2746
J. M. SANQSTER AND J. C. J. THYNNE
Reactions of Radicals Containing Fluorine. V.
The Addition of
Trifluoromethyl Radicals to Ethylene by J. M. Sangster and J. C. J. Thynne Chemistry Department, Edinburgh University, Edinburgh, Scotland
(Received January 07, 1969)
The addition of trifluoromethyl radicals to ethylene in the gas phase has been studied over the temperature range 18-201" using the abstraction of a hydrogen atom from hydrogen sulfide as the competing reaction. The Arrhenius parameters determined for the addition reaction were 1011-30 mol+ ern+ 8ec-I and 2.4 kcal mol-'.
The addition of trifluoromethyl radicals to various unsaturated compounds has been studied by Szwarc and c o ~ o r k e r s l - using ~ a competitive technique, the abstraction of a hydrogen atom from 2,3-dimethylbutane being the alternative reaction; hexafluoroazomethane was the radical source. CF3
+ CHz=CHR A CF3CHzCHR
CFa
+ CaH14 2,CFSH + CBHH
Their method suffers from the disadvantage that no product resulting directly from the addition reaction is measured or even observed, the analytical method depending upon changes in the CFIH/N2 ratio when the olefin was added. I n the case of ethylene,2 if the reported value4 for the activation energy for reaction 2 is used, the activation energy for the addition reaction is negative, which is unlikely. We have therefore studied the gas-phase addition of trifluoromethyl radicals to ethylene in the temperature range 18-201", measuring the products formed as a result of the addition reaction.
Experimental Section The source of trifluoromethyl radicals chosen was the photolysis of hexafluoroacetone since preliminary experiments with ethylene showed that this source gave a less complex set of reaction products than did trifluoromethyl iodide. A competitive technique was used, the addition reaction of the radical being measured relative to a hydrogen atom abstraction reaction. Experiments showed that abstraction from cyclohexane was too slow and that methyl mercaptan reacted directly with the ketone to form a nonvolatile complex. No such reaction was observed between hydrogen sulfide and the ketone, even a t 200°,and since hydrogen sulfide reacted readily with the radical to lose a hydrogen atom it was used as the reactive substrate. Because of the very facile addition to ethylene it was found that optimum conditions for accurate measurement of products were The Journal of Physical Chemistry
-
obtained with mixtures rich in hydrogen sulfide, namely HzS/CzH4 10. Apparatus and Procedure. The reaction cell was a quartz cylinder (volume 159 cma) which was housed in a heavy aluminum block furnace fitted with quartz side windows. The temperature of the furnace was controlled to better than h 0 . l " by a Bikini-Fenwall relay unit. The light source was a Mazda 250-W ME/D lamp, a parallel beam being arranged to fill the cell. The reaction cell was connected to the usual type of high vacuum line comprising cold traps, McLeod gauge, and gas buret. After reaction the contents of the reaction cell were expanded into the analytical line and trapped a t liquid nitrogen temperature. The noncondensable products were removed by pumping. The condensable products were then warmed to room temperature and analyzed gas chromatographically, CFBH and CFaCHzCHa being measured on an ll-m 30% diethyl adipate on firebrick column maintained at 0" and CF&H&H2CHZCHa on a 1.7-m 20% dinonyl phthalate on firebrick column maintained at room temperature. Calibrations were performed at the end of each experiment. Materials. Hexafluoroacetone was prepared by dehydration of the sesquihydrate (Koch Light) followed by low-temperature vacuum distillation a t - 60" and pumping a t -127". Hydrogen sulfide was prepared using a Kipp apparatus followed by pumping a t - 160" and several vacuum distillations at - 100". Ethylene (Matheson cylinder) was used with no further purification beyond degassing. For calibration and identification purposes the following compounds were used: trifluoromethane (Matheson cylinder), hexafluoroethane (DuPont) and trifluoropropene (Pierce Chemical Co.). Since the other (1) P. S. Dixon and M. Szwarc, Trans. Faraday Soc., 59, 112 (1963). (2) J. M. Pearson and M. Sswarc, ibid,, 60, 564 (1964). (3) G. E. Owen, J. M. Pearson, and M, Szwarc, ibid., 61, 1722 (1965). (4) G. 0. Pritchard, H. 0. Pritchard, 1%. J. Schiff, and A. F. TrotmanDickenson, ibid., 52, 849 (1956).
2747
CF3 ADDITIONTO ETHYLENE compounds (such as l,l,l-trifluoropropane, hexafluorobutane, 1,1,1-trifluoropentane, and hexafluorohexane) were unavailable commercially they were prepared by lengthy photochemical decomposition of mixtures of the appropriate compounds, followed by separation by low-temperature distillation and gas chromatography. The identity of the compounds was confirmed where possible by using two different radical sources to provide the same compound (e.g., CF3C2H5 produced from CzF6CHO-CF3COCF, and CFaCOCFaCZH6 mixtures).
Results and Discussion ( a ) Reaction of CF, Radicals with Hydrogen SulJide. Since the abstraction of hydrogen atoms from hydrogen sulfide by trifluoromethyl radicals was chosen as the competing reaction, it was necessary to know the Arrhenius parameters for reaction 3 in order to establish the addition reaction results on an absolute basis.
+ CO CF3H + SH
CFaCOCF3 h', 2CF3 C'F3
+ H2S -% 2CFa
CzFa
Reaction 3 has been studied previously5 and the following Arrhenius parameters have been reported : log cm3 sec-l) = 11.65 f 0.16 and Es (kcal mol-') = 3.88 rt 0.26. As a check on the applicability of this result t,o our system we examined the reaction briefly, measuring the rate constant ratio k3/k4'/aa t a few temperatures. Our results for log k3/~~41'2 were, a t 314, 333, and 385"K, 2.25, 2.45, and 2.68, respectively. The data of Arthur and Bell5 yield values of 2.27, 2.43, and 2.77 for the ratio a t these same temperatures. Our data are clearly in good accord with theirs and, since they examined the reaction in greater detail, we have used in this work the parameters they reported. (b) Reaction of CF3 Radicals with Ethylene. When trifluoromethyl radicals were generated in the presence of ethylene, a variety of reaction products were observed, including some high boiling substances whose identity could not be determined. The reaction was not studied extensively under these conditions because of its complexity; however, for a typical run at 81' using a 10: 1 hexafluoroacetone-ethylene mixture, the product distribution was (in micromoles) ; CFsCH= CHz, 0.117; CF,CH&H3, 0.169; CF3CHzCHZCF3, 0.110; CFsCH2CH2CH2CH3, 2.06; CF3C4H8CF3, 0.718; CF3H and CzFswere formed only in negligible amounts. The small yields of CF3H and CzFe indicate that addition of trifluoromethyl radicals to ethylene is very rapid; the extensive formation of trifluoropentane is evidence that the trifluoropropyl radical also reacts readily by addition. These observations may be interpreted in terms of the following reactions.
CF3 CF3
+ CZH4 -%
+ CFaCHzCHz
6 -3
CF3CH2CHz
+ CF&H=CHz
CFBH
A CF~CH~CHZCF~ CF&H=CHz + CF3CHZCH3
2CFaCHZCH2
-%
CF3C4H8CF3
+ C Z H-% ~ CF3C4H8 CF3 + CF3C4H8 -% CF3H + CF3C4H7 CFaCHzCHz
'2, CF3C4H8CF3
+ CFaC4Hs -% CF3CHzCH3 + CFaC4H7
CF3CHzCH2
14 +
CF,CH=CH2
+ CF3C4Hs
The negligible yield of trifluoromethane suggests that reactions 6 and 11 do not occur significantly. The results of the product distribution given above indicate that the steady-state concentration of the CF3C4H8 radicals is greater than that of the CF3CzF4species. The relatively small yield of hexafluorobutane confirms this and suggests that reaction 12 will contribute more significantly toward hexafluorohexane formation than will reaction 9. Similarly, it is likely that trifluoropropene formation by (14) is more extensive than via (8) although the respective contributions cannot be deduced. (c) Reaction of CFa Radicals with Ethylene-Hydrogen Sulfide Mixtures. When trifluoromethyl radicals were produced in C2Hd-HzS mixtures rich in hydrogen sulfide, the reaction pattern described in (b) was considerably simplified, fewer products being observed. By suitable variation of the HzS/C2H4ratio, conditions were found (ratio -10) where CF3H, CFaCH2CH3,and CFaC4Hs were the predominant reaction products, other products mentioned in (b) being absent or formed in trace quantities only. The mechanism suggested to explain product formation involves the following reactions in addition to (3), (51, and (10)
+ HzS -% CF3CHzCH3+ SH CF3C4Hs + HzS -% CF,C4Hs + SH
CFaCHzCHz
Our experimental results are reported in Table I. The absence of C Z F ~CFaCH=CHz, , CF3CH2CH2CF3, and CFGHsCF3 in this reaction system suggests that the radical-radical reactions leading to their formation are completely inhibited, and that product formation is entirely due to the reaction of trifluoropropyl and trifluoropentyl radicals with hydrogen sulfide. The (6) N. L. Arthur and T.N. Bell, Can. J. Chem., 44, 1445 (1966).
volume 73, Number 8 August 1969
2748
J. M. SANGSTER AND J . C. J. THYNNE
Table I: Product Yields and Product Ratios for the Reaction of CFI Radicals with H z S - C ~ HMixtures' ~
291.0 296.0 323.0 323.0 323.0 336.5 337.0 353.0 353.0 353.0 353.0 353.0 353.0 383.0 398.0 398.0 398.0 411.0 431.0 432.0 443.0 473.0
K
1800 1800 1320 1320 1080 1080 1080 900 900 420 900 780 900 600 480 480 480 360 420 420 300 300
175 171 171 173 171 173 175 174 173 174 173 171 870 171 160 172 166 169 171 175 170 168
= hexafluoroacetone.
15.9 15.3 15.0 15.3 15.1 15.9 15.6 15.5 14.6 15.8 16.3 16.7 16.0 15.0 14.5 15.8 15.1 15.5 15.6 16.0 15.8 15.5
11.0 11.2 11.4
11.3 11.2 10.9 11.2 11.2 5.52 5.27 21.1 20.5 10.8 11.4 11.0 10.9 11.0 10.9 11.o 11.0 10.8 10.8
All concentrations given in lo8 mol. ZCFaCzHa
continued formation of CF3C4H9,even a t high hydrogen sulfide concentrations (H2S/C2H4= 20), was rather surprising and indicated the ease with which trifluoropropyl radicals reacted with ethylene. The possibility exists that some of the trifluoropropyl and trifluoropentyl radicals may react with the mercaptyl radicals formed in the hydrogen atom abstraction reactions. We were unable to examine for this possibility by suitable variation of the hydrogen sulfide/ ethylene ratio because of experimental limitations; ratios below about 5 lead to the formation of side products and for mixtures too rich in hydrogen sulfide the abstraction obscured the addition reaction. It was noted, however, that variation in the mixture ratio by a factor of 4 a t constant ketone concentration had no effect on the rate constant ratio (see eq A below). We are therefore obliged to ignore such reactions as CF3CHzCHz
6.33 6.58 6.53 6.16 6.31 4.10 5.53 9.52 7.43 3.90 6.98 3.26 5.74 9.52 9.64 7.88 7.55 7.63 8.73 8.08 8.55 10.87
+ S H '7,CFaCHzCHs + S
18,CFSCHzCH2SH CFaC4Ha+ SH -% CFaCdH9 + S -% CFaC4HsSH and our mass balance method (discussed later) is subject to this uncertainty, although of course the occurrence of reactions 17 and 19 will have no effect on the final rate ratio. We consider that the possible radical loss by reactions 18 and 20 is not serious because, as we have noted, radical-radical reactions do not occur The Journal of Physical Chemistry
2.90 2.95 3.46 3.31 3.21 2.42 3.13 2.97 4.40 2.34 1.31 0.55 2.30 3.45 2.73 2.62 2.65 2.13 2.84 2.81 2.18 2.15
0.172 0.327 0.408 0.364 0.343 0.262 0.358 0.329 0.557 0.390 0.227 0.130 0.200 0.386 0.124 0.105 0.153 0.370 0,100 0.076 0.109 0.163
CFaH ZCFaCaH4
CFaCzHa CFaC4Hp
2.06 2.01 1.69 1.68 1.78 1.53 1.59 2.89 1.50 1.42 4.54 4.82 2.30 2.48 3.38 2.89 2.69 3.05 2.97 2.81 3.73 4.71
0.06 0.11 0.12 0.11 0.11 0.11 0.11 0.11 0.11 0.17 0.14 0.22 0.09 0.11 0.05 0.04 0.06 0.18 0.04 0.03 0.05 0.08
+
= C F ~ C Z H ~CFsC4H9.
significantly and also because the variation in reaction mixture composition (although small) showed no indication of such an effect. I n this discussion it has been assumed that the CF3CH2CH2 radical is stable and does not decompose reversibly, i.e. CF3CH2CH2 -5, CF3
+ C2I&
Szwarc6 has reported that for ethylene (but not for several other olefins), addition is not reversible. This conclusion was based upon the constancy of the CFaH/ N2 product ratio when the concentration of the radical source was altered at constant ethylene/hydrocarbon ratios. This is in accord with our observation that, a t 81", variation of the ketone concentration by a factor of 2 for a constant H2S-CzH4 mixture has no marked effect on the rate ratio. If reactions 10 and 15 represent the fate of the trifluoropropyl radicals formed by ( 5 ) , and (16) that of the trifluoropentyl radicals formed by (lo), then the following mass balance relationship holds, where R x refers to the rate of formation of X
R C F ~ C H= ~ CkS(CF3)(CA) H~
= RCFaCzHs
RCFaC4Hg
Since RCF~H = k3(CF3)(HJ3), then
(6) H. Komazawa, A. P. Stefani, and M. Szwarc, J. Amer. Chem. Soc., 85, 2043 (1963).
CFa ADDITIONTO ETHYLENE Some of the trifluoropentyl radicals formed by reaction 10 may react by further addition to ethylene to produce trifluoroheptyl radicals (21j , and neglect of this reaction could disturb the mass balance relation for trifluoropropyl radicals and affect the accuracy of k5. CFaC4Hs
+ CzH4 -% CF3CsH12
2749 I n the case of 2,3-dimeth~lbutane,~ log Azz = 11.3 (mol-l cm3 sec-l) and E23 = 7.8 kcal mol-'. It might therefore be expected that for CF3 radical attack on 2,3-dimethylbutane, Arrhenius parameters of -lollJ and 4.8 kcal mol-l would be obtained instead of the values r e p ~ r t e d . ~We therefore consider that the parameters reported for reaction 2 are incorrect, although comparison of the predicted and measured rates (at 164", 108*9 and logsa,respectively) suggest that the actual rate constants4 are not grossly in error. Hence Szwarc's data indicate log k5 = 1010.8(mol-' em3 sec-l), which is in good accord with our measured rate constant. Further support for our parameters may be obtained by considering the reported valueslo of 11.2 and 4.7 kcal mol-' for log A24 and E24.
We consider that neglect of heptyl radical formation by (21) could lead to uncertainties of about 0.2-101, and 5-10% in the formation of the CF3C2H4and CIi'GHs radicals, respectively. These errors, particularly for the trifluoroprlopyl radical, are not serious and may reasonably be neglected. It is likely, however, that the error introduced is appreciably less than these figures suggest, if the reasonable assumption is made that as the fluoroalkyl radical gets larger so the influence of the CF3 (CH&CH -% CFaH (CHI)& polar end group is "diluted" and the radical will begin It is likely that the parameters for (2) are very to react with ethylene more like an alkyl radical. If similar to this; substituting these values in Szwarc's this is so, then k21 is likely to be smaller than, say, klo 11.5 (mol-' cma sec-') and E5 rv data we find log A5 or k5. This change will not be reflected in the rate con2.35 kcal mol-', both of which values are very close to stant for the abstraction reaction since there appears to our experimental values. be little difference between the reactivities of methyl and trifluoromethyl radicals with hydrogen ~ u l f i d e . ~ * ~ (d) Reaction of CF&'H2CH2 Radicals. Our data The net effect will be for the ratio k16/k10 < k10/k21 and enable us to draw some conclusions regarding the rate a t which trifluoropropyl radicals react by addition to hence the radical loss by reaction 21 will be less than the above estimates suggest. ethylene. We may derive the mass balance ratio When our experimental data are treated by the (neglecting any heptyl radical formation) method of least mean squares we find, using values5 of 11.65 A 0.16 and 3.88 f 0.26 for log A B(mol-' cm3 sec-*) and E3 (kcal mol-'), that Neglect of trifluoroheptyl radical formation leads to an log k6 (mol-' cm3 see-') = underestimate in the formation of the pentyl radical, (2370 k 490) and so will cause our values of klO/kl5 to be too small by 11.39 0.29 2.303R T 5-10%, Because of the small quantities of trifluoropentane The error limits for reaction 3 are included in ours formed, our results for this ratio are somewhat scattered (which are standard deviations). It should be noted and no accurate value for Elo - E15may be deduced, that these are the error limits imposed on the Arrhenius although it is apparent that E15 > Elo. The rate conparameters by virtue of the least-squares treatment. stant ratio may be examined more profitably and we The absolute errors due to the neglect of reactions such find, at 164", that klo/kl~ 0.9. If we make the as (18), (20), and (21) have been neglected and may assumption [compare reaction 31 that k15 a t this temwell cause the error limits to be somewhat larger. (mol-' cm3 sec-l), At 164" (where 2.303RT is 2000), log kg (mol-' cma perature is lo9.', then klo although this value is speculative and subject to consec-l) is 10.2, so that the addition reaction clearly is siderable error. very rapid. (e) Comparison of Radical Reactivity. I n Table I1 Szwarc2 has obtained Arrhenius parameters for we show data for the Arrhenius parameters and the reaction 5 relative to those for reaction 2. Using the reported values4 for reaction 2 (log A2 = 10.17 and E2 velocity constants at 164" for the addition of various radicals and atoms to ethylene. It is apparent that = 1.7) he finds log As(rno1-I cm3 sec-') = 10.5 and trifluoromethyl radicals are by far the most reactive E5 = -0.6 kcal mol-'. species, reacting some 300 times faster than do methyl It has been observeda that, for many hydrocarbons, an activation energy difference of about 3 kcal mol-' is (7) N. Imai and 0. Toyama, Bull. Chem. Soc. Jap., 33, G52 (1960). obtained for reactions 22 and 23, little difference being (8) G.0.Pritchard, G. H. Miller, and J. K. Foote, Can. J . Chem., noted between the A factors. 40, 1830 (1962).
+
+
-
*
-
-
CHa+RH-%CH4+R CF3
+ RH -% CFaH + R
(9) A. F.Trotman-Dickensen, J. R. Birchard, and E. W. R. Steacie, J. Chem. Phys., 19, 163 (1951). (10) P. B. Ayscough and E. W. R. Steacie, Cam. J . Chem., 34, 103 (1966). Volume 79,Number 8 August 1969
2750
TAKESHI SAWAX AND WILL~AM H. HAMILL
Table 11: Arrhenius Parameters and Velocity Constants at 164’ for the Addition of Various Radicals to Ethylenea
R
+ CHz = CHz-
R
Log A
E
CFa CClS CHs CaH7 CFaCH2CH2 H
11.39 9.5 11.1 10.4
2.4 3.2 6.8 5.1
0
...
13.4
...
RCHzCHz Log IC (164O)
Relative reactivity
10.2 320 7.9 1.6 7.7 1 7.9 1.6 ... 9.6 50 3.3 11.7 104 . . . 10.7-11.8 io*-104
Ref This work
11 b b This work C
d
a A and k in mol-’ cm*see-1; E in kcal mol-*. L. Endrenyi and D. J. Le Roy, J. Phys. Chem., 71, 1334 (1967). J. H. Knox and D. G. Dalgleish, Int. J . Chem. Kinetics, in press. F. Kaufman, Progr. Reaction Kinetics, 1, 1 (1961).
radicals. The reason for this difference lies principally in the activation energy difference of 4.4kcal mol-l for
the two radicals, since the preexponential factors are very similar. Trichloromethyl radicals, although also requiring a low activation energy,l’ are much less reactive than trifluoromethyl by virtue of the very low preexponential factor, thus reflecting the steric limitations for such a bulky radical. Table I1 also shows the reaction of oxygen and hydrogen atoms with ethylene; these reactions are faster than ( 5 ) but only by about an order of magnitude. Our rate constant for the trifluoropropyl radical is of interest, since this radical also reacts readily by addition, much faster than does the propyl radical. This suggests that the effect of the polar end group is transmitted through the radical and enhances its reactivity . (11) J. M.Tedder and J. C. Walton, Trans. Faraday SOC.,62, 1859 (1966).
Electron Scavenging in Methanol-Water at 77°K by Takeshi Sawail and William H. Hamill Department of Chemistry and the Radiation Laboratory,%University of Notre Dame, Notre Dame, Indiana ,4666666 (Received January SO, 1969)
Competitive electron scavenging in CHsOH-H20 matrices, 7-irradiated at 77”K, has been examined for CaHo, CeHbOH, CeH&HzOCOCHa, (CeH&, Cd2+or Ag+ with H+ or NOS-. The mobile electron, em+,precursor of the solvated trapped electron, ea-, reacts efficiently.with aromatic compounds in polar as well as nonpolar matrices while e,- is much less reactive with these compounds. Cd2+, A@;+,and NOa- react efficiently with both ea- and e,,,-, while H+ is unreactive toward ern-, Similar results are expected in polar liquids, including water.
the effects of electron scavengers to reduce the 100-eV yields G(e,-) by trapping em-, The effects tend to be similar in organic solids8s4and in aqueous s01ids.~ Also, the relative reactivities of ClCHzC00-, NOS-, CH2CHCONH2and several other solutes are about the same in ice and in water. According to Kevan, “one may properly describe e,- as a mobile solvated electron in ice.”6 He assumed that the electron reacts with scavenger while tunneling between adjacent tetrahedral trapping sites. Because em- is, in this sense, solvated the similarity of reactivities for em- and eaq- can be accounted for.S The Jowrnal of Physical Chemistry
(1) On leave of absence from the Tokyo Metropolitan Isotope Research Center. (2) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This is AEC document number COO-38-660. (3) W. H. Hamill, “Ionic Processes in 7-Irradiated Organic Solids a t -196O,” in “Radical Ions,” E. T. Kaiser and L. Kevan, Ed., John Wiley & Sons, Inc., New York, N. Y.,1968,Chapter 9. (4) J. E. Willard, “Radiation Chemistry of Organic Solids,” in “Fundamentals of Radiation Chemistry,” P. Ausloos, Ed., John Wiley & Sons, Inc., New York, N. Y.,Chapter 9. (6) L. Kevan, “Radiation Chemistry of Frozen Aqueous Solutions,” in “Radiation Chemistry of Aqueous Systems,” G. Stein, Ed., John Wiley & Sons, Inc., New York, N. Y., 1968. (6) L. Kevan, J . Amer. Chem. Soc., 89, 4238 (1967).
