THE J O U R N A L OF
PHYSICAL CHEMISTRY Registered in U.S. Patent Office 0 Copyright, 1980, by the American Chemical Society
VOLUME 84, NUMBER 19
SEPTEMBER 18, I980
Reaction of OH with CH3CH2F. The Extent of H Abstractlon from the Positionst
cy
and ,8
Donald L. Singleton, George Paraskevopoulos,* and Robert S. Irwln Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada, K I A OR9 (Received: January 16, 1980; In Final Form: May 15, 1980)
The mechanism of the reaction of OH radicals with C2H5Fwas determined frbm the yields of the reaction products. Hydroxyl radicals were generated in the gas phase by photolysis of HzO at 184.9 nm, and products were analyzed by gas chromatographyand mass spectrometry. The reaction proceeds by abstraction of hydrogen primarily from the a position, and the major products were meso- and d,Z-2,3-C4HPz,1,3-C4HeFz,2-C4H9F, and H2 The minor products were 1,4-C4HsFz,l-C4H8,1-and 2-C3H,F, CzHs,C2H4, C2H3F,and C2H2. It was found that kinetically hot H atoms formed in the photolysis of HzO abstract hydrogen to some extent at the /3 position of C2HSFin contrast to thermal H atoms which abstract exclusively at the a position. After correction for the effects of H atom reactions, it was estimated that 85 f 3% of the abstraction by OH occurs at the a position of C2H5F.
Introduction In a recent paper reporting absolute rate constants, 12, for the reaction of OH radicals with fluoroparaffins in the gas phase, we presented a linear correlation between log k and the C-H stretching frequencies, VC-H, for several paraffins and flu0roparaffins.l The purpose of the correlation was to rationalize the trend in the observed rate constants in the absence of reliable dissociation energies for many of the fluorocarbons. In the case of CH,CH2F, there are three types of C-H bonds and hence three C-H stretching frequencies2 In the a position (i-e., the fluorinated carbon) vC-H = 2950 cm-l; in the 0 position u C - ~ for the bond trans to fluorine is 2957 cm-l; and for the two bonds in the gauche position, UC-H = 2973 cm-l. The correlation of the rate constants indicated that abstraction would occur more readily from the a position although there could be some contribution to the room-temperature rate constant by the other hydrogens. Studies of reactions of C2H5Fwith H,3 Br,4 and Hg(3P,)3 have shown that abstraction occurs exclusively at the a position, at least a t room temperature, whereas for C15v6 tNRC:C No. 18429. 0022-3654/80/2084-2339$01 .OO/O
and F7 91 and 58% of the abstraction occurs at the a position. In the present work an analysis of the products of the reaction of OH with C2H6Fin the gas phase was made in order to determine the mechanism, particularly the relative importance of abstraction from the two carbon atoms.
Experimental Section Hydroxyl radicals were generated by photolysis of H20 vapor at 184.9 nm by using a low-pressure mercury lamp. The intensity of the lamp, operated at 100 mA, was constant after a warm-up period of 0.5 h. This was established by monitoring the intensity of the transmitted 184.9-nm line with a photomultiplier and also by photolyzing several mixtures of 5.0 torr of CO + 5.5 torr of HzO for 50 min, which gave an average of 7.2 f 0.8 hmol/h of C 0 2 ,where the uncertainty is fa. Water and ethyl fluoride are transparent to the intense line at 253.7 nm emitted by the low-pressure mercury lamp. The cylindrical reaction cell had plane end windows (5-cm 0.d.) made of Suprasil quartz, and a volume of 85.8 cm3. The cell was attached to a mercury-free vacuum system in which the purification, storage, and transfer of the reactants were carried out. Pressures were measured @ 1980 American Chemical Society
2340
The Journal of Physical Chemistty, Vol. 84, No. 19, 1980
with a Baratron manometer. The photolysis cell was also connected to the mercury-containing analytical system through three liquid-nitrogen cooled traps which served to prevent mercury vapor from entering the cell and also to collect the condensable products while the noncondensable gases were collected by a Toepler pump and measured manometrically on a gas buret. Hydrogen was identified as the only noncondensable product by gaschromatographic analysis on a 13X molecular sieve column. The condensable products and the excess reactants were transferred to a mixing bulb from which several aliquots were taken for analysis. The condensable products were determined by temperature-programmed gas chromatography, using a flame ionization detector and a 5 ft, '/8 in. 0.d. column of Porapak QS. Qualitative identification was based on retention times and also on the mass spectra of the individual chromatographic peaks obtained with a gas chromatograph-mass spectrometer. Quantitative determination was based on peak area measurements with isobutane as an internal standard. The molar responses of the fluorobutanes, which were not available in pure form, were taken to be identical and equal to that of isobutane. This is a reasonable assumption since the molar responses relative to isobutane of C2H6,C2H6F,and CH3CHF2were found to be the same within 8%. The molar responses of the other products were determined experimentally. The analytical results are reported as average values from three chromatograms. The individual differences were very small, but, because of the assumptions made for the molar responses, the analytical errors for the absolute yields are estimated to be 5-10'3'0 for the more abundant products and 15-20% for the minor products. The errors in the relative yields of the fluorobutanes should be smaller. The fluorobutanes were identified by comparison of their mass spectra with those reported by Majer8 and by comparison of their retention times and mass spectra with samples of 2,3-C4H8F2,1,3-C4H8F2,1,4-C4H8F2,and 2C4HJ?synthesized as described by Edge1 and Parts.g (In all cases our yields were very poor.) The mass spectra could not be used to discriminate between the meso- and d,l-2,3-C4H&?2 isomers since they were essentially identical. Therefore the meso isomer was assumed to be eluted first.1° The elution sequence of the fluorobutanes was 2-C4H9F, 1-C4HgF,meso-2,3-C4H8F2,d,2-2,3-C4HgFz, 1,3-C4H8F2, 1,4-C4Ha2,the same as reportedlOJ1for the difluorobutane isomers. The 1,4-C4H8F2 was present in small amounts, and it was difficult to obtain a reliable mass spectrum because of the high background ion current present at high column temperatures. Except for four mass peaks which were too intense, its mass spectrum was similar to that reported by Majer8 and to that obtained from a synthesized sample having the same retention time. The remaining products-c2H6, C2H4, C2H3F,l-C4H8, 1-C3H7F,and 2-C3H7F-were identified by comparison of their mass spectra with those in the literature and, for the first three, by comparison of the retention times with those of authentic samples. Ethyl fluoride was analyzed by gas chromatography and mass spectrometry. The purity level (and major impurity) was CzHBF> 99.61% (i-C4H10, 0.38%). The helium was stated to be 99.999%, and the DzO, 99.75% D atoms. Results The product yields for various experimental conditions are given in Table I. In a series of experiments with -5.5 torr of HzO, -27 torr of C2H6F,and irradiation times
Singleton et al.
TABLE I: Product Yields of the Reaction OH t C.H.F at 297 K irradiation, min 3 15 30
60
~~~
Reactants, torr H, 0 5.53 5.59 5.52 C,H,F 28.3 27.5 27.4 total pressure 33.8 33.1 32.9 Product Yields, pmol H 0.247 1.19 2.37 meso-2,3-C,H8F, 0.054 0.275 0.531 d,Z-2,3-C,H8F, 0.053 0.266 0.511 I,3-C,H8F, 0.039 0.201 0.413 1,4-C,H8F, 0.004 0.026 0.112b 2-C,HgF 0.025 0.128 0.230 l-C,HgF 0.004 0.021 0.042 2-C3H,F 0.003 0.032 0.061 l-C,H,F trace 0.006 0.014 c 2H 6 0.008 0.034 0.061 0.017 0.020 0.020 C,H, t C,H, 0.019 0.024 0.026 C,H,F oic 0.279 1.44 2.79 0.050 0.281 0.694 pd 0.848 0.837 0.800 4. + 0)
30a
_ _
~
5.60 5.53 27.2 627 32.8 633 5.54 1.15 1.08 0.830 0.115 0.493 0.083 0.146 0.037 0.154 0.022 0.028 5.94 1.18 0.834
5.53 0.613 0.611 0.352 0.028 0.070 0.004 0.054 trace 0.022 0.058 0.080 2.92 0.412 0.876
a Average of two experiments. This value may be too large. In two duplicate experiments at slightly different light intensities, the product yields were similar except for that of 1,4-C,H8F,, which had an average value of 0.06 pmol. Sum of a radicals identifiable in the products: 2(meso-2,3-C,H8F,) t 2(d,Z-2,3-C,HsF,) + 1,3-C,HsF, t 2-C",F t 2-C,H,F. Sum of 0 radicals identifiable in the products: "2(l,4-C,H8F,) + 1,3-C,H8F, t l-C,H,F t l-C,H,F.
varying from 3 to 60 min, the major products were mesoand d,l-2,3-C4H8F2, 1,3-C4H8F2, 2-C4H9F,1,4-C4H8F2, and H2. The minor products were C2He,CzH4, C2H2,1-C4HJ7, and very small amounts of 2-C3H7F,1-C3H7F,n-C4Hlo, and a small peak eluting after the difluorobutanes which may have been a fluorinated butanol (or perhaps a fluorinated hexane) but which could not be identified because authentic samples were not available. The recombination products of a- and /3-fluoroethyl radicals account for 80-90% of the products. The Porapak QS column used for the analyses failed to separate C2H4 and C2H2,which are reported together in Table I, but additional analyses on a Porapak T column indicated that both were present. The rates of formation of most products are independent of irradiation time, as shown in Figure 1,although the rates of formation of C2H4 + C2H2and of C2H3Fdecrease with time. The rate of formation of 1,4-C4H8F2was small and, within the uncertainty of its determination, was independent of irradiation time. Because it was difficult to obtain a reproducible mass spectrum of the chromatographic peak assigned to l,4-C4H8F2,as mentioned in the previous section, the possible presence of a small amount of an unresolved product cannot be excluded. (However, the 1,4-C4H8F2contributed very little to the sum of 0fluoroethyl radicals identifiable in the products, and hence the small uncertainty in its yield has little effect on the estimate of the relative amount of abstraction by OH from the cr and /3 positions of CzH5F.) With 627 torr of CzHP,the yields of C&, 2-C4HJ7,and 1-C4HgFdecreased, whereas the yields of H2 and unsaturated Cis increased. Also, the relative contribution of C2H2 to the total unsaturated Cis increased substantially with pressure, with the ratio CzH2:CzH4 increasing from 0.9:l at 33 torr to 3:l at 633 torr. The yields of the 2,3-difluorobutanes increased slightly, while those of 1,3-C4HgF2 and 1,4-C4H8F2decreased. With 3 torr of O2 added, the yields are significantly reduced, indicating a free-radical mechanism. Also, no
The Journal of Physjcal Chemjstty, Vol. 84, No. 19, 1980 2341
Reaction of OH with CH3CH2F cn -
I from -5 pmol/h at 27 torr to -11 pmol/h at 627 torr of C2H5Fis due in part to the photolysis of the larger amounts of C2H4 and C2H3Fformed at higher pressure, as evidenced by the parallel increase of the C2H2yields at 627 torr (which is also expected to be photolyzed to some extent). Since the yield of Hz at 30 torr (5-6 pmol/h) is lower than the rate of photolysis of H20 determined in the same apparatus for H20-CO mixtures (7.2 pmol/h), some H atoms must react by addition to radicals and olefins. The decrease in rate of formation of CZH4 + C2H2 and C2H3F with irradiation time in Figure 1is probably due to their reaction with H atoms and to photolysis. The observed products can be accounted for by the following reactions: H 2 0 + hv(184.9 nm) OH + H ( 1) OH + C2H5F qC2H4F + H2O (2) H C2HSF C2H4F + H2 (3) C2H4F + *C2H4F C4H8F2 (4a) C2H5F + C2H3F (4b)
-
+
+-I n-
0
IO
20
30
40
50
60
IRRADIATION TIME (min) Flgure 'I. Rates of formation of products as a function of irradiation time. Reaction mixtures were composed of 27 torr of C,H5F and 5.5 torr of H20: (X) H,; (0) meso-2,3-C,H8F2; (m) d,l-2,3-C4H,F2; (A) C,H,; ( 0 )C2H3F. l13-C4H8F2;(01 2-C,HgF; (+) CpH,; (0) C2H4
products were formed when C2H5Fwas irradiated in the absence of water. This demonstrates that direct photolysis of C2H5Fdoes not occur and also that the reaction cell is indeed free of mercury, as otherwise products of the mercury photosensitized decomposition of C2H5Fwould have been detected. In separate experiments, 5.6 torr of DzO was photolyzed for 3.9 h (approximately equivalent to 30 min for H20, based on the measured difference in absorption coefficients) in the presence of C2H5F. For at least 24 h prior to photolysis, the reaction cell and adjacent tubing were repetitively filled with DzO, evacuated, and refilled. The noncondensable products H2, HD, and D2 were separated on a column d MnC12 on A1203at -196 "C and converted to H20, HDO, and D 2 0 which were detected by a thermistor.12 The yields of H2, HD, and D2 were found to be 2.3, 3.8, and (0.4 pmol at 625 torr of CzH5Fand 0.9, 1.0, and 0 pmol at 27 torr of C2H5F. Apparently little, if any, termolocular recombination of D atoms occurred to form
DZ. Discussion The OH radicals generated by the photolysis of HzO reacts essentially only with C2H5Fto form .CZH4Fradicals which, in the simplest case, would be expected only to combine or disproportionate. However, the chemistry is complicated by the presence of H atoms which are generated and react by various paths. Hydrogen atoms are formed along with OH radicals in the photolysis of water and also by photolysis of the olefin products, C2H4and C2H3F. Qualrtative conclusions about the origins of molecular hydrogen may be drawn from the experiments in which D20 was photolyzed. From the large yield of HD and the absence of D2, it is evident that most H atoms abstract hydrogen from CzH5Fto form Hz, rather than recombining. All of the Hz in the experiments with DzO arises from the photolysis of CzH4 and C2H3F,indicating, along with the yields of C2H2,that substantial amounts of olefin products were photolyzed. (Some of the HD in these experiments probably arises from photolysis of partially deuterated olefins created by recombination of D atoms with radicals.) The increased yield of H2in Table
+
-+
-
-- + + + + -- ++ -- ++ + + +
H
+ C2HJ
+M
[C2H5F]* C2H5F (5a) [C2H5F]* CzH4 H F (5b) H C2H4 C2H5 (6) H C2H3F CzH4F (7) C2H5 CzH4F C4HgF (84 C & & C2H3F (8b) C2H4 C2H5F (8~) C2H4 + hu(184.9 nm) CzH2 H2 (gal C,H2 H H (9b) -.CzH3 H (9c) The ratio k4b/lZ4ahas been determined13 for CH3CHF + CH3CHF to be 0.21. We make the assumption later, in estimating the distribution of cy and P radicals in reaction 2, that the same disproportionation/recombinationratio holds for CHzCH2F+ -CH2CH2Fand for CH2CH2F+ CH3CHF. Reaction 5a requires collisional stabilization of the adduct. The ratio of the rates rSa/r5b= k5aP/k5,,is 0.06 at 33 torr and 1.1at 633 torr total pressure, P, based on the data of Trotman-Dickenson and co-workers7J4and assuming the bond dissociation energy of CH3CHF-H is 98 kcal mol-I. The relative importance of the three channels for reaction 8 is apparently not known, although from the relative yields of C2H6 and 2-C4H9Fwe obtain ksb/ks, = 0.29 at 33 torr. At 633 torr the average ratio is 0.32, in good agreement. This compares with disproportionationlcombination ratios of -0.14 for -C2Ht5and 0.21 for CH3CHF13 radicals. The relative importance of the channels in reaction 9 has been determined16to be k g a / k g= 0.53, kgb/k9= 0.33, and k k / k g = 0.14. Vinyl fluoride also undergoes photolysis, but quantitative information on the primary processes at 184.9 nm is not available. The two likely processes give CzH2+ HF and .CzH2F+ H.I7 The small amounts of 2-C3H7Fand 1-C3H7Fare probably formed by combination of .C2H4Fradicals with .CH3 radicals, which could result from the reaction of atomic hydrogen with C2H5. The probability of C H 3 radicals combining to form ethane in the presence of a much higher concentration of .C2H4Fradicals is small. Secondary reactions of OH with reaction products are not expected to occur to any significant extent for the experimental conditions employed. The small peak. as-
2342
The Journal of Physical Chemistry, Vol. 84, No. 19, 1980
sumed to be a fluorinated butanol or a fluorinated hexane (which are the expected products of reaction of OH with olefins or fluorobutanes) is -4% of the total product peak area at 27 torr of C2H5Fand -10% at 627 torr. Furthermore, the rates of formation of the fluorobutanes are independent of irradiation time, as shown in Figure 1. In the experiments in which the pressure of C2H5Fis increased from 27 to 627 torr, the rate of reaction 3 increases at the expense of reactions 6 and 7 (Le,, more H atoms are removed by reaction with C2H5Fto form H2 and -C2H4Fradicals). This accounts for the decrease of the yields of CzH6and C4H9Fand the increase of the yields of H2 and C2H3F. Also, the rate of reaction 6 decreases while that of reaction 5 would be expected not to change significantly because of increased concentration of CzH4F radicals. This probably accounts for the increase of C2H4 at this pressure. The suggested mechanism accounts for the formation of the observed products and at least qualitatively for the dependence of the yields on different experimental conditions. It provides a framework for estimating the relative probability of abstraction by OH from the a and positions of C2H5F. Site of Abstraction. In Table I is listed the number of a radicals (from yields of 2,3-C4H8F2, 1,3-C4H8F2, 2-C4H3, and 2-C3H7F)and /3 radicals (from 1,3-C4Hg2,1,4-C4H8F2, 1-C4HgF,and 1-C3H7F)formed in each run. The a and p radicals are defined as those resulting by abstraction of hydrogen from the a and p positions of C2H5F. The fraction of a radical formation relative to total abstraction, a/(at p), is also given in Table I for each run. The ratio is fairly constant at 27 torr of CzH5Ffor different irradiation times and has an average value of 0.830 but is larger (0.876) at 627 torr of C2H5F,However, these ratios do not reflect abstraction by OH radicals alone but include small contributions by other reactions as well, which are discussed next. Abstraction by Kinetically Hot H Atoms. The energy in excess of the minimum required t o break the H-OH bond in the photolysis of water at 184.9 nm appears as translational energy of the fragments.l8 The H atoms have initially 33.6 kcal/mol of kinetic energy and may abstract from the 6 position in reaction 3, in contrast to thermal H atoms. I t was found that addition of helium, which thermalizes kinetically hot H atoms fairly efficiently, increased the value of a/(@t p). The effect is apparent in Figure 2 in which a / ( a fl) is plotted against the reciprocal of the helium pressure and against the average kinetic energy, ( E ) ,with which the H atoms react, calculated as described in ref 19. Neither plot is linear, but in each case a smooth curve can be extrapolated to give a / ( a t 0)= 0.880 at infinite pressure of helium, cr for completely thermalized H atoms, This value of the ratio obtained by two different extrapolations is therefore fairly well-defined. Reactions of Thermal H Atoms. At room temperature hydrogen atoms abstract exclusively at the a position in (We confirmed C2H5F, as reported by Scott and Jenning~.~ their results using mercury photosensitized decomposition of H2 to generate H atoms. The only difluorobutanes detected were the 2,3-C4H8F2 isomers.) The larger value of a / ( a + p) at 627 torr of C2H5Fmay be due in part to the abstraction by thermal H atoms formed by photolysis of the larger amounts of olefins present. Also, addition of H atoms to C2H3Fgives only a radi~a1s.l~ Reaction 7, if pressure dependent, would not be expected to contribute significantly to the increase in the values of a / ( a + p). If the pressure dependences of reactions 6 and 7 are similar, and if the deactivation efficiency of C2H5Ftoward C2H4F*
+
Singleton et al.
(E) (kcal/rnol) 0
IO
20
I
I
I
0-84
t
30 I
7
1
,. \\
I
0.83
I
0
I
I 2
I
I 3
IOO/P (torr-’
4
5
6
I
+
Figure 2. Fractional amount, a/(a b), of a radicals CH,CHF identifiable in the products as a functlon (0)of the reciprocal of the helium pressure and of the average kinetic energy, (E),with which H atoms react. PCfiIF = 27 torr; PHlO = 5 torr; lrradlatlon tlme = 30 min.
(m)
is similar to that of CzH4toward C2HS*(C2H4 is 30 times more efficient than helium20),then the rate of reaction 7 would increase less than 10% on addition of 900 torr of He to 27 torr of C2H5F. This would increase the average value of a/(. + p) from 0.83 observed in the absence of He to 0.84 yith 900 torr of He. The value of a / ( a t p) = 0.88 obtained when the H atoms are completely thermalized clearly indicates that abstraction of OH occurs to some extent at the 0position since there are no other known processes which can generate p radicals under these conditions. In order to obtain a quantitative estimate for the relative amount of abstraction by OH at the cy and p positions, the contributions of thermal H atom and OH radical reactions to the sum of a radicals identified in the products must be determined. Since the extent of formation of cy radicals by H atoms is difficult to assess, the rate of photolysis of H 2 0 is used to estimate the sum of a and 6 radicals formed by OH abstraction. Value of a/(@+ 0)for Abstraction by OH. A lower limit for the ratio a / ( a t 6) for abstraction by OH alone can be calculated for the case where H atoms are completely thermalized (Le., only abstraction by OH generates radicals). The total number of fluoroethyl radicals, both a and @,generated by OH abstraction can be obtained from the rate of photolysis of water (which is equivalent to the rate of OH radical formation). However, some fluoroethyl radicals regenerate C2H5Fin reactions 4b, 5a, and 8c, and some fluoroethyl radicals end up as products, such as C2H3F,in which they are not identifiable as a or fl. By allowing in an appropriate manner for these effects and subtracting the observed sum of /3 radicals, one can calculate the number a’of a radicals formed by reaction of C2H5Fwith OH alone (and detectable in the products) and use it to obtain a’/(a’ + 6)for OH abstraction. The value of a’is given by eq I, where AHzO is the amount of CY’ = [(a+ p)/AC,H,F](O.9AH,O - (k,,/kbb)PAHF) (1) H 2 0 photolyzed based on the rate 7.2 f 0.8 pmol/h. AC2H5Fis the net consumption of C2H5Fobtained by summation of the carbon atoms in the products, and AHF is the amount of HF formed in reaction 5b and is estimated by taking the difference between the fluorine content of the consumed C2H5F(AC,H5F) and the fluorine content
Reaction of
OH with CH,CH,F
The Journal of Physical Chemistry, Vol. 84, No. 19, 1980
of the detected products; i.e., the nonrecoverable fluorine is assumed to be HF. The expression in parentheses gives the sum of a ,f? radicals based on the rate of photolysis of water corrected (i) for the re-formation of C2H5F in reaction 4b, Le., and (ii) for the re0.9AH20 based on k4b/k4a = formation of C2H5F in reaction 5a, by subtracting (h5J k5b).PAHF.7p14 The factor ( a + @)/AC2H5Fgives the ratio of fluoroethyl radicals identifiable in the products ( a + 6 ) to tho total number of fluoroethyl radicals formed (AC2H5F). The quantity in parentheses is multiplied by this factor because only the fluoroethyl radicals identifiable as a or /3 radicals are sought. The value taken for AHF is an overestimate because HF arises from fluoroethyl radicals formed not only by OH abstraction in reaction 2 but also by reactions 3 and 7. Furthermore, some HF is formed by photolysis of C2H3F.17 Therefore CY' is a lower limit, as is the ratio a'/(a' + p) for production of a and p radicals by abstraction by OH. For the experiments in which 951,615, and 304 torr of helium were added, the observed ratios ./(CY + 6 ) plotted in Figure 2 were 0.877,0.874, and 0.872, respectively, very close to the extrapolated value of 0.880 for completely thermalized ;%toms.The application of eq I to these three data points gives values of CY' + 0)of 0.81, 0.82, and 0.83, for an average value of 0.82 as the lower limit. The upper limit is 0.88 (i.e,, if no accounting is done for reactions of thermal H atoms). Compatible with these limits is the arbitrarily taken intermediate value 0.85 f 0.03 for the probability of abstraction by OH at the a position of CzHSF, The value a/(. + p) = 0.85 estimated for abstraction by OH lies between that for abstraction by C1 (0.91)5,6and that for abstraction by F (0.581.' That this is reasonable is evident from reported activation energies for simple hydrocarbons.21 For abstraction by C1 from CH4 (DH3C-H = 104 kcal mol-') and CzH, (Dc*H&-H = 98 kcal mol-l), the difference in activation energies is 2.4 kcal mol-', but only 1.0 kcal mo1-I for abstraction by OH, indicating that OH is less sensitive than C1 to the C-H bond energy and hence less discriminating in its site of attack. The value for a/(a+ p) of 0.85 implies that the rate constant for abstraction from the a position of CH3CH,F by OH is 1.2 X lo", and from the position, 2 X 1O1O cm3 mol-1 s-l, based on the observed total rate constant1 of 1.40
+
2343
X loll. Although it would be possible to estimate the relative extent of abstraction for the trans and gauche C-H bonds in the /3 position by using the correlation between either log (k/(n)liz)or log h and the C-H stretching frequency,' it may not be advisable since the value 2 X 1O'O cm3 mol-' s-' lies 30 and 70% below the predicted values for abstraction at the trans C-H bond (the weaker of the two bonds) in the two correlations. Since the value is much closer (30%) to the prediction based on the correlation in which the number of C-H bonds, n, is taken into account, /~) appear to be the correlation involving log ( h / ( r ~ ) l would a better choice for correlating rate constants for fluoroparaffins.
Acknowledgment. We are grateful to M. E. Bednas and R. Sander for the mass spectrometric analyses and assistance in the interpretation of the mass spectra and to Dr. Y. Amenomiya for providing the apparatus for analysis of H2, HD, and D2.
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