G. 0. PRITCHARD, M. VENUGOPALAN, AND T. F. GRAHAM
1786
Graphon from n-heptane solutions with their major axis parallel to the surface. This is in contradistinction to the work of van der Waarden” on similar systems to those described here, from which the perpendicular orientation was calculated. However, van der Waarden used a carbon black with a heterogeneous surface which may give rise to different adsorption behavior and also in his analysis the adsorption of the solvent was not taken into account. The stability of dispersions of Graphon in the alkylbenzene-heptane mixtures has also been investigated.
The stability increases with chain length of the alkylbenzene which would not a t first sight appear to agree with the adsorption data. The relation between adsorption and stability will be the subject of a future publication. Acknowledgments. The authors are grateful to The Cabot Corporation for the Graphon and to the D.S.I.R. for a scholarship to E. W. (11) M.van der Waarden, J . Colloid Sci., 6, 443 (1951).
Photochemistry of the Fluoro Ketones. The Production of Vinyl Fluoride in the Photolysis of 1,3-Difluoroacetonel
by G. 0. Pritchard, M. Venugopalan, and T. F. Graham Department of Chemistry, University of California, Santa Barbara, California
(Received January d , 1964)
I n the photolysis of 1,3-difluoroacetone, CFHz radicals recombine to form CzFzH4and disproportionate to give vinyl fluoride and hydrogen fluoride. The ratio of the rates of these two reactions is a function of both the pressure and the temperature. The activation energy for H abstraction by CFHz radicals from the ketone is 8.0 kcal. mole-’.
Introduction Disproportionation reactions between CH, and higher alkyl radicals and between two higher alkyl radicals are well knowna2 Moreover, in the photolysis of CFZClCOCFzCl and CFC12COCF2C13reactions of the type
-
+ CFz CFZC12 + CFCl
--+
CH,
+ CFzCl+ CFzCl + CFCIz CFzCl
CFzClz
leading to the formation of a molecule and a substituted methylene species were found. The reaction CH3
+ CHa
+ CHz
has been postulated, but there is no direct evidence of its oc~urrence.~ The Journal of Physical Chemistry
During preliminary investigations in this laboratory on the photolysis of 1,3-difluoroacetone it was observed that vinyl fluoride is a major product. Based on the well-known scheme for acetone, it was anticipated that between 100 and 300’ only processes such as CFHzCOCFHz
+ h~ + CFHZCO + CFH,
(1) This work was supported by a grant from the National Science Foundation. (2) J. A. Kerr and A. F. Trotman-Dickenson, “Progress in Reaction Kinetics,” Vol. I, G. Porter, Ed., Pergamon Press, New York, N. Y., pp, 111-113. (3) R. Bowles, J. R. Maier, and J. C. Robb, Trans. Faraday SOC.. 58, 1541,2394 (1962). (4) E. W.R. Steacie, “Atomic and Free Radical Reactions,” Reinhold Publishing Corp., New York, N . Y., 1954,p. 651.
PHOTOCHEMISTRY OF THE FLUORO KETONES
CFHzCO
--j.
CFHz $- CFHZCOCFHZ
1787
CFHz -I- CO
Table I": Mass Spectrum of 1,3-Difluoroacetone
----f
CFH3
+ CFHCOCFHz
CFHz -k CFHZ --+ CzFzH4
-
(1) (2)
CFH2 $- CFHCOCFHz + CFHzCFHCOCFHz (3) BCFHCOCE"2
(CFHC0CFHz)z
(4)
would occur. Below looo, the participation of the CFHzCO radical may also be expected 2CFH&O
--j.
(CFH2CO)z
and CFH2
+ CFHyCO --+-CFH3 + CFHCO
The formation of vinyl fluoride could arise, therefore, only via the disproportionation reaction CFHZ
+ CFHZ +CFHzCHZ + H F
(5)
This has been further confirmed by establishing the formation'of H F during the photolysis. I n systems containing CH3 and CF3 radicals no evidence has been reported of products resulting from a reaction comparable to reaction 5.5r6 An attempt to identify the possible formation of CH2=CC12 in the cophotolysis of CH3COCH3 and CCI,COCCI, was not successful due to analytical difficulties.' The product, if present, was only minor, indicating that the reaction CH3 cCl3 --+ CJ&=CC12 HCI is relatively unimportant. Reaction 5 appears to be the first observed case of a disproportionation between two methyl (or substituted methyl) radicals resulting in the formation of two molecules, as opposed to a molecule and a methylene species.
+
+
Experimental Purification of the Ketone. Mass spectrometric and vapor phase chromatographic analysis followed by examination of the infrared spectrum indicated that the 1,3-difluoroacetone, supplied by the Aldrich Chemical Co., contained two major impurities. The purification of the ketone was effected using a Perkin-Elmer preparative column R (IJoon polyglycol LB-550-X) at 80'. The ketone, which appeared as the first major fraction, was collected and tmnsferred to the vacuum system without exposure to the atmosphere. The impurities appeared on the V.P.C. later as two minor peaks. The previously unpublished mass spectrum of 1,3-difluoroacetone is given in Table I. It indicates the occurrence of a small transference peak at m/e = 15(CH3+). This is comparable to the CC13-t ion peak which occurs in
m/e
Probable positive ion
Abundanoe
12 13 14 15 19 20 24 25 26 27 28 29 31 32 33 36 37 38 39 41 42 43 44 45 46 47 51 55 56 57 60 61 62 64 65 93
C CH CHz CHs F HF CZ CzH CzHz CzH3
14 34 65 21 5 6 5 10 26 61 26 31 66 61 1000 9 9 7 5 8 75 17 12 60 27 9 7 11 5 14 27 661 14 5 16 199
co
CHO CF CHF CHzF
cs CaH C3HZ CsH3 CzHO CzHz0 CzF CzHF CzHzF CJLF CFO CHFz C3F CaHF CsHzF CzHFO CzHzFO CzFz CzHzFz CzHsFz CzHsFz0
a Peaks at m/e < 0.5% of 33(CHzFf) and isotope p e a h are omitted, No isotope corrections have been made.
the mass spectrum of 1,3-difluorotetrachloroacetone.8 From the mass and infrared spectra, one of the impurities in the ketone appeared to be 1-fluoro-Bpropanol; the other a fluorohydroxy ketone of higher molecular weight.g Commercial samples of methyl and vinyl fluorides were obtained from Columbia Organic Chemicals Co. (5) R. A. Sieger and J. G. Calvert, J. A m . Chem. SOC.,76,5197 (1954). (6) G.0.Pritchard and J. R. Dacey, Can. J. Chem., 38, 182 (1960). (7) D.M. Tomkinson, J. P. Calvin, and H. 0. Pritchard, J . Phys. Chem., 68,541 (1964). (8) J. R.Majer, Adcan. .Fluorine Chem., 2, 55 (1961). (9) The infrared spectrum of 1.3-difluoroacetone in CCl, gave principal peaks a t 3.40,5.67,6.97,7.22,7.39,8.45,9.22,and 9.45 p .
Volume 68, Number 7
July, 1964
1788
G. 0. PRITCHARD, M. VENUGOPALAN, AND T. F. GRAHAM
and the Matheson Coleman and Bell Co., Inc., and purified by repeated low-temperature fractionation. I n Table I1 is presented the mass spectra of CHSF, CHF=
Table 11": Mass Spectra of CHsF, C Z H ~ FC, Z H ~ F Z , and Mixtures of CHaF and C2H3F
d
e
12 13 13.5 14 15 19 24 25 26 27 28 31 32 33 34 43 44 45 46 47 64 65 66
Probable positive ion
C CH CeHs/2 CHI CH3
F
cz
CzH C2Hz Cz& CZH4 CF CHF CHzF CHaF CzF CzHF CzHzF CzHaF CzH4F CzHzFz CzHsFz CZH4FZ
CHaF
CaHaF
28 46
34 45 23 42 11
79 340 7
30 64 323 377 64 64 1000 993
94 28
10 283 717 1000
M Ab
MzC
CZ&Fl
44 55 23 60 92 5 31 66 326 3 78
44 60 20 69 118
6 6
29 71 384 399
111 44 236 232 9 291 727 1000
114 50 312 307 8 328 714 1000
14 34 14 42 118 23 37 17 1000 12 23 125 84 22 12 28 40
a Peaks a t m/e < 0.57, of major peak and isotope peaks are omitted. N o isotope corrections have been made. * Mt = 21% Mz = 26y0 CHaF (mass spec. analysis). CHaF (synthetic).
CHz, CFH2CFH2,a mixture of CH3F and C2H3Fobtained in our analytical procedure (see below) a t - 160°, and a synthetic mixture of these two compounds of approximately the same composition. The ethane used was a sample obtained a t - 90' in our analysis. The ultraviolet spectrum of the ketone was recorded on a Cary Model 14 spectrophotometer. Based on logIo/l = ecZ2 E -15 1. mole-l cm.-l at 3130 8.,and a t A, (2850 A.), E 28.5 1. mole-l :m.-l. The value of the extinction coefficient a t 3130 A. is about twice as large as for hexafluoroacetone, and five times as large as for acetone at this wave length. Photolytic and Analytical Procedure. The photolysis apparatus was similar to that used in previous work in this laboratory. The light source was B.T.H. highpressure lamp (MEID, 250 watts), operated a t 4 amp. on a stabiljzed d.c. voltage. The beam was collimated and 3130 A. radiation isolated by means of a combination of glass and solution filters.1° The 10-cm. long cyThe Journal of Physical Chemistry
lindrical quartz reactor, volume 196 ml., was mounted inside an aluminum block furnace and the temperature was controlled to =k0.5%. It was completely illuminated with a parallel beam of light. The optical path, Le., the distance from the lamp to the reaction cell, was 50 cm. The pressure of the ketone was measured with a differential mercury manometer attached to the reactor. All connecting glass tubes were maintained around 60' to prevent condensation of the ketone at pressures above its vapor pressure (7 mm.) at room temperature. After photolysis, CO was collected and measured a t -195'. Its purity was checked periodically on the mass spectrometer. R!Iixtures of CH3F and CHF= CH2 were collected a t - 160' and analyzed on the mass spectrometer using pure samples as standards. The C2H4F2was collected at -go", and the mass spectrum of the compound indicated that this was a pure fraction. The ketone is not volatile enough (b.p. 101') to interfere in the low-temperature separation of the products. A series of acetone photolysis experiments at pressures 60-75 mm. and temperatures 380-410OK. was carried out using the same optical parameters as for the 1,3-difluoroacetone. The amount of CO produced was determined mass spectrometrically. Assuming €he quantum yield of CO to be unity for the above conditions, the absorbed intensity I, was calculated. The per cent absorption was measured by difference in the transmitted intensity through the cell under vacuum and when filled with acetone vapor, using an RCA 931A phototube placed at the far end of the quartz reactor, in conjunction with an Aminco photomultiplier microphotometer. From the values of partial absorption and I,, the average Io was calculated to be -91 f 4 X 10l2 &/ml./sec. Formation of H F . In an attempt to establish the formation of H F in the photclysis, a sample of the ketone was sealed in a greaseless, outgassed quartz reactor and subjected to prolonged (24 hr.) photolysis. The contents of the reactor were admitted directly into the mass spectrometer. At m / e = 85 a significant peak was obtained which could be ascribed to the SiF3+ ion.ll Also the m / e = 18(H20+)was enlarged over the background spectrum. This is reasonably conclusive evidence that H F is formed in our system and is removed by the reaction
(10) M. Kasha, J . O p t . SOC.Am., 38, 929 (1948); M. Venugopalan, G. 0. Pritchard, and G. H. Miller, Nature, 200, 568 (1963). (11) The mass spectrum of Sic14 indicates no parent peak; the major ion peak is due to SiCls+. The presence of CzH4Fz masks identification of SiFz+ and SiF+ a t m/e = 66 and 47.
PHOTOCHEMISTRY O F THE
FLUORO KETONES
1789
Table 111 Ketone, moles ml. - 1 absorption X 10'
To
T ,OK.
t , aec.
-Products, CHsF
co
moles X IO+ CZHIFZ CzHsF
-
6-7 mm. 0.599 2.047 1.398 4.631 1.422 5.373 1.308 6.526 0,627 3.995 0.517 3.302 0.833 4.287 0.:714 4.579
Pketona
-
--r
-Quantum
yield-
Yo con-
7
M. b.a
Qco
QCHaF
Q'C2ErF2
QC~H~F
0.990 1.030 0.968 0.968 0.928 0.874 0.829 0.812
0.522 0.219 0.211 1.846 2.262 2.259 0.867 1.126
...
... 0.005 0.078 0.073 0.223 0.187 0.253
0.117 0,052 0.042 0.292 0.280 0.252 0.102 0.106
0,399 0.174 0.160 1,456 1.783 1.611 0.523 0.681
3.2 6.6 8.0 11.1 7.6 8.4 11.8 16.2 2.7 4.7 4.4 2.7 4.0 4.7
version
5.0 13.0 15.0 2.0 2.0 2.0 8.0 6.0
3.370 3.540 3.570 2 990 2.660 2.207 2.409 1.867
294.7 294.7 323.7 375.7 421.7 472.2 526.7 575.2
3600 7200 7200 7200 3600 3600 3600 3600
2,673 5.839 7.098 8.275 5.069 4.631 7.105 7.569
... ... 0.153 0.349 0.163 0,457 1.533 1,700
8.0 7.33 8.67 9.33 13.13 13.33
9.879 9.228 7.354 6.822 7.386 7.674
324.7 373.7 425.3 470.2 521.2 585.2
3600 3600 2700 1800 1800 1800
6.771 10.934 8.122 4.634 7.455 8.962
0.034 0.302 0.503 0,699 2.684 2.903
0.949 0.927 0.934 0.870 0.765 0.801
0.755 1.331 1.115 0.887 1.014 1,200
0.004 0.037 0.069 0.134 0,365 0.389
0.343 0.580 0.341 0.211 0,190 0.100
0.371 0.636 0.666 0.493 0.403 0.652
354.2 2700 376.2 3600 429.7 1800 475.2 900 523.7 900 551.2 900 577.2 900
8,558 25.198 13.465 7.556 7.012 7.156 10.590
0.252 4.329 2.865 0,855 0.842 14.650 8.778 0.946 1.433 5.284 4.828 0.804 1.636 2.862 2.751 0.851 2.851 1.642 2.519 0.797 3.592 1.005 2.410 0.728 4.981 1.426 5.008 0.843
0.530 1.124 1.051 1.1.73 1.014 1.675 1.253
0.016 0.038 0.112 0.254 0.412 0.841 0.589
0.268 0.654 0.412 0.444 0.237 0.235 0.169
0.177 0,392 0.377 0.427 0.364 0.564 0.592
3.1 1.7 1.8 1.8 2.6
0.866 0.114 0.126 0.553 0,831 0.196 0.239 0.359 0.895 0.212 0.275 0.384
7.0 4.7 2.7
Pketone
Pketane
21 .oo 20.00 25.00 23.00 27,OO 16.67 33.00
19.47 20.04 17.35 17.71 15.31 15.57 16.52
6.00 13,33 16,67
3.347 479.2 3600 9.569 47'7.7 3600 11.350 480.7 1800
M.b. represents [l/&HaF
4HF
5.819 0.765 11.359 2.683 7.643 1.811
-
3.327 5.227 4.849 2.577 2.961 4.872
50 mm.
1.8 5.0
+ C2HdF2+ C2H8F]/C0.
+ Sit& -+ SiF4 + 2Hz0
2HF 3 SiF4--+ HzSiFe There was no 85 ion peak in the unreacted ketone (Table I), nor was it ever evident in the fractions collected after photolysis a t - 160 and - 90". The results of the experiments are reported in Table 111.
Discussion Variation of k5/kz with Pressure. This is shown in Fig. 1- for a-temperature of 475 & 5OK. where k5/kz = R C Z F H d /RC2F2H,. If reactions 2 and 5 proceed via a comti
20 mm.
0.846 3.719 0.850 3.263 4.905 0.837 2.347 3.284 0.855
and presumably
mon
-
3.079 4.761 2.486 1.105 1.398 0.859
an increase of pressure will favor reaction 2. Dehydrohalogenation reactions generally proceed by a molecular mechanism via a four-center transition state, with activation energies falling in the range 40-60 kcal. mole-'.12 Comparing the reaction CzF& + CZFH3
+ HF
with the similar dehydrochlorination reaction, l 3 we may estimate its activation energy as 45 3t 5 kcal. mole-', as the difference D(C-F) - D(C-Cl) is offset by the energy difference D(H-F) - D(H-Cl).14 Assuming D(FHzC-CHzF) 'v D(H3C-CH3),14the CzFzH4* species must lose about 85 kcal. mole-' of excitational energy by collision before it is in thermal equilibrium
ansition state 2CFHz
2
CzFzH4*
CzFzH4 7
k -I
C2FH3
+ HF
(2)
(5)
(12) B. G. Gowenlock, Quarl. Rev. (London), 14, 133 (1960). (13) D. H . 12. Barton and K . E. Howlett, J . Chem. Soc., 155 (1949). (14) C. 11. Patrick in "Advances in Fluorine Chemistry," Vol. 2 , Butterworth and Co., Ltd., London, 1961; T. L. Cottrell, "The Strengths of Chemical Bonds," 2nd Ed., Butterworth and Co., Ltd., London, 1968.
Volume 68, Number 7
J u l y , 1964
G. 0. PRITCHARD, M. VENUGOPALAK, AND T. F. GRAHAM
1790
will be favored. However, this interpretation has been questioned. l8 Variation of k6/kz with Temperature. There is a marked increase in kS/kZ with rising temperature at each of the three pressures studied (Fig. 2). An opposite effect would be expected, as the collision frequency increases slightly with temperature,Ig and energy transfer processes are facilitated with rising temperature. 2o
.4 5.0
e io
1 2
$ 3.0 m
c
4 1.0 I
10
I I I 20 30 40 Pressure of ketone, m m .
I 50
Figure 1. Rate constant ratio us. ketone pressure: 0 , W k z ; 8, k ~ / k z “ ~0 ; , k1/k61’2; a , kl/k,‘/Z; at 475 =k 5°K. (k/k”a in rnole-”Z rn1.1/~,sec.-1/2).
with the surrounding system. Before the loss of approximately 40 kcal. mole-l has occurred, the excited molecule may in this time reassemble its energy and split off HF. The remaining energy will then be dissipated by the two fragments. No matter how great the pressure the loss of 40 kcal. mole-’ cannot occur infinitely quickly, so that some H F will always be formed. I n Fig. 1, the pressure dependence of k6/k2 is exemplified in the pressure dependence of kl/kz‘l‘ = RCFH~/RC,F,H,‘/~ [Ket.1. The limited data (three runs) at other temperatures (approximately 375, 425, 525, and 58OOK.) show similar behavior for k6/kz and kl/kz‘/‘. At 475°K. (Fig. 1) kl/k~’” = ~ c F H , / R c , F H , ’ ” [ Kand ~~.] kl/kr1/2 = RCF,H/(RC,F,H, RC,FH,)1’2[~et. I are independent of the pressure down to 6 mm., indicating that k-, is constant over the pressure range. However, the data a t 375, 525, and 580OK. show increased values of kl/k5”2 and kl/kr’/2 at 6 mm., indicating that we may be in the pressure dependent region for CFH, radical recombination. (At 425°K. an opposite effect is observed.) The data are somewhat contradictory and too limited for a definite conclusion at this stage. We may note that a third-body dependence for CH3 radical recombination becomes apparent at about 10 mm. or higher,lb but no appreciable fall-off in the recombination ratio of CF, radicals was found a t 0.5 mm. pressure in the photolysis of CF3CH0.l6 Several groups of workers” have found variations in disproportion : combination ratios with pressure, where the radicals were formed by reaction between H atoms and an olefin. The increase in the ratio with falling pressure is ascribed to “hot” radical effects, as the radicals retain the heat from the addition of an H atom, and because the combination step is more exothermic than the disproportionation, the latter process
+
The Journal of Physical Chemistry
0.0
5.0
4.0
q“
2 3.0
2.0
1.0
300
400
500
T, OK.
Figure 2. k;/kz us. temperature: 3,6-7 mm.; 0 , 20 m m . ; 0 , 50 m m . ; 0 , 50 mm. with unfiltered radiation. (15) R. E. Dodd and E. W. R.Steacie, Proc. Roy. Soc. (London), A223, 283 (1954). (16) R . E. Dodd and J. W. Smith, J . Chem. Soc., 1465 (1957). (17) P. J. Boddy and J. C. Robb, Proc. Roy. SOC.(London), A249, 518 (1959) ; R. A. Back, Can. J . Chem., 3 7 , 1834 (1959), for example. (18) C. A. Heller and A. S. Gordon, J . Chem. Phys., 36, 2648 (1962); for a review, see R. J. Cvetanovie, Advan. Photochem., 1, 149 (1963). (19) For a particular pressure there is a small drop in the conrentration of ketone with increasing temperature, with a consequent small decrease in the rate of production of CFHzradicals. However, there will be a slight increase in the collision frequency for CFHz radicals, as it varies as T1’*,so that there is no significant variation in the rate of production of CzFzHd* with temperature. Taking the 50 mm. runs at 354.2 and 577.2“K., we may calculate the number of collisions suffered per ml. per sec. by a CZFZH4* molecule with (CFH2)zCO molecules. There ie a small increase fro? 3.8 t o 4.1 X lo8 ml.-I set.-' ( U C ~ F , H ~ = * 5.0 A., U ( C F H ~ ) > C O= 5.25 A.1. (20) H. 0. Pritchard, J . Phys. Chem., 6 5 , 504 (1961).
PHOTOCHEMISTRY O F THE
FLUORO KETONES
1791
The constancy of the k6/k2 ratio above 420OK. for the low-pressure runs is an artefact of the system, due to the fact that in these experiments there were large increases in pressure in the system during the reaction period. These points, therefore, belong more properly on curves representing higher pressures. (The percentage conversions given in Table I11 are based on CO yields.) Szwarc and his co-workersZ1have recently demonstrated a small temperature dependence for the disproportionation/combination ratio for ethyl radicals in the gas, liquid, and solid phases. Subsequent workz2 with the system CH3 -t C&, in solution indicates that this effect may be perfectly general. Contrary to our experiments, these reaults show that disproportionation is favored at lower temperatures, and the results on ethyl radicals in the gas phase2' may be interpreted &S E d l s p -- E c o m b = --0.3 kcal. mole-' 01' kdlsp/koomb T-0.7. Similar increasing disproportionation/combination ratios with decreasing temperature have been found by Klein, Scheer, and Waller,23who have pointed out that a comparison of the ratios for isopropyl radicals a t 77 and 300°K. leads to E d i s p - E o o m b = -0.3 kcal. mole-'. This difference has been substantiated by photolyzing azoisopropane over the temperature range 77-38OoK., and a similar effect is found for secbutyl radicals. 2 4 These authors maintain that the activation energy differences are real (combination requiring the higher E ) , indicating the necessity for the formulation of two distinct transition states for the two processes, a veiwpoint which is not adopted by Szwarc and his collaborators.21 I n the thermal decomposition of haloethanes
-
CZHzXy --+ GH(z- ~ ) x ( -y 1)
+ HX
the transition state will reflect an energized ethane molecule. In the reconibination of CFH2 radicals it is therefore not necessary to assume a second transition state for the vinyl fluoride-HF split, as opposed to an energized (FHZC-CHZF) * conformation, which also leads to thermally equilibrated C2F2H4. The high-temperature values of kb/k2 cause large deviations in linearity in both log k,/k2 us. 1/T and log k 5 / k z us. log T plotsjZ550 that we cannot express 8 5 EO as an activation energy difference or k6/kz as a simple function of T. Kerr and Trotman-Dickenson2e have obtained an increasing disproportionatioa/combination ratio with temperature for n-but.yl radicals in the photolysis of n-valeraldehyde, but it appears to have been due to analytical difficulties in the system.27 I n the temperature range studied in this work, there is no subsequent pyrolysis of C2F2H4,once it is formed.
Samples of the ethane heated for 2 hr. a t 587°K. showed no trace of thermal decomposition. (The ketone, under these conditions in the dark, also showed no sign of decomposition.) We are currently investigating the system in the presence of added gases and looking for wall effects, and we are seeking similar reactions in the photolysis of other fluoro ketones. We wish to reserve further comment on the temperature variation found for kb/kz, until we have performed some of these experiments. Quantuni Yields and Activation Energies. The quantum yield data are somewhat scattered and do not appear to be particularly precise. The percentage absorption in the runs a t 6-7 mm. tends to be low, and these will not be considered further. At the high pressures @CO unity above 100". (All the experiments below 100" show Cpco < unity, which may indicate the participation of the CFH2C0 radical in the photolysis mechanism.) As would be expected @CH,F rises with temperature and @c,F,H& tends to fall with temperature; (Pc,H,F shows a small increase with temperature. The mass balance (m.b. in Table 111) figures correspond to those found for the ratio ('/&H4 C2HB)/ CO obtained in the photolysis of acetone under similar conditions,28 falling below unity with increasing temperature. Between 100 and 300" a complete radical balance may be obtained by analyzing for methyl ethyl ketonez9 and the mass balance ratio in the case of acetone is close to unity. It thus appears that reactions 1-5 are an adequate description of our system above 100'. A least-squares treatment of the Arrhenius plot (Fig. 3) of the expression kl/k,'/' = RcFH,~/'/(Rc,F,H, EcpFH,)1/2[~et.] results in I C ~ / ~ &= " ~ 5.1 x 103 exp (-8000 f 100/RT) ml.*" sec. -I". The pre-exponential factor is similar in magnitude to those found in CH3 and CF3 radical H-abstraction reactions,
-
+
+
(21) P. S. Dixon, A. P. Stefani, and M . Szwarc, J . Am. Chem. SOC., 85, 2551 (1963). (22) P. S. Dixon, A. P. Stefani, and M . Szwarc, ibid., 85, 3344 (1963). (23) R. Klein, M. D. Scheer, and J. G. Waller, J . Phys. Chem., 6 4 , 1247 (1960). (24) R. Klein, M. D. Scheer, and R. Kelley, ibid.,68,598 (1964). (25) This is not so for the 6-7 mm. runs, but as mentioned previously the high-temperature values of ks/kz are incorrect. (26) J. A. Kerr and A. F. Trotman-Dickenson, J . Chem. Soc., 1602 (1960). (27) M.H. J. Wijnen, J . Am. Chem. Soc.. 83, 3752 (1961); J. C. J. Thynne, Trans. Faraday Soc., 58, 1533 (1962). (28) (a) L. M. Dorfman and W. A. Noyes, Jr., J . Chem. Phys., 16, 557 (1948); (b) A. F. Trotman-Dickenson and E. W. R. Steacie, ihid., 18, 1097 (1950). (29) L. Wlandelcorn and E. W. R. Steacie, Can. J . Chem., 32, 79 (1954); R. K . Brinton, J . Am. Chem. SOC..83, 1541 (1961).
Volume 68, Number 7 J u l y , 1964
G. 0. PRITCHARD, M. VENUGOPALAN, A N D T. F. GRAHAM
1792
.
0.6
\
I
6
Table IV
-&
F
Kcal. mole-’
Reaction
vi
+ + + +
CFHz CFHEOCFHz CH3 CH3COCHi CF3 4- CHsCOCHa CF3 CHsCOCFs CHs CHnCOCF3
O
T
-E“
\
7 -0.6 0
CFH3 + CHa -+ CFaH -+ CFiH + CHI +
+ CFHCOCFHg + CHzCOCHs + CHzCOCHi + CHzCOCFs + CHzCOCF3
8 0
9 7 6 9 6 6 8 9
Ref. Thia work 28b 6 8 5
*-
f* -1.2 8 - 1.8 2.0
2.5
3.0
1 0 3 / ~ ,O K .
Figure 3. Arrhenius plot of kl/k,lJ2 us. temperature: 0 , low-pressure runs.
indicating that CFHz radicals recombine on nearly every collision, and a “normal” steric factor for reaction 1 of 10-3-10-4. We may compare E1with the following activation energies, assuming that in each case the energy of activation for radical recombination is zero (see Table IV). Assuming the equivalence of the C-H bonds in the various ketones, it appears that the reactivity of CFHz radicals lies between those of CHa and CF3 radicals. The 6-7 mm. runs below 40OoK. have not been
The Journal of Physical Chemistry
taken into account in these calculations, as they appear to indicate a curvature in the Arrhenius plot at low pressures and temperatures. Ausloos and SteacieaO have discussed the formation of additional CH, in acetone photolysis at low temperatures as being due to two causes (a) a reaction between methyl radicals and acetone adsorbed on the wall, favored by low pressures, and (b) the occurrence of CH3 CH3C0 -+ CH, CH2C0, favored by higher intensities. It seeins that similar deviations are occurring in our system at lower pressures.
+
+
Acknowledgments. We are greatly indebted to Dr. H. 0. Pritchard for very helpful discussions, and to Dr. C. B. Anderson for aid in purification of the ketone. Thanks are also due to Mr. P. Baine for his assistance. We also wish to thank Dr. R. Klein for permission to quote his paper before publication. (30) .’l Ausloos and E. Mi. R . Steacie, Can. J . Chem., 3 3 , 4 7 (1955).
