3462
GEORGE A. DRENNAN AND RICHARD A. MATULA
Little can be said about the mechanism for propane and propene production, since we did not measure either of these compounds in the scavenging experiments. However, they may be formed by typical radical combination reactions. Polymer is probably formed by an unidentified radical reaction which is eliminated in the presence of oxygen. Elemental mercury is primarily produced by the decomposition of the methylmercury radical (reaction 3), although a direct molecular reaction such as reaction 4a or 15 may be responsible for some of the observed Hg. The relatively low yields observed from the solidphase irradiations a t -196 and -78" are not unex-
pected. It can be assumed that the yields a t low temperature (- 196") represent the maximum contributions of "molecular detachment" and/or "hot" atom reactionsaZ0 In addition, a limited amount of radical reactions can occur within the radiation spur. It is interesting to note that, with the exception of Hz, all products observed in radiolysis were either found or postulated as intermediates in the pyrolysis studies.
Acknowledgment. We wish to thank Mr. Thomas C. Harby, Jr., for his care and interest in obtaining some of the experimental data. (20) C. J. Wolf and R. H. Toeniskoetter, J . Phys. Chem., 66, 1526 (1962).
The Pyrolysis of Tetrafluoroethylene by George A. Drennan'a and Richard A. Matulalb Fluid Dynamics Laboratory, Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan (Received March 85, 1068)
The pyrolysis of tetrafluoroethylene was studied in the temperature and pressure ranges 300-455' and 25760 torr. The rate of reaction was determined to be second order with respect to the tetrafluoroethylene, and three independent numerical values of the second-order rate constant were determined by simultaneously measuring the tetrafluoroethylene concentration, the octafluorocyclobutane concentration, and the total pressure as a function of reaction time. The temperature dependence of the reaction rate constants is given by kz = 1011~07*0'0a exp[(-25,635 It 9O)IRTI cma/mol sec, Lz' = 1010~81*0~07 exp[(-25,140 f 200)/RT] cma/mol sec, and kz" = 1011,a6*0.07 exp[(-26,500 f 200)/RT] cma/mol sec.
Introduction The vapor-phase dimerization of tetrafluoroethylene (C2F4) in the temperature and pressure ranges 290-470" and 100-700 torr have been studied by Lacher, Tompkin, and Park.2 Their experiments were conducted in a 1-1. Pyrex vessel, and the kinetic results were based on total pressure measurements as a function of reaction time. The rate of consumption of C2F4 was found to be second order with respect to CZF4, and the second-order Arrhenius rate constant (ICz) was reported to have an activation energy of 26.299 kcal/mol and a frequency factor of 16.5 X 1O1O cm3/mol sec. Atkinson and coworkers3 studied the decomposition of CZF4 at temperatures from 300 to 800". At temperatures below 600", they reported that the second-order dimerization of CzF4 to octafluorocyclobutane (c-C4Fs) and the firstorder back-reaction were much faster than any of the other reactions which were taking place. I n the temperature range 600-800", hexafluoroethane (C2F6)and octafluorobutenes were also formed, and at temperatures above 800", CzFBand tars were the primary prodThe Journal of Physical Chemistry
ucts. The dimerization of CZF4 in the temperature and pressure ranges 300-550" and 200-550 torr was studied in a static Pyrex reactor. The kinetic data were based on the measurement of total pressure as a function of reaction time, and the second-order Arrhenius rate constant (kz) for the dimerization of CzF4 in this temperature range was reported to have an activation energy of 25.4 kcal/mol and a frequency factor of 10.3 X 1O'O cm3/mol sec. Butler4studied the first-order thermal decomposition of C - C ~ to F ~CzF4 in the temperature range 360-560'. These experiments were conducted in a 1-1. Pyrex flask, and initial reactant pressures between 0.003 and 600 torr were considered. Using the measured equilibrium (1) (a) Hewlett-Packard Gorp., Palo Alto, Calif.; (b) Department of Mechanical Engineering, Drexel Institute of Technology, Philadelphia, Pa. (2) J. Lacher, G . Tompkin, and J. Park, J . Amer. Chem. Soc., 74, 1693 (1952). (3) (a) B. Atkinson and A. Trenwith, J. Chem. Soc., 2082 (1963); (b) B . Atkinson and V. Atkinson, ibid., 2086 (1957). (4) J. N. Butler, J. Amer. Chem. SOC.,84, 1393 (1962).
THEPYROLYSI~ OF TETRAFLUOROETHYLENE
3463
horizontal wire-wound furnace. The vessel was apcomposition in this system and the numerical value of proximately 250 mm long and had a volume of 455 em3. the first-order rate constant for the decomposition of cPrior to instillation in the furnace, the reactor was C4Fs, Butler calculated that the second-order rate concleaned with a 5% HF-H20 solution. Power was stant (kz) for the rate of decrease of CzF4 had an acsupplied to the furnace from a commercially available tivation energy of 24.0 kcal/mol and a frequency factor temperature controller which was capable of controlling of 1010.4 crna/mol sec-l. Butler also found that during the temperature to within +0.5" over a period of the course of c-C4Fs decomposition a slow parallel several hours. I n order to ensure that temperature decomposition forming perfluoropropene (C3F6) ocgradients along the furnace cavity were negligible, a curred with a first-order rate constant ( k 3 ) . The Arrhenius parameters for k3 were determined to be 1017.2 manually controlled guard heater was installed at each end of the furnace cavity. The temperature of the sec-1 and 87.2 kcal/mol. Atltinson and A t l ~ i n s o nalso ~~ reactor vessel was monitored by four chromel-alumel showed that the formation of C3F6 from c - C ~ is F ~a firstthermocouples which were placed in contact with the order reaction. Their experiments were conducted in reactor wall and were equally spaced along the longia nickel pyrolysis tube and covered the temperature tudinal axis of the vessel. The sampling tube, which range 550-650'. Based on these experiments, the firstextended into the geometric center of the vessel, and order rate constant (k3) was reported to have an activathe pump tube, which was sealed flush with the reactor tion energy of 79.0 kcal/mol and a frequency factor of wall, were both made of 6-mm Vycor tubing. Both the 3.9 X 10l6 sec-'. Lifshitz, et aL,6 have studied the thermal decomposisampling and the pump tubes were terminated outside tion of c-C4Fs behind reflected shock waves in a singleof the furnace by 2-mm greaseless vacuum stopcocks. pulse shock tube over the temperature range 770The dead volume between the furnace and the stopcocks 930". During the course of these experiments, shock was approximately 0.6 cm3. Since large gas samples waves were driven into highly diluted c-C4Fs-argon were extracted from the reactor, this small dead volume mixtures, and the initial partial pressures of c - C ~ F be~ did not have a significant effect on the results. hind the reflected shock waves were approximately 7 An Aerograph h!todel 202-B dual-column, hot-wire, mm. The rate of decomposition of c-C& was found thermal conductivity gas chromatograph was used to to be first order with respect to c-C4Fs, and the firstidentify and qualitatively to determine the gaseous prodorder Arrhenius rate constant (kl) was reported5 to ucts as a function of reaction time. A Beckman have an activation energy of 74.300 kcal/mol and a IR-10 infrared spectrophotometer with a spectral frequency factor of 2.1 X l O l 6 sec-l- These results range 300-4000 cm-l was used as a backup instrument can be used in conjunction with the known temperafor the identification of any species which escaped detecture-dependent equilibrium constant for the c-C4Fs tion by the gas chromatograph. A 4-ft column of 2C2F4 equilibrium system (ie,,see ref 4) to estimate Ic2. 50-80 mesh Poropak (Waters Associate, Inc.) Type N I n order to evaluate the second-order rate constant maintained at 100" was used to separate the C2F4 (h)for the dimerization of CzF4 based on the experi- pyrolysis products. The column was packed in 0.25-in. mental determination of total pressure as a function of 0.d. Type 316 stainless steel tubing, and the helium reaction time, the previous investigator^^^^^ were forced carrier gas flow rate was maintained at 75 cm3/min. to make certain assumptions. The present investigation Prior to final installation in the chromatograph, the was undertaken in order to determine three indepencolumn was activated by heating it to 200" while dent numerical values of the rate constaiit (kz). The purging with helium (75 cm3/min) for 2 hr. The conthree independent values of kz were determined by centrations of the various products were determined by simultaneously measuring the CzF4 concentration, the comparing the electrical output of the chromatograph c - C ~ Fconcentration, ~ and the total pressure as a funcfrom the unknown sample with the output from a calition of the reaction time. The concentrations of CZF4 bration mixture of known component concentrations. and c-C4Fs were determined with the aid of gas-solid The CzF4 used in this study was purchased from chromatography. The numerical values of kz based on Columbia Organic Chemicals, Inc., Columbia, S. C., the total pressure are compared directly with the values and it was stored in a steel cylinder as a liquefied gas reported in ref 2 and 3a, and the values of ICz based on the under its own vapor pressure of approximately 20 atm concentration measurements were used to check the a t 20". The manufacturer stabilized the liquid phase assumptions which were made in order to determine kz by adding 1wt % a-pinene to the liquid. The supplier from the total-pressure measurements. The present specified that the minimum purity of the gas phase was series of experiments were conducted in the tempera99%. Subsequent .gas chromatographic analysis of ture and pressure ranges 300-455" and 25-760 torr. the C2F4 indicated that the major gas-phase impurity was c-C4Fs and that traces of COz, CF4, and CzFewere Experimental Section
*
Appayatus. The experiments were conducted in a cylindrical Vycor reactor which was enclosed in a
(5) A. Lifshits, H. F. Caroll, and 8. H. Bauer, J. Chem. Phys., 39,
1661 (1963) Volume 79,Number 10 October 1968
3464
GEORGE A. DRENNAN AND RICHARD A. MATULA
also present. The mole fraction of the c-C4F8 impurity was determined to be approximately 9 X I n order to determine if the rate of C2F4pyrolysis was affected by residual inhibitor which may have been present in the gaseous CZF4 supplied from the cylinder, a number of preliminary experiments were conducted in which both purified C2F4 and C2F4 taken directly from the cylinder were pyrolyzed. Purified CZF4 was obtained by withdrawing a sample of C2F4 from the cylinder and collecting that fraction of the sample which 196". was volatile a t - 126" and condensable at Heicklen and Knighta reported that this purification technique yields C2F4 with less than 0.1% of any impurity. I n all cases the experimental results were identical for both purified and cylinder C2F.4. Therefore, in all experiments the C2F4was taken directly from the cylinder and was used without further purification. The c-C4Fs and C3F6 used in these experiments were purchased from the Pt4atheson Co., East Rutherford, N. J., and Air Products and Chemicals, Inc., Allentown, Pa., respectively. Both of the gases had impurities of less than 1% and were used directly without further purification. Procedure. Reactants and calibration mixtures were introduced into the reactor and gas chromatograph through a glass manifold equipped with greaseless vacuum stopcocks. A gas-sampling valve was used in conjunction with a 2-ml sample volume to inject samples into the gas chromatograph. All pressure measurements were made with a Wallace and Tiernan Type 145 precision dial manometer which has a lowpressure range of 0-30 in. and a least count of 0.05 in. A mechanical vacuum pump, vented through a standard laboratory fume hood, was capable of evacuating the system to a pressure of approximately lov3torr. During any series of experiments, the reactants were introduced into the reactor, which was maintained at a controlled temperature, and the time dependence of the total pressure and the concentration of both reactants and products were determined by withdrawing a large sample from the reactor at various reaction times and analyzing the sample with the aid of the gas chromatograph and the ir spectrophotometer. The experimental data at the lowest temperature were determined for reactions in which the extent of reaction, with respect to c-C4Fs, ranged from approximately 10 to 35%. At the highest temperature the extent of reaction varied from approximately 20 to 75y0',.These data which were obtained with temperature, initial pressure, and reactant composition as independent variables were used for the evaluation of the necessary rate equations and the appropriate Arrhenius parameters.
temperature range 300-550", can be represented by
Butler4 has shown that if the temperature is less than 500" the second term on the right-hand side of eq 1 is insignificant with respect to the first term. Therefore, the rate expression for the pyrolysis of CZF4 a t temperatures below 500" can be represented by the equation
-
Results D uta Analysis. The results of previous investigators have indicated that the rate of C2F4 pyrolysis, in the The Journal of Physical Chemistry
The integrated form of eq 2 yields the rate constant (k2) as a function of parameters, which were experimentally determined
where t is the reaction time in seconds and [C2F4],and [C2F4Ioare the measured concentrations of C2F4 (mol/ cm3) a t time t and zero, respectively. Assuming that the back-reaction is insignificant and that the only important products are CzF4 and c-C~FS,eq 1 can be rewritten in terms of the c-CdFs concentration
d[C-C4F81= &[([C2F& - ~ [ c - C ~ F ~-I O ) dt 2 ~ [ C - C I F Ecm3/mol ]~] sec (4) The numerical value of the second-order rate constant for CzF4 pyrolysis (kz') can be determined by integrating eq 4
cm3/mol sec ( 5 ) The numerical value of kat as calculated from eq 5 should be equal to the value of k2 calculated from eq 3. However, since the two k2's are based on independent experimental measurements, the "prime" nomenclature is used to differentiate between the two experimental values of the rate constant kz. Since the stoichiometry of the reaction has been assumed and the only products are CZE'4 and C-GFs,the numerical value of k2 can also be determined by measuring t'he total pressure of the products as a function of time. The numerical value of k 2 based on totalpressure measurements is given the symbol kz" k 2 / ,=
RZ[ t
1
2pt -
(PCtF4)O
(pC2Fa)O
cm3/mol sec (6) where R is the universal gas constant, T is the reaction (6) J. Heicklen and V. 'Knight, U. 8. Air Force Report No. SSD-TR66-105,Aerospace Corp., EI Segundo, Calif., June 1966.
3465
THEPYROLYSIS OF TETRAFLUOROETHYLENE temperature, P , is the total system pressure a t time t , and Po is the initial pressure. CzF4 Pyrolysis. The order of the C2F4 pyrolysis reaction was determined at 365" by applying the half-life method. During the course of these experiments, the initial CzF4 pressure was varied from 740 to 175 torr, and the C2F4 half-life was approximately 2 X lo3 sec when the initial CZF4 pressure was 175 torr. A leastmean-squares fit to the data indicated that the order of reaction was 1.98, and hence for all practical purposes the rate equation for the pyrolysis of CZF4 is given by eq 2. Once the reaction was established t o be second order, the pyrolysis of CzF4 was studied over the temperature and initial pressure ranges 300-460" and 50-200 torr. The three independent rate constants given by eq 3, 5, and 6 were calculated from the experimentally determined time dependence of the C2F4 concentration, the c-C4Fs concentration, and the total pressure. The numerical values of these rate constants based on a series of experiments with a reaction temperature and initial C2F4 pressure of 452" and 50.8 torr are listed in Table I. Arrhenius plates of the three second-order rate constants kz, 1c2', and lc2" are given in Figures 1-3. Each of these curves is based on 31 data points. The Arrhenius curves for k2" as calculated from the results given in ref 2 and 3a are also given in Figure 3. The Arrhenius expressions for the temperature dependence of the three rate constants obtained by a leastmean-squares fit of the experimental data are given by
Table I : Second-Order Rate Constant for Tetrafluoroethylene Pyrolysis at 725'K kn", oma/mol 880
300 300 600 600 900 900 1200 1800
2240 2278 2379 2355 2295 2260 2250 2381 Av 2305
1947 1904 1791 1865 1871 1886
2520 2520 2380 2380 2250 2250 2502 2561 Av 2420
Av 1877
cm3/mol see (7) k2'
(
1.40
lO"J.Sl10.07exp -25,1rTh 200)
1.60
1.50 X
IO3,
l,70
(OK$
cm3/mol sec (8)
Figure 1. Arrhenius temperature dependence of kz.
cm3/mol see (9)
ing the course of c-C4Fspyrolysis. These unimolecular reactions are given by
c-C~FSP y d y s i s . The thermal decomposition of cC4Fs was studied in the temperature and pressure ranges 452-552" and 100-200 torr. These experiments were conducted in order to confirm the results of previous investigators concerning the relative rates of the second-order production of c-CdFs from CZF4 and the first-order decomposition of c-C4Fs. The results of these experiments confirmed the validity of the assumption that the term representing the first-order back-reaction in eq 1 is insignificant with respect to the contribution of the second-order term for temperatures less than 460". It has been previously shown3bp4that both CzF4 and C3F6 are formed by parallel, unimolecular reactions dur-
c-CIF~-% 2czF4 c-C4Fs-% C3Fe
+ CF2
(10) (11)
Butler4 has shown that both ICl and IC3 can be determined if k2 is known, and the concentrations of c-C~FS, C2F4, and C3F6 are measured during the course of cC4Fs pyrolysis. The numerical values of kl and k3 based on our limited c-C4Fspyrolysis data a t both 452 and 552" are in reasonable agreement with the results given by B ~ t l e r . ~
Discussion The pyrolysis of C2F4 has been studied in the temperature and pressure ranges 290-470" and 100-700 Volume 7.9, Number IO October 1968
GEORGEA. DRENNAN AND RICHARD A. MATULA
3466
t
3 '
1
I
1
1.40
I
1.60
1.50 X
1
1.70
IO3, pK1-I
Figure 2. Arrhenius temperature dependence of kz'.
7
6
4
, IAO
I
I
I
I.50
+
I
1.60
I
1.70
x 03,PKrI
Figure 3. Arrhenius temperature dependence of lea": , ref 2 ; - - - -, ref 3a; ___ , present study.
-_.-
torr. This reaction was shown to be second order with respect to the concentration of C2F4, and the numerical value of the second-order rate constant based on three independent experimental measurements has been determined. These results were obtained by simultaneously measuring the CZF4 concentration, the c-C~FB concentration, and the total pressure as a function of reaction time. The Journal of Physical Chemistry
The temperature dependence of the second-order rate constant (kz"), based solely on total-pressure measurements, has been previously reported in the literat ~ r e . ~In, ~the~ temperature range of interest, the values of kz" reported in ref 2 and 3a vary by approximately 26%. The numerical values of k2 and kz" evaluated in the present study are within approximately 5-l2Y0 of each other and they generally fall between the results of the previous investigators. However, the second-order rate constant (kz") based on the measured c-CeF8 concentration is approximately 20% lower than ICz. A number of possible reasons for the variance between kz and ICz' and kz" were considered. An error analysis was made in order to determine if the variance in the rate constants was due to experimental errors. This analysis revealed that the expected deviations in the rate constants based on the estimated experimental errors were not large enough to account for the measured deviations. The experimental determinations of the second-order rate constants kz, kz', and k2" were based on two assumptions: (1) CZF4 and c-C4F8were the only important products; (2) the first-order rate of decomposition of c-GFs was negligible with respect to the second-order rate of formation of c-C4F8. Careful gas chromatographic analysis of the reaction products indicated that no significant side reactions were occurring when the reaction temperature was less than or equal to 452". As discussed earlier, the validity of the second assumption was confirmed by studying the thermal decomposition of c-CiF8 a t 452". The possible effects of any heterogeneous effects were also considered. The previous studies2g3awere conducted in Pyrex reactor vessels in which the surface-to-volume ratios were varied by a factor of 100, with no significant effect on the rate of reaction. The present investigation was conducted in a Vycor reactor with yet another surface-to-volume ratio. Since heterogeneous reactions are strongly influenced by both the reactor material and the surface-to-volume ratio, it was concluded that the consistency of the results obtained by similar methods over a broad range of experimental conditions preclude any significant surface effects. The frequency factor associated with the secondorder rate constant k2 can be estimated from either simple kinetic theory Considerations or the theory of absolute reaction rates. The molecular diameter of C2Fd,which is required for the kinetic theory calculations, is assumed to be 5.12 A.' The molecular diameter reported in ref 7 was determined by correlating viscosity measurements with the aid of the LennardJones (6-12) intermolecular potential energy function. The calculated frequency factor, based on kinetio theory, at 455" is 1.35 x 1014 cms/mol sec. This (7) 3. H. Simmons, "Fluorine Chemistry," Vol. 5 , Academic Press Inc., New York, N. Y., 1964, p 150.
THEPYROLYSIS OF TETRAFLUOROETHYLENE result is approximately 1000 times the experimentally determined frequency factor, and hence the steric factor for this reaction is approximately Since CzF4 has a number of internal degrees of freedom, a steric factor of is reasonable. The theory of absolute reaction rates can be employed to estimate the theoretical bimolecular rate constants
where & is Boltzmann’s constant, h is Planck’s constant, Eo is the activation energy at absolute zero, and Q* and Qc2p4are the partition functions of the activated complex and C&, respectively. The relationship between the experimental activation energy, Ea, and EO is determined by equating the logarithmic differential of eq 12 to E,,/RT2
Combinjng eq 12 and 13 with the temperature dependence of the reactant and the activated-complex partition function yields
3467 eq 16 at 455”, is approximately 1.5 X 10’ cm3/mol sec. This result, is approximately lo4 times lower than the experimentally observed frequency factor. The statistical evaluation of the equilibrium constant for the reaction c-C4Fa& 2CzF4 requires that the partition functions of both CzF4 and c-C4F8be known. Since the partition functions of both CzF4and c-C4Fg had been evaluated at 455”,the equilibrium constant of the reaction mentioned above is readily evaluated at 455”. The calculated equilibrium constant is in excellent agreement with the experimental value,4 and hence the discrepancy between the measured and calculated frequency factors cannot be attributed to numerical details. Since the partition function of CZF4 is accurately known, the theoretical frequency factor can only approach the measured value if the partition function of the activated complex is increased by approximately 104. The rotational and translational contributions to Q e - ~ 4 ~ 8are relatively well known, and hence any increase in the total partition function would most probably have to come from vibrational contributions. The discrepancy between the experimental and calculated frequency factors could be reduced significantly if the actual activated complex is geometrically similar to c-C4Fs and is loosely bound. The loosely bound complex could have a number of low vibrational frequencies which would increase Q significantly. The hydrocarbon system analogous t o the fluorocarbon system of interest has also been studied
*
c-C~HR+2CzH4 hcw xzZT
where w is the molecular vibration wave number and xi and xo are calculated from the known vibrational wave numbers of the activated complex and C2F4, respectively. The experimental frequency factor can be estimated by combining the results of eq 12 and 14
*
The partition function for CZF4 is readily calculated with the aid of the molecular parameters listed in the JANAF tables.9 The partition function of the activated complex is calculated based on the assumption that it) has the same geometrical arrangement as cC4Fs. The 23 fundamental vibrational frequencies of c-C4Fs are available in the literature.l0 However, the numerical value of a number of these frequencies are not known accurately. Electron diffraction studies1’ have been used to evaluate the spatial configuration of c-C~F~ and , these results have indicated that c-C.jFg is nonplanar. Therefore, the symmetry number of this molecule is 2 . The frequency factor, as calculated from
(17)
The heat of reaction at 427” and the equilibrium constant at 455” for the dissociation of cyclobutane, see eq 17, are 18.92 kcal/mol and 7.43 X mol/cm3, respectively.12 The corresponding heat of reaction and equilibrium constant for the dissociation of c-c4l?S, see eq 10, are 49.92 kcal/mol and 4.12 X lo-” mo1/cm3, respectively.12 These thermochemical data indicate that the c-C4Fs is considerably more stable than c-C4Hs. The pyrolysis of C2H4 has been studied by a number of in~estigators.l~-~6 Since the pyrolysis of C2H4 is considerably more complex than the analogous fluoro(8) S. Glasstone, K. J. Laidler, and H. Eyring, “The Theory of Rate Processes,” McGraw-Hill Book Co., Inc., New York, N. Y., 1941. (9) “JANAF Interim Thermochemical Tables,” Dow Chemical Co., Midland, Mich., 1964. (10) H. H. Claassen, J . Chem. Phys., 18, 543 (1950). (11) H.P. Lemaire and R. L. Livingston, ibid., 18, 569 (1950). (12) D.R.Stull, E. F. Westrum, Jr., and G. C. Sinke, “The Chemical Thermodynamics of Organic Compounds,” John Wiley and Sons, Inc., New York, N. Y.,1968,in press. (13) F. Fischer and H. Pichler, Brennstof-Chem., 13, 381,406,435 (1932). (14) R. E. Burke, B. G. Baldwin, and C. H. Whitacre, Ind. Eng. Chem., 29, 326 (1937). (15) E.F. Greene, R. L. Taylor, and W. L. Patterson, Jr., J . Phya. Chem., 62,238 (1958). (16) G.B.Skinner and E. M. Sukolski, ibid., 64, 1028 (1960). Volume 72,Number 10 October 1968
3468
B. STEVENS AND B. E. ALGAR
carbon reaction, a direct comparison between the present results and the C2H4 system cannot be made. The thermal decomposition of c-C4Hs has been investigated by Walters and coworkers17-1a and Pritchard and coworkers.20 The first-order Arrhenius rate constant, ko, for the decomposition of c-C4Fs is reported to have a frequency factor of 1015-71 sec-' and an activation energy of 62.8 kcal/mol.l4 Butler4 has reported that the frequency factor and activation energy of the unimolecular rate constant for the decomposition of cC4FS are 10l6J sec-l and 74.3 kcal/mol, respectively. The two unimolecular frequency factors are in good agreement. However, the activation energy for the perfluoro compound is 11.5 kcal/mol higher than the
analogous hydrocarbon. Since the c-CIF~ is more stable than c-C4Hs,this result is reasonable. Acknowledgment. This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant No. AF-AFOSR-1144-67. (17) Fa Kern and W. D. Walters, Proo. Nat. Acad. Sei. U.S., 43, 937 (1952). (18) C. T . Genaux and W. D. Walters, J . Amer. Chem. SOC.,73, 4497 (1951). (19) C. T . Genaux and W. D. Walters, ibid., 75, 6196 (1953). (20) H. Pritchard, R. Sowden, and A. Trotman-Dickenson, Proc. Roy. SOC.,A218,416 (1953).
The Photoperoxidation of Unsaturated Organic Molecules. 11. The Autoperoxidation of Aromatic Hydrocarbons by B. Stevens1 and B. E. Algar Department of Chemistry, Shefield University, Shefield, England
(Received March 86, 1068)
The quantum yields of autoperoxidation of 9,10-dimethylanthracene19,10-dimethyl-l,2-benzanthracene, naphthacene, and rubrene have been measured as a function of the dissolved oxygen concentration down to 2.6 X M in benzene at 25". An analysis of the data in terms of the participation of an 0 # A g ) intermediate provides estimates of the triplet-state formation yields of the substrates and leads to the conclusion that Oz(1A,) is produced solely by oxygen quenching of the aromatic hydrocarbon triplet state, which is a yield-limiting process at very low oxygen concentrations.
Introduction The over-all photosensitized peroxidation2 of an unsaturated organic molecule XI in the presence of molecular oxygen may be represented by the process
where the sensitizer S absorbs the incident radiation but remains chemically unchanged, and the peroxide or hydroperoxide NO2 is the sole product. Three types of substrate, 31, subject to this process are: (a) those molecules containing an isolated double bond with a @-hydrogen atom,3 e.g.
CHa
CHs
The Journal of Physical Chemistry
CH3
CH3
(b) cyclic conjugated he~adienes,~ of which a-terpinene is a classic example5
I
CH3 CH3
CH3 CH,
(c) the larger catacondensed aromatic hydrocarbons, (1) Department of Chemistry, University of South Florida, Tampa, Fla. 33620. (2) The term peroxidation is used here to distinguish the addition of molecular oxygen from oxidative electron and H atom transfer processes. (3) G. 0. Schenck, Angew. Chem., 69, 579 (1957). (4) R. N. Moore and R. V. Lawrence, J . Amer. Chem. Soc., 80, 1438 (1958). (5) G. 0. Schenck and K. Ziegler, Naturwissenschaften, 32, 157 (1944).
