WILLIAM A. SAXDERS AND R. E. REBBERT
I70
VOl. 67
THE REACTIONS OF METHYL RADICALS WITH AROMATIC COMPOUNDS. 11. THE XYLENES BY WILLIAM A. SANDERS AND R. E. REBBERT~ Chentistry Department, Georgetown Univeisity, Washington 7 , D. C Received July 30, 1962 Acetone was photolyzed in the presence of the p-, a-, and m-xylenes between 100 and 200'. The activation energies for the reactions of the type CHI REI 4 CHd R are 7.4 f 0.2, 7.8 f 0.3, and 8.5 f 0.3 kcal./mole, respectively. The observed order of the activation energies for the three xylenes is consistent with a hyperconjugation mechanism. It should be noted that the pre-exponential factors for the three xylenes are not constant but vary by a factor of four.
+
+
Introduction As in part I,, this investigation was undertaken so that a better representation of the rates of hydrogen atom abstraction by methyl radicals from aromatic compounds would be available. The xylenes were chosen so that the effect of the para-, ortho-, and metasubstitutions could be determined. The observed activation energies should give some indication of the C-H bond strength of the molecule from which the hydrogen atom is abstracted. Szmarc4 has determined the bond dissociation energies for the hydrogen atom 011 the methyl groups of the p-, 0-, and m-xylenes to be 75.0, 74.0, and 77.5 kcal./mole, respectively. Experimental The same apparatus and general procedure as described in were used. The p- and a-xylenes were standard samples part 1% from the National Bureau of Standards. The m-xylene was Phillips Petroleum research grade.
Results The results of the photolysis of acetone in the presence of the three xylenes are given in Tables I and 11. The following equations obtained by the least squares method give the best straight lines through the experimental points
For p-xylene: 13
1619 + log kz7% = 5.262 - T
For o-xylene: 13
+ log k2'/2 = 5.566 - 1712 T
k3
For m-xylene: 13
pre-exponential factors were calculated from the average deviation of the experimental points. TABLE I Im/k2'/z
hcetone pres-
Temo., .. OK.
sure, mm.
x Xylene pressure. T h e . mm. sec.
372.0 395.6 417.7 447.5 469.5
44.3 46.7 49.0 52.3 53.9
49.3 45.6 48.1 48.5 44.9
900 900 900 900 900
374.3 397.2 421.5 428.8 452.5 467.2 477.6
43.1 46.3 45.9 47.6 51.3 57.0 52.1
46.1 50.2 40.7 25.7 42.7 44.7 28.8
900 900 900 900 900 900 900
373.1 399.0 421.5 447.4 469.6
44.8 46.1 48.7 52.6 54.3
56.6 47.2 53.0 51.9 43.8
900 900 900 900 900
+ log
z/'
k2
+
molecules'/n-
~
CO
CHI
p-Xylene 2 . 7 3 0.56 3 . 1 1 0.99 3 . 4 7 1.62 3 . 8 2 2.72 4.01 3.47 a-Xylene 2.50 0.60 2.86 1.10 3 . 5 3 1.69 3.81 1.69 3 . 5 5 2.79 3 . 7 1 3.37 3 . 5 7 3.14 m-X ylene 2 . 0 0 0.52 2.39 0 . 9 3 2 . 6 4 1.54 2.87 2.39 2 . 7 6 2.70
C?Hs
CO
sec.'/z
1.91 2.12 2.04 1.62 1.27
0.91 1.00 1.05 1.14 1.18
8.3 14.8 23.5 44.0 06.7
1 . 7 7 0.95 1.85 1.80 2.23 1.39 0.96 1.03
1.03 0.99 1.03 1.18 1.17 1.17
10.1 16.8 32.0 40.3 56.4 79.4 98.1
1.34 1.45 1.19 0.91 0.56
0.93 1.00 1.03 1.15 1.18
8.0 16.5 27.9 51.7 86.3
h/k?l/Z
1863 = 5.886 - ___ T
All rate coiistants are in molecules, cubic centimeters, and seconds; IC3 and k 2 refer to reactions 2 and 3 of part I CH3
CH4 C2Hs
cc.-see.
1013,
cc.1/2
TABLE I1
k3
I _
k3
Product molecules x 10-13
2CI-I, +C2H6
(2)
+ RI-I +CH, + R
(3)
The activation energies for the hydrogen abstraction reactioiis are 7.4 f 0.2, 7.8 f 0.3, and 8.5 f 0.3 kcal./mole, respectively, for p-, o-, and m-xylenes. for these three The pre-exponential factors, A,: reactions are 1.8 f 0.5 x 10-8, 3.7 f 1.5 x 10-8, and 7.7 f 1.6 x molecule-1/2 C C . ' / ~ sec.-1/2. The limits of precisioil of the activation energies and the (1) Abstracted from a thesis submitted to t h e Graduate School of Georgetown Univeisity in partial fulfillment of the requirements of the M.S. degree. (2) Division of Physical Chemistry, National Bureau of Standards, Washington 2 5 + D. C. (3) I. B. Burkley and R. E Rebbert, J. Phys. Chem., 67, 168 (1963). (4) M. Szwarc, J . Chem. Phya., 16, 128 (1948); 16, 637 (1948).
Compound
x in13 ( a t 182")
p-X ylene a-Xylene m-Xylene Toluene3
50 8 63.6 62.1 22
As X lo8/ AS'/=
1 8 f0.5 3 7 f 1.5 7 7 f 1 . 6 0.8
Ea
- 1/zE2
7.4 f 0 . 2 7.8 f . 3 8 5 f .3 7.4
Discussion Insofar as the activation energies of the hydrogen abstraction reactions by methyl radicals are related to the C-H bond dissociation energies, the results indicate that the bond dissociation energies of the hydrogen atom on the methyl group of the xylenes should follow the order para, oytho, meta from the lowest to the highest. Sewarc4 has studied the kinetics of this pyrolysis of the xylenes by his toluene carrier technique. He reported the following values of the bond dissociation energies of a hydrogen atom from the methyl group: 75.0, 74.0, and 77.5 kcal./mole, respectively, for p-, 0-, and m-xylenes. I n support of the order indicated by his results, Szwarc cited the work of Dobryanskii and Saprykin,6 who studied the isom( 5 ) 4.F. Dobryanskii and F. Y . Saprykin, J . Gen. Chem. ( U S S R ) ,9, 1313 (1939).
Jan., 1963
REACTIOXS O F bxETHYL RADICALS WITH XYLESES
erization of the xylenes. They found that the ortho isomer isomerized most readily, while the meta isomer showed no tendency to isomerize and the para isomer mas between the two. Although the experimental error in our results (about 0.2 kcal./mole) leaves the para-ortho order somewhat uncertain, two other investigators in our Laboratory6J have found this same order with the fluorotoluenes and the eth,yltoluenes. The order for the activation energy of the hydrogen abstraction reaction increasing from the para to meta isomer clan be explained on the assumption that a hyperconjugation mechanism is involved in the stabulization of the transition state complex. Szwarc4 originally put forth this explanation in his study of the kinetics of the pyrolysis of the xylenes. Thus, for p- and o-xylenes we can write the resonance structures
H .*CHz
No such structure can be written for the meta-isomer. Moreover, there is the possibility that there is some steric hindraiice in the transition state complex of the o-xylene structure which lessens the full effect of the hyperconjuga tion. In this regard, it is interesting to note that for the fluorotoluenes the activation energies for the ortho and para isomers are relatively closer to one another and farther from the meta isomeras I n the case of the ethyltoluenes just the opposite is true, that is, the activation energies for the ortho-meta isomers are closer together and farther from the pala isomer." If bleric hindrance is the reason for the difference in activation energy between the ortho and para isomers, a smaller difference would be expected with the fluorotoluenes and a larger one with the ethyltoluenes. It is interesting to compare the results of this investigation with the data obtained from studies of the analogous reaction with toluene. I n Table I1 a surn(6) F. J. Wundrrlich and R. E Rebberi., to be published. (7) I B Bulkley, Ph.D Thesis, 1962, Georgetown Vniveisity
171
mary of these results is given. The ratio of rate constants of the xylenes to toluene amounts to somewhat more than the statistical factor of two which may have been expected if the only change were the presence of an additional methyl group. Throughout this paper we have assumed that it is the hydrogen atom on the methyl groups which are abstracted and not the hydrogen atoms 011 the benzene ring. The reasons for this assumption were given in part I s 3 It should also be mentioned that for all three xylenes a positive deviation was noticed above 200'. However, the amounts of ethane produced at these high temperatures are very small. This makes accurate measurements difficult, since a small absolute error in the ethane measurement would produce a large error in the rate constant. A more recent investigation7 of the 0- and m-xylenes with acetone-ds did not show any curvature in the Arrhenius plots up to 275'. In this case, the ratio of CD3H/CD4 was measured on a mass spectrometer and the results, therefore, should be more accurate. Finally, it should be pointed out that the preexponential factors for these three very similar reactions are quite different. As seen in Table 11, the pre-exponential factors vary by a factor oE four fmm p- to m-xylene. Again this same variation in preexponential factors was obtained by two other investigators with the fluorotoluenes6 and the ethyltoluene^.^ The consistency of these three investigations rules out experimental error as the cause of these differences in the pre-exponential factors. Although at the present time we have no explanation for these differences, it is important to note that the pre-exponential factors are different since in the past many authlors have assumed constant pre-exponential factors for similar reactions. Thus, they have calculated activation energies from rate constants at one temperature. Activation energies so obtained may be considerably in error. It is interesting to note that the activation energy and the pre-exponential factor for p-xylene are almost , ~ the the same as those obtained for toluene i t ~ e l fwhile ortho and meta isomers have higher activation energies and also higher pre-exponential factors. It is not immediately obvious why this should be.
