]ratio is varied. The circles represent the data from Table IV plotted according to eq V. The sum of [acetone] and [isobutene], is plotted as squares, and the horlzontal line is kl[C4H,0] [O]t .
reaction channels for the t-Bu radicals. Then if Q is the acetone formed as a fraction of the total product
TABLE 111: Conditions and Results of 02/O(3P) Competition for the tert-Butyl Radical reactotal [isobution press., tanel, [OI,, [ O ] , time, torr mtorr mtorr mtorr ms
Washida and Bayes
f k 3 P 1 + sk6P2l = k3[01 + k6[021 rearrangement gives
k,/k,, 2.51 * 0.13 2.97 t 0.24 2.75 * 0.12 2.680 c 0.083
a The first line in the table is for the experiment shown in Figure 2. If the weighted average is used, the 95%confidence limits are calculated to be k J k , = (2.68 * 0.36) x
Q(l
+ x ) = f + sx
(VI
where x is k6[02]/k3[O]. The results given in Table IV are plotted in this way in Figure 3, and they give an excellent straight line having an intercept f = 0.20 f 0.08 and a slope s = 0.73 f 0.03. This means that t-BU + 0
* * (80
S)%
(20 & 8)%
changed (see Table IV and the upper part of Figure 3). In addition, this sum agreed well with the calculated loss of isobutane, kl[C4Hlo][O]t, using the value of kl determined above and the average oxygen-atom concentration (see upper solid line in Figure 3). This agreement between calculated isobutane reacted and the sum of acetone and isobutene indicates that there are no other major stable products formed as O2 is added to the system. A quantitative treatment of the variation of acetone and isobutene with O2 addition is possible. Assume that in reaction 3 a fraction f mol of acetone and (1- f ) mol of isobutene are formed; similarly assume in reaction 6 s mol of acetone and (1 - s) mol of isobutene are eventually formed; and assume that there are no other significant
t-Bu + 02 --*
(27
(73
CHZ=C(CH3)2 CH3COCH3
3)%
Lk
3)%
(3)
CH2=C(CHJ2 CH3COCH3
(6) The mechanism of these reactions will be discussed below. The methyl radical signal also changed as O2 was added to the system, as shown in Figure 4. Since the reaction of methyl with O2 is slow at these low pressures: the rate of removal of CH3 was primarily determined by its reaction with oxygen atoms, which remained relatively constant. Therefore the increase in methyl signal by more than a
TABLE IV: Variation of the Acetone and Isobutene Products as 0, is Addeda [ 0 , ]mtorr ,
7.6 26.1 49.0 72.2 90.7 113.5 145.9 159.8 210.5 242.7 284.1
[0
2 1
/ [ol
5.0 17.2 32.2 47.5 59.7 74.7 96.0 105.2 138.5 159.7 187.0
(IV)
[acetone], ptorr
[isobutene], ptorr
7.19 10.06 11.41 13.30 12.71 13.66 13.66 15.36 13.93 16.89 15.81
4.72 4.01 3.30 2.89 2.59 3.04 2.48 2.23 2.08 2.38 1.98
[isobutene] T, [acetone] t fitorr [isobutene] T ,ptorr 20.12 17.10 14.07 12.32 11.04 12.96 10.57 9.51 8.87 10.15 8.44
27.3 27.2 25.5 25.6 23.8 26.6 24.2 24.9 22.8 27.0 24.3
a The experimental conditions were as follows: [isobutane] = 1.20 mtorr, [O], = 1.68 mtorr, [O] = 1.52 mtorr; total pressure, 4.48 torr; reaction time, 4.17 ms.
Reactions of Isobutane and the tertdutyl Radical
factor of 2 must be caused by a proportional increase in ita rate of production. The increase in methyl production parallels the increase in acetone formation, although the growth of CH:, is less than that of CH3COCH3 The kinetic interpretation of this data will be delayed to the next section.
Discussion The direct observation of butyl radicals at the earliest stage of'the reaction of O(3P)with isobutane is only compatible with hydrogen abstraction as the initiating step. The fact that (CH3)3CDgives 88% C4H9radicals and only 12% C4H8Dshows that abstraction of the single tertiary deuterium is strongly favored over abstraction of the nine primary hydrogens. With nondeuterated isobutane the formation of the tert-butyl radical should be even more dominant. Because of the difference in zero-point energies, abstraction of H is favored over abstraction of D: for 0 Hz and 0 Dz, hydrogen abstraction15 is faster by a factor of 4 at 400 K; for OH deuterated methanes,l6 hydrogen abstraction is faster by a factor of 11; for Br + CHzDCGH5,hydrogen abstraction is 12 times faster than deuterium ab~tracti0n.l~Thus for 0 isobutane, the tert-butyl radical should be formed in well over 88% of the reactive encounters. It is not possible to set an exact percentage since the ratio of H and D abstraction by O(3P) from alkanes has not been measured. Using the rate constants for O(3P)attack on individual H bonds proposed by Herron and Huie,l one would predict 96% tert-butyl and only 4% isobutyl radicals formed in reaction 1; the present results are compatible with that prediction. The absolute value for kl determined above (1.0 X cm3 molecule-l s-l) also agrees with the predicted value based on Herron and Huie's generalized rate constants (1.1 X 10-13). From the competition experiments of Paraskevopoulos and CvetanoviE18 one can calculate a value of kl of 5.8 X 10-14,which is fair agreement considering that both determinations are indirect. The value determined by Wrightlg (6 X 10-ls) using final product analysis is suspect since it is so far below the rate constants for other similar alkanes. Using the present value for k6/k3 together with an absolute value for k6 determined previously20((2.3 f 0.3) X 1O-l1) gives an absolute value for k3 of (8.6 f 1.3) X 10-lo. This is a very large rate constant, probably even larger than the hard-sphere collision frequency. This large rate constant may be caused by the long-range attraction between the electronegative 0(3P)and the easily ionizable t-Bu radicalaZ1The large value for k3 is consistent with the observed products, which show that all parts of the tertbutyl radical are reactive. The isobutene that is observed as the major stable product of reaction 3 can be formed by a simple hydrogen abstraction from one of the methyl groups of the tert-butyl radical. AH = 41 kcal/mol (CHB)3C-+ (CH3)zC=CHz + H
+
+
+
+
This H bond is weak, much weaker than the O-H bond being formed.zz The weakness of this C-H bond and the fact that there are nine equivalent H atoms that can be attacked combine to make this reaction extremely fast. If the incoming Of3P) does not impinge on one of the methyl hydrogens, it should encounter the central carbon atom with its1 unpaired electron. This encounter would form a vibrationally hot tert-butoxy radical (eq 3b) which O(3P) + (CHJ3C (CH3)3C0 AH = -94 kcal/mol (3b) has about 77 kcal/mol in excess of the activation energyz3 +
The Journal of Physlcal Chemistry, Vol. 84, No. 11, 1980
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necessary to decompose (eq 3b'). This fast decomposition CH3COCH3 + CH3 (3b') (CHJ3CO would form acetone and methyl radicals, both observed products. The above description of the reactions of O(3P) with isobutane is very different from the interpretation of who investigated only the stable products. He rejected the possibility of forming a hot tert-butoxy radical which subsequently decomposes to acetone and CHBbecause he observed no effect of pressure on the tert-butyl alcohol/acetone ratio over the range 3.6-760 torr. However, it seems likely that the very hot tert-butoxy radical formed in reaction 3b always decomposes, even at 760 torr, and that the tert-butyl alcohol observed by Wright was formed by other routes. The acetone observed by Wright accounted for 27% of the reacted isobutane, in fair agreement with the 20% observed in this study. Wright observed only 23% of the reacted isobutane as isobutene. In view of the behavior of isobutene shown in Figure 1, it is clear that Wright's isobutene product was reacting further with the high concentrations of O(3P) to form a multitude of products. Another observation by Wright,lg the formation of 1,ldimethylcyclopropane from the reaction of O(3P) with neopentane, can be explained by a similar sequence of reactions. The initiating step would be hydrogen abstraction to form OH and the neopentyl radical. Then another O(3P)would attack the radical, predominantly abstracting a H from one of the other methyl groups to form OH and simultaneously 1,l-dimethylcyclopropane. This unusual product accounted for at least 38% of'the neopentane reacted. This high yield, which is difficult to explaii by other schemes, supports the interpretation given above that O(3P) can readily abstract H from an alkyl radical. When varying amounts of Oz were added to the system, the sum of the two products, acetone and isobutene, remained constant and no new products were observed. However, the ratio of products changed, from 80% isobutene with only O(3P)present to 73% acetone when O2 was in excess. At first it was thought that with O2 in excess, the isobutene could come from the direct reaction (eq 7). However Thomas and Calvertz6carefully looked 02 + (CH3)SC -+ HOz + (CH3)zC=CHz (7) AH = -6 kcal/mol for isobutene in this reaction and found none, concluding that reaction 7 does not occur at room temperature. The major difference between their study and the present one is the presence of oxygen atoms, so it is logical to associate the formation of butene to a reaction of O(3P). When [ O z ] / [ O is ] large, the tert-butyl radicals will react predominantly with Oz to form tert-butyl peroxy radicalsz6 (reaction 6). Then isobutene could be formed in the subsequent reaction of O(3P)with the peroxy radical. (eq 8). Here the Ozmolecule has served only as a chaperon 0 + (CH3)3COO --t OH + (CHJzC=CH2 + 02 (8) AH = -34 kcal/mol for the alkyl radical until the more reactive O(3P) can abstract a hydrogen. Another channel in the reaction of O(3P)with tert-butyl peroxy radicals could explain the acetone formation in this system: 0 + (CH3)3COO 0 2 + (CH,),CO (9) AH = -64 kcal/mol +
followed by the fragmentation of the hot tert-butoxy
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The Journal of Phydcal Chemistty, Vol. 84, No. 11, 1980 A 0
Washlda and Bayes
are released simultaneously so that they could recombine as they are leaving the vicinity of the acetone molecule. If this occurred for approximately one-third of the reactive collisions, the results shown in Figures 3 and 4 would be compatible. Thus the kinetic behavior of the methyl signal supports the mechanism given above for acetone and isobutene formation, but only if one additional postulate, the formation of some CH3O2 in reaction 9, is added.
-30
0
n
Acknowledgment. We thank the National Science Foundation for support of this work under grants CHE74-18646 and CHE-7823867. References and Notes
n
X
+
Figure 4. Dependence of the CH, signal on the [ O , ] / [ O ] ratio. The relative methyl radical signals are plotted as triangles for the conditions: 0.48 mtorr; [C4HI0] = 0.97 mtorr;reaction time, 5.51 ms; total [O], pressure, 3.83 torr. In this experiment the oxygen atoms were generated by the N NO titration. The soli curve is a plot of eq VI. The circles and straight line represent the same data fitted to eq VII.
+
radical to give acetone and CH3 (reaction 3b’). Thus the same two products are formed whether O(3P)or O2 is in excess, but the proportions are different because of the different mechanisms. A quantitative test of this proposed mechanism is successful, as shown by the straight line in Figure 3. Another test of the above mechanism is possible by examining the methyl signals. As shown in Figure 4, the CH3 signal increases as O2 is added, in parallel to the increase in acetone formation. Using reactions 1 , 3 , 6 , 8 , and 9 together with the two loss processes for CH3 (eq 10 and 11)and assuming steady-state concentrations for the CH3 + 0 H2CO + H (10) CH3 + 02 CH300 (11)
--
CH3 and t-Bu radicals, it is possible to derive eq VI where (CHJ/(CHJo = (1 + sx/f)(l + x)-’(1 + kii[021/ki0[01)-~ (VI) (CH,), is the methyl signal with no 02, (CH3) is with O2 present, and s, f , and x have the same definitions as above. For the conditions of Figure 4, the ratio of kll/klo is aboutg 3X Inserting this value and rearranging gives eq VII. (1 x ) ( l 0.0112~)(CH3)/(CH3)0= 1 + s x / f (VII)
+
+
The left-hand side of this equation is plotted against x in Figure 4, resulting in a good straight line with an intercept of unity as required. However, the slope of this line is 2.55, whereas the value of s l f determined from the measurements shown in Figure 3 is 3.75. This means that as 02 is added to the system the acetone yield increases by a factor of 3.75, but the methyl radical yield increases by only 2.55 times. This difference in stoichiometry might be explained if during reaction 9 the methyl radical and 0 2
J. T. Herron and R. E. Huie, J. Phys. Chem., 73, 3327 (1969). F. E. Biacet, T. C. Hall, and P. A. Leighton, J . Am. Chem. Soc., 84, 4011 (1962). I.T. N. Jones and K. D. byes, J. Am. Chem. Soc., 94,6869 (1972). J. R. Kanofsky and D. Gutman, Chem. Phys. Leff., 15, 236 (1972). I. T. N. Jones and K. D. Bayes, Symp. (Int.) Combust. [Proc.], 14, 277 11973). J. R.’Kandsky, D. Lucas, and D. Gutman, Symp. (Int.) Combust. [Proc.], 14, 285 (1973). N. Washkla, R. I. Martinez, and K. D. Bayes, 2.Nafurforsch. A, 29, 251 (1973). I.R. Slagle, J. F. Pruss, Jr., and D. Gutman, Int. J. Chem. Kinet., 6, 111 (1974). N. WashIda and K. D. Bayes, Int. J. Chem. Kinet., 8 , 777 (1976). N. Washida and K. D. Bayes, to be submitted. I. T. N. Jones and K. D. Bayes, Roc. R. Soc. London, Ser. A , 335, 547 (1973). There was a small but significant consumption of O(3P)during the reaction. For quantitative measurements on [ f-Bu],, the oxygen atom concentration at the pinhole, [O], was calculated from the original value, [O],,by the expression
101 = [OIOexp(-nkl [C4Hj01t ) where nrepresentsthe number of @P) consumed per C,H,o reacted. Although the value of n varies from 4 to 7 or more, depending on the extent of secondary reactions of isobutene, t-Bu02,etc., a value of n = 6 was used for all conditions. As can be seen in Tables I11 and I V the correction for oxygen-atom consumptlon was 15% or less. For measurements on the stable products, the average oxygen-atom concentration, [ b ] was , calculated by
J. T. Herron and R. E. Huie, J. phys. Chem. Ref. Data, 2,467 (1973). For the conditions used in the 0,-addition experiments, the ratio [O]/[C,H,,] was 1.4 or larger, sothat at least 95% of the OH radicals generated In reactions 1 and 3 were destroyed by reacting with O ( 9 ) in reaction 4 and did not generate additional t-Bu radicals in reaction 5. Thus reaction 1 is the dominant source of t-Bu, with or without
02.
A. A. Westenberg and N. de Haas, J. Chem. Phys., 47, 4241 (1967); 50, 2512 (1969). S. Gordon and W. A. Muhc, Int. J. Chem. Kinet., Symp. No. 1, 289 (1975). R. B. Timmons, J. de Guzman, and R. E. Vamerin, J. Am. Chem. Soc., 90, 5996 (1968). G. Paraskvopoulos and R. J. CvetanoviE, J. Am. Chem. Soc., 91, 7572 (1969). F. J. Wrlght, Symp. (Int.) Combust. [Proc.], 10, 387 (1965). T. M. Lenhardt, C. E. McDade, and K. D. Bayes, J. Chem. Phys., 72, 304 (1980). R. P. Ruiz and K. D. Bayes, to be submitted. Thermodynamic data and ionization potentials have been taken from H. M. Rosenstock,K. Draxl, B. W. Stelner, and J. T. Herron, J. phys. Chem. Ref. Data, 6 , Supplement No. 1 (1977) and from ref 20. S.W. Benson and H. E. O’Neal, “Kinetic Data on Gas Phase Unimolecular Reactions”, Nab. Stand. Ref. Data Ser., Nab. Bur. Stand., No. 21, 1970, 597. F. J. Wright, J. Chem. Phys., 38, 950 (1963). S. S. Thomas and J. G. Cahrert, J. Am. Chem. Soc., 84,4207 (1962). 0. McKay, Prog. Energy Combust. Sci., 3, 105 (1977).
