THOMAS SMAILAND F. S. ROWLAND
1866
The Insertion Reactions of Mono- and Difluorocarbene with Hydrogen Halides by Thomas Smail and F. S. Rowland Department of Chemistry, University of California, Irvine, California 08664 (Received November 11, 1060)
Both monofluorocarbene (CHlBF) and difluorocarbene(CF18F)react in direct insertion reactions with hydrogen halides to form CH218FXand CHF18FX, respectively. The fluorocarbenes are produced by secondary decomposition reactions of excited 18F-labeledmolecules formed by hot '8F atom reactions with various precursor molecules, including CF4, CHFI, CHzF2, and C2F4. Competitive studies show rapidly decreasing reactivity of HX toward CF18Fin the order HI > HBr > HCI. The reaction of CH18Fwith HI forms CH218FI* with sufficient excess energy to decompose these molecules to CH218Fand I in gas-phase experiments. The scavenging of CH18F by t w o successive reactions with HI thus results in the formation of CHalSF. The exothermicity of the other reactions forming CH218FX and CHFlEFX is insufficient for appreciable secondary decomposition.
Introduction The study of the chemical reactions of the fluorocarbenes, CHF and CF2, furnishes fruitful comparisons and contrasts with the known reactions of CH2, and has received increasing attention as convenient sources of these interesting species have become available. Difluorocarbene has been readily generated by several synthetic routes, including the pyrolysis of (CF3)3PF2,2 the decomposition of C2F4 by flash photolysis3 or Hg-sensitized p h o t ~ l y s i sand , ~ by the pyrolysis or photolysis of difluorodia~irine,~ and its reaction products in various systems have therefore been available in macroscopic amounts. Monofluorocarbene, on the other hand, has been prepared only at radioactive tracer levels by a hot-atom technique involving the decomposition of a vibrationally excited precursor formed by an energetic substitution reaction. This technique has been used to form both CTF and CH18F,initiated by energetic T and 18Fatoms, respectively, as well as CF18F.6-8 The reactions leading to the formation of CH18F and CF18F from 18F reactions with CHzF2 are illustrated in eq 1-4.g The stereospecific nature of the fluorocarbene '8F
+ CH2Fz
--+
CH2F18F*
CH2F1*F*+CH'8F
"F
+ CHzFz
--t
+ HF
CHF218F*
CHFZ18F* ---+ CF'sF
+F
+H
+ HF
(1) (2)
(3)
+
CF2 HC1+ CHF2C1 (5) The experimental approach through hot atom reactions is relatively free from complications: the formation of F atoms and H F molecules at tracer levels from a relatively inert parent, such as CH2F2, permits simple gas-handling techniques-Pyrex glass-without physical deterioration. Since the carbenes are formed by hot reactions, the yields are not altered significantly by the inclusion in the samples of small amounts of potential reactants such as ethylene or a hydrogen halide. This relative independence of carbene formation to detailed chemical composition enables the absolute yield of CH18Fand CF18Ffrom a given precursor to be easily established, leading directly to measurements of the relative efficiencies of various "scavenger" molecules for the carbenes.
Experimental Section Sample Preparation and Irradiation. The Matheson Co. supplied, with the purities listed: CF4 (99.7%), CHFa (98%), HC1(99%), HBr (99.8%) and HI (96%). CHzFzand CzF4 were supplied by Peninsular Chemresearch and were shown to be free of significant impurities (< 0.1%) by vpc. All compounds were de-
(4)
addition to olefins has led to the suggestion that both CF25 and CHF' have a singlet electronic ground state. The stoichiometric equivalent of insertion of CF2 into HC1, as in (5), or HBr has been previously reported.2t10 We have utilized hot-atom generated CH18F and CFlsF for a further study of reactions of this kind with HC1, HBr, and HI, and have measured the competitive rates of such reaction with those for scavenging by olefins. The energetic '*F atoms have been formed by the 19F(n, 2n) lSFnuclear reaction in CH2F2, CHFa, CF4, or C2F4, and have reacted by (1) and The Journal of Physical Chemistry
(2) to form CHIEFand by (3) and (4) to form CFlsF (or by similar reactions with the other molecules),
(1) This research was supported by A.E.C. Contract No. AT-(11-1)34, Agreement No. 126. (2) W. Mahler, Inorg. Chem., 2, 230 (1963). (3) W. J. R. Tyerman, Trans. Faraday Soc., 65, 1188 (1969). (4) J. Heicklen and V. Knight, J. Phys. Chem., 70, 3901 (1966). (6) R. A. Mitsch, J. Amer. Chem. SOC.,87, 758 (1965). (6) Y.-N. Tang and F. 8. Rowland, ibid., 88, 626 (1966). (7) Y.-N. Tang and F. S, Rowland, ibid., 89, 6420 (1967). (8) Y.-N. Tang, T. Smail, and F. S. Rowland, ibid., 91, 2130 (1969). (9) Both reactions 2 and 4 can also give the isotopically alternate reaction in which the 18F appears as Hl*F with no radioactive label for CHF or CFz. (10) J. W. Edwards and P. A. Small, Nature, 202, 1329 (1964).
1867
REACTIONS OF MONOAND DIFLUOROCARBENE WITH HYDROGEN HALIDES ~
Table I: Yields of 18F-Labeled Products from 18F Reactions in CH2Fz Mixtures with HI, HC1, 02,and/or CzHa Reactants
CH9z HI HC1 0 2
C2H4 18F-labeled product CH2F''F CHF2'8F C H F l8FI CHFIeFC1 CHPFI CHPFC1 CHs'8F c-C~H~"F
--4000 83
Pressure, Torr-----
4160 84
...
... *.. ...
1.48 1 0 . 0 9 0.79 1 0 . 0 3 2.57 1 0 . 1 0
2.90 f 0.19 0.74 1 0 . 0 8 2.78 1 0 . 2 0
...
382
...
...
3860
...
380 190
*..
...
...
,..
Nd
...
.,.
1.83 1 0 . 1 3
0.26 1 0 . 0 4
3.3 1 0 . 1 3
...
380 .,.
1.30 1 0 . 0 3
0.11 =k 0.07
...
. . I
Absolute yields, % 1.64 1 0 . 0 5 Nd 0 . 7 1 f 0.02 0.72 1 0 . 0 7
Nd
...
3800
...
...
gassed a t -196" prior to use. Slow warming from - 196" removed traces of Iz from HI. After degassing at -196") CzF4was stored in a -80" bath and was removed as required, leaving behind the traces of polymerization inhibitor added by the manufacturer. Samples were prepared in the routine manner described elsewhere." The hydrogen halides were measured in a calibrated, grease-free, mercury-free vacuum line, using a spiral gauge and mirror arrangement for pressure monitoring. Fast-neutron irradiations were performed with a Kaman A 711 neutron generator and samples were analyzed by radio chromatography.11v12 In a typical irradiation such as the left-hand column of Table I (4000 Torr of CHzFz in a 10.7 ml bulb, irradiated for 10 min), 1900 i 110 counts of 18Fwere observed in a 25-ml external flow counter during the passage of the CHzFz peak (flow rate 0.5 ml/sec). The absolute product yields, i.e. per cent of the total 18Fformed which appears in a particular chemical product, were determined using a calibrated Teflon-sleeve monitor, and are accurate to i l O o J , . l l The observed volatile 18F radioactivity among the listed products never accounted for more than 10% of the total 18Fformed in any of these systems. Large amounts of 18F appear as inorganic compounds, presumably almost entirely H18F, or as CH2=CH18F in CzH4-scavenged systems. Since our interest in these experiments has been focused upon the reactions of CF18Fand CH18Fand not upon the primary hot reactions of 18F,we have not been concerned with the assay of these other 18F products and have not listed the yields, when measured, in the data tables. Our experimental procedures have taken these other products into account only insofar as necessary to avoid interference by them in the assay of the 18Fproducts involving fluorocarbene-18Fprecursors. The temperature of the samples during irradiation was 10-15O-the temperature maintained by the cooling system of the neutron generator. This temper-
.*.
2.02 1 0 . 0 7 0 . 3 f0.07
...
I _ _
3800
3800 16
3840 101
46 190
...
,,.
... ...
1.91 f 0.05 0.76 1 0 . 0 7
... ... ... ... ...
2.09 f 0.08
...
190
...
190
Nd Nd Nd
Nd Nd Nd
Nd
Nd
...
...
..*
.,.
1.68 1 0 . 0 3 1.72 f 0.05
3.24 0 . 0 5 0.75 1 0 . 0 4
ature is neither precisely known, nor easily varied outside this range. Irradiations in the fast neutron beam of this neutron generator produce very little radiation damage in the system, usually unobservable with our standard measurement of macroscopic composition by thermal conductivity. l1 Radio Gas Chromatography. Several different chromatographic columns were used, often in combination in a column-switching technique" for rapid one-aliquot analysis of both low- and high-boiling components. Dimethylsu lfolane columns of varying lengths were used for the analysis of 1,l-difluoro-2, 2-dimethylcyclopropane, as well as for CHzFCl and CHFzC1. With a 25-ft column at room temperature and a helium flow rate of 0.5 ml sec-', the retention times in minutes were (boiling points in parentheses): air, 9; CF3I (-22.5"), 18; CHFJ (21.6'), 55; CH3I (42.5"), 87; CHZFI (53.4'), 114. CHF2Br and CHzFBr were separated by a 50-ft tri-m-tolyl phosphate column operated at 55". Low-boiling fluorocarbons were separated by either a 50-ft column of 10% propylene carbonate on alumina (order of elution: CF4, CH3F, CHZFZ,CHR), or a 100-ft column of 30% di-n-butylphthalate on Chromosorb P (order of elution: CF4, CHFB,CHBF,CzH3F,CH2Fz). Whenever possible, the columns were calibrated with authentic ~amp1es.l~ l,l-Difluoro-2,2-dimethylcyclopropane was identified by comparison of peak sizes and locations in mixtures containing CF18F sources and either isobutene, trans-2butene, or cis-Zbutene. The CF18F-olefin adduct shifts to progressively greater retention times in that order, as expected on the basis of earlier work.6J Additional confirmation was provided by the observa(11) T. Smail, G. E. Miller, and F. S. Rowland, J. Phys. Chem., submitted. (12) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. S. Rowland, Anal. Chem., 30, 903 (1962). (13) Authentic samples of CHnFCl and CHnFBr were supplied by Dr. H. L. Jackson of DuPont Laboratories.
Volume 74, Number Q April 80,1QYO
1868
THOMAS SMAIL AND F. S. ROWLAND
tion that the l,l-Fls-2, 2-Me2-c-C3Hzwas only found in 18Fhot atom systems in which CFl8F can be generated (e.g., CH2F2, CHFI, and CF,), but was not present among the I8F-labeled products found in an irradiated mixture containing SFe, isobutene, and 02. The calibrations of CHF21 and CH2FI rest on the following points. (a) Macroscopic amounts of the compounds, detectable by thermal conductivity response, were formed by IZscavenging of radiolysis products in CH3F (only CHZFI observed), CH2F2 (both CHF2I and CHzFI found), and CHF3 (only CHF21 found). (b) The retention times were approximately those expected on the basis of the boiling points of the compounds (see above). (c) The yield of CF18F from several precursors determined as CHF18FI was in good agreement with the yield of 1,1-F18F-2,2-Me~-c-C3Hz in isobutene scavenged systems (Table I). (d) As with the CF18F-olefinadducts, CHFlsFI was only found in those samples in which CF18F is generated and was not observed in an irradiated sample containing only SFeand HI. While all of this evidence is indirect, taken together it strongly suggests that the identification of CHF21and CHzFIhas been correctly made. Results and Discussion Thermochemistry of Carbene Reactions with Hydrogen Halides. The exothermicity of the addition of lCH2 to H I can be directly estimated from the heats of formation t o be at least 91 kcal/mol (plus any vibrational excitation energy of lCHZ),l4as indicated in eq 6. Since 'CH2 AH* 90
+ HI 6
--f
CHaI* 5 AH = -81 kcal/mol
(6)
this excitation energy is 1 35 kcal/mol greater than the 56 kcal/mol bond dissociation energy of the C-I bond, almost complete dissociation of CHJ into CH3 and I would be anticipated for gas-phase experiments.15 The exothermicity for 'CHZ addition to other hydrogen halides is also approximately 90 kcal/mol, while the energies required for decomposition are progressively larger for CH3Br, CH8C1, and CH3F. Clearly, similar estimates of the stability toward decomposition of the corresponding reaction products of CHF or CF2with HX are pertinent to the understanding of gas-phase experiments with the fluorocarbenes. Unfortunately, the heats of formation of the corresponding fluorinated species-both fluorocarbenes and fluorohalomethanesare much less accurately known than for methylene and the halomethanes. We have estimated the following heats of formation in kcal/mol (the uncertainty in each is about f 5 kcal/ mol) CHF21, -90; CHzFI, -43; CHF2Br, -100; CHFzC1, -113; CHzFCl, -66.l6 From the heat of formation of CF2 (- 39 f 3 kcal/mol)17and the assumption that CF2 has no vibrational excitation energy, the internal energies for CF2 insertion into H X are (kcal/ mol): CHFzI, -57; CHFzBr, -53; and CHF2C1, The Journal of Physical Chmiatry
-52. Comparison of these excitation energies with assumed bond dissociation energies for these molecules would indicate that CHFzCl and CHF2Br would never be sufficiently excited t o decomposels a t any pressure. The calculation for CHFzI is somewhat uncertain since the error margin on the excitation energy is quite large-some decomposition might be observed a t low pressures, or the molecule might be completely stable against decomposition a t pressures of 0.1 atm or greater. Thus, one can conclude from thermochemical estimates that the insertion reactions of CFz with the hydrogen halides would lead to stable, observable yields of CHF2C1, CHF2Br, and CHFJ, respectively. No thermochemical estimate of the heat of formation of CHF appears to be available, so we have made estimates of excitation energies by assuming that the heat of formation of CHF is the arithmetic mean19 of the heats of formation of CH2 and CF2: 25 kcal/mol. The internal excitation energies so obtained for CHF reaction with HX are: CH2F1, -74 kcal/mol; CHzFC1, -69 kcal/mol. Comparison of these excitation energies with the assumed C-I and C-Cl bond energies indicates that CH2FCl should also be completely stable against C-C1 bond break. However, the estimated 74 kcal/mol excitation energy of CH2FI is about 19 kcal/mol in excess of the activation energy required for C-I bond dissociation, more than sufficient to cause extensive secondary decomposition for 5-atom molecules a t these pressures.ls Consequently, the thermochemical estimates suggest that the observable product expected from the insertion reaction of CHIEF HCI is the stabilized CH2l8FC1,while the reaction of CH18F with H I should lead to CHPFI*, and then to CH21sFplus I. In the absence of other
+
(14) The heat of formation of methylene in the singlet electronic state is not accurately known, and may be several kcal/mol higher than the 90 used in this calculation. Singlet methylene also is known frequently to react rapidly enough that excess vibrational energy (when formed by uv photolysis) is still present a t the time of chemical reaction. (15) J. C. Hassler and D. W. Setser, J . Amer. Chem. Soc., 87, 3793 (1965). The rate constant for C-I bond break in CHaI with 25 kcal/mol excitation energy is k 2 5 X 1011 sec-1. (le) The heats of formation of CHsF and CHzFa are quoted as - 55.9 f 0.8 kcal/mol and - 108.2 f 0.2 kcal/mol, respectively. [J. A. Lacher and H. A. Skinner, J. Chem. Soc., A , 1034 (1968)l. Assuming C-H bond dissociation energies of 101 It 4 kcal/mol for each [J. A. Kerr, Chem. Rev., 66, 465 (1966)], the heats of formation of CHzF and CHFz are estimated as - 13 f 4 and - 60 f 4 kcall mol, respectively. The heats of formation of thec hloro, bromo, and iodo combinations with these radicals are then obtained by assuming C-Cl, C-Br, and C-I bond dissociation energies of 82, 67, and 55 kcal/mol, respectively. (17) H. F. Zmbov, 0.M. Uy, and J. L. Margrave, J. Amer. Chem. Soc., 90, 5090 (1968). (18) Alternate decomposition paths for excited CHFzCl* would include the intramolecular elimination of H F forming CFCl or the back reaction to CFz + HCl. There is, of course, just sufficient energy for the back reaction to occur, implying a rate constant sufficiently long that collisional stabilization would intervene before it could occur; the energetics and conclusion are presumably very CFC1. similar for the alternate path to H F (19) This assumption is frequently valid-see Skinner and Lacher, quoted in ref 16.
+
REACTIONS OF MONO-AND DIFLUOROCARBENE WITH HYDROGEN HALIDES competing processes, these C H P F radicals should abstract H from another molecule of HI, with C H P F the final observable product from the initial reaction of CHlsF HI. DiJluorocarbene Reactions. The inclusion of isobutene as a scavenger in systems containing CFlsF results ,in the formation of easily measurable yields of 1, l-F1sF-2,2-dimethylcyclopropane.8When H I is substituted for isobutene in the same systems, substantial yields of CHF'SFI are isolated in amounts essentially comparable to that scavenged by isobutene, as shown in Table I1 for lsF reactions with CF4, CHFa, and CZF4.
+
Table 11: Formation and Trapping of CF18F from Different
CFlEF
CF4 CHF3 CZF4 a
Mel-c-CaHz
CHF18FI
CHF21*Fand
0 . 5 3 f 0.06 2.4 k 0 . 3
0.60 f 0.06 2.0 f 0 . 2
CF318F C2Fa"F
7.0 f 0 . 7
8 . 5 *0.6
CFa'8F
Total pressure:
3-4 atm.
It is clear, too, from Table I1 that precursor molecules vary greatly in their stability toward secondary decomposition: CF2=CF18F, for which the split into CF2 CF18F is 76 kcal/mol endothermic, is stabilized only in about 15% of the substitutions; CFa18F, for which the decomposition to CFlsF F2 is 184 kcal/ mol endothermic (and the more probable CF'SF 2F is 224 kcal/mol endothermic) is stabilized for about 50% of the primary products.
+
+
CF18F
+ HI
+
CHF18FI
(7) Two questions can then be raised about the formation of CHF18FI. What is the efficiency of scavenging of H I for CFlsF? Is the mechanism of the reaction simply the one-step addition, or insertion, of CFlsF into the €11 bond shown in eq 71 A series of experiments with CF4 as the source of CF18Fshowed that H I is an efficient trap, and that no additional CF18F is recovered when the concentration of H I is raised above the lowest concentration used, as in Table 111. The reduction in yield of CHF218F with increasing C2H4:HI ratio found in Table I11 is a real effect, reflecting competition between H I and C2H4 in scavenging C F p F . Although the reaction of CF218F with H I has a low activation energy,2oso also has the addition of CFz18Fto ethylene2' and the direct competition is quite reasonable, as in (8) and (9). The reaction of CF2 with C2H4is known t o be extremely slow ($.e. unobservable) under these CFzl*F
+ HI
--t
---f
CHF2"F
+I
--.)
CF2"FCHzCHz
(8)
(9)
condition^,^ so that the introduction of C2H4 into a CF4-HI system should not be expected to influence the formation of CHFlsFI, in agreement with the observations of Table 111. An alternative mechanism to reaction 7 for the formation of CHF18FI could involve a two-step reaction with two molecules of HI, in one of which the reaction proceeded by I-abstraction. Since there is a considerable body of evidence that monoradicals react with H I almost exclusively by H abstraction,22the possible two-step sequence is that shown in (10) and (11). Reaction 10 is somewhat analogous to the known reaction of singlet methylene with methyl halides via abCF18F
Yield, ratio, CFlBF adduct/ stabilized precursor With isohutene: 111-F18F-2,2With HI:
precursor
+ C2H4
+ H I CF18FI + H CHF1*FI+ I CF18FI + H I --+
Target Compounds Following 19F(n, 2n)lsF Reaction
Targeta molecule
CFz"F
1869
--t
(10) (11)
straction of halogen However, if reaction 10 occurred, the following monoradical reaction 11 should be sensitive to the inclusion of 0 2 in the system; Table I shows that the formation of CHFlsFI and CHF18FC1is essentially unaffected by the presence of O2 in the system. At the same time, the yields of CHzF'*F and CH3l8Fin the same systems are depressed by the inclusion of 02,indicating monoradical precursors to some of the reaction yields of these molecules in H I scavenged systems. Consequently, we conclude that the major mechanism for the formation of CHF18FI in these systems is the direct insertion reaction 7. Two of the 18F-labeledproducts in Table I are formed by direct substitution reactions of energetic lsF atoms with CH2Fz: CHzF18F and CHF2'8F by replacement of F or H, respectively. The yield of C H F P F is independent of any other additives, suggesting that the direct reaction is its sole mechanistic source. The yield of CH2F18F,on the other hand, is much higher in the presence of H I and absence of 02,implying that some CHF18F monoradicals will abstract H from H I if not first removed by reaction with 02. The possible sources of CHFQFradicals are discussed below. Two of the products in Table I, CHF18FI and CHF18FC1, are formed by direct insertion reactions of CF18Fwith H I and HCl. The lower yield of CHFl8FC1 implies inefficient scavenging of CF18Fby HC1. The last four products in Table I are all assigned to mechanisms involving CHlsF as a reactant, and are discussed later. (20) Log A = 11.9 (M-1 sec-I), Es = 0.68 h 0.45 kcal/mol. N. L. Arthur and P. Gray, Trans. Faraday Soc., 65, 434 (1969). (21) Log A = 11.39 (M-1 sec-I), EQ = 2.4 koal/mol, J . M . Sangster and J. C. J. Thynne, J. Phys. Chem., 73, 2746 (1969). (22) D. M. Golden and S. W . Benson, Chem. Rev., 69, 126 (1969). (23) R. L. Johnson and D. W . Setser, J. Phys. Chem., 71, 4366 (1967). (24) P. S.-T. Lee and R. S. Rowland, presented at the Fifth International Conference on Photochemistry, Yorktown Heights, N. Y., Sept 1969. Volume 74, Number 9 April $0, 1970
THOMAS SMAILAND F. S. ROWLAWD
1870 Table 111: Variation in Yields of 18F-Labeled Products following 18F with Variations in Concentration of Ethylene and HI Scavengers Reactants
--__--
Pressure, Torr---
2470 232 52
1.95 f 0.20 0.34 f 0.03 1.35 f 0.08
CHFaUF CHF18FI Q
I
2500 232 131 Absolute Yields,
CFa'8F
7
I -
2470 232 14
CF4 CZH4 HI
+ CFa 2470 232 260
%
*
1.97 0.20 1.23 f 0.04 1.26 f 0.07
Nda
1.09 f 0.06 1-11 f 0.05
1.91 f 0.19 1.19 f 0 . 0 5 1.10 f 0.05
Nd means not determined.
Table IV: Yields from the Scavenging Reactions of Hydrogen Halides for CF18F
----
Reaotant CHF3
HI HBr HC1 CzHa
2480 95
... ...
228
2840
3800
84
...
29 196
... ... .
.
I
Q
...
... ...
0.98 f 0.02 1.40 f 0.03
,. . *
3.84 f 0.06 .
.
I
3800 16 159
...
57
. * a
...
*.. Absolute Yield,
0.91 f 0.07 1.24 f 0.07 3.63 rt 0.11
1
3500
"F-Product CF818F CHFPF CHF18FI CHFlsFBr CHF18FCl
Pressure, Torr-------------
...
%
0.91 f 0.02 0.92 f 0.05
0.96 f 0.02 1.25 f 0.02
0.52 f 0.04
3.79 f 0.08 0.48 f 0.07
... ...
...
Nd" Nd 3.49 f 0.08 0.49 f 0.11
*..
Nd means not determined.
Relative Eficiencies of CF2 Scavengers. Earlier competitive studies of CF2 plus hydrogen halides had qualitatively shown that CHF2Br was formed in preference to CHF2Cl.'o Quantitative data for CF'8F reactions with HI, HBr, and HC1 (using CHF3 as the reactant for energetic 18Fatoms) are shown in Table IV. The yields of CHF18FX are in the range 3.6-4.39;'o for samples containing HI or HBr, while HC1 again fails to trap all of the CF18F. The results of direct competition between HI-HBr and HBr-HC1 (Table IV) show that H I is about 70 times as efficient as HBr, which in turn is about 50 times as efficient as HCl in scavenging CF'8F. The order of increasing reactivity HCl < HBr < H I can be understood in terms of the reaction mechanism proposed by Sim0ns,~6in which the initial interaction between CF2 and HC1 was represented by donation of electrons from a nonbonding 3p orbital on the C1 atom into the vacant 2bl orbital of CF2. This process should become more favorable in the sequence HC1, HBr, HI. I n other experiments, the scavenging of CFlsF by H I has been shown to be so much more efficient than by CFZ=CFZ that no C - C ~ F is ~ ~observed ~F in HI-CFz== CF, scavenged systems." The ultimate fate of the "missing" CF18F when HCl is the only scavenger present is uncertain. DimerizaThe Journal of Physical Chemistry
tion of CF2, which is frequently observed in macroscopic studies of CF2 reactions is not a major process in these experiments, since the total number of CF18F fragments produced is -106/cm3 and the instantaneous concentrations must be 5 103/cm3.26 Perhaps a slow wall reaction to give an oxygenated species such as CF18F0 is involved; if such a product were present, we would expect it not to emerge in a measurable molecular form from our particular chromatographic analysis systems. Monofluorocarbene Reactions. Monofluorocarbene has been prepared by hot atom techniques previously6-* and the addition to olefins has been extensively studied for CTF.6 The present experiments have used CHISF CH'8F
+ CH2=CH2
--it
c - C ~ H ~ ' ~ F (12)
prepared by reactions 1 and 2, and have shown that CH'8F reacts efficiently with both H I and HCl. The olefin addition reaction 12 is known to occur readily for (25) J. P. Simons, J . Chem. Soc., 5406 (1965). (26) The total 18F activity produced in the sample indicates that the number of 1*F-containingmolecules is -108 in a 10-ml bulb after a typical irradiation of 15 min. Assuming a 4% yield for CFiaF and a lifetime no longer than 10 sec for diffusion to the walls, the X instantaneous concentration of CF1aF is not more than the total CFlaF production.
1871
REACTIONS OF MONO-AND DIFLUOROCARBENE WITH HYDROGEN HALIDES
5 X 10'' sec-' corresponds to 15-25 kcal/mol excitation energy above the bond dissociation energy, the higher value corresponding to the more rigid of two models used for the transition complex. This range of numbers is in satisfactory agreement with the 19 kcal/ mol estimated from the thermochemical arguments and presumably provides the not surprising confirmation that our assumption that the heat of formation of CHF can be approximated by the arithmetic mean of those of CF2 and CHe is within 5-10 kcal/mole of the correct CH'8F HC1 --+ CHz"FC1 (13) value. I n HI-C2H4scavenged systems, the yield of C - C ~ H ~ ' ~ F Other Sources of 18F-Monoradicals. Quantitative is reduced below that found in the presence of OZ-C~H~, interpretation of the CH818F yields in this system is complicated by the fact that hot reactions with both consistent with the competitive removal of CH18F by CHzF2 and CH2=CH2 can form CH2'8 F radicals which reaction 14 with HI. The ratio of rate constants, will be indistinguishable from those produced via i&4/kl2, can be roughly estimated as about 3 at -10". reaction 15.27 However, direct measurement with H I as the only The diminution of CH2FI8Fyield by addition of O2 scavenger shows only 0.11 A 0.07% yield of C H P F I . to a CH2F2-HI system indicates the presence of CHF'sF As indicated above by the thermochemical calculations, monoradicals. Although the abstraction of H from CH218FI is formed with vibrational energy about 19 H I (or other molecule) by CF18F could hypothetically kcal/mol in excess of the C-I bond dissociation energy, be the source of CHF18F radicals, related experiments and the decomposition-scavenging process of (15) with C2Fd-HI mixtures demonstrate that abstraction and (16) should result in the observation chiefly of of H from H I by CF'8F is very unlikely. The distribuCH818F instead of CH218FI in gaseous systems. The data of Table I show several per cent yields of CHa18F tion of 18Fproducts in such a system includes the following, in absolute percent yields : CF2=CF18F, under these conditions-more than adequate to account 1.5%; CHF18FI, 12.8%; CH2F18F,0.25%. Thus, more for the diverted 2% yield of CH18F, while simultathan 98% of the CF18F from the decomposition of neously indicating the necessity for a second mechanism vibrationally excited CF2=CF1*F appears as CHF'SFI, for the production of CH218Fradicals. with less than 2% as CH2FI8F. Even this small CH'*F H I --+ CHZ'~FI* (14) amount of CH2F18F may be formed via a small partial decomposition of CHF'BFI (see thermochemical estiCHz18FI*--+ CH2I8F I (15) mates) and subsequent scavenging of the CHF18F CH2"F H I --+ CH8"F I (16) radical. The most IikeIy sources of the excess CHFL8Fradicals Comparison of the total yield of CH'8F when scavare by the secondary decompositions of excited moleenged by HC1 or C2H4with the stabilized yield of cules formed by direct substitution reactions, as in C H P F I , when scavenged by HI, indicates that approxi(17) and (18). mately 5 f 301, of the H I adduct is stabilized a t these pressures. Since the collision frequency for stabilizing CHF2l8F* --+ CHF'8F F (17) collisions is about 3 X 1O'O sec-' at 4000 Torr, the average rate constant for decomposition must be about 5 X CHzF"F* --.t CHl[i"'F H (18) lo1'sec-l. A crude estimate of the relationship between decomposition rate and excitation energy for CH2FI (27) 18F + CHz=CHz + CzH418F* + CHa + CHz'SF, '*F + CHZFZ + CHaFi8F* + F; CHzFIsF* + CHd8F + F. In each case, the can be obtained by comparison with the calculations for pathway shown is not the major decomposition pathway for this CHa1.l6 For the latter molecule, a rate constant of excited species, but does occur.
monofluorocarbene,6 and the yield of C-CaH618F (i.e., the yield of CH18F)has been measured as 2.09 f 0.08% (Table I) in an 02-C2H4 scavenged sample. Replacement of CzH4 by HC1 results in the formation of an equivalent amount of CH2'8FC1, suggesting that HC1 is much more efficient in trapping CHJ8Fthan it is for CF18F. The independence of yield of CH218FC1upon the presence of 0 2 indicates that reaction 13 also takes place via a direct insertion mechanism.
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Volume 74,Number 9 April SO, 1970
