THERMAL DECOMPOSITION OF 1,1,1-TRIFLUOROACETONE
1007
The Kinetics of the Thermal Decomposition of 1,l,l-Trifluoroacetone
by Chas. E. Waring and Alexander J. Feketel Department of Chemistry, Universitg of Connecticut, Storrs, Connecticut 06868 (Received August 19, 1969)
The thermal decomposition of l,l,l-trifluoroacetone (TFA) has been investigated in a static system between 545 and 600" over initial pressures from 10.6 to 134.8 mm. The order of the reaction was found to be 1.3 both by initial rate measurements and by the rate of disappearance of TFA itself. The order for the rate of formation of such products as CFsH, CF&Hs, and CFzCH2,however, was found to be unity. The Arrhenius parameter for the TFA decomposition is given by ko = 3.6 X 101be-86,2OO/RT M-OJ sec-1 while that for CFsH is 1.5 X 1012e-691400'R*sec-l, that for CFsCHs is 1.4 X lOl1e-6@,mO/RT sec-l, and that for CF~CHZ is 1.8 X e-72voOo/RT sec-l. The decomposition of TFA was also investigated in the presence of various foreign gases, one of which inhibited the reaction while the others produced marked catalysis. From a knowIedge of the products from the uninhibited and inhibited decompositions a, mechanism is proposed to account for the predominant reactions. The activation energies of several of these reactions are evaluated by use of the Arrhenius equations given above.
Introduction The photolysis of l,l,l-trifluoroacetone (TFA) was first investigated by Sieger and Calvert12who found the major reaction products to be CO, CH4, CFaH, CzHs, CF3CH3, and C2F6. No CHaF, CF4, HF, or Fz was observed. More recently, Dawidowicz and Patrick3 reinvestigated the TFA photolysis and while they were not in agreement with the mechanism proposed by Sieger and Calvert, they reported the same products of reaction in essentially the same amounts as the earlier investigators. Although Clark and Pritchard4 determined the Arrhenius parameters of TFA by thermally decomposing a mixture of 1% of this compound in toluene at 840", they made no attempt to study the kinetics and mechanism of the pyrolysis. The purpose of this research, therefore, was to study the kinetics and mechanism of the thermal decomposition of TFA and to compare the reactions of the CFa radical with those of the CH3 radical under similar conditions. It was also thought worthwhile to examine the effect of well-known methyl radical inhibitors on the trifluoromethyl radical.
Experimental Section A. Apparatus. The thermal decomposition of TFA was studied in a static system employing a spherical quartz reaction flask of 176-ml capacity. This was heated in an electric furnace whose temperature was controlled to within k0.1"in the area of the reaction zone. To prevent condensation, the lines from the furnace, including stopcocks, were wrapped with electrical heating tape and maintained at a constant temperature of 75". The reaction system was evacuated by the usual techniques and no run was made unless the pressure in the system was mm or less, as measured by a McLeod gage. Pressures were measured by an Atcotran transducer unit, also maintained at 75", in con-
junction with a modified Leeds-Northrup Speedomax recorder. Before each run the vacuum line was flamed out to eliminate the possibility of oxygen contamination. A Toepler pump attached directly into the line was employed to remove the gaseous reaction products. Analyses were made on a vapor fractometer and a mass spectrometer. B. Materials. The l,l,l-trifluoroacetone used in this study was obtained from Merck Sharp and Dohme. Its purity, as established by a Perkin-Elmer Model 154D vapor fractometer, was 99%. Tetrafluoromethane, 1,l-difluoroethylene, 1,1,l-trifluoroethane, trifluoromethane, and hexafluoroethane, obtained commercially, were found to have a purity of 99%-except tetrafluoromethane which was 95%. Since these compounds were used for identification purposes, they were employed without further purification. 1,1,l-Trifluoropropane and 1 ,1,l-trifluoropropene were synthesized by the method of Hasek, Smith, and Engelhardt5 by allowing the corresponding carboxylic acids to react with sulfur tetrafluoride in a 75-ml stainless steel container for 8-48 hr. The products were removed and treated with KOH solution to remove the excess SF,. Final purification and isolation was accomplished with the vapor fractometer.
Results 1. Nature of the Decomposition. TFA m7as found to decompose at a conveniently measurable rate between 545 and 600" and a t initial pressures from 10.6 to (1) Preaented in partial fulfillment of the requirements for the Ph.D. degree a t the University of Connecticut, June 1967. (2) R. A. Sieger and J. G. Calvert J. Amer. Chem. Soc., 7 6 , 5197 (1954). (3) E. A. Dawidowicz and C . R. Patrick, J. Chem. SOC.,4250 (1984). (4) D. Clark and H. 0. Pritohard, ibid., 2136 (1956). (5) W. R . Hasek, W. C. Smith, and V. A. Engelhardt, J . Amer. Chem. Soe., 82, 543 (1960). Volume ?'4pNumber 6 March 6, 1970
CHAS.E. WARINGAND ALEXANDER 5. FEKETE
1008
Table I : Change in Pressure with Time for Various Initial Pressures of TFA a t 546.2' pi! mm-
t , sea
30 60 120 180 240 300 540 780 1020 1260 1500 1740 1980 2220
23.0
47.7
54.6
69.3
99.5
115.2
134.8
141.3
23.3 23.7 24.0 24.7 25.0 26.1 27.4 29.8 30.9 33 .a 34.0 35.0 36.7 37.7
48.4 49.4 51.1 52.5 53.9 55.2 58.7 61.7 64.8 67.6 70.0 72.4 74.8
55.6 56.6 59.0 61.1 62.4 63.5 68.3 71.7 75.1 78.2 81.3 83.7 85.4
70.3 72.0 74.8 76.8 78.5 80.3 86.1 90.2 94.3 98.1 101.2 103.9 107.0 109.1
101.5 103.9 108.7 111.8 114.9 117.6 126.2 133.4 139.3 144.1 149.2 153.3 157.4 161.2
117.3 120.4 125.5 129.7 133.8 136.9 147.8 156.8 164.6 171.2 177.2
137.5 141.3 147.5 151.9 156.4 159.5 170.8 179.4 187.3 194.5 201.7 207.5 214.0
146.1 150.2 156.1 160.5 164.6 168.1 179.1 188.7 196.9 205.1 212.7 219.5
...
...
134.8 mm. After the reaction bulb was conditioned by the TFA products of reaction for 2 days, the rates of decomposition were readily reproducible and the reaction was homogeneous. The ratio of the final to the initial pressures, pt/pi, was found to be constant a t 2.17 over the pressure range from 23 to 128 mm. 2 . Order of Reaction. Table I presents typical data for the change in pressure with time for various initial pressures of TFA at 546.2'. These data, and all similar data, were programmed on an IBM 1620 computer to solve for k i J the initial rate constant, and n, the order of reaction, in the equation log (dp/dt)o = log k i
+ n log pi
(1)
A value of n = 1.3 was obtained a t each of four different temperatures from 546.2 to 599.1O, indicating that the order of the decomposition lay between 1.0 and 1.5. Visual evidence for the consistency of the data can be seen in Figure 1. The order of the reaction was also determined by measuring the amount of TFA that decomposed in 1 min. This study was conducted over an initial pressure range from 25 to 150 mm at four different temperatures. Figure 2 is a typical plot of the logarithm of the rate of disappearance of TFA as a function of the logarithm of the initial pressure. The rate constants and order of reaction at the various temperatures were evaluated and are given in Table 11.
... *..
....
. . I
... ...
i
0'4
-0.8
1.50
1.70
1.90
2.10
2.30
Log pI, mm.
Figure 1. Variation of the logarithms of the initial rate constants with the logarithms of the initial pressures of TFA a t 599.1'.
Figure 2. Log of initial rate of disappearance of TFA us, the log of the initial pressure at 599.1". Table I1 : The Order and Rate Constants Based upon the Amount of TFA Disappearing in 60 Sec 104 ko, mm-O,a
Temp, O C
Order
sea-I
546.7 555.7 576.6 599.1
1.4 1.4 1.3 1.1
5.83 9.25 23.3 65.3
Av The Journal of Physical Chemistry
1.3
3. Formation of CFgH, CFEEHI, and C F y C H 8 . The initial rates of formation of these three products of the decomposition of TFA were also determined by measuring the number of millimeters of each formed in 60 sec reaction time over an initial pressure range between 17 and 150 mm. The orders of reaction and initial rate constants at various temperatures are presented in Table 111.
1009
THERMAL DECOMPOSITION OF 1,1,1-TRIFLUOROACETONE Table 111: Rate Constants and Orders for the Formation of CFaH, CFaCHs,and CFzCHzat Various Temperatures -104
ki, aeo-1
Temp, OC
CFaH
CFaCHa
CFzCHz
546.7 555.7 576.6 599.1
2.17 3 -45 9.12 19.5
1.10 1.67 3.76
0.86 1.37 3.76 12.9
. I .
- -
Order-
Av
lo3/
