MARKCHER
1316
The Reaction of Methyl Radicals with Toluene
by Mark Cher North American Aviation Science Center, Canoga Park, California (Received October 88, 1958)
Gas phase photolyses (A = 3660 k )of mixtures of azomethane with toluene-a-da, toluenedR, and toluene-d5 were carried out a t 60°, and the isotopic composition of the product methane was determined in order to assess the importance of the methyl free radical reaction with the aromatic ring. It is concluded that methyl radicals abstract hydrogen directly from the aromatic ring in toluene at a rate which is 0.17 as fast as the rate of abstraction from the side chain. Deuterium isotope effects of about 6.4 for both the side chain and ring abstraction reactions are shown to be extremely important in determining the course of the reaction. Analyses by gas chromatography showed that ethylbenzene is a major nongaseous product, and xylene and bibenzyl are barely detectable. I n the liquid phase photolysis, o-xylene is a major product, while m-xylene, p-xylene, and ethylbenzene are formed in smaller yield, and bibenzyl is found only in trace amount. The difference in the products implies that methyl radicals do not permanently add to the ring in the gas phase reaction, but they do so in the liquid phase reaction. These results are discussed in terms of the different mechanisms that operate in each phase.
Introduction The reaction of methyl radicals with toluene is of particular interest because of the possibility of comparing relative rates of reaction with the aliphatic and aromatic parts of the toluene molecule. I n the liquid phase, evidence has been presented for both addition of the free radical to the ring' and abstraction of hydrogen from the ring to form i ~ ~ e t h a n e . I~nJ the gas phase it is usually assunied,4~5on the basis of bond strength arguments, that methyl radicals do not interact with the ring to any great extent, but only abstract hydrogen from the side chain. However, the recent work of Meyer and Burr6suggests that in the gas phase at 750' abstraction from the ring may indeed be very important. Therefore, we have undertaken to test directly for the possibility of ring involvement in the reaction producing methane by photolyzing gaseous mixtures of azomethaiie and deuterated toluene and by studying the isotopic composition of the methane. We reasoned that if addition of methyl radicals to the ring were indeed occurring, the reaction would be favored at low temperatures. Consequently, we chose to u7ork a t 60°, where the vapor pressure of toluene is adequately high. The production of methyl radicals in the photolysis of azomethane has been well established.' The Journal of Physical Chemistry
Experimental Apparatus. A conventional glass vacuum line was used to handle gases. I n order to prevent condensation of toluene vapor, the reaction cell, the toluene storage vessel, a mercury manometer, and a McLeod gage were enclosed within a heated box maintained at 60.0 f 0.5'. Metal Hoke valves, Type 413, were used inside the box. The cylindrical Pyrex reaction cell (50 mm. i d . , 176 mm. long) had flat Pyrex windows and was provided with a magnetically operated stirrer for gas mixing. The gas pressures were measured directly with the McLeod gage or mercury manometer. The light source was a Hanovia Type A medium pressure mercury arc. The light beam, which filled the entire cell, was collimated roughly with a quartz condensing lens. A Corning 7-83 glass filter was used to isolate the 3660 A. region. (1) M. Levy and M.Pzwarc, J . Am. Chem. Soc., 76,5981 (1954).
S. H. Wilen arid E. L. Eliel, ibid., 80, 3309 (1958). M. Levy, M. Steinberg, and M. Szwarc, ibid., 76, 3439 (1964). A. F. Trotman-Dickenson and E. W. R. Steacie, J . Chem. P h y s . , 329 (1951). I. B. Burkley and R. E. Rebbert, J . Phys. Chem., 67, 168 (1963). R. A. Meyer and J. G. Burr, J . Am. Chem. Soc., 85,478 (1963). (7) M. H. Jones and E. W. R. Steacie, J . Chem. Phys., 21, 1018 (1953).
(2) (3) (4) 19, 1.5) (6)
REACTION OF METHYL RADICALS WITH TOLUENE
Analysis. After photolysis all vapors were tranaferred to a series of traps a t -196'. The noncondensable fraction, composed mainly of nitrogen and methane, was transferred by means of a toepler pump into a calibrated volume where the pressure was read, transferred to a container, sealed, and subsequently analyzed with a C.E.C. 21-103 mass spectrometer. .A second fraction volatile a t - 150" (isopentane slush) was similarly collected and the pressure-volume product measured. Analysis of a few early samples showed this fraction to be mainly ethane, and so the mass speotral analysis in most, cases was not performed. Qualitative analyses for nongaseous products were carried out in a Perkin-Elmer Model 226 gas chromatograph equipped with a flame ionization detector, using a 30-m. "R" capillary column. This column is very satisfaatory for resolving the three xylenes and ethylbenzene from each other. Materials. Azomethane was prepared from 1,2-dimethylhydrazine hydrochloride by the procedure of Renaud and LeitchsB It was dried by bulb-to-bulb distillations through a Drierite trap and a final distillation a t -78'. Methyl chloride contamination was reduced to less than 1% by extended pumping at -150'. The azomethane was stored a t -196', and all operations involving its volatilization were carried out in a semidarkened room. Toluene-a-d3 and toluene-de were purchased from Merck Sharp and Dohme of Canada, Ltd. Toluene-& was prepared by J. G. Burr in our laboratory bly methylation of benzene-de. Each of these compounds was purified by means of preparative gas chromatography. Their low voltage mass spectra showed 6-7% toluene-d,-l impurii,y, where n is the number of :D atoms in each of the parent compounds. Carbon dioxide (Matheson, Bone Dry) was purified by bulb-to-bulb distillation a t - 78'.
Results The results of the photolysis of mixtures of azomethane with toluene-a-da (CeH&D8), toluene-de (CaD6CD3),and toluene-& (CaD6CH3)are summarized in Table I. In all cases the major gaseous products are nitrogen, ethane, methane, and methane-&. Trace quantities of hydrogen are also observed. The fraction of asomethane decomposed, based on the yield of nitrogen, ranged from 16 to %yo,.Since ethane is the major hydrocarbon product, and toluene is present in very large excess, the fraction of toluene decomposed is less than 0.1%. Figure 1 shows the CHJCH3D yield ratio plotted as a linear function of the initial azomethane-toluene concentration ratio for the photolyses involving toluene-a-d3 and toluene-de. We notice that
1317
0
I
I
1
I
I
I
0.5
1.0
1.5
2.0
2.5
100X [ AZOMETHANE] / [ TOLUENE] Figure 1. The isotopic composition of methane in the photolysis of azornethane-toluene mixtures.
the relative yield of CH4increases with increasing &BOmethane concentration, which is to be expected since a fraction of the methane originates via hydrogen abstraction from azomethane itself. In the case of toluene-d6, the CHJCH3D ratio is essentially independent of the azomethane-toluene concentration ratio within the range of concentrations studied, as shown in Table I. Addition of a large excess of COz gas (last two runs in Table I) had no effect on the CH.,/CH3D ratio, but resulted in a significant decrease in the (C2Ha CH4 CH3D)/N2ratio. I n view of the liquid phase findings1 that methyl radicals add to the aromatic ring, it seemed worthwhile to examine the nature of the nongaseous products in the gas phase photolysis and compare them with the products of the liquid phase photolysis. For this purpose a mixture containing about 10% azomethane and 90% toluene was condensed and sealed into a small ampoule made of 6-mm. Pyrex tubing, the vapor phase region was masked off, and the exposed solution was photolyzed at room temperature for about 110 min. The total weight of liquid was comparable to the weight of gas in the gaseous experiments. A similar mixture
+
+
(8) R. Renaud and L. C . Leitch, Can. J . Chem., 32, 545 (1964).
Volume 68, hrumber 6 June, 1964
1318
MARK
Table I : Photolysis of Azomethane-Toluene Mixtures-Gaseous
-__
Pressure, mm.----Azomethane Toluene
pmoles---
CHiD
C Ha
-C2H6
CHaD
+ CH4 + CzHa 1c-2
C H4 CHaD ~
0.056 0.068 0,095 0.110 0.113 0.120 0.124
1.97 1,78 3.65 3.90 5.59 3.80 5.45
0.89 0.78 0.97 0.97 0.96 1.06 0.94
1.27 1.30 1.36 1.47 1.74 1.70 1.74
9 8 8
0 60 0 82 1 48
8 0
1 75 2 25
148 147 131 117 105
Toluene-ds 1.95 0.084 2.84 0.106 4.15 0,105 4.45 0.104 5.52 0.121
0.017 0.023 0.041 0.044 0.066
1.95 2.65 3.83 4.11 4.97
1.05 0.98 0.96 0.96 0.93
0,202 0.217 0,390 0.423 0.545
5 7 6 2 3
1 1 2 1 1
126 123 80 126 130
Toluene-& 4.07 0.016 4.01 0,023 3.92 0.019 4.21 0.025 4.00 0.023
0.648 0.896 0.730 0,969 0,919
3.96 3.03 3.12 2.18 1.62
1.14 0.99 0.99 0.76 0.64
1 30 1 57 2 00
94 97 87 89 89
1 1 2 1 1
71 81 83 84 83
... ...
... ... ... , . .
...
297 641
I
FEDC
sz
Toluene-a-ds 2.33 0.044 2.44 0,083 3.93 0,071 4.21 0,075 5.98 0.066 3.78 0.060 6.02 0.071
0 57 0 80
49
,-.----
121 133 131 116 120 136 97
79.9 81.0 83.9 84.2 76.9 47.9 79.8
51
Time, min.
Yields
0.71 0.73 1.20 1.69 2.16 2.32 2.51
0.57 0.59 1.01 1.42 1.66 1.11 2.00
29 47 26
100 X [.4l/[TI
Con
CHER
B
f- Time.
(a)
80 80
io 79 79
L B
t- Time. (b)
Figure 2. Gas chromatograms of liquid residues: ( a ) gas phase photolysis; ( b ) liquid phase photolysis. The identified peaks are: A, azomethane ( X2000); B, toluene ( X50,OOO ( a ) and X20,000 ( b ) ) ; C, ethylbenzene; D, p-xylene; E, m-xylene; F, o-xylene. The attenuations of the product peaks are X.50 for ( a ) and X 5 for ( b ) . The slight difference in retention times is due to the fact that ( a ) and ( b ) were run a t different temperatures. The apparent peak between B and C is caused by the toluene tail as a result of the change in attenuation.
was also photolyzed at 60' for 104 niin. in the gas phase in the usual maimer. The condensable liquid residue in The Journal of Physical Chemistry
40.5 39.0 38.4 38.8
40.0
each experiment was analyzed by gas chromatography. The chromatograms obtained, Fig. 2 , clearly show that in the liquid phase o-xylene is a major product, while m-xylene, p-xylene, and ethylbenzene are produced in smaller yield. I n the gas phase, however, ethylbenzene is the only major nongaseous product, and the xylenes are barely detectable. Bibenzyl is observed only in trace quantities in both the gas phase and the liquid phase residues. Blank analyses of the nonphotolyzed material demonstrated absence of any of these products in the starting material. Except for smaller yields of products, analysis of the residue from a gas phase run involving toluene-a-do gave similar results.
Discussion The striking increase in the CH, yield relative to that of CH3D in changing from toluene-& to tolueneCY-& (Fig. 1) can only mean that hydrogen from the ring appears in the product methane, for otherwise the two lines would coincide. This conclusion is substantiated by the presence of CH3Din the photolysis involving toluene-&. Although the yield of CHaD in this case is quite miall, about 2.5y0of the total methane, it is nonetheless very significant in view of the isotope effect tending to retard reactions of deuterium relative to those of hydrogen, as discussed below. I n order to estimate the relative importance of ring abstraction us. side-chain abstraction, we assume the
REACTION OF METHYLRADICALS WITH TOLUENE
following simple mechanism for the formation of methane CH3n”CHa
+ h~
---j
2CH3
+ Nz
+ A CH4 + .CHZNNCH3 CHB+ T ---%CH4 + .CHZC&T.~ C& + T A CH, + CH3CeH4.
CH3
+-
RCH RCH~D rd [TI -L
rh
Rate constant ratio
(2)
rh/sd
1.07’
TdSh
0.026‘ 11 3.8 0.O98’ 2.3
(3)
sh
ah/&
rh/Td
(4)
(toluene-a-d3)
(5)
(toluene-d5)
(7)
Sd
+-
Table I1
(1)
where A and T denote azomethane and toluene, and a, s, and r are specific rate constants pertaining in the case of toluene to the reaction with the entire side chain or the entire aromatic ring. This mechanism leads to eq. 5-7 for the rateir of production of CH, and CH3D from each of the deuterated toluenes
RCH 2RCH~D Sd [TI
1319
rd
In these equations the subscript d or h indicates whether the abstraction involves deuterium or hydrogen. The justification for writing reaction 4 as a direct abstraction from the aromatic ring in place of a more complicated sequence of events involving a Breliminary addition to the ring is given below. However, eq. 5, 6 , and 7 are ind,ependent of this assumption. These equations predict straight lines when the CH,/CHSlD yield ratio is plotted against the azomethane-toluene concentration ratio,3 as shown in Fig. 1. The small intercept in the case of toluene-& is due, of course, Lo the hydrogen impurity in the toluene-ds. Equating the observed slopes and intercept in Fig. 1 to the corresponding ratio of rate constants, eq. 5 and G , and using the observed CHJCHaD yields in the toluene-d6 experiments (Table I) in conjunction with eq. 7, we obtain after some simple aJgebraic manipulations the set of rate constant ratios shown in Table 11,column (a). The first two ratios are obtained more or less directly from the experimental measurements and are probably accurate to f10%. The last four ratios depend on the small difference between the two slopes in Fig. I. ; an error of + l o % in each slope could lead to an error of a factor of two in the calculation of these numbers. If we assume that the two isotope effect ratios r h / r d and Sh/Sd are equal within an uncertainty of, say, 10% then we obtain the values shown in Table 11, column (b), in which case tbe limits of error are reduced to about 30%. We shall use these latter values for fur-
rh/Sh
a/(rh
+
Sh)
~--Csloulated value(a) a (b)Ir
6.4 6.4 0.17’ 3.3
Remarks
Toluene-oc-d8 Toluene-ds Side chain isotope effect Ring isotope effect Toluene-do
‘Calculated
from the slopes and intercept of Fig. 1 using eq. Calculated assuming equal isotope effects; i e . , r h / r d = Sh/sd. ’ These values are to be multiplied by the factor 3//b to obtain rate constants erpressed in terms of “per C-H bond.” 5-7.
ther discussion, as they are considered more reliable. (However, see below.) The significance of these ratios may be seen in the following summary. The ratios r h / S d = 1.07 and r d / S h = 0.026 are measures of the relative rates of abstraction from the ring and the side chain in toluene-a-d3 and toluene-d6, respectively. Thus with toluene-a-dr, half of the methane originates via reaction with the ring, whereas with toluene-de almost none comes via this process. The ratio r h / s h = 0.17 indicates that about 15% of the time hydrogen abstraction from toluene occurs a t the ring. The activation energy difference between ring abstraction and side-chain abstraction corresponding to this ratio and assuming a steric factor ratio of 5/3 is 1.5 kcal./mole, in very good agreement with the measured activation energy difference for the hydrogen abstraction reactions of methyl radicals with benzene4 and tolueneS4,5 We note that this difference is nearly identical with the zero point energy difference between C-H and C-D bond stretching vibrations. It is not surprising, therefore, that the substitution of deuterium for hydrogen is so important in determining the course of the reaction, as is shown most vividly by comparing the cases of toluene-a-d3 and toluene-da. If we consider either Polanyi’s universal linear relationship” or Johnston’s recent it be(9) Because the A/T concentration ratio changes with time, the proper procedure would have been t o plot in Fig. 1 average A/T ratios rather than initial A/T ratios. The error introduced by making this simplification is relatively minor. (10) This conclusion is not in conflict with previous observations4~6 of straight lines in plots of In kai,e/k1/200mt,inat,on us. 1/T. Over the temperature range available for study, a plot of the function In (Ale-E1’RT A ~ e - ~ a IJS. ’ ~ ~1 /)T shows no discernible curvature if E1 and b differ by 1-2 kcal./mole and AI and APare of the same order of magnitude. Normal scatter in experimental data of this sort would completely obscure any curvature. (11) N. N. Semenov, “Some Problems of Chemical Kinetics and Reactivity,” Vol. I, Pergamon Press, New York, pi. Y . , 1958, p. 27. (12) H. 8. Johnston and C.Parr, J. Am. Chem. SOC.,85, 2544 (1963).
+
Volume 68, h’umber 6
June, 1964
1320
MARK
conies evident that this observed difference in activation energy of 1.5 kcal./mole is too small and not consistent with the difference of about 21 kcal./niole in the bond dissociation energies of the aromatic and sidechain C-H bonds in toluene. A plausible explanation is that the transition state for the ring reaction has a different geometrical configuration from that of the sidechain reaction, the latter corresponding most assuredly to the linear three-center type discussed by Johnston. l 2 For aromatic systems, one may conceive a transition state such that the methyl radical is somehow directly attached to the ring, perhaps by the formation of a diniethylcyclohexadienyl radical, in which case Polanyi’s relationship or Johnston’s calculations would not necessarily apply. The isotope effect ratios r h / r d = 6.4 and S h / s d = 6.4 are measures of the relative rates of abstraction of hydrogen and deuterium from the ring and the side chain, respectively. A detailed calculation of these ratios is not now feasible because of the uncertainty in the transition state for the ring reaction. Nonetheless, if we still apply the simplest possible theory and assume the loss of a C-H stretching mode and possibly the loss of two C-H bending modes in the transition state, we obtain, l 3 using reasonable vibrational frequencies, l 4 kinetic isotope effects of about 6 or 14, depending on whether we consider the bending modes or not, in fair agreement with our experimental results. The important consideration in these calculations is that if the kinetic isotope effects depend specifically on the shifts in the vibrational frequencies of C-H and C-D bonds of toluene, the two isotope effects should be of comparable magnitude, apart from the unknown tunneling corrections, l 5 This would justify our previous assumption of equal isotope effects. However, because of the probable difference in transition states this question remains unsettled, pending the direct measurement of the temperature coefficient of the isotope effects. Finally the ratio a / ( r h sh) = 3.3 refers to the relative rate of hydrogen abstraction by methyl radicals from azomethane and toluene. Using data compiled sh) = by Trotman-Dickenson,16we calculate a/(% 3.6 at 60°, in excellent agreement with our results. By considering the rates of steps 2-4 as well as the known rate of recombination of methyl radicals, it is possible from our data to evaluate each of the rate constants on an absolute basis. For example, in the case of toluene-ds
+
+
a = kt”2R~~,/R~,~ 1 , 1 / 2 [ A (8) sd
-k
rd
=
J ~ t ” Z R ~ ~ , ~ / R ~ , ~ d ” (9) a[TI
where k+is the rate constant for recombination of methyl The Journal of Physical Chemistry
CHER
radicals. Other rate constants are obtained in a similar manner by reference to the data from the other deuterated toluenes, except that in the calculations of s h and r h the previously computed value of a/kt1la (eq. 8) is utilized. The results using kt = 1013.34 cc. mole-’ set.-' l7 are shown in Table 111. Values of a and s h r h computed from data in the literature are also shown for comparison purposes. The excellent agreement displayed here, although very pleasing, must be considered to some extent fortuitous because the absolute rates of formation of methane and ethane were measured with considerably less precision than the CHb/CHZD ratios. Because of this reservation, we prefer not to compute ratios of rate constants from these data but use instead the values in Table 11. A question that arises now is whether the reaction of methyl radicals with the aromatic ring results in the immediate abstraction of a hydrogen atom or whether a permanent addition to the ring occurs, followed by hydrogen abstraction from the dimethylcyclohexadienyl radical via a second methyl radical, as postulated by Szwarc3 for the liquid phase. The fact that the ratio (C2H6 CH, CH3D)/N2 is unity within experimental error (Table I)’* and that xylene is not an important product of the reaction are convincing evidence that permanent addition of methyl radicals to the ring does not take place. We would propose instead that direct abstraction from the ring results in
+
+
+
Table 111: Absolute Values of Rate Constants a t 60°a Rate constant
a sd
+
rd
Sd Th rd
6h Sh
+
Th
C-
Calculated value-----This work Literature
( 1 . 0 f 0.1) x (3.7 f 0.2) x (2.6 i0 . 2 ) x (2.3 f0.4) X (1.0 f 0 . 2 ) x (3.8 f 0.8) X ( 4 . 0 f 0.8) x
106 io4 104
0.9
lO6*
...
... .‘.
lo4 104
... ...
106
lo6
x
4.7 4.1
x x
lo6’ 106d
Source of data
Toluene-& Toluene-& Toluene-d3 Toluene-& TOlUene-ds Toluene-da Toluene-da Toluene-&
The quoted error limits are a All units are cc. mole-’ sec.-1. average deviations. ‘See ref. 7. See ref. 4. See ref. 5 .
(13) L. Melander, “Isotope Effects on Reaction Rates,” Ronald Press Go., New York, N. Y., 1960, pp. 20-22. (14) R. N . Jones and C. Sandorfy, “Technique of Organic Chemistry,” Vol. I X , A. Weissberger, Ed., Interscience Publishers, Inc., New York, N . Y . , 1961, pp. 387-398. (15) H. S. Johnston, “Advances in Chemical Physics,” Vol. 111, Interscience Publishers, Inc., New York, N. Y . , 1961, p. 131. (16) A. F. Trotman-Dickenson, “Gas Kinetics,” Butterworths Scientific Publications, London, 1955, pp. 201 and 202.
REACTION OF
IbIETHYL
the short-lived tolyl free radical, which then undergoes a metathetical reaction with toluene to form the relatively stable benzyl radical. Recombination of the methyl radical with the benzyl radical produced in this way, as well as by direct abstraction from the side chain in toluene, accounts for the ethylbenzene product and also for the mass balance requirement that two methyl radicals be urred up for each methane produced. Recombination of any radical R to form the dimer product RB (e.g., bibenzyl) would cause the ratio (CzHe CH, CH3D)/Nz to become greater than unity. Such a process is unlikely since the concentration of CH3 radicals is large compared to that of any other R radical, as evidenced by the small yield of CH4 relative to C2H6. Indeed no significant yield of bibenzyl was found. The decrease in the ( C Z H ~ CH4 CH~D)/NB ratio caused by the introduction of COB may have to do with the competing reactions of methyl radicals with azomethane in abstraction and addition.? It is possible that the addition reaction requires a third body, and thus it is accelerated by the COz. Thi~s point requires further study. Perhaps the most interesting facet of this system ILS the striking difference in the reaction mechanisms of the liquid and gas phases insofar as the participation of the aromatic ring is concerned. We can formally illustrate this difference by writing down the partial reaction scheme
+
+
+
CH3
+ CeH6CH3 -2
y
free radical addition complex
1321
RADICALS WITH TOLUENE
CH4
\ RH
+
+ CHaCeH4.
+ xylene (liquid phase)
(Ia)
Ob)
where R is a free radical. Reaction I a is the unimolecular decomposition of the addition complex, and it represents the reaction path for the gas phase reaction which involves the ring. Reaction I b is a bimolecular disproportionation reaction between free radicals, and it describes the predominant process in the liquid phase. Several factors may be suggested to explain the difference between the mechanisms in the two phases. Because of the lower recombination rate constant and much higher rate of initiation, the steadystate concentration of radicals R is higher in the liquid phase, and this undoubtedly accounts for the enhancement of reaction I b in the liquid phase. A second consideration relates to the lower equilibrium constant between reactants and addition complex in the gas phase reaction, this being a free volume or entropy effect. Thus in the gas phase the addition complex is more likely to fly apart, and reaction 3 becomes the major process. An alternative but related point of view is that the initially highly excited addition complex has a longer lifetime in the liquid phase, because of efficient collisional deactivation, and thus the rate of reaction I b is enhanced. However, an attempt to increase the yield of the ring reaction in the gas phase experiments by the addition of COz so as to increase the lifetime of the complex was not successful, as shown by the constant CH4/CH3D yield ratio in the experiments with toluene-ds.
Acknowledgment. The author is indebted to Prof. Harold 8. Johnston for very helpful discussions, and to Mr. R. A. Meyer for the mass spectroscopic analysis. (17) A. Shepp, J . Chem. Phys., 24, 939 (1956). (18) A similar good material balance was obtained by Burkley and RebberV in their photolysis experiments with acetone as the source of methyl radicals.
V o l u m e 68, Number 6
June. 1964.
