REACTION OF THERMAL HYDROGEN ATOMSWITH FLUORINATED ETHYLENES
2065
The Addition and Abstraction Reaction of Thermal Hydrogen Atoms with Fluorinated Ethylenes
by R. D. Penzhorn and H. L. Sandoval Chemistry Department, Faculty o j Physical and Mathematical Sciences, University o j Chile, Santiago, Chile (Received September 16, 1969)
The photolysis of HBr was studied in the presence of ethyleneand of each of the six fluoroethylenes,with overall conversions below 0.1%. The results were interpreted in terms of a thermal hydrogen atom addition and :CzFH3: abstraction reaction from these compounds. The increasing relative rates of addition are c~s-CZFZH~ C~H~:trans-CzFzH~:l,l-C~FzHz:C~F3H:CzF~ = 1.00:1.12:1.43:1.67:2.08:2.36:2.42, respectively, and those of abstraction are CZFHs:CzF3H:cis-CzFzHz:trans-CzFzHz: 1,l-CZFzHz= 1.00:1.78:1.94:3.61:6.60,respectively.
Introduction A review of the literature reveals that, whereas the hydrogen atom addition to ethylene has been extensively investigated, little attention has been paid to the study of the reaction of these atoms with halogen-substituted ethylenes. I n an early investigation, Allen, Melville, and Itobb' obtained for the reaction of hydrogen atoms with tetrafluoroethylene a collision efficiency of 0.3 X More recently12 thereact ion between H atoms and vinyl chloride was investigated by photolyzing H I in 1,he presence of C2H3C1. The following reactions were suggested
A chlorine atom abstraction from an olefinic bond had not been reported previously. Further, gas chromatographic evidence strongly suggested that (c), a hydrogen atom abstraction reaction, took place and, moreover, was faster than (b), the corresponding addition to the double bond. This is interesting in view of the generally accepted fact that abstraction by hydrogen atoms from even the higher olefins is usually quite small when compared with the addition reaction, at least a t room temperature. I n view of the scarcity of quantitative data for the reaction of H atoms with halogenated olefins and of the fact that substitution of hydrogen by halogens in olefins seems to have a great effect on the reaction mechanism, it seemed of interest to examine the relative rates of addition and, if occurring, abstraction of hydrogen atoms with the complete series of fluoroethylenes.
Experimental Section Apparatus. The apparatus employed for this investigation was essentially the same as one described elsewhere.a Reaction took place in a 195-cm3 cell provided with a Vycor window, attached to a 3501 cm3 volume. The gases were mixed and circulated with an approximately 7 em3 X sec-l unidirectional circulating pump. Only greaseless Viton A diaphragm valves were used in the reaction and gas measuring section of the apparatus. Hydrogen bromide and the fluoroethylenes were measured with a very sensitive dibutyl phthalate differential pressure manometer, which had the mercury side covered with a dibutyl phthalate film to avoid the mercury contamination of the system. The manometer was capable of detecting a pressure differential greater than 1/200 mm of mercury and was not temperature sensitive at our experimental conditions. During the later stages of this work, this manometer was replaced by a more convenient to operate and of similar sensitivity KernSpringham glass spiral manometer. The overall reproducibility was not affected by this change. GO2 was measured with a Wallace and Tiernan absolute pressure gauge. As a light source, a Hanovia SH type medium pressure mercury lamp was employed, operated at 1.2 A with the help of a voltage stabilizer. Reagents. Hydrogen bromide, carbon dioxide, vinyl fluoride and 1,l-difluoroethylene were from the Matheson Go. and cis- and trans-difluoroethylene were from Penninsular Chem Research. Trifluoroethylene was prepared by alcoholic zinc dehalogenation of l-chloro2-bromo-1,2,2-trifluoroethane(kindly supplied by ICI). (1) P. E. M. Allen, H. W. Melville, and J. C. Robb, Proc. Roy. Soc., A218, 311 (1953). (2) A. M. Rennert and M. H. J. Wijnen, Ber. Bunsenges. Phys. Chem., 7 2 , 222 (1968). (3) R. D.Penzhorn and B. deB. Darwent, J. Phys. Chen., 7 2 , 1639
(1968). Volume 74, Number 10 May 1.4, lor0
2066 Thirty-six grams of the starting material was added dropwise to a vigorously stirred mixture of 20 g of zinc powder and 50 cm3 of ethanol at 60" over a period of 4 hr. The product was removed via a condenser and a vacuum-jacketed column packed with Benske helices and condensed into two traps, the first one in Dry Ice and the second in liquid nitrogen. The trap with Dry Ice retained most of the trifluoroethylene plus some alcohol and starting material, whiIe the second trap retained a small amount of pure product. The first fraction was distilled with a vacuum-jacketed column to separate the unchanged starting material and the alcohol from the trifluoroethylene. The product obtained was purified by gas chromatography on a 20-ft column of 30% dibutyl phthalate on firebrick (60-80 mesh). Tetrafluoroethylene was obtained by thermal decomposition of Teflon, following a technique suggested in the l i t e r a t ~ r e . ~ All gases were repeatedly purified by trap to trap distillations and their purity controlled on a gas chromatograph and with an AEI MS2 mass spectrometer. Total impurities never exceeded 0.5%. Procedure. The gases were successively expanded and condensed into the reaction system, isolated by a stopcock, again expanded, mixed for at least 30 min, and finally photolyzed for periods between 5 and 30 min. The time of photolysis was varied in order to obtain a quantity of hydrogen measurable in a gas buret, never below 15 and usually of about 40 prnol. This was possible because the rate of hydrogen formation for a certain HBr pressure proved to be directly proportional to the time. After photolysis the gases were condensed and the resulting molecular hydrogen measured by PVVT procedures on a gas buret. A special precaution was necessary with ethylene because of the relatively high vapor pressure of this gas a t liquid nitrogen temperatures. For ethylene, the Toepler-collected total noncondensables were compressed into a U-shaped capillary and frozen down with liquid nitrogen. The remaining noncondensables were then expanded into the comparatively large volume of the Toepler pump, isolated from the U tube by a stopcock, and measured as in previous experiments. Mass spectrometric analysis of the noncondensables separated by this method showed that they were about 99% Hz. The ethylene remaining in the U tube contained less than 0.5% Hz.A large number of experiments were carried out using a solid nitrogen trap between the reaction system and the Toepler pump. Both techniques gave consistent experimental results. In most experiments the COz pressure was kept between 250 and 300 mm. This gas was used to facilitate the operation of the circulating pump, improve the gas mixing, and assure proper thermalization. lllercury was kept away from the reaction system at all times with the help of at least two liquid nitrogen T h e Journal of Physical Chemistry
R . D. PENZHORN AND H. L. SANDOVAL traps per entrance. Total conversion for a series of runs was always below 0.1%. All fluoroolefins could be photolyzed without producing any measurable noncondensables, an indication that no hydrogen producing impurities were present. When the uv absorption spectrum above 2000 A of 1 atm of CzFH3 and 1 atm of 1,1-CzF2Hz in a 5-cm cell was taken, no absorption was detected with vinyl fluoride and only a very slight absorption, starting a t 2300 A, was observed with 1,1-CzFzH2. When the HBr absorption coefficients (in good agreement with those ,Of Rommand6) determined between 2000 and 2300 A were compared with those of 1,1-C2FzHz,it was found that the former were larger by an average factor of about lo3. The uv absorption of C2F4was and can also be considered studied by Lacher, et negligible in comparison to HBr. It was felt that this observation can be extended to the rest of the fluoroolefins. N o signs of polymerization were detected.
Results Experimentally this investigation was based upon measurements of hydrogen quantum yields, 9, of Hz. These were determined by comparison of the hydrogen yield obtained by photolysis of HBr (GH2 = 1 for conversions below 1%) in the presence of GOz, with the hydrogen yield from the same mixture but with added fluoroolefin. Since in all experiments the conversion was kept very low, it was possible to carry out several photolyses with the same mixture by only adding increasing amounts of either fluoroethylenes or HBr. The validity of this procedure was verified by two types of experiments: (a) repeated photolysis of the same mixture which gave comparable results within the experimental error; (b) addition of HBr to a certain mixture which reproduced the results expected for the new ratio, from previous experiments with the same mixture; see, for example, experiments 122 and 125. The latter results suggest, furthermore, that no important secondary reactions interfered in the photolyses of HBr-fluoroethylene mixtures. Several calibration photolyses with the pertinent HBr-C02 mixtures as well as frequent calibration curves of the HBr pressure us. hydrogen yield were carried out before and after each series of runs to take care of small variations in the lamp intensity and so reduce the experimental error. When a mixture of 10 mm of CpF4with 1.5 mm of HBr (C2F4:HBr ratio 6.67) or a mixture of 8.05 mm of CZFH, with 4 of mm HBr (C2FH3:HBr ratio 2.01) were repeatedly photolyzed at room temperature, adding (4) J. Heicklen, V. Knight, and S. A. Greene,
J. Chem. Phys.,
42,
221 (1965).
(5) J. Rommand, Ann. Phys. (Paris), 4, 527 (1949). (6) J. R. Lacher, L. E. Hummel, E. F. Bohmfalk, and J. D. Park, J . Amer. Chem. SOC.,72, 5486 (1950).
2067
REACTION OF THERMAL HYDROGEN ATOMSWITH FLUORINATED ETHYLENES
t
4 0
4
ql.-
ID)
iH2
4 C2FH,/HSr.?,01 0
:o,oss
I2/HI
1661
C~F@BI
0 +/HEr
,
200 ,.
,
Mo ,
.
4W 1
,
.
5W
,
s0,0&72
6W ,
,
.
7W
CO2
Figure 1. COZ pressure effect on the hydrogen quantum yield from: D, 0.167 mm of Iz with 3.02 mm of HBr; 0 , 0.175 mm of 1 2 with 4.00 mm of HBr; 0, 8.05 mm of GFHa with 4.00 mm of HBr; and 0,10.0 mm of CzFl with 1.5 mm of HBr mixtures.
each time increasing amounts of C 0 2 as an inert diluent, no pressure effect on the hydrogen quantum yield up to a pressure of about 700 mm could be observed (see Figure 1) This is in marked contrast with two series of experiments in which a mixture of 0.167 mm of Iz with 3.02 mm of HI (Iz:HI ratio 0.055) and 0.175 mmLof I2 with 4.00 mm of HBr (I2:HBr ratio 0.044) were photolyzed, also a t room temperature and again with increasing addition of COZ. This time, a pronounced initial decrease of the quantum yield was found, which leveled off after a pressure of approximately 50 and 100 mm of C02, respectively (see Figure 1). When HBr was replaced by HI and a 2.08 mm of H I with 4.16 mm of C2FH3mixture was photolyzed, again no pressure effect was observed, but when the ratio of HI:C2FH3was kept constant and the total pressure of the reactants reduced to about 3 mm the quantum yield decreased with pressure. This decrease was even larger when the reactants pressure was further lowered (see Figure 2). A study of the effect of the partial pressure variation of the fluoroethylenes (C2F,H, where x; y = 4) and hydrogen bromide upon %I .% gave a straight line relationship between the HBr :C2F,H, ratios and (1/%1, l)-l. This type of investigation was carried out for C2H4, C2FH3, 1,1-C2F2H2, c ~ s - C ~ F ~ Ir~ns-CzFzHz, H~, C2F8H, and C2F4. All experiments were performed a t room temperature and in the presence of COz a t pressures generally above 250 mm. The data of these experiments are listed in Table I and plotted in Figures 3 and 4. As can be observed, most compounds show an intercept appreciably higher than zero when the HBr : C2F,H, ratio approaches this same number. The only exceptions are ethylene and perfluoroethylene, whose intercepts were found to be -0.056 and 0.074, respectively, by the least-squares method. The slopes were found to be only a function of the HBr :C2F,H, ratio, as can be observed from some experiments in which only
+
Oh 0.5
mrn Hg
5
''
30
40
C02
50
Figure 2. Effect of COZpressure, at room temperature, on the hydrogen quantum yield 0 of Hz from the photolysis of 0, 2.08 with 4.16; D, 1.10 with 2.14; and 0, 0.61 with 1.17 mm of HI-C%FHsmixtures.
one reagent (C2F,H, or HBr) was varied and the other kept constant (see, for example, experiments 217-225 or 150-153), and from some others in which the ratio was maintained constant but the partial pressures of HBr and C2F,H, about quadrupled (experiments 147 and 153). I n addition, the slope was found completely pressure independent as is apparent from some experiments with Iruns-CzF2H2 (experiments 191-192 and 205-206) where the C 0 2 pressure was increased from about 250 to 550 mm. Photolysis repeated under constant conditions gave satisfactory reproducibility. The following slopes and intercepts were calculated by the least-squares method with an IBM 360 computer: C2H4,11.87 f 0.11 and -0.056 f 0.035; C2FHa,15.12 A 0.60 and 0.447 f 0.273; truns-C2FzH2,10.18 f 0.28 and 1.088 f 0.068; cis-C2FzHz,16.96 f 0.40 and 0.977 f 0.197; 1,l-C2F2H2,8.18 f 0.60 and 1.603 f 0.329; C2F3H, 7.19 i 0.19 and 0.379 f 0.0831; and CzF4, 7.02 f 0.18 and0.074 0.072.
*
Discussion The photochemistry of HBr has been discussed recently.' Upon photolysis with a medium pressure arc the HBr molecule will dissociate giving essentially a normal electronic state (2P,,J bromine atom and a ground state (2Ss,,)hot hydrogen atom, according to HBr
+ hv
---t
H*
+ Br
(e>
(asterisk denotes a hot radical). The produced hydrogen atoms may possess energies from 23 up to 39.5 kcal/mol, considering the bond energy of HBr as 88 kcal/mol, the HBr absorption onset as 2660 A, and the (7) R. M. Martin and J. E. Willard, J. Chem. Phgle., 40, 3007 (1964). Volume 74, Number 10 M a y 14, 1970
R. D. PENZHORN AND H. L. SANDOVAL Table I Expt
no.
HBr, mm
CPzHu, mm
[HBrl ICPZHVI
RH,'
a
mm
RHx
CaH4 158 159 160 161 229 230 232 297 298
2.10 2.10 2.10 2.10 1.51 1.51 1.51 1.53 1.53
2.75 10.07 10.07 16.39 4.52 9.10 24.16 10.20 18.30
0 763 0.208 0.208 0.128 0.334 0.166 0.0625 0.150 0.0837 I
9.00 2.34 2.34 1.50 4.00 2 .oo 0.70 1.71 0.917
30.0 30.0 30.0 30.0 22.5 22.5 22.5 15,33 15.33
27.0 21 .o 21 .o 18 .O 18.0 15 .O 9.25 9.67 7.33
277.5 277.5 277.5 277.5 265.0 265.0 265.0 227.5 227.5
31 .O 31 .O 31.0 31 .O 31 .O 31 .O 38.5 51 .O 67.0 15.17 15.17 15.17 15.17
29.0 29 .O 28.0 26.0 24.5 21 .o 30.0 43 .O 60.0 10.33 9.33 9.0 9.0
14.50 14.50 9.34 5.20 3.75 2.10 3.53 5.37 8.58 2.14 1.60 1.46 1.46
257.5 257.5 257.5 257.5 257.5 257.5 257.5 257.5 257.5 153 .O 153.0 153.0 153.0
17.0 17.0 16.0 15.0 14.0 13.5 13.5 13.5 13.5
17.0 17.0 8.0 5.0 3.5 3.0 3.0 3.0 3.0
272.5 272,5 272.5 272.5 272.5 272.5 272.5 272.5 272.5
CzFHa 145 146 147 148 149 150 151 152 153 279 280 281 282
2.00 2.00 2.00 2.00 2.00 2.00 3.30 5.30 8.50 1.50 1.50 1.50 1.50
2.12 2.12 4.32 6.62 9.02 15.35 15.35 15.35 15.35 12.85 15.96 19.40 22.58
0.944 0.944 0.463 0.302 0.222 0.130 0.215 0.345 0.554 0.117 0.094 0.077 0.067
cis-CzFzHz 217 218 219 220 221 222 223 224 225
1.03 1.03 1.03 1.03 1.03 1.03 1.03 1.03 1.03
1.08 1.08 2.58 4.10 5.60 7.12 8.62 10.24 12,54
0.952 0,952 0.400 0.251 0.184 0.145 0.120 0,101 0.0822
18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0
t~ans-CeFaHz 211 212 213 214 199 200 20 1 203 204 205 206 182 183 184 185 186 187 188 189 190 191 192
1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60 1.60
3.30 5.70 7.32 7.32 7.60 10.70 10.70 12.70 15.10 16.50
1.60
16.50
1.90 1.90 1.90 1.90 1.90 1.90 1.90 1.90 1.90 1.90 1.90
3.00 5.20 7.20 9.20 12.00 15.00 15 .OO 18.00 18.00 22.70 22.70
The Journal of Physical Chemistry
0.483 0.281 0.218 0.218 0.211 0.150 0.150 0.126 0.106 0.097 0.097 0.633 0.368 0.264 0.207 0.158 0.127 0.127 0.106 0.106 0.084 0 084 I
23.3 23.3 23.3 23.3 24.0 24.0 24.0 24.0 24.0 24.0 24.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29 .O 29.0
20.0 19 .o 18.0 18.0 18.5 17.0 17.1 16.5 16 .O 16.0 16.0 25.5 24.0 23.0 22.0 21 .o 20.0 20.0 20.0 19.5 20.0 20.0
6.07 4.42 3.40 3.40 3.36 2.43 2.48 2.20 2.00 2.00 2.00 7.29 4.80 3.83 3.14 2.62 2.22 2.22 2.22 2.05 2.22 2.22
255 255 255 255 262.5 262.5 262.5 262.5 262.5 262.5 525 255 255 255 255 255 255 255 255 255 255 547
2069
REACTION OF YrHERMAL HYDROGEN ATOMSWITH FLUORINATED ETHYLENES Table I (Continued) Expt no.
HBr,
CzFzHv,
mm
mm
IHBrl ICzFzHwI
134 136 137 138 139 140 141 142 334 335 336 337 338 339 329 330 422 423 424
2.10 2.10 2.10 2.10 3.20 4.30 4.30 4.30 2.18 2.18 2.18 2.18 2.18 2.18 1.59 1.59 1.43 1.43 1.43
4.25 6.40 8.50 11.60 11.60 11.60 34.10 34.10 1.20 2.34 3.44 10.33 42.33 42.33 6.15 6.15 4.03 14.02 43.09
0.495 0.329 0.247 0.181 0.275 0.370 0.126 0.126 1.817 0.932 0.634 0.212 0.052 0,052 0.259 0,259 0.355 0.102 0.033
167 168 169 171 172 173a 1736 174a 174b 175 176
2.10 2.10 2.10 2.10 2.10 2.10 2.10 2.10 2.10 7.00 7.00
2.10 4.10 6.50 9.10 9.10 12.40 12.40 15.70 15.70 15.70 15.70
1.000 0.513 0.322 0.231 0.231 0.169 0.169 0.133 0.133 0.447 0.447
119 120 122 123 124 125 126 127 271 272 273 274 275 276
2.10 2.10 2.10 2.10 2.10 3.25 4.45 7.50 1.50 1.50 1.50 1.50 1.50 1.50
2.10 4.50 6.65 8.80 11.10 11.10 11.10 11.10 10.00 12.10 14.25 18.15 23.25 23.25
1.ooo 0.468 0.317 0.238 0.189 0.301 0.400 0.676 0.150 0.124 0.105 0.083 0.065 0.065
(k-
RH^' 1,l-CeFaHa 36 .O 36.0 36.0 36.0 44.0 51 .O 51 .O 51 .O 33.625 33.625 33.625 33,625 33,625 33.625 33.20 33.20 31 .OO 31 .OO 31.00
coz, mm
1)-
31 .O 30.0 28.0 27.0 35.0 43.0 38.0 35.0 31.86 30.75 29.50 26.37 18.36 18.36 27.30 27.30 25.50 21.50 19.10
6.20 5.00 4.00 3.00 3.89 5.38 2.92 2.19 18.10 10 69 7.151 3.635 1.20 1.20 4.63 4.63 4.63 2.26 1.60
265 265 265 265 265 265 265 265 253 253 253 253 253 253 270 270 255 255 255
30.50 30.50 30.50 30.50 30.50 30.50 30.50 30.50 30.50 57.50 57.50
27.00 24.00 22.50 21 .oo 21 -00 19.00 19.00 17.00 17 .OO 45.00 45.00
7.70 3.70 2.81 2.21 2.21 1.65 1.65 1.26 1.26 3.60 3.60
252.5 252.5 252.5 252.5 252.5 252.5 252.5 252.5 252.5 252.5 252.5
38.00 38.00 38.00 38.00 38.00 48.00 50.00 65.00 16.50 16.50 16.50 16.50 16.50 16.50
33.25 30.00 27.00 25.00 23.00 32.00 36.00 54.00 9.00 7.66 7.33 6.60 5.56 5.53
7.00 3.75 2.45 1.92 1.53 2.00 2.58 4.90 1.20 0.87 0.80 0.67 0.51 0.51
147.5 147.5 250 .O 250.0 250.0 250.0 250.0 250.0 312 .O 312.0 312.0 312.0 312.0 312.0
I
C8aH
CaFa
a
RH^' expressed in units of mm/5-min photolysis X 0.126 cm5.
spectral energy distribution of the radiated mercury lines of the lamp employed in this work. No clear statement can be made with regard to the relative importance of the various possible hydrogen atom energies. When a mixture of a fluoroethylene and HBr is photolyzed, only the HBr molecule will decompose. The fate of the produced hot hydrogen atom is to react either
with HBr or else with the existing fluoroolefin, by one of the following reactions
H*
H*
+ HBr +Hz + Br
+ CZF,H,
--+-
CZF,-lHff
(1)
+ HF
Volume 74, Number 10 May 1.4, 1070
R. D. PENZHORN AND H. L. SANDOVAL
2070
7.5
5
2.5
I
~
7-
6.5
0.25
0.75
1.0
1.25
Figure 3. Graphical representation of the data from Table I according to eq I1 for trans-l,Z-CzFzHz, 1,1-CZF2HzJ cis-1,2-CzFzH2, and CZFI.
or to be thermalized by collisions H*
+ M +H + R/I
(5)
with an inert diluent M (COz or any of the other reagents). The thermal hydrogen atoms can then undergo a set of reactions, similar to those of the corresponding hot atoms H HBr .--) Hz Br (6)
+
+
H H
+ CzF&,
+ C2F,H,
----+
CzF,H,+i
-3 CZF,H,-i
+ €12
(7)
(8)
+
+
H C2F,H, CzFz-lH, HF (9) All possible fluoroethylene radical reactions were neglected in view of the low conversions employed in this work. Applying the usual steady-state considerations, the following relation can be derived
where RH2' corresponds to the hydrogen produced from HBr alone or with C02, and RH^ is the hydrogen produced when the same pressure of HBr is photolyzed in the presence of fluoroolefin. Only one important assumption was made in the derivation of eq I. It was considered that under the prevailing conditions all of the reactions were thermal, ie., k&II >> k3 HBr (kz k3 k4)C2F2Hy.
+
The .Journal of Physical Chemistry
+ +
Clearly the hydrogen atoms produced during the primary photolytical act are hot as can be deduced from some experiments of the pressure effect of COz on @HZ for HX-I2 mixtures. Initially small and increasing up to a certain presamounts of COz will reduce the sure, after which the hydrogen production becomes pressure independent. It is interesting to note that it takes about half the COz pressure to moderate equal ratios of I z : H X when X is Br as when it is I. This indicates that, with the same incident A 2537 8 light, the energy of the H atoms produced from HI is about double that from HBr which, considering the bond energy of 7 0 kcal/mol of H I and 88 kcal/mol of HBr, is in agreement with expectations (see Figure l), provided little energy goes into excitation of the X atom. It is then reasonable to infer that HBr alone, at least at pressures of 1.5 mm, will not completely moderate the hot H atoms. When the same type of experiments were carried out with CZFH, or C2F4 and HBr no such pressure effect on the hydrogen quantum yield could be detected, which seems to suggest that the fluoroethylenes play an important)role in the moderation process and that these binary systems are in thermal conditions at least at total fluoroethylene pressures above 4 mm. Some experiments were carried out using HI. The H :toms produced from this hydrogen halide at 2537 A will have essentially 41 kcal/mol,s about 16 kcal/mol (8) L. E. Compton and R. M. Martin, J. Phys. Chem., 73, 3474 (1969).
REACTION OF THERMAL HYDROGEN ATOMSWITH FLUORINATED ETHYLENES
2071
7.5
5
L5
0
:CiFHa
0
CZHI
0=
C,F,H
HBr C2h% 0.25
Figure 4.
a5
0.75
1.25
1.0
Graph.ical representation of the data from Table I according to eq I1 for CaFHa, C Z H ~and , CtFsH.
higher than those produced from HBr at the same wavelength. With HI a COZ pressure effect on h2 was noticed, but only a t very low reactant pressure (see Figure 2). The extent of the reduction increased as the total pressure was lowered. This is in contrast with a study of the photolysis of a 9 :1 and a 5.5 : 1 mixture of CzDdwithabout, 1mm of HI, where a plateau was reached only after a total pressure of the order of 200 mma9 A similar observation was made recently by Wooley and Cvetanovi6,lO who used H2S as the H atom source. These results clearly indicate that the fluoroolefins are more effective €I*atom moderators than ethylene or iodine and that therefore the collisions of these compounds with the hot hydrogen atoms are quite inelastic. The higher efficiency of the fluorinated olefins can be attributed to the greater density of vibrational levels of the C-F bonds and the related increase in the number of effective oscillators of these molecules. Furthermore, considering that a hydrogen abstraction occurs with the fluoroolefins, it can be suggested that the average reaction probability per hot collision is low because no structure was observed in the moderation curves of Figures 1 and 2. When a hot €1 atom collides with a fluoroolefin molecule, a superhot alkyl radical will be formed with an exH* CzF,H, +C2FzH,+1** cess energy given by the sum of the exothermicity of the reaction and the energy carried by the H atom. This radical may decompose according to C2li’,Hg+1** * C2FZH, H
+
+
even a t total pressures of a few hundred millimeters. A thermalized H atom, on the contrary, will either abstract or otherwise give rise to a much longer-lived fluoroalkyl radical
H
+ C2F,H,
-
CzFzH,+i *
whose fate will be to disappear by reaction with HX or C2F,H,. Upon rearrangement, eq I becomes
(k- ’> -1
(HBr) +---+ Le (C2F,H,)
ks
k6
=9 -
(11)
k7
which can be seen plotted for the various fluoroethylenes in Figures 3 and 4. The slopes of the straight lines can be identified with k 6 / ( k 7 kg) and the intercepts with k8/(k7 he). The behavior predicted by eq I1 was observed for all fluoroolefins with the exception of C2H4 and CzFd which show only a negligible intercept, attributable to the experimental error. From the previous treatment it is expected that perfluoroethylene should not show an intercept, because this compound has no hydrogen and therefore reaction 8 cannot take place. With ethylene, on the other hand, only reaction 9 can be a priori disregarded and therefore an intercept is possible, but it is a well established fact that hydrogen abstraction from ethylene does not occur at room temper-
+
+
(9) R.D.Penzhorn, Diss. Abstr. B , 28, 2379 (1967). (10) G.R.Wooley and R. J. Cvetanovi;, J. Chem. Phys., 50, 4697 (1969).
Volume 74, Number 10 M a y 14, 1970
R. D. PENZHORN AND H. L. SANDOVAL
207 2 Table 11: Ratios of Rate Constants kdke
Compd
0.0843 f 0.0008 0.0662f0.0026 0.1223f0.0091 0.0589f0.0014 0.0983 rt 0.0027 0.1390f0.0039 0.1425 f 0.0037
C2H4 CzFH3 1,l-CzFzHa C~S-CZFZHZ trans-CzFzHz CZFIH CZF4
ature and is of negligible importance even for the higher olefin." Up to now, with the sole exception of some very hot hydrogen atom studies,12 no conclusive evidence for fluorine abstraction reactions has been reported, not ~ ~ ' ~ and Jennings's even for the f l u ~ r o a l k a n e s . ~ Scott suggested that a reaction for fluorine abstraction
H
+ CHI-CH2F
+CH3-CH2
+ HF
takes place during the Hg-photosensitized decomposition of ethyl fluoride. The presence of ethyl radicals explained the formation of such products as n-C4Hlo, C2H6, and CzH4. Nevertheless, the evidence given is not conclusive. The possibility that hydrogen atoms, generated by the interaction between mercury and the substrate, could react with the other radical produced in the primary process, CH3-CHF, cannot be ruled out. I n that event the following sequence of reactions would also explain the observed products
+ H +CH,-CHZF* CH3-CHzF* +C€I2=CHz + H F CHz=CH2 + H +CH1-CH2, etc CH,y-CHF
Ethylene could also come from a primary unimolecular decomposition of an exited ethyl fluoride molecule. Finally, Wijnen, et ~ l . could , ~ not detect fluorine atom abstraction from CH2=CHF by H atoms. In view of all the previous considerations, it seems reasonable t o assume that kl and IC8 >> ICg. Table I1 summarizes the kI/k6, k&l, and k8/k6 ratios for all fluoroethylenes. From the data it is apparent that hydrogen abstraction is of considerable importance for the fluoroethylenes and that for 1,1-CzH2Fzand for trans-C2F2Hz it is even faster than the corresponding addition, indicating that replacement of hydrogen by halogen atoms weakens the C-H bond strength of the corresponding olefins. Although there is a very regular variation in the fluoroethylene structures, there seems to be no obvious correlation with any trend in the addition and abstraction reaction. With the exception of cis-CpFzHz and ethylene, it can be observed that there is an increase in the rate of addition with increasing fluorine substitution. The relative rate of addition of methyl radicals t o ethylene and tetrafluoroethylene has been investigated T h e Journal of Physical Chemistry
kdki
-0.056fO.035 0.45 f:0.27 1.60 10.32 0.98 f 0.20 1.09 f 0.07 0.38 f 0.08 0.074 10.072
kdks
... 0.0297 f 0.0178 0.196 1 0 . 0 5 4 0 0.0577f0.0132 0.1070f0.0077 0.0528 f 0 . 0 1 2 8
...
in the gas phase.16 It was found that the reactivity of the fluorinated compound is about ten times faster than that of ethylene. It is a well established fact that hydrogen atoms and the methyl group, both electroneutral radicals, show great similarities in their trends and chemical behavior. Accordingly, we find that tetrafluoroethylene has a 1.69 times affinity for hydrogen atoms than ethylene. Robb, et al., found that tetrafluoroethylene reacts with €1 atoms about 25 times slower than ethylene. This surprisingly cliff erent result could be explained by a mercury-photosensitized decomposition of the reagents, which takes place quite readily.'7,ls It is interesting to notice that both the H and the CH3 radical were found to be unreactive toward C2C14. Some studies of the addition of electrophilic radicals like CCl3l9and CF3,20collected in Table 111, have been compared with the data for H and CH3 addition. These radicals will also add preferably t o the less halogen substituted side of the olefin but, being electrophilic, will add increasingly slower with fluorine substitution. However, to explain all the observed trends, more data on activation energies and preexponential factors are needed, as well as information with respect to the site of the abstraction. ratios for the fluoroethylene Table I1 shows the series. Again, no regular trend is observed. The abstraction rate increases in the order CzFH3, C2F3H, cis-CzFzHz, trans-CzFzHz,and 1,1-C2F2Hz. Evidently, the most favorable molecule will be the highly polarized 1,1-CzF2Hz. The surprisingly low CzF3H rate can (11) R. J. Cvetanovi6, Advan. Photochem., 1, 149 (1963). (12) R. Odum and R. Wolfgang, J . Amer. Chem. Soc., 85, 1050 (1963). (13) H. N. Chadwell and T. Titani, ibid., 5 5 , 1363 (1933). (14) J. R. Dacey and J. W. Hodgins, Can. J . Res. Sect. B , 28, 173 (1950). (15) P. M. Scott and K. R. Jennings, Chem. Commun., 700 (1967). (16) R. P. Buckley and M.Szwarc, J . Amer. Chem. Soc., 78, 5696 (1956). (17) J. Heicklen, V. Knight, and S. A. Greene, J. Chem. Phys., 42, 221 (1965). (18) A. B. Callear and R. J. Cvetanovib, ibid., 24, 873 (1956). (19) J. M. Tedder and J. C. Walton, Trans. Faraday Soc., 6 2 , 1859 (1966). (20) A. P. Stefani, b. Herk, and M. Sawarc, J . Amer. Chem. Soc., 83, 4732 (1961).
REACTION OF THERMAL HYDROGEN ATOMSWITH FLUORINATED ETHYLENES kl
Table 111: Relative Rate Constants for Different
CzHaCl CzHsF
Addition Reactions to Pluoroolefins" Compd
C2H4 CzFHs 1,l-CzFzHa cis-CzFzHz
t~~ns-CzFzHz CzFaH CZF4 a
CFab
CClaC
CHad
H'
1.000
1.0000 0,8610 0.0998
1.0
*..
1.00 0.79 1.45 0.70 1.16 1.65 1.69
... *..
... ...
...
0.151
... ...
0.0108 0.0010
Data expressed relative to ethylene.
ref 19.
See ref 16.
e
2073
. t .
... ...
... 10.0
See ref 20.
See
Present work.
partly be attributed to the decrease in the number of available C-H bonds. As mentioned before, Wijnen found that R hydrogen abstraction rea,ction occurs between H atoms and vinyl chloride. In a earlier study of the photolysis of HIC2H4 mixture^,^ the ratio of rate constants k(H CzH4)/k, was found to be 0.0593 at room temperature. When this result is combined with the k(H C2H4)/k6 ratio found in the present investigation, a value of 1.43 can be calculaied for k,/ke. With these values, the rates of addition and abstraction of H atoms with vinyl chloride and fluoride with respect to k6, can be calculated as in the following table.
+
+
O?
kb/k3
0.0508 0.0662
ks or W k a
0.109 0.0297
It seems reasonable to suggest that the inductive effect of electron attraction, which is greater for fluorine than for chlorine,21will favor addition of H atoms to C2FH3. Nevertheless, the experimental rate difference would have been expected to be greater. The existence of a resonance effect between the T electrons of the double bond and the nonbonding electrons of the h a l o g e n ~ could ~ ~ , ~explain ~ the hydrogen abstraction. However, contrary to observation, the rate with C2FH3would have been expected t o be faster than that with CzClH3considering that, from chemical properties of the ground states of halogenated molecules, the mesomeric effect (electron release) is greater for fluorine than for chlorine. Acknowledgment. The financial support from FORGE (The Funds for Overseas Research Grants and Education) and €rom the Comisi6n Nacional de Investigaciones Cientfficas y Tecnol6gicas de Chile is gratefully acknowledged. (21) D. T. Clark. J. N. Murrell, and J. M. Tedder, J. Chem. SOC., 1260 (1963). (22) L.Pauling, "Nature of the Chemical Bond," Cornel1University Press, Ithaca, N. Y., 1945. (23) J. R. Majer, Advan. FZuorine Chem., 2 (1961).
Volume 74, Number 10 May 14, 1070
