J . Phys. Chem. 1988, 92, 672-676
672
Gas-Phase Thermal Reactions of ''F Atoms with cis-I-Chloropropene and trans- 1-Chloropropene Patricia J. Rogers and F. Sherwood Rowland* Department of Chemistry, University of California, Irvine, California 9271 7 (Received: March 10, 1987; In Final Form: September 8, 1987)
The gas-phase reactions of fluorine atoms with cis- and t r a w l -chloropropenehave been investigated with I8F atoms moderated to thermal energies after formation by the 19F(n,2n)18Fnuclear reaction in SF6. Addition to the olefinic position occurs preferentially on the central carbon atom, with a terminal/central ratio of 0.46 & 0.03, in contrast to the terminal addition preferences > 1.3 found with propene and 3-chloropropenesubstrates. Terminal fluorine addition to CH3CH=CHC1is followed on a picosecond time scale by loss of C1, with no observed stabilization of the CH3CHCHCll8F*radical even at 3100 Torr of pressure. In contrast, only about 1% of the CH3CHl8FCHCI*radicals formed by central addition decomposed (by CH3 loss) at 3100 Torr. The observed cis/trans product ratios of CH3CH=CHI8F after terminal addition to CH,CH=CHCI and C1 loss were 1.93 0.10 from the cis substrate and 1.29 f 0.04 from the trans. These ratios are consistent with about 10% I8F/CI substitution with retention of the substrate geometry before rotation can occur in the newly formed radical and 90% loss of CI from a rotating radical giving an equilibrated product favoring the more stable cis product in the ratio of 1.65/1.0. The overall rate constant for reaction of fluorine with the cis- and trans-CH3CH=CHC1 substrates is about 1.5 X lo-'" cm3 molecule-' s-' and approximately 1.0 X lo-'' cm3 molecule-' s-I for the addition process by itself. The addition rate constant is not significantly different than that for CH3CH=CH,, indicating that the lower yield of terminal addition with 1-chloropropene comes from internal directional preferences after bond-forming olefinic addition is already assured.
*
Introduction The gas-phase thermal reactions of 18F atoms have been investigated with cis- 1-chloropropene and trans- 1-chloropropene over a total pressure range from 500 to 3400 Torr at 287 f 2 K, utilizing a sensitive radiotracer technique for analysis of the fluorinated reaction products. cis-1 -Chloropropene and trans1-chloropropene were chosen as the reactive substrates because they offer comparisons and contrasts with the corresponding results found in previous investigations of the reactions of thermal 18F atoms with and with 3-chl0ropropene.~ The most important new mechanistic information arising in these systems is facilitated by the availability with the isomeric 1-chloropropene targets of geometric product isomers following the substitution of I8F/C1, and from the presence of the electronegative C1 atom at the n-bond site during the olefinic addition of fluorine. The main reaction routes for atomic fluorine with l-chloropropene are given in (1)-(3): (a) terminal addition to the n-bond, 18F + CH3CH=CHCl CH3CHCHC1I8F* (1)
+ CH3CH=CHCl I8F + CH,CH=CHCl
18F
--
CH3CHI8FCHCl*
(2)
+
Hl8F C3H4Cl (3) (b) central addition to the n-bond, and (c) hydrogen abstraction. The radicals formed in (1) are highly excited from the formation of the C-F bond and can either decompose by loss of C1 in (4) or be stabilized by collision with a bath molecule (usually SF6) in (5). Potentially competitive decomposition reactions involving loss of H atoms from the C-1 or C-3 positions are energetically unfavored in competition with C1 loss. The excited radicals from (2) similarly have two available reaction routes: decomposition by loss of CH3 in (6) or stabilization in ( 5 ) , with the H atom loss CH3CHCHCl18F* CH3CH=CHI8F (cis or trans) + CI (4) C3H5C1I8F* M C3H,C118F+ M (5) -+
+
CH3CH'8FCHC1*
CHI8F=CHC1 (cis or trans)
+ CH,
(6) channel again energetically unlikely. The stabilized species from (1) Williams, R. L.; Iyer, R. S.; Rowland, F. S. J . Am. Chem. SOC.1972,
94, 7192.
(2) Rogers, P. J. Ph.D. Thesis, University of California, Irvine, CA, 1986. (3) Rogers, P. J.; Rowland, F . S. J . Phys. Chem., in press. (4) Iyer, R. S.; Vasi, L.; Rowland, F. S.J . Phys. Chem. 1985, 89, 5051.
0022-3654/88/2092-0672$01.50/0
(5) can for both terminal and central 18Fatom addition be converted into observable, volatile products by the inclusion of the scavenger molecule HI in the reaction system, as in (7). However,
+
-
C3H,C118F H I
+
C3H6Cl18F I
(7)
with HI present as an additional substrate, some of the thermal 18Fatoms are diverted into HI8F through reaction 8. 18F
+ HI
HI8F + I
(8)
The relative reaction rate constants for each of these processes can be directly inferred from the ratios of measured product yields. The absolute total reaction rate for fluorine atom reaction with either of the 1-chloropropene targets can then be determined from a direct competition with another substrate for which the rate constant is well-known, as with H I in (8). We have also studied the observable reactions of atomic fluorine with the l-chloropropene isomers in mixtures with O2as the reactive radical scavenger.
Experimental Section The radioactive I8F atoms have been generated via the usual I9F(n,2n)l8F reaction in SF6 exposed to a 14-MeV neutron A moderator/substrate mole ratio, Le., [SF6]/[C3H5Cl], exceeding 20 is sufficient to reduce the reactions of still energetic 18F atoms with CH3CH=CHC1 to negligible yields., The radio gas chromatographic analytical technique used in these experiments has been described in detail previou~ly.~-~ The chemicals were supplied as follows (purity levels in parentheses): SF6 (98.6% or >99.9%), Matheson Gas Co.; HI (98%), Matheson Gas Co.; cis- (98%) and trans-1-chloropropene (98%), Chemical Samples Co.; O2 (99.4%) and helium (99.99%), Liquid Carbonic. The unwanted opposite isomer of 1-chloropropene and impurities identified as 2-chloropropene, 3-chloropropene, 1-chloropropane, and 2-chloropropane, were removed from both the cis- and trans-1-chloropropene by gas chromatography. Hydrogen iodide was vacuum distilled at 77 and 195 K to remove H2 and I, impurities. Both HI and the purified chloropropene isomers were ( 5 ) Smail, T.; Miller, G. E.; Rowland, F . S. J. Phys. Chem. 1970, 74, 3464. (6) Rowland, F. S.;Iyer, R. S. Atomic Energy Commission Report; University of California: Irvine, CA, 1973; No. UCI-1973-1. (7) Iyer, R. S. Ph.D. Thesis, University of California, Irvine, CA, 1973. (8) Rowland, F. S.;Rust, F.; Frank, J. P. ACSSymp. Ser. 1978, 66, 26.
0 1988 American Chemical Society
Reactions of I8F with cis- and trans-CH3CH=CHC1
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 673
TABLE I: Observed Product Yields from the Reaction of Thermal '*F Atoms with cis-1-Chloropropene and Oxygen at 287 f 2 K press., Torr % yields SF6
cis-C3H5C1
776 1960 3520 56 1 718 1150 2260 2280 3290 1190 1190
19.4 49.4 87.3 13.4 17.3 29.8 75.2 57.8 85.5 230 23
0 2
8.2 21.0 33.8 51.9 57.8 89.1 120 120
SF5I8F 0.93 1.07 1.48 0.93 1.00 1.20 1.21 1.27 1.39 nmb nmb
f 0.13 f 0.08 f 0.04 f 0.17 f 0.14 f 0.09 f 0.07 f 0.07 f 0.05
C ~ ~ - C , H ~ ~ ~trans-C,H518F F 12.2 f 0.3 9.46 f 0.17 12.5 f 0.1 12.4 f 0.4 14.2 f 0.4 13.4 f 0.2 12.8 f 0.2 13.2 f 0.2 13.1 f 0.1 9.71 f 0.17 11.0 f 0.2
5.95 4.95 6.71 6.09 6.59 6.92 6.86 6.73 6.88 5.38 5.69
f 0.23
i 0.21 f 0.09 f 0.31 f 0.37 f 0.19 f 0.22 0.17 f 0.11 f 0.16 f 0.15
*
cis/trans" 2.05 f 0.09 1.91 f 0.09 1.86 f 0.03 2.04 f 0.12 2.15 f 0.14 1.94 f 0.06 1.86 f 0.07 1.96 f 0.06 1.90 f 0.04 1.80 f 0.06 1.93 f 0.06
" [ C ~ S - C , H ~ [~t~r ~F n] /s - C , H ~ ~ ~nm, F ] . not measured. TABLE II: Observed Product Yields from the Reaction of Thermal '*F Atoms with trans-1-Chloropropene and Oxygen at 287 f 2 K press., Torr % ' yields C ~ S - C ~ H ~ ~ ~t rF~ n s - C , H ~ ' ~ F cis/trans" F6 truns-C,H,CI 0 2 SF518F 1.20 f 0.09 11.2 f 0.2 8.64 f 0.17 1.30 f 0.03 1030 21.0 9.13 f 0.23 97 1 20.0 100 1.05 f 0.08 11.8 f 0.2 1.29 f 0.04 91.9 0.99 f 0.08 11.0 f 0.2 8.54 i 0.26 1.29 f 0.05 1010 21.1 1030 1200 1190 1200 1210 1190 1190
22.2 23.0 23.3 23.6 23.0 230 23
99.2 116 129 122 124 120 120
0.97 f 1.12 f 1.17 f 1.14 f 1.08 f nmb nmb
0.08 0.07 0.08 0.09 0.07
11.3 f 0.5 10.9 f 0.2 12.1 f 0.2 10.7 f 0.2 11.5 f 0.2 8.28 f 0.16 9.14 f 0.15
9.18 8.28 9.11 8.10 8.66 6.58 7.06
f 0.66
f 0.16 f 0.17 f 0.18 f 0.17 f 0.16 f 0.04
1.23 f 1.32 f 1.33 f 1.32 f 1.33 f 1.26 f 1.29 f
0.10 0.03 0.03 0.04 0.03 0.04 0.06
[cis-C,H5I8F]/[tr~ns-C,H,~~F]. nm, not measured. stored in the dark at low temperatures to prevent any light-induced isomerization or decomposition prior to the filling of samples. The ampules were filled by the standard vacuum-line techniques described earlier.2-8 Several chromatographic column combinations were used in the separation of the various fluorinated products. The cis- and product isomers found in either the trans- 1- [18F]fluor~pr~pene unscavenged or OZ-scavenged samples were assayed after separation with a 20- or 30-ft dimethylsulfolane (DMS) column at 0 "C. In HI-scavenged mixtures, two columns in sequence were used: a 25-ft silicone oil column operated at 95 OC and a 50-ft DMS column kept at 24 "C. Samples were injected into the gas flowing through the silicone oil/DMS series, and SF5I8Fwas allowed to elute completely through both and then elute into the two-detector sequence. After 50 min, the DMS column was shunted aside, allowing 1-chloro-2-fluoropropaneto elute from the silicone oil column directly into the proportional-counter detector. After an elapsed time of 120 min since the original gas injection, the silicone oil column was removed from the gas train, and the gas flow was then directed through the DMS column to elute the 1-fluoropropene isomers being held there. A 50-ft tritolyl metaphosphate (TTMP) column was used at 50 "C for observation of 1-chloro-2-fluoroethyleneproducts. No radioactivity corresponding to the expected elution time for 1,l -chlorofluoropropane was ever observed. The carbon-bonded I8F yields do not add to loo%, with the remainder assumed to have formed H18Fby either (3) or (8). Experiments with other substrates have confirmed that complementary yields of 18Fcan be found on the walls of the glass ampules, as expected for tracer quantities of HISF.
Results and Discussion The measured absolute percentage yields for the observed 18F-labeled products from a series of either unscavenged or Oz-scavenged samples are listed in Table I for cis- 1-chloropropene as the substrate present in dilute mole fraction. The corresponding results frem trans-I-chloropropene are given in Table 11. Only two major products, cis- and trans-CH3CH=CHI8F,were found from reaction with the substrate in the presence of O2 or in the absence of any scavenger, corresponding to terminal addition in (1) followed by decomposition in (4). The pressure-dependent 1-2% yields of SF,I8F from energetic reaction with SF6 are
consistent with those recorded earlier in the presence of many different substrate molecules.2-8 The quoted standard deviations are one-a errors derived solely from the random statistical errors of radioactive decay. The variation in absolute yields is indicative of the positioning and monitoring errors in the nuclear reactor irradiations. Most of the conclusions are based upon the relative yields of two products, e g , the cis/trans ratio, and are not affected by the variations in absolute yield. The most significant observations from Tables I and I1 are (a) Only about 20% of the '*F radioactivity has been found in these products, with 80% presumed to have reacted to form H18F by hydrogen atom abstraction in (3), or to radicals from (1) or (2) stabilized by collisions in ( 5 ) . (b) Both the cis- and transCH3CH=CHI8F products are found from reactions of '*F with either cis- or trans-CH,CH=CHCl substrates. (c) The quantitative cis/trans product ratio for CH3CH=CHI8F is about 1.9 from the cis substrate and only 1.3 from the trans substrate. Any deviations from these average values are statistically marginal, and we conclude that no significant variations in the cis/trans product ratios have been established except for the difference attached to the cis or trans geometry of the reacting substrate. (d) The presence of O2 in minor concentrations relative to SF6 has no significant effect on the observed I8F product yields. Variations in the total pressure provide no changes in the product yield percentages within the limitations of the statistical reproducibility of the absolute yields. The absence of any Oz scavenger effect on the observed yields requires that none of the I8F atoms are permanently stabilized as I8FO2or as products incorporating this group as an entity. The unchanged distribution of individual product yields presumably means that I8FO2when formed reacts almost entirely by reversible dissociation back to I8F and 02,because it is unlikely that both IsF and '*F02would provide the same products in the same ratio. Two additional product molecules, cis- and trans-CHI8F= CHC1, were observed in very minor yields, summing to less than 0.4% at 3100 Torr total pressure, in experiments with each substrate in which analysis with the long TTMP column was focused only on them. These products represent the loss of CH3 groups from the excited radicals formed by central addition of I8F and, by comparison with the yield of stabilized radicals (see below), indicate no more than about 1% decomposition in competition with
674 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Rogers and Rowland
TABLE III: Observed Product Yields from the Reaction of Thermal **FAtoms with CiS-l-ChlOrODrOWIIe and HI at 287 f 2 K
SF, 568 558 1150 1880 1920 2830 3080 3100 3100 3090 3110 3100 3140
press., Torr C,HSCI
HI
SFSi8F
14.5 30.0 29.5 46.8 51.6 73.7 10.1 10.3 10.3 16.2 9.8 9.4 10.7
13.5 29.4 28.4 50.4 48.0 73.0 11.8 5.5 21.5 33.5 29.8 41.4 31.1
1.06 f 0.12 1.14f 0.14 1.28 f 0.08 1.23 f 0.06 1.19f 0.07 1.17 f 0.06 1.27f 0.05 1.36 f 0.04 1.30 f 0.04 1.23 f 0.04 1.22f 0.05 1.32f 0.05 1.33 f 0.06
% yields CH3CH18FCH2CI CZ'S-C~HS'~F
nmc nm' nm' nmc nm' nm' 30.0f 0.2 36.1 f 0.2 22.0f 0.2 25.0 f 0.2 20.4f 0.2 15.3 f 0.2 20.2 f 0.2
8.88 f 0.24 8.89f 0.25 9.66f 0.20 8.60f 0.13 9.36f 0.18 8.43 f 0.14 9.44f 0.17 11.9 f 0.2 7.20f 0.16 7.17 f 0.15 5.72f 0.15 4.77f 0.14 6.22 f 0.18
r r a n ~ - C ~ H ~ ' ~ F cis/trans" 4.82 i 0.23 5.28 f 0.26 4.99 f 0.18 4.68 f 0.12 4.75 f 0.17 4.40f 0.12 4.90f 0.17 5.99 f 0.17 4.00 f 0.18 3.42f 0.14 3.17 f 0.16 2.56 k 0.16 3.08 f 0.18
1.84f 0.10 1.73 f 0.10 1.94f 0.08 1.84f 0.05 1.97 f 0.08 1.92 f 0.06 1.93 f 0.08 1.99 f 0.06 1.80f 0.09 2.10f 0.10 1.80f 0.10 1.86 f 0.13 2.02 f 0.13
Rb
0.48 0.50 0.51 0.42 0.44 0.48 0.46
[cis-C3H5l8F+ ~~U~~-C~H,~~F]/[CH~CH'~FCH~CI]. cnm, not measured. [~is-C~H~~~F]/[truns-C~H~'~F]. stabilization. Because the yields of the CH"F=CHCl isomers were so low when specifically sought, the sample analyses were usually conducted to favor determination of the products in major yield, and the minor products were not measured. None of the possible IsF/H decomposition products from the radicals of (1) and ( 2 ) were detected, consistent with the expected negligible yields for decomposition by higher energy pathways. The measured product yields for HI-scavenged experiments are given in Tables 111 and IV, respectively, for the I8Fatom reactions with cis- and trans- 1-chloropropene. Only one additional compound, CH3CHlsFCH2C1,was found as a major product in the HI-scavenged systems. The total observed radioactivity in these three organofluorine compounds summed to as much as 5 5 % despite the loss of some I8F to reaction 8 with HI. The undetected 40+% is assigned to HisF formed by either (3) or (8), and the individual importance of these two routes can be assessed by extrapolation of the measurements to zero concentrations of HI. The most significant additional observations from the HIscavenged experiments are (a) The cis/trans product ratio is unchanged by the presence of HI, although the yields of both are reduced. The progressively lower yields are consistent with the removal of I8Fatoms by direct reactions with H I in competition with the olefinic substrate. The rate constant for thermal reaction of I8F with the 1-chloropropene isomers can be measured by quantitative evaluation of this competition. (b) The stabilized central addition product has a yield which is approximately double that of the two decomposition products resulting from a terminal addition reaction. The only significant statistical variations in the yields are their dependence on the identity of the geometrical substrate isomer and the parallel diminution in all three lsF-labeled organic yields as the [HI]/ [substrate] ratio is increased. In sharp contrast to the observation that decomposition is only a minor pathway following central addition of I8Fto CH,CH= CHCI, loss of C1 is the only reaction route found following terminal addition, with no observation of CH3CH2CHC1I8F,expected if stabilization by (8) and subsequent reaction with HI had occurred. We estimate from our limits of detection for CH3CH2CHC1I8F that decomposition is at least 99% complete even at 3100 Torr total pressure for the excited radicals formed by terminal addition of thermal I8F to the 1-chloropropene isomers. The absolute total reaction rate for thermal I8F with the 1-chloropropene isomers was determined by observation of the diminution in measured product yields as the HI/C3H5C1ratio increased. A simple rate constant expression can be written in the form of eq 9, which provides estimates from the intercept at zero [HI] 0.98 km11 = -kT+ (9) yield[CH3CH1*FCH2C1] k2 ~~[C~HSC~I of the fractional yield for the central addition product CH3CH"'FCH2CI and from the slope of the straight line for the relative reaction rates with H I and C3H,Cl. Corresponding equations can be written for each of the CH3CH=CH"F products. The coefficient 0.98 in the left-hand numerator makes
im 0.98 150
I8F+cis-CH3CH=CHCl
intercept=454f023
10 0
50
00
IO
20
30
40
[ H I ] / [cis-C3H5CI]
Figure 1. Reciprocal yields of products from I8F addition to cis-l-
chloropropene graphed versus [HI]/ [C3HSC1]ratio.
I= 100
0.98
-
intercept = 2 2 O f O 3 1
?
00 00
10
20
30
40
-
[ H I I/[ t r a n s C3H5CI]
Figure 2. Reciprocal yields of products from I8F addition to trans-1chloropropene graphed versus [HI] /[C,HSCI] ratio.
allowance for the loss of about 2%of the IsF atoms by reaction with SF,, and kT = k l + k2 + k3. The fractional yields for the observed volatile products are graphed versus the [HI]/[C3H5Cl] ratios for terminal and central addition reactions in Figure 1 with cis-1-chloropropene as the reactive substrate and in Figure 2 for reactions with trans-lchloropropene. The intercepts and slopes from the best fits to these data are collected in Table V. With the known value for k8 = (6.65 & 0.66) X lo-" cm3 molecule-] s-l chosen as the standard for placing these reaction rates on the absolute scale,9 the rate constants for terminal addition, k l , and central addition, k2, were determined for both 1-chloropropene substrates. The slopes and intercepts are very similar with each and are not significantly
Reactions of 18F with cis- and trans-CH3CH=CHC1
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 675
TABLE I V Observed Product Yields from the Reaction of Thermal "F Atoms with trans-1-Chloropropene and HI at 287 f 2 K press., Torr 7% yields SF, C3H5CI HI SF51sF CH3CHlsFCH2CI c ~ ~ - C ~ H , ' ~ Ftrans-C3H5'sF cisftrans" 455 23.7 12.9 1.25 i 0.20 nmc 9.54 f 0.30 7.24 f 0.32 1.32 f 0.07 792 1210 1210 3100 3100 3100 3100 31 I O 3100 3090 3100 3130 3100
39.7 24.6 24.8 10.3 10.1 15.4 25.9 16.0 25.9 24.5 156 154 155
19.3 122 122 40.3 39.8 45.0 51.4 46.4 51.0 79.3 80.0 81.7 80
nmc
nmc
1.09 f 0.08 0.98 f 0.06 1.16 f 0.04 1.25 f 0.05 1.32 f 0.05 1.31 f 0.05 1.30 4 0.05 1.31 f 0.05 1.39 f 0.06 1.26 f 0.05 1.12 f 0.03 nmc
nmc nmc 16.4 f 0.2 14.5 f 0.2 17.3 f 0.2 25.4 f 0.2 18.4 f 0.2 22.7 f 0.2
nmc 34.3 f 0.2
nmc
nmc
8.54 f 0.18 3.33 0.11 3.34 f 0.10 3.58 f 0.12 3.79 f 0.15 4.73 0.13 5.83 f 0.16 4.66 f 0.14 5.81 f 0.16 4.58 f 0.07 8.43 f 0.16 8.14 f 0.08 6.59 f 0.20
*
*
6.43 f 0.14 2.50 f 0.1 1 2.67 f 0.1 1 2.88 f 0.14 3.00 f 0.16 3.79 f 0.15 4.65 f 0.18 3.65 f 0.16 4.77 f 0.17 3.41 f 0.07 6.60 f 0.17 6.26 f 0.07 4.60 f 0.18
Rb
1.33 f 0.04 1.33 f 0.07 1.25 f 0.06 1.24 f 0.07 1.26 f 0.08 1.25 f 0.06 1.25 f 0.06 1.28 f 0.07 1.22 f 0.05 1.34 f 0.03 1.28 f 0.05 1.30 f 0.02 1.43 f 0.07
0.39 0.47 0.49 0.41 0.45 0.47 0.44
[ ~ i s - C ~ H ~ ' ~ F ] / [ t r u n s - C ~ H ~[ c' ~~F~]-. C S H+~ 'tr~.ans-C3H5'8F]f F [CH3CH1sFCH2C1].cnm, not measured.
TABLE V Estimated Reaction Rate Constants for Competitive Thermal I8F Reactions with cis - and trans-1-Chloropropene in Mixtures with Hydrogen Iodide at 287 f 2 K
substrate cistransCH,CH=CHCl CH,CH=CHCI ~
rate const ratio kS/kl kTfkI kS/k2
kdk2
~~~~
* *
2.06 f 0.09 4.54 f 0.23 0.93 f 0.06 2.20 f 0.17
2.34 0.14 5.09 f 0.38 1.07 0.11 2.20 f 0.31
3.2 f 0.4 7.2 0.9 15.2 f 2.9
2.8 f 0.3 6.2 f 0.9 14.1 f 3.4
absolute reaction rate const," lo-'' cm3 molecule-' s-' kl k2 kT
*
"All rate constants have been calculated relative to the value (6.65 f s-l for k8.
0.66) X lo-" cm3 molecule-I
different from one another within the accuracies of our data. The absolute reaction rate for addition of thermal 18F to cis-lchloropropene has been measured to be (10.4 f 1.O) X lo-" cm3 molecule-' s-l, and that for addition to the trans compound is (9.0 f 0.9) X lo-" cm3 molecule-' s-l. In each case, the total absolute reaction rate with the inclusion of H abstraction in (3) is (15 f 3) X lo-" cm3 molecule-' s-l. These absolute reaction rate constants are given in Table V. A comparison is made in Table VI of the absolute reaction rate constants and the ratios of terminal/central addition for and 3-chloropropene," as well as for the two 1-chloropropene isomers studied here. The rate constants for olefinic addition to the 1-chloropropeneisomers are essentially the same as for addition to propene, indicating that the chlorine substituent adjacent to the double bond appears not to have any effect on the overall probability of the olefinic addition of atomic fluorine. On the other hand, the more distant, out-of-plane C1 atom in the methyl position of 3-chloropropene appears from the data in Table VI to suppress slightly the overall probability for fluorine addition to the olefin. This conclusion is not firm, however, because the competitive study of the thermal I8F reactions with 3-chloropropene was carried out separately with each of four different competitor molecules, with not wholly consistent quantitative results in the estimates of the reaction rate constant^.^ The competitive reactions with C2H2, CH4, and H2 all indicated overall reaction rates higher than the (10.8 f 1.2) X lo-" cm3 molecule-' s-I measured with HI as the competitor. If the actual sum of the reaction rate constants for addition is somewhat higher than indicated by using H I as the standard, then the absolute reaction rates for all of the olefinic additions of Table VI are essentially the same within an error margin of 10-15%. The measured absolute reaction rate constants in the range of cm3 molecule-' s-' for olefin addition correspond to successful bond formation on almost every collision of the fluorine atom with the a-electrons of the substrate molecule.
TABLE VI: Absolute Reaction Rate Constants Measured for Thermal '*F Reactions with CH3CH=CH2,CH2CICH=CH2, cis -CH,CH=CHCI, and trans-CH3CH=CHC1 in Competition with Reaction with Hydrogen Iodide substrate cis-CH3CH=CHC1 rruns-CH3CH=CHCI CH,CICH=CHi CH$H=CH2'
kTU 15.2 f 14.1 f 10.8 f 19.0 f
kADDN"
terminal/central addn ratio
2.9 10.4 f 1.0 3.4 9.0 f 0.9 1.2 7.9 f 0.8 3.0 10.4 f 1.6
"All rate constants in units of lo-" cm3 molecule-' 4. 'References 1-3.
0.47 f 0.44 f 1.55 1.35 f
*
0.03 0.04 0.05 0.04
bReference
SKI.
While not affecting the summed rate constants for addition to the various double bonds, the presence of a chlorine substituent adjacent to the double bond clearly affects the ratio of terminal to central addition. The terminal/central ratio of 0.46 f 0.03 for the 1-chloropropene substrates shows a definite shift of the addition process toward the central atom, in contrast to 1.35 f 0.05 for and 1.55 f 0.05 for 3-chl0ropropene.~Steric and electronegativity effects should be at a minimum for propene, and fluorine atoms show a small preference for addition to the terminal position. Substitution of C1 in the 3-position enhances this preference for terminal addition slightly, but C1 in the 1position directs the addition away from the 1-position toward the central atom. Because addition appears to occur just as readily with each substrate, the observed differences in the ratio of terminal to central addition appear to be the consequence of directional influences acting after bond formation has been assured-preferences for permanent bonding to the C-1 and C-2 carbon positions competing for a fluorine atom already trapped into addition to the a-system. The CH3CHI8FCH2Clmolecules formed by the successive reactions of central addition, stabilization, and then reaction with HI, are measured only in the presence of the scavenger. Extrapolation of the fractional yields of this product to zero H I concentration shows that about 45% of the I8F released in this system followed the stabilization route, with no more than 0.4% decomposition. This decomposition/stabilization ratio of about 0.01 at 3100 Torr total pressure is several times smaller than that found for propene (D/S = 0.03)'-3 at the same pressure and indicates an average lifetime toward loss of C H 3 by CH3CHl8FCHC1*of about s. However, the substitution of the heavy C1 atom for H provides more low-frequency molecular vibrations for storage of internal energy and a slower decomposition rate from RRKM calculations. The ratio of the yields of cis-CH3CH=CH18F to transCH3CH=CHI8F is given in Tables I-IV for a wide variety of experimental conditions and competition among substrates and has an average value of 1.29 f 0.04 from reaction mixtures involving the trans-CH3CH=CHC1 substrate and 1.93 f 0.10 from the cis substrate. The relative stabilities of the cis- and trans- 1-fluoropropene isomers have been determined by Abell and
676 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 AdolphIo and by Whangbo et al.” In these experiments, the cis isomer has been determined to be more stable than the trans isomer by 0.75 kcal/mol, and the barrier against rotation of the methyl group from eclipsed to staggered conformation has been estimated as 0.65 kcal/mol. In the experiments of Abell and Adolph, the cis/trans isomer ratio was determined over a temperature range from 150 to 250 “ C and pressures of 5-50 Torr for the HBrcatalyzed photoisomerization from CH2FCH=CH2. The cis/ trans CH3CH=CHF product ratio ranged from 2.45 at 150 “ C to 2.09 at 250 “C. We reject the possibility that our observed product isomer ratios of 1.93 f 0.10 from the cis compound and 1.29 f 0.04 from the trans are directly related to temperatures (e.g., 300 “ C for 1.93), which would be calculated from an extrapolation to higher temperatures of the results of Abell and Adolph. The excitation following thermal addition of I8F is greatly in excess of that estimated for thermal reactions at 300 “ C and hence cannot explain through temperature alone the observed isomer ratios. Instead, the disparity in cis/trans product ratios from one substrate to the other suggests an origin other than simple differences in total energy content of the decomposing CH3CHCHI8FCl*radicals. The enhanced preference for “cis from cis” and reduced preference for “cis from trans” is consistent with a mechanism in which the incoming thermal 18Fatom tends to add to the olefinic position while displacing the outgoing C1 atom with retention of the original geometry. This yield is then coupled with a much larger yield of CH3CH=CHI8F product resulting from addition of the I8Fatom to the olefin, followed by rotational motion about the C-C bond to erase any memory of the initial geometric orientation. Our qualitative interpretation is that a preference exists during approach of the fluorine atom s) departure to the r-system such that immediate (Le., of the C1 atom would leave the fluorine in the position of the original geometric isomer. However, if the departure of the C1 atom is delayed for a few picoseconds, rotation about the new single bond ensues and both geometric product isomers are formed in a ratio consistent with a very highly excited radical, much more excited than the radicals involved in the experiments of Abell and Adolph. Quantitatively, the ratio of 1.93 f 0.10 represents 65.8 f 1.2% cis and 34.2 f 1.2% trans product, and 1.29 f 0.04 indicates 56.3 f 0.8% cis and 43.7 f 0.8% trans. A composite isomer distribution with 10% of the CH3CH==CH18F product retaining the geometric configuration of its particular parent because of rapid C1 loss, and 90% being distributed between cis and trans in the ratio 56%/34%, then produces a 66%/34% product ratio from cis-CH3CH=CHC1 and a 56%/44% ratio from trans-CH,CH=CHCl, consistent with the observations. The 56/34 (=1.65) ratio of the equilibrating product is consistent by extrapolation of the Abell-Adolph data with the isomer ratio expected from a thermal system at 750 K, favoring the more stable isomer but by less than in their experiments conducted at 250 “C. Our experiments were, of course, carried out at 287 K, but the excited radicals formed in (1) (a) are highly energetic; (b) are not in thermal equilibrium with the system; and (c) never reach equilibrium because they all decompose by CI atom loss before a stabilizing collision can occur. Geometric isomerization might perhaps be possible if an extremely excited molecule is left behind as the C1 atom exits. We do not know of any quantitative information about the activation energy toward rotation about the incipient C=C bond in a very excited molecule; in this situation, the calculated “equivalent temperature” of 750 K for the geometric isomers could be a (9) Mo, S.-H.; Grant, E. R.; Little, F. E.; Manning, R. G.; Mathis, C. A,; Werre, G. S.; Root, J. W. A C S S y m p . Ser. 1978, 66, 59. (IO) Abell, P. I.; Adolph, P. K. J . Chem. Thermodyn. 1969, 1 , 333. ( 1 1 ) Whangbo, M. H.; Mitchell, D. J.; Wolfe, S. J . Am. Chem. SOC.1978, 100, 3698.
Rogers and Rowland measure of the average of the residual energies still present when a &/trans ratio gets “frozen in” during the progressive removal of such high excitation energy through collisions with the bath molecules. Because our data do not indicate any appreciable variation in the cis/trans ratio except with the geometry of the parent molecule, we are unable to distinguish whether the 10% retention component of the proposed mechanism is a direct displacement of C1 by F or an addition of F to the olefinic position followed by loss of C1 from the excited radical so quickly that less than half of one rotation has been completed. Direct displacement of 18Ffor C1 has been regularly found with saturated substrates such as chlorofluorocarbons1z~13 and CH3C1,14x’5but normally such reactions occur in much lower percentage yield even when initiated by energetic “hot” I8F atoms and in barely detectable yields with thermal 18Fatoms.I6 On the other hand, the C1 atom in an olefinic position is probably more sterically exposed than in the saturated systems. The distinction between retention of the original geometry by the direct and by the addition/decomposition mechanisms could be so blurred as to be mostly a semantic division. In any case, a three-way kinetic competition appears to be occurring after fluorine atom addition to a terminal CHCl olefinic position: (a) stabilization of the excited radical by collision with the SF, bath molecules; (b) loss of C1, leaving behind the components of 1-fluoropropene labeled with I8F; and (c) rotation about the nascent C-C bond formed by addition to the original C=C position. The highest pressures of our experiments were more than 4 atm, corresponding to a collision time for excited radicals with SF6 of about s. Because stabilization with retention of the C1 atom within the radical fragment was not observed at all (upper limit < l%), the reaction rate for loss of C1 appears to be at least 100 times faster than collisional stabilization, or about s. The rotational period of a CH,CHCHClF* radical would also be expected to fall into the 10-% range, so that competition between loss of the C1 atom and rotational loss of the “memory” of the original geometry is not unreasonable. The loss of the C1 atom from the excited CH3CHCHFC1* radical on the 10% time scale implies a negligible energy barrier in the exit channel and consequently a very small activation energy for the reverse process of addition of atomic C1 to the terminal C H F position in CH3CH=CHF. No measurements seem to have been made of the activation energy for this particular terminal chlorine atom addition to CH3CH=CHF. (It is not even clear without measurement of the kinetic energy of the exiting chlorine atom that the correct reverse reaction is the thermal C1 atom addition.) Other experiments in this laboratory1’ have shown that thermal 38Clatoms are able to add with significant yields to either end of CH,=CHF, and the resulting 2/1 preference for addition to the CH2 end is consistent with an energy barrier as small as 500 cal/mol toward addition to C H F by thermal atomic chlorine. Acknowledgment. This research was supported by Department of Energy Contract No. DEFG-03-86AR-13469 and formed part of the Ph.D. thesis of P.J.R. Preliminary experiments in this laboratory by Dr. Gerald F. Palino provided useful initial information. Registry No. (2)-CICH=CHMe, 16136-84-8; (E)-ClCH=CHMe, 16136-85-9; “F, 13981-56-1. ( 1 2) Rogers, P. J.; Rowland, F. S., unpublished experiments. (13) Palino, G. F.; Rowland, F. S. Radiochim. Acta 1971, 15, 5 7 . (14) Spicer, L.; Todd, J. F. J.; Wolfgang, R. J . Am. Chem. SOC.1968, 90,
7T -7 A.--.
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nail, T.; Iyer, R. S.; Rowland, F. S. J . Phys. Chem. 1971, 75, 1324. lyer, R. S . ; Rowland, F. S. J . Phys. Chem. 1981, 85, 2488. (17) Iyer, R. S . ; Chen, C.-Y.; Rowland, F. S . J . Phys. Chem. 1985, 89, 2042. (16,
