486
J. M. SCARBOROUQH AND R. B. INQALLS
concentration in moles per lo00 cms. The apparent molar volume has the value 105.72 over the whole concentration range. Achnowledgment. We wish to thank Professor G. V.
Schultz of the Science Faculty of the Johannes Gutenberg University, Maina, West Germany, for kindly making available to us the thesis of Dr. O-M. H. von Gierke.
Pyrolysis and High=TemperatureRadiolysis of o=Terphenyll
by J. M. Scarborough and R. B. Ingalls Atomics International,A Division of North American Aviation, Inc., Canoga Park, California (Received April 91,1966)
The pyrolysis and high-temperature radiolysis (400482") of liquid o-terphenyl have been studied. The pyrolytic decomposition was found to follow simple first-order kinetics with an activation energy of 71.7 A 1.3 kcal/mole. The radjolytically induced decomposjtion occurred with an apparent activation energy of about 22 kcal/mole. The ratio of biphenyl formation to terphenyl disappearance is about 0.35 for pyrolysis but increases with increasing temperature during radiolysis with l-Mev electrons. A thermal spike model of low LET radiolysis originally derived to explain the radiolysis of toluene a t low temperatures is shown to account for the major features of the data.
Introduction I n order to rationalize the occurrence of chemical reactions which require high activation energies during low temperature, low LET (linear energy transfer) radiation, regions of high-energy density called thermal spikes have been suggestedS2 I n ref 2a it was estimated that radiation-induced reactions take place at "temperatures" on the order of 500" above ambient. If ambient temperature is sufficiently high, it is to be expected} then, that (radiation-induced) pyrolytic decomposition will occur in the thermal spike in addition to the decomposition induced more directly by the ionizing radiation. Thus the theory predicts a kind of synergistic effect of high temperature and ionizing radiation rather than the simple addition of the two effects. Such synergism has been observed in the hightemperature radiolysis of o-terphenyl which is the subject of the present paper.
contained in evacuated stainless steel capsules were pyrolyzed at 419, 428, 455, and 482". Other samples of the same o-terphenyl were irradiated with l-Mev electrons at controlled temperatures in special irradiation cells which were designed to operate tit high temperatures and pressures. The samples were thoroughly degassed and the cells were sealed under vacuum in a manner similar to that used for the pyrolysis capsules. A cross-sectional drawing of the irradiation cell is shown in Figure 1. The unique feature of this cell is the reinforcement of the thin window with a stainless steel honeycomb structure. The completed cell withstood a pressure of 1600 psi at 480" in air and passed virtually all of the electron beam that would be transmitted by the unsupported window. Precautions were taken to minimize thermal degradation of the samples during heating to experimental
Experimental Section Samples of chromatographically pure o-terphenyl
(2) (a) R. B. Ingalls, P. Spiegler, and A. Norman, J . Chem. Phys., 41, 837 (1964); (b) J. M. Scarborough and J. G. Burr, ibid., 37, 1890 (1962).
The Journal of Phyaical Chemistry
(1) This work was supported by the U. 8.Atomic Energy Commission.
PYROLYSIS AND RADIOLYSISOF 0-TERPHENYL
487
419 O C
=. t
-I
0.80 o.eoC
z
sL
0.90
6
0.70
Y
428OC
80
40
O
,
& I . O O r \ ,
I
I
120 ),
I
I60 I
,
I
455%
..-F"-.-a]
i; 1.00
$ W
0.80
I
0
'
1
'
1.0
1
'
1
3.0
2.0
'
1
-
1
1
4.0
TIME (hr)
Figure 2. Weight fraction of o-terphenyl vs. time.
Figure 1. Irradiation cell.
temperature, and corrections were made for the slight damage which did occur during initial heating and cooling. This correction was significant only at 482" and even at 482' it was less than 2%. All irradiated and pyrolyzed samples were analyzed by standard methods of gas chromatography to determine the major components, o-terphenyl and biphenyl. Dosimetry was based on the hydrogen yield (GH,= 0.038) from benzene.
Results Pyrolysis. Experimental results for the pyrolysis experiments are shown in Figure 2. The concentration (weight fraction) of o-terphenyl remaining as a function of time at a given temperature was found to be best represented by a first-order equation of the form (1
- y)
= e--kt
where (1 - y) is the weight fraction of o-terphenyl remaining after t hours, and IC is specific rate constant (initial decomposition rate). Figure 2 shows the semilog plots of concentration vs. time in hours.
Specific reaction rates, k, for each temperature were determined by the method of least squares. Thus the specific reaction rates at 419, 428, 455, and 482" were 0.00067, 0.00154, 0.0103, and 0.0588 hr-l, respectively. The initial thermal decomposition rate for o-terphenyl as a function of temperature is given by the expression In R(-+8)= -(36,100/T)
+ 44.9
where R is in weight fraction per hour and T is temperature in degrees Kelvin. An Arrhenius plot of initial rates is shown in Figure 3; the corresponding activation energy is71.7 f 1.3 kcal/mole. Owing to the scatter in the data for the formation of biphenyl, it is not possible to establish clearly whether biphenyl formation is a zero or first-order reaction. However, a somewhat better fit of the data was obtained from a first-order equation of the form
42 = c(1
- e--kt)
where &t is the weight fraction of biphenyl present in the sample, t is the time in hours, k is the specific rate constant for terphenyl disappearance, and c is a constant. The initial formation rates for biphenyl at 428, 455, and 482" are thus 0.00040, 0.0028, and 0.0149 weight fraction/hr, respectively. The temperature dependence of biphenyl formation is plotted in Figure 3 and is given by the expression Volume 71, Number 9 Februury 1967
J. M. SCARBOROUQHAND R. B. INGALLS
488
Table I : Radiolysis of o-Terphenyl -Composition
of sample, w t fraction-
Dosage rate, w/g
terphenyl,
Total decompn products,
1-Y
Y
Temp, OC
Time, hr
Total dossge, w hr/g
400 & 3
0.517 1.00 1.58 3.00 4.08 5.0 12.0 20.0
0.419 0.862 1.296 2.473 3.342 4.438 9.91 16.45
0.810 0.862 0,820 0.824 0.819 0 * 888 0 826 0.823
0.923 0.900 0.862 0.789 0.749 0.696 0.589 0.477
0.077 0.100 0.138 0.211 0.251 0.304 0.411 0,523
0.0056 0.0075 0.0238 0.0372 0.0454 0.0557 0.0678 0.756
428 zk 3
0.333 0.750 1.25 1.50 3.00 6.75 10.0
0.241 0.541 0.940 1.071 2.140 4.766 7.170
0.723 0.721 0.752 0.714 0.713 0.706 0.717
0,931 0.806 0.827 0.823 0.709 0.568 0.447
0.069 0.134 0.173 0.177 0.291 0.432 0.553
0.0178 0,0348 0.0405 0.0389 0.0649 0.0856 0.103
454 f 3
0.50 0.80 1.53
0.431 0.689 1.310
0.861 0.861 0.856
0.815 0.760 0.673
0.185 0.240 0.327
0.0448 0.0585 0.0805
481 =!z 3
0.50 1.00 1.50 2.3
0.445 0.893 1.252 1.830
0 * 889 0.893 0.834 0.796
0,752 0,643 0.595 0.500
0.248 0.357 0.405 0.500
0.0639 0.0904 0.103 0.123
I
0-
Biphenyl, 99
where R is in weight fraction per hour and T is in degrees Kelvin. The activation energy is therefore about 71 kcal/mole. This is, within experimental error, the same as the activation energy for terphenyl decomposition. Radiolysis. Radiolysis data are shown in Table 1. Since simple first, second, or third-order kinetic equations did not provide satisfactory representation of the data, the differential method3 was used to determine the initial rates (specific rates) graphically. Initial G values (molecules of o-terphenyl destroyed per 100 ev of energy absorbed) were calculated from initial decomposition rates. The temperature dependence of initial G values is shown in Figure 4 and is represented by the expression 1nG = (-ll,lOO/T) 0.0001
1.32
1.40
1.35
1.44
+ 17.2
Thus the radiolytic decomposition of o-terphenyl occurs with an activation energy of 22 kcal/mole. Biphenyl Formation. The differential method was
I/T(OK)x103
Figure 3. Initial rates us. inverse of temperature.
The Joulnal of Phyaical Chemistry
(3) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1950, p 14.
PYROLYSIS AND RADIOLYSIS OF O-TERPHENYL
489
also used to determine the initial formation rates (specific rates) for biphenyl from data in Table I. Initial G values for the formation of biphenyl were calculated from initial rates. The temperature dependence of biphenyl formation based on initial G values is shown in Figure 4. It may also be seen from Figure 4 that the Arrhenius plot for biphenyl formation is linear and corresponds to an activation energy of about 40 kcal.
Discussion Pyrolysis. The activation energy (71.7 f 1.3 kcal) observed for the pyrolysis of o-terphenyl is significantly lower than the dissociation energy of the weakest bond; Le., about 95 kcal for the G C bond between rings. This suggests that a chain mechanism is involved in the thermal decomposition. The pyrolysis of the three isomeric terphenyls has been studied quite extensively by Juppe, Alvarenga, and Hanr~aert.~ Their results indicate that the molar yields of benzene and biphenyl are approximately equal, that the moles of hydrogen formed equal roughly the number of moles of higher molecular weight products such as quaterand quinquephenyl, and that there is significant isomerization of o-terphenyl by a first-order reaction. These findings suggest that displacement reactions of the type +3
+
-
‘ ~ 3
H
+
c~e
--,++a -+2+Y)4 - + + 4 + n
-++S+V
are common. Furthermore, C-H bond breaking will probably occur to an appreciable extent at these temperatures, so that the reaction mechanism is probably quite complicated. However we do not need to understand this mechanism in detail in order to use the thermal decomposition of o-terphenyl as a measure of local temperatures, which is what we do in effect below. Radiolysis. The average LET of cobolt-60 y-ray radiolysis is so low as to suggest that no track effects should be observed. However, there are 6 rays produced which may deposit 1 kev or more of energy6 in a very small volume. This energy is deposited largely as excitation and ionization of the molecules of an organic liquid. Such electronic excitation will usually lead directly to vibrational excitation since the intranuclear distance at minimum energy is in general different in an excited state than it is in a ground state. Thus, Franclr-Condon transitions lead to vibrational
0.10 1.M
1.35
1.40
I45
1.54
IIT ( O K ) I 103
Temperature dependence of initial G values.
Figure 4.
(and rational) excitation which is quickly equilibrated with its surroundings and forms the basis for assigning a “temperature.” The lifetimes of ions and electronically excited states are probably quite short in an organic liquid so that most of the energy originally deposited as electronic energy is converted into the form of vibrational and rotational energy (“heat”) in a short time. Magee6 has estimated that after about 10-l2sec it is possible to assign temperatures with some validity. Thus, these 6 rays constitute thermal spikes which are qualitatively different from the rest of the solution. The LET in the 6 rays is quite high’ even though the average LET is low. Most of the chemical reactions induced by y or electron irradiation occur in the low LET portion of the electron path (i.e., outside of the thermal spike) but the higher temperatures and higher concentrations of excited molecules in the &ray thermal spikes allow reactions to proceed which cannot otherwise occur at measurable rates. Thus, certain products may be formed in the thermal spikes exclusively. We propose that this thermal spike model is applicable to the high-temperature electron irradiation of o-terphenyl, and we will compare the properties of this model with the experimental results. ~~
~~
~~
~~
~~~
~
(4) G . Juppe, A. Alvarenga, and H. Hannaert, European Atomic Energy Community. Euratom publication EUR 1647e (1964). (5) The number of such events is small end difficult to determine but the energy deposited in these high-energy I rays may be as high as 8% of the total absorbed from Coao radiation. See A. M. Rauth and J. A. Simpson, Radiation Res., 22, 643 (1964). (6) J. L. Magee, Discussions Faraday Soc., 36, 232 (1963). (7) 1 kev in 6 X 10-7 cm = 17 ev/A.
Volume 71, Number 3 Febrmrg 1967
490
The decomposition of o-terphenyl induced by ionizing radiation at room temperature is more than an order of magnitude lower than that induced by radiation at the temperature used in this investigation. Thus to a first approximation the decomposition induced directly by the ionizing radiation may be neglected by comparison with the decomposition which is induced by both heat and radiation. Presumably, most of the energy of the ionizing radiation is lost in processes which are quite independent of temperature. However, temperatures in the thermal spike have been estimated*&to be in the order of 500” above the temperature of the bulk of the sample. Although these thermal spikes are of short duration, encompass a small number of molecules, and account for only 8% of the energy of the ionizing radiation, they may be responsible for initiating most of the decomposition observed during radiolysis at high temperatures. Thermal spikes attaining “temperatures” of several hundred degrees above the temperatures at which the irradiations reported in this paper were carried out result in the thermal dissociation of substrate molecules as well as free radicals and other intermediates. This initiation is followed by a chain and thereby enhances the decomposition markedly. Thus, we propose that the only chemically important step in the “radiation-induced pyrolysis” of o-terphenyl which occurs in the thermal spike is the initial dissociation of the molecules into free radicals which requires about 95 kcal/mole, since there is not time for the chain reactions involved to take place before the thermal spike cools off. Chain propagation probably requires a sufficiently low activation energy that it occurs readily outside the thermal spike in these experiments. Thus the thermal spike “temperature” referred to in this paper is “vibrational temperature,” since translational and rotational degrees of freedom do not play a major role in the dissociation of molecules. The conclusion that there is a chain reaction occurring outside the thermal spike is supported by the magnitude of the yields observed. At 481 ” G( - o-terphenyl) is 12.7, which requires that about 1500 molecules be decomposed per thermal spike. There are only about 300 molecules in a volume with a radius of 3 X lo-’ cm,8 however, so that the spikes are simply not large enough to account for the yields if all the reaction were to take place in the spike. Furthermore, at 1200-1500°K only a fraction of the molecules in the spike are expected to decompose in lo-” sec8 This time is long compared to the lifetime of excited states in a liquid and is long compared to the time required for a molecule to dissociate, but a t 1200-1500°K only a small fraction of the molecules have sufficient energy to dissociate. Thus, it is clear that the model requires The Jaurnal of Physical Chemistry
J. M. SCARBOROUGH AND R. B. INGALLS
that a chain reaction continue outside the thermal spike in general agreement with the accepted interpretation of the pyrolysis of o-terphenyl. Therefore, the energy required to break the weakest bond ( ~ 9 5kcal) and initiate the chain is taken as the approximate activation energy of the “radiationinduced pyrolysis” and a value of 1430°K is obtained for the temperature in the thermal spike calculated on that basis.9 This is some 700” higher than the bulk temperature which is higher than the 500” above ambient estimated in the case of toluene. Quite obviously these estimates do not take into account all the observed decomposition such as the temperature dependence of chain length, so that 500 and 700” temperature rise for these two systems is satisfactory agreement. The fact that pyrolytic reactions are not the only reactions involved in the high-temperature radiolysis of o-terphenyl is demonstrated by the fact that the ratio of biphenyl yield to o-terphenyl disappearance is constant in pyrolysis but varies with temperature in radiolysis (Figure 5). Thus, the model must not only account for a variation of the ratio of biphenyl formation to o-terphenyl disappearance, but it must account for a higher value of the ratio of biphenyl formation to terphenyl disappearance in high-temperature radiolysis than in pure pyrolysis. These results are accounted for by the model because of the formation of a high concentration of phenyl and biphenyl radicals as a result of thermal dissociation in the thermal spike. These radicals are then available for reaction with the biphenyl-cyclohexadienyl radical produced via radiolysis. lo The products of these reactions are probably benzene, biphenyl, terphenyl, and quaterphenyl. Another reaction which may occur in the thermal spike which would result in the formation of benzene and biphenyl is the direct combination of hydrogen atoms with phenyl and biphenyl radicals (produced pyrolytically). By either of these mechanisms or by a combination of the two, the relative yield of biphenyl will increase with increasing temperature, and can exceed the yields, relative to terphenyl disappearance, which occur in pure pyrolysis. However, it should be pointed out that this is by no (8) The dimensions of the thermal spike were estimated theoretically in ref 2s. (9) The estimate of the local temperature in the thermal spike is made as in ref 2a by solving for the temperature T at which a reaction with an activation energy E = 95 kcal will change its rate by the observed factor p = 12.7/2.14 = 5.94 in a change of temperature A T = 400° - 481O = -81°, as observed. This temperature is 143OOK. (10) Hydrogen atoms, a major intermediate in the radiolysis of aromatic systems, add to the aromatic substrate to produce cyclohexadienyl radicals.
PYROLYSIS AND RADIOLYSIS OF O-TERPHENYL
Figure 5 . Ratio of biphenyl formation to terphenyl disappearance us. temperature.
means the only possible kind of explanation for our results. Other workers” have suggested that the thermal decomposition of free radicals produced by the ionizing radiation leads to a chain reaction and is r e sponsible for the small variations of yield with temperature. Such a mechanism is possible in saturated hydrocarbon systems. However, aromatic radicals in the o-terphenyl will not dissociate with an activation energy near 20 kcal/mole. Therefore, this explanation is not applicable to the O-terphenyl system. “Hot” atoms or radicals may provide the basis for still another explanation. l2 Radiolytic instability of some pyrolytic decomposition products13 or pyrolysis of radiation-induced have also been suggested for aromatic systems. However, the success of the thermal spike model in accounting qualitatively for the variation of yields of products formed by high activation energy processes (those products produced mostly in the thermal spike) in at least two systems makes it attractive even if other explanations exist for each system separately. Thus the thermal spike model provides the basis for explanation of the large yields observed and temperature dependence of the data presented, and this same model has been used to explain the low-temperature dependence and low yields of hydrogen observed in the low-temperature radiolysis of toluene.2a I n the earlier example of its application the bulk temperature was so low and the LET so small that no significant pyrolysis occurred. I n the experiments reported here, the bulk temperature is apparently high enough so that
49 1
the energy density in the thermal spike is sufficient to induce significant thermal dissociation. This initiation is followed by a chain reaction which leads to the high yields of products observed. If the combined pyrolytic and radiolytic decomposition yields are compared to the pyrolytic yields and lowtemperature radiolysis yields (assuming that radiolysis and pyrolysis are additive), it is found that, separately, they each contribute only slightly to the total decomposition rate. Even at 482” pyrolysis alone accounts for less than 6% of the combined radiolytic and pyrolytic rate, at a dosage rate of 0.8 w/g and the low-temperature radiation yield is lower by more than an order of magnitude. Thus high temperatures and ionizing radiation are synergistic, as predicted by the thermal spike model. The thermal spike model is admittedly crude and in need of refinement. It is inherently approximate so long as it uses the concept of “temperature” to describe a region which is obviously not in thermodynamic equilibrium. Also, an exact description of the shape of the spike (Le., energy density as a function of time and space) is not easily determinable. However such limitations may not impair the usefulness of the model in correlating observations and predicting results.
Summary and Conclusions The pyrolytic decomposition of o-terphenyl follows simple first-order kinetics. Specific reaction rates were established at 419, 455, and 482” and the activation energy for the thermal decomposition of o-terphenyl was found to be 71.7 f 1.3 kcal/mole. Biphenyl formation in pyrolysis also follows apparent first-order kinetics and may occur with the same ratedetermining step as the decomposition of o-terphenyl; the activation energy for biphenyl formation is equal within experimental error to that for o-terphenyl disappearance. This work establishes a temperature dependence for the radiolysis of o-terphenyl not clearly indicated in previous work. An activation energy of 22 kcal/mole was observed in the temperature range studied which is significantly lower than that expected for freeradical dissociations in aromatic systems. Thus, this (11) A. V. Topchiev, “Radiolysis of Hydrocarbons,” English ed, R. A. Holroyd, Ed., Elsevier Publishing Co., Amsterdam, 1964, p 204; and P. L. Lucchesi, B. L. Tarmy, R. B. Long, D. L. Baeder, and J. P. Longwell, Ind. Eng. Chem., 5 0 , 879 (1958). (12) J. Y. Yang and J. G . Burr, J . Chem. Phys., 44, 1307 (1966); R.B. Ingalls, ibid., 44, 1308 (1966). (13) J. Weiss, C. H. Collins, J. Sacher, and N. Carciello, I d . Eng. Chem., 3 , 73 (1964). (14) P. Leveque, F. Franaetti, M. Van der Venne, and M.Guilani, U.N . I t e r n . Conf. Peaceful Uses At. Energy, Srd, A, 28/P/53,May 1964.
Volume 7I,Number 8 February 1967
E. GRUNWALD, C. F. JUMPER,AND M. 5. PUAR
492
result suggests that one or more of the rate-controlling steps in the high-temperature radiolytic decomposition involves thermal dissociation in a thermal spike. Acknowledgment. The authors wish to thank Dr. Richard Holroyd of our laboratory for extremely
helpful discussions and criticism of this work. We wish to acknowledge the help of Dr. Trent Tiedeman, also of this laboratory, for his assistance in calculating the activation energies of certain free-radical processes which may occur during pyrolysis of o-terphenyl.
Rates and Solvent Participation in Acid-Base Reactions of Substituted
Phenols and Phenoxides in Methanol'"
by Ernest Grunwald,'b Charles F. Jumper, and Mohindar S. Purlb Bell Telephone Laboratories, Inc., Murray Hill, New Jersey, and Lecks Chemical Laboratories, Brandeis University, W d t h a m , Massachusetts (Received M a y 31, 1966)
The kinetics of proton exchange between methanol (the solvent) and p-nitrophenol or p bromophenol has been investigated by the nmr method in buffered solutions containing phenol and phenoxide and in acid solutions containing phenol and HC1 at -80". Rates of proton exchange are derived from measurements of the CH3- and OH- proton resonances of methanol and of the OH-proton resonance of the phenol. The following processes involving the given numbers of solvent molecules have been identified: (1) base dissociaHO(CH3) HOCHI i=2 tion of ArO-, which involves two solvent molecules, ArOArOH (CH3)OH -0CH3; (2) a symmetrical process that involves one solvent molecule, HO(CH3) HOAr + ArOH (CH3)OH -0Ar; and (3) a process that involves ArOone methyloxonium ion, one ArOH, and an unknown number of solvent molecules. Rate constants for these processes and base dissociation constants KBfor reaction 1 are reported. Substituent effects on K B at -80" are nearly equal to those at 25" and are largely entropy effects. Reversal of reaction 1 is fast enough so that this rate could be diffusion controlled even at -80". The self-association of p-nitrophenol was measured at -80" by means of nmr chemical shifts and was substantially that to be expected on the basis of volume fraction statistics. A noteworthy and unsolved problem in the interpretation of the CH3-proton resonance of methanol is that plausible rate laws can be obtained only if values of 1l.r based on the Bloch equations are first corrected by means of an empirical function, + ( T ) , as described in the paper.
+ +
+ +
I n previous papers from these laboratories, nuclear magnetic resonance (nmr) was used to measure rates and solvent participation in acid-base reactions in We now apply water and other hydroxylic the nmr method to the proton exchange reactions of phenols and their conjugate bases in methanol. Our The Journal of Phyeicial Chemistry
+
+
+
+
substrates are p-bromophenol and p-nitrophenol, the reaction temperature is -80", and the pH of the reac(1) (a) Work supported in part by the Petroleum Research Fund of the American Chemical Society. Grateful acknowledgment is made to the donors of that fund. (b) Brandeis University Waltham, Mass.
