THE REACTION OF TRIFLUOROMETHYL RADICALS WITH HYDROGEN SULFIDE 4239

48.6 eu is consistent with the values for Mn304 (35.5 eu), Fe304 (35.0 eu), and Pb304 (50.5 in light of the mass diff erencee of the metals.

Acknowledgment. The support of the U. S. Atomic Energy Commission (COO-716-037) is gratefully ac- knowledged.

The Kinetilcs of the Reaction of Trifluoromethyl

Radicals with Hydrogen Sulfide1

by Jayavant D. Kale and Richard B. Timmons Department of Chemistry, The Catholic University of America, Washington, D. C. 80017 (Received June 9, 1068)

The gas-phase reactions of trifluoromethyl radicals with hydrogen sulfide have been studied over the tempera- ture range of 95-161". The trifluoromethyl radicals were generated by photolysis of hexafluoroacetone. The rate constant for the reaction CF3 + HzS --f CF3H + SH is found to be k = 101l*z*O.l exp(-1200 It 100)/RT cc mol-' sec-'. This value is calculated using the reported value for trifluoromethyl radical recom- bination of 2.3 x 10la cc mol-' sec-l.

Introduction The photocliemistry of hexafluoroacetone (HFA)

has been the subject of many papers since the original work of Ayscough and Steacie.2 Recently, much emphasis has appeared on fluorescent measurements in HFA photochemistry. From these studies a rather detailed picture of the photochemistry of this molecule is emerging. 3 , 4 In addition, the importance of chemical quenching of excited HFA molecules has been raised.

The photolysis of HFA has been used extensively as a source of CFa radicals for kinetic studies. One of the recent papers on CF3 reactions is the work of Arthur and who studied the kinetics of abstraction of hydrogen atoms from HzS by CF3 radicals. At the time this paper appeared we were in the process of investigating this reaction over somewhat different reaction conditions and under conditions such that any chemical quenching of HFA by H2S would be mini- mized. The kinetic parameters determined in our studies differ from those reported in ref 5, and as such, our results are of interest.

Experimental Section The HFA (Allied Chemical Corp). was

degassed initially a t -196" and was then allowed to distil to a trap a t -145" to --150" (2-methylbutane slush) where further degassing took place. It was found necessary to carry out degassing at a temperature of --150° in order to remove sizable C2Fs and CF3H impurities in the HFA. The HFA was then distilled from a trap a t -80" to a storage bulb with only the middle fraction being retained for reaction.

Materials.

Hydrogen sulfide, research grade (Matheson), was degassed a t -196" and then distilled from a trap at -80" to the storage bulb. Mass spectrometric analysis failed to reveal any impurities.

Reactions were carried out in a 200-cc cylindrical quartz reaction vessel. The total reaction system actually comprised a volume of 5.1 1. and in- cluded an all-glass circulation pump.6 The gases circu- lated through the quartz reaction vessel which was suspended in a tubular furnace. Photolysis was carried out with a Hanovia medium-pressure mercury ltmp. The effective wavelengths were limited to X >3100 A by insertion of a Pyrex filter between the lamp and the reaction cell. With this filter arrangement, no H2 was produced by circulating 20 mm of HzS through the cell for 30 min with the lamp turned on.

Reactions were carried out over a temperature range of 95-161". After a run, the products noncondensable a t -196" were collected and measured in a combina- tion Toepler pump-gas buret. The per cent conversion of the H2S vas never allowed to exceed 5% in any given run. The temperature of the traps containing the

Apparatus.

(1) Abstracted, in part, from the Ph.D. Thesis of J. D. Kale, The Catholic University of America, Washington, D . C. 20017. (2) P. B. Ayscough and E. W. R. Steacie, Proc. Roy. SOC., A234, 476 (1956). (3) R. K. Boyd, G. B. Carter, and K. 0. Kutschke, Can. J . Chem., 46, 175 (1968). (4) J. S. E. McIntosh and G. B. Porter, Trans. Faraday SOC., 64, 119 (1968). (5) N. L. Arthur and T. N. Bell, Can. J . Chem., 44, 1446 (1966). (6) J. 8. Watson, Can. J . Technol., 34, 373 (1956).

Volume 78, Number 18 November 1968

4240 JAYAVANT D. KALE AND RICHARD B. TIMMONS

~~

Table I : Product Yields in the Photolysis of Hexafluoroacetone with Added Hydrogen Sulfide

1 0 3 / ~ , 108[HFAI, 108IHzS1, 1 0 1 t R ~ ~ s ~ , ~ O ~ ~ R C ~ F ~ , OK-1 rnol/oc mol/cc mol/oc seo mol/co sec

2.701 2.699 2.699 2,543 2.546 2.538 2.539 2,415 2.413 2.411 2.410 2,304 2.301 2.297 2.296

1.74 1.79 1.74 1.72 1.69 1.72 1.69 1.64 1.64 1.67 1.67 1.60 1.60 1.60 1.60

1.81 2.90 3.00 2.43 2.38 4.28 4.15 3.02 3.35 2.07 2.05 1.48 2.16 1.40 2.36

6.25 6.67 6.67 6 .83 6.91 7.71 7.58 7.33 7.75 7.16 7.25 8.16 8.16 8 .08 8.42

25.16 10.83 9.58

14.20 15.83 5 .74 6.25 8 .50 8 .00

17.50 15.83 40.83 18.33 44.16 16.66

10'2RC0, mol/oo sec

4.41 4.41 4 . 4 3 4 .58 4 .42 4.49 4.54 4.58 4.54 4 .62 4.58 4.74 4.83 4.83 4.91

log (k3/k2' /2)

3.838 3.844 3.856 3.872 3.863 3.886 3.864 3.920 3.913 3.925 3.950 3.940 3.945 3.938 3.941

condensable products were then raised to --150", and a second fraction was collected in the gas buret. This fraction consisted mainly of CzFs and CF3H with small amounts of HZS. The absolute amounts of CF3H and CzFe were determined by gas chromatographic analysis using a 2-m silica gel column and a flame ionization detector. The chromatography column was 6 mm 0.d. aluminum tubing and was packed with 120-200-mesh silica gel (Fisher Scientific Co.). The separation was carried out at a column temperature of 100".

In some runs, gas chromatographic analysis was carried out on the reaction products in a search for CH3CHO as a reaction product. Both the fractions collected at - 146' and a second fraction taken a t -80" were analyzed for CF3CHO. In these experiments 20% Carbowax on a Chromosorb W column was used, the column being 2.2 m long and 6.0 mm 0.d. aluminum tubing. The retention time of pure CFsCHO (a sample of CF3CHO-HzO obtained from the K & K Laboratories, Inc. was dehydrated over P205) was determined on this column. In none of our experi- ments were we able to detect CF3CHO as a reaction product. From our calibration curves we estimate that as little as 0.2% CF3CH0 compared with the CF3H would have been detectable.

Results and Discussion Typical reaction pressures in our experimental work

were HFA pressures of 30-60 mm and HZS pressures of the order of 0.1-0.3 mm. The experimental results showing the rate of production of GO, CFIH, and C2F6 for various reactant pressures and reaction temperatures are shown in Table I. The reaction mechanism sug- gested b y Arthur and Bell5 is as follows

CF3COCFg + h~ + 2CF3 + CO (1)

2CF3 + CzFa (2)

CF3 + H2S CFDH + HS (3)

SH + SH + Hz + Sz SH + SH--tH2S + S

( 4 4

(4b) This mechanism predicts a mass balance of CO =

'/z CF3H + CzFtj. It is obvious from Table I that this mass balance is not obtained in our experimental work, there being a CF3 deficiency. An 18% CF3 deficiency was observed in the work of Arthur and Be1L6 How- ever, it is to be noted that this CF3 deficiency has been observed in all other systems, including the photolysis of HFA in the absence of added reactants.? With re- spect to the CF3 deficiency, it should be pointed out that sulfur is produced in our reaction system, and, under the circumstances, some CF3 might irreversibly be removed by reaction with sulfur.

Presumably, some of the CF3 deficiency in our work could arise from a reaction such as

CFI + SH 4 CF3SH

The importance of CF3SH formation would be a com- plex function of HzS and HFA pressure if one includes reaction 6 together with reactions 1-4. The analysis for CFBSH in a reaction mixture containing large quan- tities of HFA and HzS is difficult and our attempts to analyze for CF3SH were inconclusive. It is to be noted that the occurrence of reaction 5 would not change the value of k3/k2'la obtained in our work.

A more serious complication would arise from the dis- proportionation reaction

(5 )

CF3 + SH + CF3H + 8 (6)

Formation of CF3H via this reaction would alter the ratio CFaH : CzFs and thus introduce an uncertainty into our reported value of k3/k2'", I n order to check on the possible importance of reaction 6, runs were carried out a t several temperatures with varying pressures of

(7) A. Gordon, J . Chem. Phys., 36, 1330 (1962).

The Journal of Physical Chemistry

THE REACTION OF TRIFLUOROMETHYL RADICALS WITH HYDROGEN SULFIDE 4241

H2S but constsnt HFA pressure. Application of the usual steady-state treatment to reactions 1-6 leads to hopelessly complex functions for the CF3 and SH radi- cals. Nevertheless, if reaction 6 is of importance, one anticipates a complex functional dependence on H2S pressure as well as on absorbed light intensity for the ratio of lc3/k2'/', Within the limited pressure range of our experiments, this rate-constant ratio was indepen- dent of the ratio HFA:HzS. I n addition, two experi- mental runs were carried out a t reduced light intensity. Insertion of wire filters decreased the absorbed light intensity by a factor of 10 as determined from the CO yield. No change in the ratio of k3/kz1l2 was observed under these conditions. In view of the low activation energy for reaction 3, i t seems reasonable to expect the CFa steady-state concentration to be low, and, as such, reaction 6 does not contribute significantly to CFaH production.

If we assume that reactions 2 and 3 are the only source of C2F6 and CFIH, respectively, least-squares treatment of thle data in Table I yields the result

log ("> = 3.896 f 0.013 J c 2 1 / z

at 400°K. Using the reported value of kz = 2.3 X 1013 cc mol-' sec-18 and assuming that . E 2 = 0, we calculate the value k3 = 1011.2+0.1e-1700*100)~R~ cc mol-'sec-' for reaction 3. Our value does not agree with the previous report of Arthur and Bell,5 who obtained

In Figure 1, we have plotted our data along with the data of Arthur and Bell. We attempted to extend our investigation to lower temperatures : however, the data reproducibility a t the lower temperature was very poor, and, as such, these points were not included in our cal- culations of the activation energy of reaction 3. The reason for the difference in activation energy of reac- tion 3 measured in our work compared with that of Arthur and Bell is difficult to determine. Experi- mental procedures in the two cases were comparable with the main exception that our work involved the use of an all-glass circulation pump, whereas their work was carried out in a static system. The two sets of experi- mental results differ in another important aspect con- cerning H, production. Under no reaction condition do we observe more than trace quantities of Hz forma- tion (always less than 0.3% of the CO production). In the work of Arthur and Bell, up to 7.6% Hz was reported. Hydrogen production is considered to arise from reaction 4a, which is in competition with reaction 4b. The ratio originally proposed, k4s/k4b = -O.lZ,g has been modified recently to say that k d a << Our results are in agreement with this latter conclusion. We carried out some reactions in a static system, but hydrogen prodluction was not observed under these con- ditions.

In view of the recent reports of reactions of electroni-

the value ks = 1011.65f0.16e(--3880+280)/RT cc mol-' seC-1.

3.6

' 3.2 Y"

0 0 -t

2.2 2.4 2.6 2.8 3.0

1 0 ~ 1 ~ (OK- '

Figure 1. work; 0, Arthur and

Arrhenius plot of log (k3/kt1/') vs. 1/T: B, this

cally excited HFA molecules with Brz,4 with CH3CFz- Br," and hydrocarbons,lZ we were concerned with the possible importance of reactions of the type

CF3COCF3* + HzS + CFaCHO + CFBH

CFsCOCF3* + H2S + CF3H + CO + CF3SH

CF3COCF3* + HtS --j CFSH + CF3COSH

(7)

(8)

(9)

where CF3COCF3* represents an electronically excited HFA molecule either in the first excited singlet state or in a triplet state. If reaction 7 occurs then CFaCHO would appear as a reaction product. No evidence for CF3CH0 formation was ever observed in our work. The occurrence of reaction 8 predicts a dependence of CO production on HzS pressure for a fixed HFA concen- tration. We have carried out experiments with a fixed HFA pressure of 80 mm and varied the HPS pressure over the pressure interval 0.0-3.8 mm. The yield of CO was found to be constant, within experi- mental error, and thus was independent of HZS pres- sure. The quantum yield for GO production in the photolysis of HFA under our experimental conditions is approximately 0.25.13 Thus, in our experiments, chemical quenching of the HFA* by H2S must compete with physical quenching of HFA* by HFA molecules. Finally, the occurrence of reaction 9 to any extent would be revealed in differences in the k3/kz1/' ratios obtained for different ratios of H2S:HFA a t a fixed temperature. As mentioned previously, this was not observed experimentally. We are led to conclude that

(8) P. B. Ayscough, J . Chem. Phys., 33, 1566 (1955). (9) D. DeB. Darwent and R. Roberts, Proc. Rag. Soc., A216, 344 (1953). (10) B. DeB. Darwent, R. L. Wadlinger, and Sr. -M. J. Allard, J . Phys. Chem., 71, 2346 (1967). (11) L. M. Quick and E. Whittle, Can. J. Chem., 45, 1902 (1967). (12) S. N. Khanna and K. 0. Kutschke, ibid., 44, 1465 (1966). (13) P. G. Bowers and G. B. Porter, J . Phys. Chem., 70, 1622 (1966).

Volume 78, Number 12 November 1968

4242 JAYAVANT D. KALE AND RICHARD B. TIMMONS

under our reaction conditions of low HzS pressure, the chemical quenching of HFA by H2S is not impor- tant. In view of the lack of Hz formation in our reac- tions, i t also means that a reaction such as

"FAo + HzS --.t HFA + H + HS (10) does not occur, since the fate of H atoms produced would be reaction with HZS to form H2. An analogous reaction in the HFA-plus-Br2 photolysis has been shown to occur3 to produce 2Br or Br2*. The bond strength of H-SH is reported to lie between 89 and 93 kcal/mol, the most recent value being D(H-SH) I 90.6 kcal/ m01.l~ As such, the above reaction would be slightly endothermic, which perhaps accounts for the lack of importance of reaction 10.

The lack of CF3CH0 formation even at the lower temperatures is of interest in view of the suggestion that at temperatures below 80" the primary step in HFA photolysis should be written as15

CFaCOCF3 + h~ + CF3CO + CF,

We had anticipated that the CF3CO radicals would abstract an H atom from HzS to form CF3CHO. A comparable reaction has been reported for the reaction of CH&O radicals with H2S.16 The lack of CF3CHO

parison of CF3 and CH8 radical reactions with hydro- carbons has revealed a consistently lower activation energy for CF3 reactions, usually 2-3 kcal/mol, than the corresponding CH, reaction.17 On the other hand, in- version of this order has been reported for reactions of these radicals with the polar compounds HBrlSb and HzS.5 It has been suggested that the polarity of these molecules results in a repulsion between CF3 and the molecule leading to higher Eact for the CF3 reactions relative to those of CH3 reactions. The Eact for CH3 reactions with H2S has been reported to lie between 2.6 and 4.0 kcal/mol.l* Quite clearly, our results are con- trary to this notion of repulsion between CF3 and H2S as leading to a higher activation energy. The general importance of such electron repulsion exerting an in- fluence on activation energies remains an open question.

Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.

Acknowledgment.

(14) T. F. Palmer and F. P. Lossing, J. Amer. Chern. SOC., 84, 4661 (1962). (15) (a) B. G. Tucker and E. Whittle, Proc. Chem. Soc., 93 (1963); (b) B. G. Tucker and E. Whittle, Trans. Faraday SOC., 61, 484 (1965).

formation indicates that if CF,CO is formed in the primary step its lifetime is short with respect to its unimolecular decomposition. This COnCluSiOn agrees with the recent work of McIntosh and Porterq4

our experiments is worth further mention.

(16) ?r. M. J. Allard, Ph.D. Thesis, The Catholic University of America, Washington, D. C., 1965.

Bimolecular Gas Reactions," National Bureau of Standards, Ref. (17) cf, A, F, Trotman-Dickenson and G. 8, Milne, "Tables of

No. NSRDS-NBS 9, U. S. Government Printing Office, Washington, D. C., 1967. (18) (a) N. Iinai and 0. Toyama, Bull. Chen. SOC. Jap., 33, 652 The Of lm2 kcal/mol for reaction determined in

The com- (1960); (b) N. Imai and 0. Toyama, ibid., 33, 1120 (1960).

The Journal of Physical Chemistry