2944
G. 0. PRITCHARD AND M. J. PERONA
The Decomposition of Vibrationally Excited
1,1,2,2-Tetrafluoro-l-chloroethane~ by G. 0.Pritchard and M. J. Perona Department of Chemistry, University of California, Santa Barbara, California 93106
(Received February 6 , 1969)
The decomposition of the ground-electronic state vibrationally excited molecule CF,CICF,H, formed in the photolysis of CFzClCOCFzH in the 3130-A region and at room temperature, is found to produce C2F4by HCl elimination. The production of CzF&l by HF elimination is very minor. CzF4 is also produced in another unspecified reaction, and is removed from the system, presumably by radical and/or atom attack. The inherent experimental complications in this and related systems are discussed. The disproportionation/combination ratio for CF2Cl radicals is found to be 0.17. Introduction
ment with a similar calculation6a yielding a value of El = 51 kcal mol-’. Further substantiation for the There has been much recent interest shown in the delow value of El is given by Trotman-Dickenson who hydrohalogenation of “hot” haloethanes formed by the observes that the eliminationsc of HF from CH3CF2C1* chemical activation technique.2 Setser and his co(formed via CH3 CF2C1)is four times faster than for workers have investigated, for the most part, “hot” HC112 and about 30 times faster for CH2FCHzC1*. chloroethanes formed in systems involving triplet This “hot” molecule was formed in the system Fz methylene and the appropriate ~hloromethane.~We CHsCH2C1.6. CH&HFCl* was formed preferentially have examined, in the main, “hot” fluoroethanes formed in the same system; however, the loss of HCl from it by methyl- and fluoromethyl-radical c~mbination.~ was found to be nine times greater than for HF, and the Some parallel studies have been carried out by Trotqualitative data of Tomkinson and Pritchard9on CFC12man-Dickenson and his coworkersnb The investigaCHa* and CF2ClCHa* indicate preferential HC1 elimtion of the fluoroethanes has been important in that no of CH2BrCH2C1* ination. Data on the decomposition quantitative kinetic data are available on the pyrolysis shows3@ that HBr is eliminated more readily than HCl, of alkyl fluoridess and in fact, so far, they have defied investigation.’ The first well-authenticated observation of the dehydrofluorination of a “hot” fluoroethane (1) This work was supported by the National Science Foundation, Grant No. GP 8069. molecule was made by us,4at6,8,9 and a Rice-Ramsperger(2) B. S.Rabinovitch and M. C. Flowers, Quart. Rev. (London), 18, Kassel (RRK) treatment of this molecule CFH2CFH2* 122 (1964). (asterisk denotes vibrational excitation) was made by (3) E.g., (a) J. C. Hassler, D. W. Setser, and R. L. Johnson, J . Chem. Phys., 45, 3231 (1966); (b) J. C. Hassler and D. W. Setser, Benson and Haugen16resulting in a reasonably precise ibid., 45, 3237 (1966); (c) R. L. Johnson and D. W. Setser, J. Phya. evaluation of the activation energy of the normal pyChem., 71, 4366 (1967). HF, of rolysis reaction, CFHzCFHz .-* CFH=CH2 (4) E.g., (a) G. 0. Pritchard, M. Venugopalan, and T. F. Graham, J . Phya. Chem., 68, 1786 (1964); (b) G. 0. Pritchard and R . L. 62 kcal mol-’. Thommarson, ibid., 71, 1674 (1967); (c) J. T. Bryant and G. 0. Maccoll has extensively d i s c u ~ s e d ~the , ’ ~pyrolysis of Pritchard, &id., 71, 3439 (1967); (d) G. 0. Pritchard and J. T. Bryant, ibid., 72, 1603 (1968). alkyl halides and it is well established that the rate of (5) E.g., (a) J. A. Kerr, A. W. Kirk, B. V. O’Grady, D. C . Phillips, dehydrohalogenation is in the order of HI > HBr > and A. F. Trotman-Dickenson, Discussions Faraday Soc., 44, 263 HCl for the ethyl, i-propyl, and t-butyl halides. Ex(1967); (b) D. C. Phillips and A. F. Trotman-Diekenson, J . Chem. SOC.,A , 1144 (1968); (c) ibid., 1667 (1968); (d) J. A. Kerr, D. C. perimental activation energies (log A values are 13.5 in Phillips, and A. F. Trotman-Dickenson, ibid., 1806 (1968). each case) are 50.8, 53.3, and 57.9 kcal mol-‘ for ethyl (6) S. W. Benson and G. Haugen, J . Phys. Chem., 69,3898 (1965). iodide, bromide, and chloride, respectively. Based on (7) A. Maccoll, Discussions Faraday SOC.,44, 288 (1967), a comment on ref Sa. his representation of the transition state as an intimate (8) R. D. Giles and E. Whittle, Trans. Faraday SOC.,61, 1425 ion pair1’ M a c c 0 1 P ~has ~ ~predicted the above activation (1965). energies and calculates that the process (9) D. M. Tomkinson and H. 0. Pritchard, J . Phys. Chem., 70,
+
+
+
CZHrjF +C2H4
+ HF
(1)
will have an activation energy, El = 63.5 kcal mol-’. This is in reasonable agreement with the value of E1 = 59 kcal mol-’ obtained by us from a RRK treatment4b of the “hot” molecule CzHsF*, but in severe disagreeThe Journal of Physical Chemistry
1579 (1966). (10) A. Maccoll, Adcan. Phys. Org. Chem., 3, 91 (1965); J. Ridd, Ed., “Studies of Chemical Structure and Reactivity,” Methuen and Go., London, 1966, pp 53-72; Chem. Rev., 69,33 (1969). (11) A semi-ion pair has also been suggested, S. W. Benson and A. N. Bose, J . Chem. Phys., 39, 3463 (1963). (12) Admittedly a weighting factor of two should be taken into account.
THEDECOMPOSITION O F 1,1,2,2-TETRAFLUORO-1-CHLOROETHANE as would be expected. Similarly, Quick and Whittle13 observe HBr elimination in the photosensitized decomposition of CH8CFzBr*. I n view of the contradictory nature of the datal4 we have made a careful examination of the competitive dehydrohalogenation of CF2HCFZCl.* Experimental Section Most of the reactions were carried out a t low pressures to promote elimination reactions. The mercuryfree apparatus and procedure has been described elsewhere.4d The ketone CF2HCOCFzCl was supplied chromatographically pure by the Hynes Chemical Research Corp., and it was used as a photolytic source of CFzH and CF&l radicals. The mass spectrum of the ketone is given in Table I. The predominant formaTable I : Mass Spectrum of CF2C1COCF2H mle
12 13 28 29 31 32 35 36 37 50 51 60 66 67 68 69 78 79 85 87 100
117 119 129
Probable positive ion
C CH
co
CHO CF CFH c1 HCl cia7 CFz CFzH CiFHO CFCl CFHCl CFCP CFHCla7 CFzCO CzFzHO CFzCl CFzC18' CzF, CzF8HCl C2FsHC13' CsF4HO
Relative abundanoe
18.8 17.3 23.0 15.8 141 69.2 22.0 2.89 5.76 93.7 1000
26.0 2A.5 101 7.20 39.0 44.7 312 190 59.1 98.0 13.0 2.88 101
tion of the alkyl-ion fragments, CFzH+ (parent) and CF2C1+, and the large abundance of only the CFzHCO+ acetyl-ion fragment, is similar to the cracking pattern of pentafluoroacetone and other unsymmetrical fluoro I~etones.'~These mass spectra are of interest as it is possible to correlate them with the primary act in the respective ketoae photodecompositions at 3130 .k.4b Product analysis was accomplished by vpc using 30/, squalane on alumina columns. (Varying column lengths and conditions were used dependent upon the separations desired.) A number of early experiments were carried out on a conventional apparatus4& to establish reaction products. At about 130" and 10-cm ketone pressure, C2F4,CF,Cl,, CF2C1CF2C1,CF2HCF2H,
2945
CF2HCFzCl,CFzH2,and CF2HCl were obtained after photolysis. All of these products were trapped separately, and identified by their mass-spectral cracking patterns. Retention times were checked and calibrations made, by injection of known samples, where this was possible.16 Some experiments were carried out a t lower pressures4d in the range 0.2 to 2 mm, at about 120" (see Table 11)and 200". The above-reported products were obtained, plus C2F8H and traces of C2F3C1. A number of runs carried out at room temperature are reported in Table 11. This temperature was chosen to promote radical-radical interactions by reducing the rates of any reactions with positive temperature coefficients. The main purpose of this work was to observe the comparative rates of dehydrohalogenation of CF2HCFzC1* under the least complex conditions; to further this end most of the experiments were carried out in the presence of an inert gas addend, 2-perfluorobutene. At room temperature only trace amounts of CzFaCland C2F3Hwere obtained, and CF2HC1was absent. CF2H2was not measured in runs with added C4Fs as the retention times were similar, and CzF4Hz was not measured in all experiments due to its long retention time. We were never able to isolate sufficient of the C2F3C1 product for mass spectral confirmation; the identity of the trace peak is solely based upon the comparison of its retention time with an authentic sample. When C2F3C1 was added to the ketone in the reaction cell before photolysis, it was clearly evident on the product chromatograms. Discussion We will assume that the primary photolytic process a t room temperature is" CFlClCOCFzH ---f CO
+ CFzCl + CFzH
(2)
(13) L. M. Quick and E. Whittle, Can. J . Chem.. 45, 1902 (1967). These authors used hexafluoroacetone at 3130 A as a sensitizer. The Hg(3P1)-photosensitized decomposition of CZHSF[P. M. Scott and K. R. Jennings, Chem. Commun., 700 (1967)l leads to CzH4F H with no HF-elimination process. Similarly, we have observed no HF-elimination products in mixtures of benzene and CFsCHs (or CFzHCHs) irradiated at 2537 A. (14) From a RRK treatment of CHsCFzCI* Trotman-DiokensonlC calculates activation energies of CHaCFsCl + CHs=CFz HCI and CHz=CFCl H F of 68 and 56-57 kcal mol-', respectively. However, an investigationhd of the "hot" molecule CHaCFZH* leads to E = 47-48 kcal mol-' for the process CHsCFzH * CHa=CFH HF. These results are the reverse to what would be expected from a-halogen substitution.l o (16) J. B. Hynes, R. C. Price, W. 9. Brey, Jr., M. J. Perona, and G. 0. Pritchard, Can. J . Chem., 45, 2278 (1967). (16) Samples of CFzClCFaCl and CFzHCFzCl were not available; CFsCFClz was used for calibration purposes for the former, and the sensitivity of the latter was estimated by comparison with the other haloparaffins. (17) Acetyl radicals probably play a minor role in this system, particularly at low pressures. le,, > 0.85 in the photolysis of CFzClCOCF2C1, 10 cm at Z O O 1 * and Qeo Y 1.0 for CFzHCOCFzH photolysis, 0.7-10 cm at room temperature.'o (18) R . Bowles, J. R. Majer, and J. C. Robb, Trans. Faraday Soc., 58, 1541 (1962). (19) G. 0. Pritchard and J. T. Bryant, J . Phys. Chem., 70, 1441 (1966).
+
+
+
+
Volume 73, Number 9 September 1969
2946
G. 0. PRITCHARD AND M. J. PERONA
Table 11: CFzClCOCF2HPhotolysis Data Products, mol X 108
Q
CZF4
CFzClz
CzFiCIz
CzFiHCl
15.5 8.30 16.6 30.2 8.85 92.6 8.73 96.0 9.07 8.08 7.70 9.20 205 4.92 7.08 6.52 9.76 2.85 6.83 14.9 8.34 16.2
7.04 3.70 6.66 10.5 4.15 27.9 5.08 27.3 5.34 5.49 4.81 5.74 140 4.75 7.70 6.05 10.2 3.89 10.4 8.75 4.48 9.64
49.2 25.4 52.4 102 26.6 244 24.5 367 27.2 24.8 21.8 26.6 920 23.4 43.4 36.0 67.1 9.24 34.6 49.9 24.8 55.2
70.7 38.3 77.0 155 40.2 518 26.2 520 19.0 30.7 27.5 33.2 1228 37.6 70.6 54.0 103 12.8 48.7 79.5 37.4 84.5
CzFiHz
-
CFzHz
213 190
P(ket),= mm
P(CiF8),a
0.157 0.157 0.157 0.310 0.157 1.05 0.071 1.2 0 080 0.072 0.068 0 072 4.7 0.156 0.156 0.156 0.156 0.156 0.156 0.165 0.165 0.165
0.155 0.155 0.155 0.310 0.155 1.05 0.058 1.2 0.082 0.072 0.070 0.070 4.7
I
13.1 679 12.3 25.6 19.1 34.6 14.1 30.2
I
3.33 6.96 5.28 11.1 5.92
mm
0.042 0.042 0.042
Photol. time, rnin
Temp,
OC
20 10 20 20 10 20 20 20 20 20 20 20 20 10 21 15 30 5 20 20 10
23 23 23 23 23 23 23 23 23 23 23 23 22 24 24 22 22 123 123 22 22 22
20
Reactant pressures were measured on a thermocouple gauge calibrated for both compounds.
The subsequent radical-radical reactions of interest are
+ CF2C1+
CF2H
% -,
CF2HCF&l*
(3)
CFzHCFzCl (a)
e
CFi=CF2
CF2HCF2C1*
CFz=CFCl
+ HC1 (b)
+ H F (c)
(4)
and the indistinguishable co-disproportionation CFzH
+ CFzCl +CFzHCl + CFz
(5)
We also have CF2H
+ CF2H M
CFzHCFzH*
(’--+
---f
CFzHCFzH*
d33”H ( 4 cF
CFFCFH
CFzH
+ CFzH
CF&l
+ CFzCl+
4
CFzHz
+ H F (b)
+ CF2
CF&lCF&l*
CFzClCFZCl* -% CFzClCFzCl CFzCl
The mechanism predicts that
+ C F z C 1 4 CFzClz + CFz
(6)
(7 1 (8) (9) (10) (11)
where M is any quenching molecule. Reactions 6 ~ 8 ~salient ~ ~ feathrough 11 are well e s t a b l i ~ h e d . ~ d *The ture of this work is that C2F4was found under all conditions, while CzFaClwas only a trace product under the most favorable low-pressure conditions. This verifies that klb > I’h0in accord with Maccoll’s observations that HC1 elimination is more facile than H F elirnination.‘*’O The Journal of Physical Chemietry
where R is rate of formation and P is the total pressure of quencher M in mm. An examination of the data in Table I1 shows that for a constant P,20 the ratio R c ~ F ~ / RC~F~ECI decreases with time. This time dependence was only removed by the addition of 50 mole % of C4Fs (see the first three experiments). Giles and Whittles originally commented on the possibility of olefin loss by radical addition in these systems, but time-dependence studies on the rate ratios are generally not reported. This could be particularly important in methylene sysolefin is very rapid. tems, where the reaction CH2 Setser21 has reported an investigation of ‘ ‘ h ~ t ”chloroethanes formed in the Hg(3P1)-photosensitized decomposition of CH2C12. Loss of olefin was observed unless the C1 atoms present in the system were scavenged with propene.21 There is a possibility that C1 atoms are present in our system, due t o a primary photolytic decomposition mode of the ketone into CFzHCOCFz C1, and that the C1 atoms are partially responsible for the loss of CzF4 unless they are consumed by the C4FS.22 How-
+
+
(20) With added CkF8, P is taken as the total pressure of it and the ketone. Both are polyatomic molecules which remove large increments of excess vibrational energy on collision, acting with virtually unit deactivation efficiency. (21) D.W. Setser, J. Amer. Chem. Soc., 90,582 (1968).
THEDECOMPOSITION O F 1,1,2,2-TETRAFLUORO-1-CHLOROETHANE
2947
ever, Cl-atom production was found to be unimportant in the photodecomposition of CFzClCOCFzCl in the 3130-w regionr8 (the exciting wavelength used by us from a high-pressure Hg arc), and this was assumed to 0.2 be the case by T r o t m a n - D i c k e n ~ o n . ~Nevertheless, ~*~~ it would seem that the possible presence of C1 atoms, which will rapidly remove olefin, should be taken into 0.1 account.24 It is evident that these systems require careful calibration, before valid kinetic data can be ob1 .o 3.0 5.0 7.0 tained from them. 1 mm-l A plot of R c ~ B ~ / R c ~us. F ~1/P, H c ~to establish the validity of eq 12, is given in Figure 1. The curve is given Figure 1. Plot of the product ratio RO~F~/RC~F,HOI us. reciprocal for the data in which 50 mol % of C4F8was added, and pressure for 50 mol yo ketone and C4F8mixtures. The slope it is linear. However, the nonzero intercept indicates of the line gives k4b/kca.= 1.4 x l o b a mm at room temperature. an additional source of CzF4 (other than reaction 4b) in the high-pressure limit when all the “hot” molecules in peratures. The absence of CFzHCl at room temperareaction sequence 4 are quenched (reaction 4a). Adture rules out C1 abstraction by CFzH and H abstracditional olefin formation has similarly been observed by tion by CFGl from the ketone (the product was obSetser in one of his “hot” chloroethane sy~tems.~bAlso served at higher temperatures). Also either disprothe data in Table I1 (e.g., at room temperature, -0.16The interportionation path, reaction 5 , is ruled mm ketone pressure and 20-min duration) show inaction of CFzCl with ketone creasing Rc~F~/RC~F~HCI ratios with increasing (0, 20, and up to 50 mol %) CzF8pressure. This apparent conCF&1 CFzClCOCFzH 4 tradiction may be rationalized on the basis that the CFzCl, CFzCOCFzH (14) C4F8serves t o protect the CZF4 formed (both in reaction to give CF2C12may well have a lower activation energy 4b and the pressure-independent source) from radical or (3-5 kcal/mol-1),18 but if we assume its unimportance atom attack. A possible source of CzF4 is the recoma t room temperature and very low pressures, the data bination of CF2 formed in reactions 8 and 11. HowC ~beF ~used C J Ito~ give a in Table I1 on R C F ~ C ~ ~ / Rcan ever, there is good evidence that CF2 is rapidly removed of k l l / l ~ 9 . ~ 8 For the 20 experiments a t room temvalue by mono-radicals in similar systems to form haloproperature the ratio is 0.17 (standard deviation 0.038). panes, and CzF4 was not observed as a product in them.16~18J6 The latter authorsz5sensitized the decomThe ratios at 123” are 0.42 and 0.30 indicating the onset of reaction 14. The present value of 0.17 is intermeposition of CFzRCl with Hg (3P1)to generate CFzH and diate between the previously quoted values of 0.0418and CF2Cl radicals; the pressures employed, however, were < 0.5.26 too high to observe any dehydrohalogenation reactions following radical combination. Values of the dispro(22) The attack on any CsFaCl formed is presumably equally as portionation/combination ratios were found25 to be rapid, but no significant increase in this trace product was observed kdka = R c F ~ H ~ / R c=~ F 0.19 ~ Hand ~ W k g = R C F ~with added C4F8. R C ~ F5~ 0.5; C ~ ~k-6
