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A long-lived C2H3ClBr* radical from the reaction of atomic chlorine with vinyl bromide
CHEhiICAL
Volume 103, number 3
A LONG-LIVED C,H,ClBr*
PHYSICS
30 December
LETTERS
1983
RADICAL FROM THE REACTION
OF ATOMIC CHLORINE WITH VINYL BROMIDE
R. Subramonia Department
IYER and F.S. ROWLAND
of Chemistry, University of Gdifomia. Irvine. Cidifomia 92717,
Received 28 July 1983; in final form 17 October
USA
1983
The reaction of Cl with CHz=CHBr produces both CHz=CHCl and (with HI present) CHzCICH2Br in pressure-dependent yields, indicating a long-lived (~lF’” s) CzHsClBr* precursor to CH2=CHCI. The precursor structure(s) may be CH&ICHBr* and/or CH+ZHClBr*, probably the latter with a 1,2-Br shift included in the mechanism. The addition reaction itself may not be very “anti-Markownikoff ‘, gaining that appearance by the subsequent 1,2-halogen shift.
The stoichiometry of the addition of atomic chlorine to gaseous vinyl bromide at low pressure is simply the displacement of Br by Cl as in [l] Cl + CH2=CHBr
h CH,=CHCl + Br,
(1)
but the mechanistic details of this reaction are not well established. Such addition is usually postulated to be “anti-Markownikoff” with Cl attaching initially as in Cl + CH,=CHBr
--, CH,CICHBr*
(2a)
to the CHz end of the olefm, forming the energized radical CH2ClCHBr* [2]. This qualitative preference for formation of CH,ClCHBr* rather than CH-JHCiBr” from Cl + CH2=CHBr
+ CH,CHClBr*
(2b)
is rarely formulated quantitatively, and both radicals may very well occur initially. The appearance of CH,=CHCI has been rationalized through the postulate of the 1,2&lorine atom shift in CH2ClCHBr* followed
--f CH,CHCiBr*,
(3)
rapidly by the loss of Br as in [3--61
CH,CHClBr*
--, CH,=CHCl + Br.
(4)
The lifetime of the CH$HClBr* radical has been calculated with the RRKM formalism to be only 0.1 X lo-l2 s [6j, much shorter than the estimated rotational period (3 X lo-l2 s) for the radical. Forward-back0 009-2614/83/S
(North-Holland
03.00 0 Elsevier Science Publishers B.V.
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Division)
ward asymmetry was observed in one molecular beam study [6], but an earlier investigation gave a symmetrical distribution which was attributed to a lifetime greater than 5 X 10-12 s for reaction (3) [3] _ Infrared ChemiIuminescence has provided information about the distribution of excitation energy within the modes of the product CH,=CHCI [4,5] _Very recently reaction (1) has been proposed as a titration reaction for Cl in fast-flow systems, ad a reaction rate constant of (1.4 ? 0.3) X lo-lo cm3 molecule-l s-1 measured for it [ 11. We have previously used reaction (1) as a comparison standard in several intermolecular competitive chlorine atom systems [7,8], and have observed that (1) alone is not sufficient to describe the full mechanism at pressures in the vicinity of 1 atm [9] _We now report further experiments on the reaction of Cl with CH,=CHBr over the pressure range from 50 to 4000 Tori in a bath gas of CClF,. These experiments show pressure-dependent yields of CHZ=CHC1 and CH2CICHZBr, providing evidence for collisional stabilization of C2H3ClBr* radicals on a time scale longer than 10-10 s. Our experiments have utilized radioactive 38~1 atoms formed in situ by neutron irradiation of CCIF, and thermalized by multiple collisions wiLLhCCIF, prior to reaction with the vinyl bromide substrate. Gtieous samples containing >90% CCIF,, 20-40 Torr of Ar (as a neutron flux monitor) [lo] and small fractions of CH,=CHBr were prepared using standard vacuum 213
CHCMICAL PHYSICS LETTERS
Volume103. number 3
lie techniques. Hydrogen iodide was frequently added as a radical scavenger to convert C,H,CIBr radicals
into C,H,ClBr,
although it thereby became a competi-
tor with vinyl bromide for the initial reaction of atomic chlorine. L_essthan 5% of the total 38C1 produced in
these samples reacts with CClF, in energetic reactions,
30 December 1983
and the remaining 95% are available as thermalized
atoms for reaction with the other substrate molecules. The radioactive products were analyzed by the standard procedures of radio gas chromatography [ 10,111, and included CH,=CH38Cl from CH,=CHBr plus C38C1F3, etc., from the hot reactions with the bulk molecule.
Table 1 Percentageproduct yields from “Cl reactions with CHa=CHBr and HI Pressure(Torr) CClFs
Product yields a)
CHz==CHBr
HI
CHa3’CICH2Br observed
3980 3990 3920 3930 3950 3890
3850 2990 2900
10.6 47.2 475 40.9 39.8 20.2 19.9
936 952 954 933 921 466 474 205 205 199 191 191 181 98 90 80
10.1 29.9 9.9 10.7 10.4 11.5 9.9 5.6 5.8 5.1 5.0 5.4 5.4 5.4 55 4.1 5.0 5.9
93 47 25
5.5 4.1 2.2
CH2=CH3*CI
(C)
(D)
-
-
-
7355 47 27 17 -
65 45
46 -
42
42 -
10.7 21.6
32 22
30 20
34.9 39.6 O_ 6.2 0 b) 0 5.8 17.1 216 27.7 0 5.2 17.7 22.0 0 0
17 12 -
15 12 -
29 19 15
15
19 -
18 -
10 5.0 5.3 3.3 -
10 4.8 4.0 3.3
10 4.8 4.1 3.3 -
4.5 2.2 1.7
4.8 2.4 2.2 -
4.8 2.5 2.3 -
0 0 b) 0 10.2 40.0 61.2 101 0 30.3 -
-
-
24
17 -
65 46 25 17
12 -
observed
(C)
(D)
22.6
19 19 19 16 11 6.2 4.3 22 13 40 21 14
19 19 19 16 11 6.2 4.3
23.9 22.9
17.7 11.7 6.0
4.2 24.5 13.7 47.1 24.0 15.6 12.9 9.7 63 32.7 81* 3 831 3 36.6 19.4 14.2 11.4 92 * 5 36.3 17.0 11.8 885 8 1105 15
11 9 56 27
73 73 33 16 13 11 84 37 18 14 90 92 -.
23 14 44 24 16 13 10 59 30 74 74 35 17 14 12 84 38 19 15 90 92
a) The yields in columns (C) and (D) correspond to the kinetic mechanisms described In tire text with these parameters: (C): k5/k2 =0.68;k2bcO.lka; k4>>>ks[M]; ks/k6=0.67;andks=k,[M] at 900Torr.@): ks/k2=0_62; k2b=0.49k2;k3=0; k4 = ks[M] at 1850 Torr and k6 = k7]M] at 650 Torr. b, 11 Torr 02 present in these samples.
214
CHEMICAL
Volume 103, number 3 r\_E
8-
“.JiJ
C‘I
f
CH..=
30 December
PHYSICS LETTERS
[CH$ICH2Btj
P
kH2
CHBr
= CHCU
10.0 40_-
6-
.
100
0
2 IO
0
1.000
0
4.000
2.0
:
I .o
-
03
-
4
2
[CHz
0
.
0.1
Cl
210 100
I
1
6
1
/
[ctit
=
CHB~]
’
Fig. 2. Ratio of product yields for CHa3aClCH2Br and CH2=CH3’C1 from reactions of thermal 38C1 with CHa=CHBr
= CHBr]
from table 1.
When HI was present, CHz38ClCHzBr was observed, but no radioactive peak corresponding to CH3CH38ClBr (
I .ooo
0
summed yields of CH2=CH3aCI and CHa3%ZICHaBr from reactions of thermal 3sC1 with CHa=CHBr as a function of [HI]/[CHa=CHBr] ratio and total pressure. -Calculated values with mechanism (D) for total pressures ranging from 2 Torr to infinite pressure, using p-eter values
reactivity
.
8
Fig. 1. Reciprocal
The relative
4.000
0
.
torr
0
r-l
”
FtiI]
[HI] /
1983
can be
evaluated through the fractional diminution in the yields of CH,=CH38Cl and/or CH,38C1CH,Br as 38C1 atoms are diverted away from CHz=CHBr by reaction with HI. The inverses of the sum of these observed yields are graphed in fig. 1 after allowance for 5% hot reaction. The increased slopes at the lower pressures demonstrate that the apparent reactivity of CH2=CHBr relative to HI decreases with decreasing total pressure. Although the ratios of the yields [CH,38ClCH,Br] / [CH,=CH38C1] vary by a factor of 60 between 50 and 4000 Torr total pressure, they are essentially independent of the [HI] / [CH2=CHBr] concentration ratios at any given total pressure, as shown in fig. 2. With increasing [HI] /[CH2=CHBr] ratios, both products are proportionally reduced in yield as 38C1 atoms compe-
as a function of [HI]/[CH2=CHBr] ratio and total pressure. -Calculated values with mechanism (D) for 100, 2 10. 1000 and 4000 Torr total pressure, using parameter values from table 1. titively
react
instead
with
the alternate
substrate
HI.
The most important observations from these experiments are: (a) the yield of CH2=CH3”CI from (1) is pressure dependent, varying from about 23% at 4000 Torr to 95 f 5% at 50 Torr; (b) another product, CH138C1CH,Br. is observed from j8Cl atom reaction in mixtures of CH?=CHBr and Hi, and is attributed to H-atom abstraction from HI by a stabilized C,H338C1Br radical; (c) the yield of CH23BC1CH1 Br is also pressure dependent, ranging as high as 75% at 4000 Torr, and as low as 10% at 100 Torr both yields being extrapolated to zero HI concentration; (d) the relative yields of CH238ClCH,Br and CH,=CH38C1 vary with pressure, but not with [HI] / [CH2=CHBr] ratio; (e) the yield of CH3CH38CIBr, for which the radical CH,CH38ClBr is a logical precursor, is always
Volume103, number 3
CHEMICAL
38Cl + HI + H38C1 + 1
PHYSICS LETTERS
(5)
is-accepted [8] as 1.3 X lO-‘O cm3 molecule-1 s-l, then the absolute reaction rate constant for net removal of 38C1 atoms by CH,=CHBr varies from about 1 X lo-10 cm3 molecule-l s-l at low pressure to about 2 X 10WIOcm3 moleculeS1 s-l at the high-pressure limit. These observations place some strong constraints on the mechanisms applicable to the reactions of WI with CH,=CHBr: (a) the pressure dependence of the yields for CH2=CH3*C1 and for CH,38CICH,Br implies some intermediate radical with a lifetime comparable to collision frequencies in this system, i.e. 1O-g-lO-1o s; (b) yields of CH, 38C1CH2Br >70% at high pressure and of CH2=CH38CI >90% at low pressure required the inclusion of a 1 J-halogen atom shift in the mechanism, with reaction (3) an obvious possibility; (c) the reduction in reactivity of CH,=CHBr at low pressure implies a step such as CH238 ClCHBr* + CH2=CHBr + 38C1,
(6)
which is capable of returning pressure-dependent amounts of 38C1 to the system. The mechanisms which are potentially applicable to this system can include various combinations of reactions (2)--(6), together with the stabilization reactions CH2 38ClCHBr* + M + CH,38ClCHBr + M,
(7)
CH2CH3*ClBr* + M + CH2CH3*CIBr + M,
(8)
and the reaction of stabilized radicals with HI by R+Hl+RH+l
(9)
to provide stable molecules for subsequent measurement_ We have found it very useful to add one more reaction, the 1,2-Br atom shift of CH2CH3*ClBr
+ CH2BrCH38CI,
(10)
to provide the complete set from (2) to (10). One mechanism (A) which avoids the 1,2halogen atom shifts of (3) or (10) and appears qualitatively appropriate is the predominant formation of CH238CICHBr* initially (kza > k.&), with a lesser original formation of CH,CH38ClBr* followed by irnmediate Br loss (k4 S ka [M] ). Relatively rapid loss of 216
30 December
1983
38C1 by (6), i.e. K, > k, [M], could permit the recycling of 38Cl through the competitive reaction system, with the yield of CH2=CH3*C1 accumu!ating to a high percentage after many such cycles in low-pressure systems. While such a mechanism would qualitatively produce pressure dependence for both CH,=CH38CI and CH238CICH2Br, quantitative formulation shows a strong dependence of their yield ratio on the [HI] / [CH?=CHBr] ratio, in disagreement with fig. 2, and mechanism (A) cannot account for the actual observations. Mechanism (B), also unsuccessful, involves a minor initial pathway tc CH2=CH3*C1 by (2b) + (4) with k, + k8 [Ml, plus predominant addition (kza > k,,) to form CH238C1CHBr*. The pressure-dependent isomerization (3) furnishes additional CH2=CH3*CI at low pressures (i.e. k competitive with k, [MI). In this formulation all 3 gC1 atoms reacting with CH2=CHBr lead to either CH2=CH3*C1 or CH238ClCHZBr at all pressures, and the removal of 38C1 atoms is pressure independent in contradiction to the observations of fig. 1. Mechanism (C) is a modification of(B) which includes both isomerization (3) and loss of 3gCl by (6) for the kinetic disappearance of CH,38CICHBr*, and can successfully reproduce the observations. The necessary quantitative kinetic parameters include about S--90% initial formation of CH,3gClCHBr* (kzb = 0.1 kh) and roughly comparable reaction rates for k, , k6 and k7 [M] at 1000 Torr. However, another quite different mechanism (D) also exists which fits the observations equally well. Chlorine atoms are postulated to react to form both CH,38ClCHBr* and CH2CH38CIBr* in generally comparable initial yields (kza = k,,). No 1,2-&orine isomerization is included (k3 = 0) and the only reactions available to CH?38ClCHBr* are collisional stabilization (IQ [M]) or t&loss of 38~1 by (6) The lifetime of CH2CH3*C1Br* is assumed in (D) to be in the 10mg10-l” s range (k = k8 [MI), such that reaction (4) provides more C&1=CH38CI at low pressures and CH2CH38ClBr* radicals tend to be stabilized at high pressures. The critical postulate in mechanism (D) is the additional assumption that the stabilized CH2CH38ClBr radicals undergo the 1,2-bromine atom shift of (10) prior to reaction with HI for form C2H438C1Br. This 1,2-bromine shift satisfactorily explains the absence of CH,CHsaCIBr as an observed
Volume 103, number 3
CHEhlICAL PHYSICS LETTERS
product, while the measured CH,38ClCH2Br is expected from reaction with HI by either CH2 38C!CHBr or CHZBrCH3*C1 radicals. Quantitative formulation of mechanisms (C) and (D) requires knowledge of several parameters, including: (a) the relative reactivity of HI versus CH,=CHBr (kz versus k,); (b)the distribution of initial addition between CHZ3*ClCHBr* and CH&H38ClBr* (kza versus k2J; (c)therate constants for loss of Cl from CH,38ClCHBr*, and for 1,2-U isomerization by (3) versus collisional stabilization (k3 and k, versus k, X
WI); (d) the rate constant for loss of Br from CH2CH38ClBr* versus collisional stabilization versus k, [M]); and
(X4
(e) the average energy loss for these excited radicals in collision with CClF, _ None of these parameters has been fured from any independent experiments, and satisfactory fits to the observations can be obtained with various combinations of such values. Reasonable parameters are found with mechanism (C) for 85-90% reaction at the CH2 end of CHz=CHBr, k, =Sk,, the rate of collisional stabilization (k7 [MI) equal to k, -i-k, at about 400-600 Torr in CClF,, reaction (4) assumed to be too rapid (k4 3 k8 [Ml) for any collisional stabilization to occur, and intermediate energy losses (a few kcal/mole) in stabilizing collisions. Reasonable combined sets of parameters for mechanism (D) are found with SO-60’5% initial reaction at the CH, end, e.g. k?b = (0.7-l .O)kZa, with k4 = kg [hi] at about 500-750 Torr and k, = k8 [M] at 1500-2000 Torr, with k3 taken as zero, and intermediate collisional energy losses. The limiting total reactivity of CH?=CHBr at high pressure is approximately 1.7 times that of HI for both mechanisms (C) and (D). The calculated variations in yields of CH,=CH38Cl and CHZ38C1CH?Br for one such set of parameters with mechanism (D) a;e graphed in fig. 1 and 2, and listed in table 1_ A set of parameters which provides comparable fits to the observations for mechanism (C) is also listed in table 1. Some circumstantial evidence exists which is more easily consistent with mechanism (D) than (C). The comparable data for s8C1 reactions with CH2=CHF are readily satisfied by the analog of mechanism (A) with-
30 December 1983
out recourse to any step involving a 1 &halogen shift [ 121. The isomeric 1.2 shift of a halogen atom in a radical is frequently postulated for bromine. sometimes for chlorine, and almost never in an authenticated system for F [ 131. The postulate of a 1,2-bromine shift in (D) provides the needed differential behavior when applied to CH,=CHBr versus CH?=CHF. A second observation is the known isomerization of halogen atoms QWYZ~ from a multiply-halogen-substituted carbon atom toward the CH? end of the molecule [Is, 14]_ An example of such a shift is the 1,2Cl atom shift from CH,CCl, to CH?ClCC12, such that the radical with term&al Ccl, is usually not observed_ The 1.2Br shift of (10) is consistent with the direction of these other observed halogen atom shifts. The absence of any yield for CH,CH3*ClBr introduces some lower limits on the needed rapidity of 1 ,I?-Br isomerization in order to avoid its formation, i.e. kilo versus k9 [Hl] _On the other hand, if bromine atom isomerization were too fast. then no loss of Cl could be observed, so that k,. for the excited radical cannot be much faster than k 4, and probably is slower. Not very
much information is available concerning the rate constants for radical reactions with HI, and obviously none can be available for the reaction rate constant of the not-yet-detected CH,CHClBr radical. Experiments with another haloethyl radical (CH?FCH,) in 4000 Torr SF6 have shown essentially equal reaction rates with HI and with O2 [ 151, while the high-pressure rate constants for R + O2 + M to fomr RO, are given as 2 X 10-l’ and 7 X lo-l1 cm3 molecule-1 s-l for CH3 and C2H5. respectively [ 161. By analogy, the approximate estimate can be made that the rate constant for reaction (9) with CH2CHC1Br is also about 4 X 1O-1z cm3 molecule-l s-l. The rate constants for other small organic radicals with HI are similar in magnitude [ 17, IS] _The needed rate constant for the 1,2-Br isomerization in mechanism
(D) to satisfy the various observations falls in the log1011 s-1 range, and appears to be a plausible hypothesis [13,14]. Our experiments demonstrate the existence of an intermediate long-lived relative to the rotational period of CZH3Br38C1* radical. but do not clearly identify whether that long-lived radical is CH238ClCHBr* in mechanism (C) or CH2CH38C1Br* in mechanism (D). Comparison with product analysis in a variety of other systems leads us to believe that mechanism (D) with a long-lived CH,CH38ClBr* radical is probably the dom217
Volume103, number3
CHEMICALPHYSICSLETPEES
inant mechanism. An interesting corrollary to mechanism (D) is the observation from the quantitative parameters that there is little or no (i.e. k3b 7 (0.7-l .O)k2Q “anti-Markownikoff’ preference in the initial addition of Cl to CH?=CHBr. If(D) is correct, then the product distribution which gives the appearance of “anti-Markownikoff ‘, i.e. a high yield of CHZClCH2Br and an absence of CH-JHCIBr, is the consequence not of the initial addition process, but of the subsequent postulated 1,2-bromine atom shift. The generality of such an explanation for “anti-Markownikoff’ observations would certainly be worth exploring if mechanism (D) is shown to be operative here. While mechanisms (C) and (D) are not distinguishable in our present experiments with CH2=CHBr, they will lead to different products if Cl atoms are reacted in the gas phase with olefins tagged on one end, as with CHt=13CHBr, CH,=CFBr, CH2=CBrCH3, etc. Thjs research has been supported by Department of Energy Contract No. DE-AT-3-76ER-70126.
References [l] J.-Y_ Park, 1-R. Slagle and D. Gutman, J. Phys. Chem. 87 (1983) 1812. [2] J_D. Roberts and MC. Camrio, Basic principles of organic chemistry (Benjamin, New York. 1977) p_ 390.
218
30 December1983
[3] J.T. Cheung, J.D. McDonald and D.R. Herschbach, J. Am. Chem. Sot. 95 (1973) 7889. [4] J.G. Moehlmann and JD. McDonald, J. Chem. Phys. 62 (1975) 3052. [S] JE. Durana and J.D. h&Donald, J. Chem. Phys. 64 (1976) 2518. [ 61 R J. Buss, M J. Coggiola and Y _T. Lee, Faraday Discussions Chem. Sot. 67 (1979) 162. [7] FSC. Lee and FS. Rowland, J. Phys. Chem. 81 (1977) 86. [8] R.S. Iyer, PJ. Rogers and FS. Rowland, J. Phys. Chem.. to be published_ [9] D. Herschbach, Discussions Faraday Sot. 67 (1979) 250251. [IO] FSC. Lee and FS. Rowland. J. Phys. Chem. Sl(l977) 1229. [ 111 FSC. Lee and F.S. Rowland, J. Phys. Chem. 81(1977) 1222. [ 121 C--L. Chen and FS_ Rowland, unpublished remits. [I 31 PS. Skell, RX Pavlis, DC. Lewis and K J. Shea, J. Am. Chem. Sot. 95 (1973) 6735. [ 141 RXh. Friedlina, in: Advances in free radical chemistry, Vol. 1, ed. GH_ Williams (Academic Press, New York, 1965) p. 211. [IS] RL. hlilstein, R.L. Williams and FS. Rowland, J. Phys. Chem. 78 (1974) 857. [ 161 DL. Baulch, RA. Cos, PJ. Crutzen, Rf. Hampson, JA. Kerr, J. Troe and R.T. Watson, J. Phys. Chem. Ref. Data 11 (1982) 327. [ 171 D.M. Golden, R. Walsh and SW_ Benson, J. Am. Chem. Sot. 87 (1965) 4053. [ 181 DM. Golden, AS. Rodgers and S.W. Benson, J. Am. Chem. Sot. 88 (1966) 3196.
Volume 103, number 3
A LONG-LIVED C,H,ClBr*
PHYSICS
30 December
LETTERS
1983
RADICAL FROM THE REACTION
OF ATOMIC CHLORINE WITH VINYL BROMIDE
R. Subramonia Department
IYER and F.S. ROWLAND
of Chemistry, University of Gdifomia. Irvine. Cidifomia 92717,
Received 28 July 1983; in final form 17 October
USA
1983
The reaction of Cl with CHz=CHBr produces both CHz=CHCl and (with HI present) CHzCICH2Br in pressure-dependent yields, indicating a long-lived (~lF’” s) CzHsClBr* precursor to CH2=CHCI. The precursor structure(s) may be CH&ICHBr* and/or CH+ZHClBr*, probably the latter with a 1,2-Br shift included in the mechanism. The addition reaction itself may not be very “anti-Markownikoff ‘, gaining that appearance by the subsequent 1,2-halogen shift.
The stoichiometry of the addition of atomic chlorine to gaseous vinyl bromide at low pressure is simply the displacement of Br by Cl as in [l] Cl + CH2=CHBr
h CH,=CHCl + Br,
(1)
but the mechanistic details of this reaction are not well established. Such addition is usually postulated to be “anti-Markownikoff” with Cl attaching initially as in Cl + CH,=CHBr
--, CH,CICHBr*
(2a)
to the CHz end of the olefm, forming the energized radical CH2ClCHBr* [2]. This qualitative preference for formation of CH,ClCHBr* rather than CH-JHCiBr” from Cl + CH2=CHBr
+ CH,CHClBr*
(2b)
is rarely formulated quantitatively, and both radicals may very well occur initially. The appearance of CH,=CHCI has been rationalized through the postulate of the 1,2&lorine atom shift in CH2ClCHBr* followed
--f CH,CHCiBr*,
(3)
rapidly by the loss of Br as in [3--61
CH,CHClBr*
--, CH,=CHCl + Br.
(4)
The lifetime of the CH$HClBr* radical has been calculated with the RRKM formalism to be only 0.1 X lo-l2 s [6j, much shorter than the estimated rotational period (3 X lo-l2 s) for the radical. Forward-back0 009-2614/83/S
(North-Holland
03.00 0 Elsevier Science Publishers B.V.
Physics Publishing
Division)
ward asymmetry was observed in one molecular beam study [6], but an earlier investigation gave a symmetrical distribution which was attributed to a lifetime greater than 5 X 10-12 s for reaction (3) [3] _ Infrared ChemiIuminescence has provided information about the distribution of excitation energy within the modes of the product CH,=CHCI [4,5] _Very recently reaction (1) has been proposed as a titration reaction for Cl in fast-flow systems, ad a reaction rate constant of (1.4 ? 0.3) X lo-lo cm3 molecule-l s-1 measured for it [ 11. We have previously used reaction (1) as a comparison standard in several intermolecular competitive chlorine atom systems [7,8], and have observed that (1) alone is not sufficient to describe the full mechanism at pressures in the vicinity of 1 atm [9] _We now report further experiments on the reaction of Cl with CH,=CHBr over the pressure range from 50 to 4000 Tori in a bath gas of CClF,. These experiments show pressure-dependent yields of CHZ=CHC1 and CH2CICHZBr, providing evidence for collisional stabilization of C2H3ClBr* radicals on a time scale longer than 10-10 s. Our experiments have utilized radioactive 38~1 atoms formed in situ by neutron irradiation of CCIF, and thermalized by multiple collisions wiLLhCCIF, prior to reaction with the vinyl bromide substrate. Gtieous samples containing >90% CCIF,, 20-40 Torr of Ar (as a neutron flux monitor) [lo] and small fractions of CH,=CHBr were prepared using standard vacuum 213
CHCMICAL PHYSICS LETTERS
Volume103. number 3
lie techniques. Hydrogen iodide was frequently added as a radical scavenger to convert C,H,CIBr radicals
into C,H,ClBr,
although it thereby became a competi-
tor with vinyl bromide for the initial reaction of atomic chlorine. L_essthan 5% of the total 38C1 produced in
these samples reacts with CClF, in energetic reactions,
30 December 1983
and the remaining 95% are available as thermalized
atoms for reaction with the other substrate molecules. The radioactive products were analyzed by the standard procedures of radio gas chromatography [ 10,111, and included CH,=CH38Cl from CH,=CHBr plus C38C1F3, etc., from the hot reactions with the bulk molecule.
Table 1 Percentageproduct yields from “Cl reactions with CHa=CHBr and HI Pressure(Torr) CClFs
Product yields a)
CHz==CHBr
HI
CHa3’CICH2Br observed
3980 3990 3920 3930 3950 3890
3850 2990 2900
10.6 47.2 475 40.9 39.8 20.2 19.9
936 952 954 933 921 466 474 205 205 199 191 191 181 98 90 80
10.1 29.9 9.9 10.7 10.4 11.5 9.9 5.6 5.8 5.1 5.0 5.4 5.4 5.4 55 4.1 5.0 5.9
93 47 25
5.5 4.1 2.2
CH2=CH3*CI
(C)
(D)
-
-
-
7355 47 27 17 -
65 45
46 -
42
42 -
10.7 21.6
32 22
30 20
34.9 39.6 O_ 6.2 0 b) 0 5.8 17.1 216 27.7 0 5.2 17.7 22.0 0 0
17 12 -
15 12 -
29 19 15
15
19 -
18 -
10 5.0 5.3 3.3 -
10 4.8 4.0 3.3
10 4.8 4.1 3.3 -
4.5 2.2 1.7
4.8 2.4 2.2 -
4.8 2.5 2.3 -
0 0 b) 0 10.2 40.0 61.2 101 0 30.3 -
-
-
24
17 -
65 46 25 17
12 -
observed
(C)
(D)
22.6
19 19 19 16 11 6.2 4.3 22 13 40 21 14
19 19 19 16 11 6.2 4.3
23.9 22.9
17.7 11.7 6.0
4.2 24.5 13.7 47.1 24.0 15.6 12.9 9.7 63 32.7 81* 3 831 3 36.6 19.4 14.2 11.4 92 * 5 36.3 17.0 11.8 885 8 1105 15
11 9 56 27
73 73 33 16 13 11 84 37 18 14 90 92 -.
23 14 44 24 16 13 10 59 30 74 74 35 17 14 12 84 38 19 15 90 92
a) The yields in columns (C) and (D) correspond to the kinetic mechanisms described In tire text with these parameters: (C): k5/k2 =0.68;k2bcO.lka; k4>>>ks[M]; ks/k6=0.67;andks=k,[M] at 900Torr.@): ks/k2=0_62; k2b=0.49k2;k3=0; k4 = ks[M] at 1850 Torr and k6 = k7]M] at 650 Torr. b, 11 Torr 02 present in these samples.
214
CHEMICAL
Volume 103, number 3 r\_E
8-
“.JiJ
C‘I
f
CH..=
30 December
PHYSICS LETTERS
[CH$ICH2Btj
P
kH2
CHBr
= CHCU
10.0 40_-
6-
.
100
0
2 IO
0
1.000
0
4.000
2.0
:
I .o
-
03
-
4
2
[CHz
0
.
0.1
Cl
210 100
I
1
6
1
/
[ctit
=
CHB~]
’
Fig. 2. Ratio of product yields for CHa3aClCH2Br and CH2=CH3’C1 from reactions of thermal 38C1 with CHa=CHBr
= CHBr]
from table 1.
When HI was present, CHz38ClCHzBr was observed, but no radioactive peak corresponding to CH3CH38ClBr (
I .ooo
0
summed yields of CH2=CH3aCI and CHa3%ZICHaBr from reactions of thermal 3sC1 with CHa=CHBr as a function of [HI]/[CHa=CHBr] ratio and total pressure. -Calculated values with mechanism (D) for total pressures ranging from 2 Torr to infinite pressure, using p-eter values
reactivity
.
8
Fig. 1. Reciprocal
The relative
4.000
0
.
torr
0
r-l
”
FtiI]
[HI] /
1983
can be
evaluated through the fractional diminution in the yields of CH,=CH38Cl and/or CH,38C1CH,Br as 38C1 atoms are diverted away from CHz=CHBr by reaction with HI. The inverses of the sum of these observed yields are graphed in fig. 1 after allowance for 5% hot reaction. The increased slopes at the lower pressures demonstrate that the apparent reactivity of CH2=CHBr relative to HI decreases with decreasing total pressure. Although the ratios of the yields [CH,38ClCH,Br] / [CH,=CH38C1] vary by a factor of 60 between 50 and 4000 Torr total pressure, they are essentially independent of the [HI] / [CH2=CHBr] concentration ratios at any given total pressure, as shown in fig. 2. With increasing [HI] /[CH2=CHBr] ratios, both products are proportionally reduced in yield as 38C1 atoms compe-
as a function of [HI]/[CH2=CHBr] ratio and total pressure. -Calculated values with mechanism (D) for 100, 2 10. 1000 and 4000 Torr total pressure, using parameter values from table 1. titively
react
instead
with
the alternate
substrate
HI.
The most important observations from these experiments are: (a) the yield of CH2=CH3”CI from (1) is pressure dependent, varying from about 23% at 4000 Torr to 95 f 5% at 50 Torr; (b) another product, CH138C1CH,Br. is observed from j8Cl atom reaction in mixtures of CH?=CHBr and Hi, and is attributed to H-atom abstraction from HI by a stabilized C,H338C1Br radical; (c) the yield of CH23BC1CH1 Br is also pressure dependent, ranging as high as 75% at 4000 Torr, and as low as 10% at 100 Torr both yields being extrapolated to zero HI concentration; (d) the relative yields of CH238ClCH,Br and CH,=CH38C1 vary with pressure, but not with [HI] / [CH2=CHBr] ratio; (e) the yield of CH3CH38CIBr, for which the radical CH,CH38ClBr is a logical precursor, is always
Volume103, number 3
CHEMICAL
38Cl + HI + H38C1 + 1
PHYSICS LETTERS
(5)
is-accepted [8] as 1.3 X lO-‘O cm3 molecule-1 s-l, then the absolute reaction rate constant for net removal of 38C1 atoms by CH,=CHBr varies from about 1 X lo-10 cm3 molecule-l s-l at low pressure to about 2 X 10WIOcm3 moleculeS1 s-l at the high-pressure limit. These observations place some strong constraints on the mechanisms applicable to the reactions of WI with CH,=CHBr: (a) the pressure dependence of the yields for CH2=CH3*C1 and for CH,38CICH,Br implies some intermediate radical with a lifetime comparable to collision frequencies in this system, i.e. 1O-g-lO-1o s; (b) yields of CH, 38C1CH2Br >70% at high pressure and of CH2=CH38CI >90% at low pressure required the inclusion of a 1 J-halogen atom shift in the mechanism, with reaction (3) an obvious possibility; (c) the reduction in reactivity of CH,=CHBr at low pressure implies a step such as CH238 ClCHBr* + CH2=CHBr + 38C1,
(6)
which is capable of returning pressure-dependent amounts of 38C1 to the system. The mechanisms which are potentially applicable to this system can include various combinations of reactions (2)--(6), together with the stabilization reactions CH2 38ClCHBr* + M + CH,38ClCHBr + M,
(7)
CH2CH3*ClBr* + M + CH2CH3*CIBr + M,
(8)
and the reaction of stabilized radicals with HI by R+Hl+RH+l
(9)
to provide stable molecules for subsequent measurement_ We have found it very useful to add one more reaction, the 1,2-Br atom shift of CH2CH3*ClBr
+ CH2BrCH38CI,
(10)
to provide the complete set from (2) to (10). One mechanism (A) which avoids the 1,2halogen atom shifts of (3) or (10) and appears qualitatively appropriate is the predominant formation of CH238CICHBr* initially (kza > k.&), with a lesser original formation of CH,CH38ClBr* followed by irnmediate Br loss (k4 S ka [M] ). Relatively rapid loss of 216
30 December
1983
38C1 by (6), i.e. K, > k, [M], could permit the recycling of 38Cl through the competitive reaction system, with the yield of CH2=CH3*C1 accumu!ating to a high percentage after many such cycles in low-pressure systems. While such a mechanism would qualitatively produce pressure dependence for both CH,=CH38CI and CH238CICH2Br, quantitative formulation shows a strong dependence of their yield ratio on the [HI] / [CH?=CHBr] ratio, in disagreement with fig. 2, and mechanism (A) cannot account for the actual observations. Mechanism (B), also unsuccessful, involves a minor initial pathway tc CH2=CH3*C1 by (2b) + (4) with k, + k8 [Ml, plus predominant addition (kza > k,,) to form CH238C1CHBr*. The pressure-dependent isomerization (3) furnishes additional CH2=CH3*CI at low pressures (i.e. k competitive with k, [MI). In this formulation all 3 gC1 atoms reacting with CH2=CHBr lead to either CH2=CH3*C1 or CH238ClCHZBr at all pressures, and the removal of 38C1 atoms is pressure independent in contradiction to the observations of fig. 1. Mechanism (C) is a modification of(B) which includes both isomerization (3) and loss of 3gCl by (6) for the kinetic disappearance of CH,38CICHBr*, and can successfully reproduce the observations. The necessary quantitative kinetic parameters include about S--90% initial formation of CH,3gClCHBr* (kzb = 0.1 kh) and roughly comparable reaction rates for k, , k6 and k7 [M] at 1000 Torr. However, another quite different mechanism (D) also exists which fits the observations equally well. Chlorine atoms are postulated to react to form both CH,38ClCHBr* and CH2CH38CIBr* in generally comparable initial yields (kza = k,,). No 1,2-&orine isomerization is included (k3 = 0) and the only reactions available to CH?38ClCHBr* are collisional stabilization (IQ [M]) or t&loss of 38~1 by (6) The lifetime of CH2CH3*C1Br* is assumed in (D) to be in the 10mg10-l” s range (k = k8 [MI), such that reaction (4) provides more C&1=CH38CI at low pressures and CH2CH38ClBr* radicals tend to be stabilized at high pressures. The critical postulate in mechanism (D) is the additional assumption that the stabilized CH2CH38ClBr radicals undergo the 1,2-bromine atom shift of (10) prior to reaction with HI for form C2H438C1Br. This 1,2-bromine shift satisfactorily explains the absence of CH,CHsaCIBr as an observed
Volume 103, number 3
CHEhlICAL PHYSICS LETTERS
product, while the measured CH,38ClCH2Br is expected from reaction with HI by either CH2 38C!CHBr or CHZBrCH3*C1 radicals. Quantitative formulation of mechanisms (C) and (D) requires knowledge of several parameters, including: (a) the relative reactivity of HI versus CH,=CHBr (kz versus k,); (b)the distribution of initial addition between CHZ3*ClCHBr* and CH&H38ClBr* (kza versus k2J; (c)therate constants for loss of Cl from CH,38ClCHBr*, and for 1,2-U isomerization by (3) versus collisional stabilization (k3 and k, versus k, X
WI); (d) the rate constant for loss of Br from CH2CH38ClBr* versus collisional stabilization versus k, [M]); and
(X4
(e) the average energy loss for these excited radicals in collision with CClF, _ None of these parameters has been fured from any independent experiments, and satisfactory fits to the observations can be obtained with various combinations of such values. Reasonable parameters are found with mechanism (C) for 85-90% reaction at the CH2 end of CHz=CHBr, k, =Sk,, the rate of collisional stabilization (k7 [MI) equal to k, -i-k, at about 400-600 Torr in CClF,, reaction (4) assumed to be too rapid (k4 3 k8 [Ml) for any collisional stabilization to occur, and intermediate energy losses (a few kcal/mole) in stabilizing collisions. Reasonable combined sets of parameters for mechanism (D) are found with SO-60’5% initial reaction at the CH, end, e.g. k?b = (0.7-l .O)kZa, with k4 = kg [hi] at about 500-750 Torr and k, = k8 [M] at 1500-2000 Torr, with k3 taken as zero, and intermediate collisional energy losses. The limiting total reactivity of CH?=CHBr at high pressure is approximately 1.7 times that of HI for both mechanisms (C) and (D). The calculated variations in yields of CH,=CH38Cl and CHZ38C1CH?Br for one such set of parameters with mechanism (D) a;e graphed in fig. 1 and 2, and listed in table 1_ A set of parameters which provides comparable fits to the observations for mechanism (C) is also listed in table 1. Some circumstantial evidence exists which is more easily consistent with mechanism (D) than (C). The comparable data for s8C1 reactions with CH2=CHF are readily satisfied by the analog of mechanism (A) with-
30 December 1983
out recourse to any step involving a 1 &halogen shift [ 121. The isomeric 1.2 shift of a halogen atom in a radical is frequently postulated for bromine. sometimes for chlorine, and almost never in an authenticated system for F [ 131. The postulate of a 1,2-bromine shift in (D) provides the needed differential behavior when applied to CH,=CHBr versus CH?=CHF. A second observation is the known isomerization of halogen atoms QWYZ~ from a multiply-halogen-substituted carbon atom toward the CH? end of the molecule [Is, 14]_ An example of such a shift is the 1,2Cl atom shift from CH,CCl, to CH?ClCC12, such that the radical with term&al Ccl, is usually not observed_ The 1.2Br shift of (10) is consistent with the direction of these other observed halogen atom shifts. The absence of any yield for CH,CH3*ClBr introduces some lower limits on the needed rapidity of 1 ,I?-Br isomerization in order to avoid its formation, i.e. kilo versus k9 [Hl] _On the other hand, if bromine atom isomerization were too fast. then no loss of Cl could be observed, so that k,. for the excited radical cannot be much faster than k 4, and probably is slower. Not very
much information is available concerning the rate constants for radical reactions with HI, and obviously none can be available for the reaction rate constant of the not-yet-detected CH,CHClBr radical. Experiments with another haloethyl radical (CH?FCH,) in 4000 Torr SF6 have shown essentially equal reaction rates with HI and with O2 [ 151, while the high-pressure rate constants for R + O2 + M to fomr RO, are given as 2 X 10-l’ and 7 X lo-l1 cm3 molecule-1 s-l for CH3 and C2H5. respectively [ 161. By analogy, the approximate estimate can be made that the rate constant for reaction (9) with CH2CHC1Br is also about 4 X 1O-1z cm3 molecule-l s-l. The rate constants for other small organic radicals with HI are similar in magnitude [ 17, IS] _The needed rate constant for the 1,2-Br isomerization in mechanism
(D) to satisfy the various observations falls in the log1011 s-1 range, and appears to be a plausible hypothesis [13,14]. Our experiments demonstrate the existence of an intermediate long-lived relative to the rotational period of CZH3Br38C1* radical. but do not clearly identify whether that long-lived radical is CH238ClCHBr* in mechanism (C) or CH2CH38C1Br* in mechanism (D). Comparison with product analysis in a variety of other systems leads us to believe that mechanism (D) with a long-lived CH,CH38ClBr* radical is probably the dom217
Volume103, number3
CHEMICALPHYSICSLETPEES
inant mechanism. An interesting corrollary to mechanism (D) is the observation from the quantitative parameters that there is little or no (i.e. k3b 7 (0.7-l .O)k2Q “anti-Markownikoff’ preference in the initial addition of Cl to CH?=CHBr. If(D) is correct, then the product distribution which gives the appearance of “anti-Markownikoff ‘, i.e. a high yield of CHZClCH2Br and an absence of CH-JHCIBr, is the consequence not of the initial addition process, but of the subsequent postulated 1,2-bromine atom shift. The generality of such an explanation for “anti-Markownikoff’ observations would certainly be worth exploring if mechanism (D) is shown to be operative here. While mechanisms (C) and (D) are not distinguishable in our present experiments with CH2=CHBr, they will lead to different products if Cl atoms are reacted in the gas phase with olefins tagged on one end, as with CHt=13CHBr, CH,=CFBr, CH2=CBrCH3, etc. Thjs research has been supported by Department of Energy Contract No. DE-AT-3-76ER-70126.
References [l] J.-Y_ Park, 1-R. Slagle and D. Gutman, J. Phys. Chem. 87 (1983) 1812. [2] J_D. Roberts and MC. Camrio, Basic principles of organic chemistry (Benjamin, New York. 1977) p_ 390.
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30 December1983
[3] J.T. Cheung, J.D. McDonald and D.R. Herschbach, J. Am. Chem. Sot. 95 (1973) 7889. [4] J.G. Moehlmann and JD. McDonald, J. Chem. Phys. 62 (1975) 3052. [S] JE. Durana and J.D. h&Donald, J. Chem. Phys. 64 (1976) 2518. [ 61 R J. Buss, M J. Coggiola and Y _T. Lee, Faraday Discussions Chem. Sot. 67 (1979) 162. [7] FSC. Lee and FS. Rowland, J. Phys. Chem. 81 (1977) 86. [8] R.S. Iyer, PJ. Rogers and FS. Rowland, J. Phys. Chem.. to be published_ [9] D. Herschbach, Discussions Faraday Sot. 67 (1979) 250251. [IO] FSC. Lee and FS. Rowland. J. Phys. Chem. Sl(l977) 1229. [ 111 FSC. Lee and F.S. Rowland, J. Phys. Chem. 81(1977) 1222. [ 121 C--L. Chen and FS_ Rowland, unpublished remits. [I 31 PS. Skell, RX Pavlis, DC. Lewis and K J. Shea, J. Am. Chem. Sot. 95 (1973) 6735. [ 141 RXh. Friedlina, in: Advances in free radical chemistry, Vol. 1, ed. GH_ Williams (Academic Press, New York, 1965) p. 211. [IS] RL. hlilstein, R.L. Williams and FS. Rowland, J. Phys. Chem. 78 (1974) 857. [ 161 DL. Baulch, RA. Cos, PJ. Crutzen, Rf. Hampson, JA. Kerr, J. Troe and R.T. Watson, J. Phys. Chem. Ref. Data 11 (1982) 327. [ 171 D.M. Golden, R. Walsh and SW_ Benson, J. Am. Chem. Sot. 87 (1965) 4053. [ 181 DM. Golden, AS. Rodgers and S.W. Benson, J. Am. Chem. Sot. 88 (1966) 3196.