Arthur J. Frank and Robert J. Hanrahan
1532
Gas Phase Radiation Chemistry of Ethyl Bromide Arthur J. Frank and Robert J. Hanrahan' Department of Chemistry, University of Florida, G'ainesville, Florida 326 I 1 (Received December 19, 1975) Publication costs assisted by the University of Florida
The y radiolysis of ethyl bromide has been investigated at 100 Torr pressure and 23 OC. In the pure system between an absorbed dose of 1.0 X lozoand 1.5 X 1020eV/g the major products and their respective G values are as follows: hydrogen bromide, 3.89; ethane, 2.70; ethylene, 2.17; acetylene, 0.31; hydrogen, 1.39; 1,l-dibromoethane, 0.88; 1,2-dibromoethane, 0.12; vinyl bromide, 0.32; methane, 0.083; methyl bromide, 0.080; and bromoform, 0.0078. When oxygen is added, the G values in this dose range become the following: hydrogen bromide, 4.89; ethane, 0.31; ethylene, 0.78; acetylene, 0.27; hydrogen, 1.38; 1,l-dibromoethane, 0.028; 1,2dibromoethane, 0.56; vinyl bromide, 0.0; methane, 0.03; methyl bromide, 0.32; and bromoform, 0.0034. Bromine is also formed with a G value of 2.4 when oxygen is added. The presence of hydrogen and acetylene in the radiolysis indicates that these species must be formed from higher energy processes not accessible in the 253.7-nm photolysis, which was studied in a parallel investigation. The product distribution indicates that the probabilities of single bond rupture in the primary event are approximately C Z H ~ - B ~ : C ~ H ~ B ~ - H : C H ~ CH2Br = 1.00:0.40:0.06. Either a hot hydrogen atom abstraction reaction or direct molecular Hz elimination accounts for about 16%of the hydrogen yield. Strong similarities in dose-yield plots suggest that many of the secondary processes involved in the photolysis are important in the radiolysis of ethyl bromide as well. The high pressure mass spectrometry of the system indicates the role of ionic species. Differences in radiolytic behavior of ethyl chloride, bromide, and iodide can largely be explained in terms of the energetics of the primary and secondary processes in each system.
Introduction Although the gas phase radiolyses of ethyl chloride1 and ethyl iodide2 have been investigated, no work on ethyl bromide has been reported. Prior to the advent of gas chromatography, however, the fast electron and x-ray decomposition of liquid phase ethyl bromide was ~ t u d i e d . ~ The investigation of the y radiolysis of ethyl bromide at room temperature in the gas phase was undertaken to study the primary and secondary processes leading to its decomposition. Parallel studies of the 253.7-nm photolysis4 and high pressure mass spectrometry of the system provided supplementary information on the decomposition mechanism. Oxygen was added to identify free radical processes and to determine their contribution to the observed stable products. An additional goal of this investigation was to compare the radiation chemistry of ethyl bromide with that of ethyl chloridel and ethyl iodide2 since the chemical kinetics and product distributions of these systems differ dramatically. In contrast to ethyl iodide, carbon-hydrogen rather than carbon-halogen bond rupture is the major primary event in the chloride. Also, formation of C4 products is important in the ethyl chloride system but not in the ethyl iodide system. These observations as well as others can be explained in terms of bond-energy arguments. Such comparisons are considered only briefly here; further discussion is given e l ~ e w h e r e . ~
Experimental Section Methods used for purification of the ethyl bromide, preparation of samples, and analysis of organic products, HBr, and Br2 will be described in a companion paper on the photolysis of ethyl b r ~ m i d efurther ;~ details are given e l ~ e w h e r eHy.~ drogen was measured using a thermal conductivity gas chromatograph, or alternatively using a combined Toepler pumpMcLeod gauge, making corrections as necessary for the The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
volatile organic component (mostly CH4) based on analysis of the gas by flame ionization gas chromatography, In all irradiations, ethyl bromide was a t a pressure of 100 Torr except in those experiments intended for HBr analysis where the pressure was 300 Torr. In the majority of scavenger experiments, added oxygen was 5% of the total pressure; in the HBr determination runs, it was 12%. To accommodate the analytical methods employed in sample analysis, two types of radiolysis vessels were employed. Each was made of Pyrex and equipped with a single breakseal. For spectrophotometric analysis of bromine, an annular-shaped 355-cm3 vessel with a 10-cm path quartz optical cell was used. Samples intended for gas chromatography and HBr analysis were irradiated in 30-cm3 cylindrical vessels. Irradiations were performed a t room temperature using a cobalt-60 y source.6 At maximum dosage the net amount of ethyl bromide consumed was less than 1%.The energy absorbed per unit mass of ethyl bromide was calculated relative to G(H2) = 1.2 in ethylene7 applying the correction factor 0.729 for the difference in the massstopping powerse8 For a brief reinvestigation of ion-molecule reactions we utilized a Bendix Model 14 time-of-flight mass spectrometer modified as described by F ~ t r e l lAll . ~ source potentials were as describedQexcept that the innermost ion grid (no. 1)was pulsed a t +23 V rather than +25 V.
Experimental Results A total of 29 products were observed from the gas phase radiolysis of ethyl bromide; the yields of the 11 most important, which we designate as major or semimajor, are listed in Table I. These include HBr, CzH4, CH4, CHsBr, CzHsBr, l8l-C2H4Br2,1,2-C2H4Brz, and CzH6, all of which were also significant products in the photolysis. In addition, Hz and C2H2 are important products in the radiolysis, although they are not formed in photolysis at 253.7 nm. As in the photolysis, Br2 is observed in the presence of oxygen, but not in its ab-
1533
Gas Phase Radiation Chemistry of Ethyl Bromide
TABLE I: G Values for Major and Semimajor Radiolysis Products from Ethyl Bromide Vapor at 100 Torr as a Function of Radiation Dose Range Pure system 02 scavenged system Absorbed doselg X 10-20 Absorbed doselg X Producta
0-0.5
Convex upward HBrc Brz C2H4 C2Hz Concave upward CH4 CH3Br CZH3Br lJ-CzH4Br2 1,2-CzH4Brz Linear H2 CZH6
1.0-1.5
-6.02
3.89
3.5-4.0
4.5-5.0
0-0.5
1.0-1.5
3.5-4.0
4.5-5.0
4.89 2.4 0.78 0.13
4.89
4.89 0.12
4.89
0
0
0 0
0 0
0.78 0.16
2.17 0.31
2.07 0.31
0.86 0.13
0.083 0.080 0.32 0.88 0.12
0.083 0.080 0.32 0.88 0.12
0.26 0.24 0.75 1.66 0.31
1.39 2.70
1.39 2.70
1.39 2.70
0.78 ~0.27
0
0.06 1.30
0.028 0.56
0.030 -0.32 0 0.028 -0.56
1.38 0.31
1.38 0.31
0.030 0.32
0.26 0.24 0.75 (51.66) 0.31
0
1.39 2.70
2.4
0.33
0
0.31 1.22 1.38 0.31
d d d d
1.30 0 0.31 1.22
1.38 0.31
a The dose-yield plots for the listed products are found in Figures 1-7. The G values for the minor radiolysis products are given in Table VII, ref 5. Oxygen comprised 5% of the total pressure except in the case of samples intended for HBr analysis, where it was 12%. Irradiated ethyl bromide was at 300 Torr. d No data points were taken in this absorbed dose regime. e The compound was not detected.
/
50
I
dole, ..v.iprsm Jose,
c."./~ry1
or c , I
2 5
YT
x .c-,C
Figure 1. Production of hydrogen bromide (pure, 0:12% 02, e)and hydrogen (pure, 0 ;5% 02,B) as a function of dose.
sence. Since many of the radiolytic yields vary throughout each experiment, Table I records differential yields for several dose ranges, from (0-0.5) X lozoto (4.5-5.0) X lozoeV/g, both with and without added oxygen. The yields are also displayed graphically in Figures 1-7. Further details on 18 additional products which were observed in the radiolysis are given el~ewhere.~ Fourteen of these were definitely identified, three tentatively identified, and one high boiling species remains unidentified. None of these species represents a significant pathway for decomposition of ethyl bromide, however, since all are formed with G values of 0.03 or less. A few have some mechanistic interest, including CHBr3 (G = 0.031, CH2Br2 (G = 0.00721, C4H10 (G = 0.002), and 1,1,2-C~H3Br3 (G = 0.007). The latter was somewhat more prominent among photolytic products, being classified as a semimajor product in that case. As in the photolysis of ethyl bromide: the various major
O?
c2P,Br x
- " Id
Figure 2. Production of bromine (5% 02,D) and 1,2-dibromoethane (pure, 0;5% 02,e)as a function of dose.
m e , a . v . / p u or C ~ H 1~ B ~
Flgure 3. Production of ethane (pure, 0, 5% 0 2 , e)as a function of dose. The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
1534
Arthur J. Frank and Robert J. Hanrahan
k s e ,c.v./grlun
Or c2n5ar
m e .
Figure 4. Production of ethylene (pure, 0; 5 % 02,.)as a function of dose.
e.v./aa.
or C ~ H ~ 1B0 ' ~ ~
Figure 6. Production of 1,l-dibromoethane (pure, 0; 5% function of dose.
02,0 )as a
2.50
2.00 rn
h .
1.5C
P5 6
1.00
c.0:
7 1.3
-
1
,,a
2." "OB*
r
"
Iqrw
oc
Y.J
c 2 7 5 Ir
5.0
6.3
x .L-2-
Figure 5. Production of methane (pure, 0; 5 % Op,0 )and acetylene D) as a function of dose. (pure, 0 ;5 % 02,
and semimajor products can be categorized with respect to the shape of their yield-dose curves. In the absence of added oxygen, G values for HBr and CzH4 are maximum at low dose, and fall to zero at higher doses (Figures 1and 4). The products l,l-C2H4Brz,1,2-C2H4Br2, CH4, CzH3Br, and CH3Br all show an induction period andlor a G value which increases with dose, although only slightly, in certain instances (Figures 2, 5,6, and 7 ) .Within the limits of our experimental precision, the yields of Hz and CzHc are strictly linear, both in the presence and absence of oxygen (Figures 1and 3). The yielddose graphs for CzH4 and CzH2 do not quite extrapolate through the origin (Figures 4 and 5) but this effect may well be an artifact of the analytical procedure. Our interpretation of the mechanism, as discussed below, indicates that C2H4 and CZHPare direct radiolysis products which should show maximum G value at zero dose. A decrease of G value with dose is reasonable for these unsaturated species. This is clearly seen for CzH4 (Figure 3) and suggested by the data for C2H2 (Figure 4). As in the photolysis of ethyl bromide, the most notable effect of added oxygen is net production of Brz; the initial G value is 2.4, falling off to zero at high dose. Other major yields which are enhanced in the presence of oxygen include those of 1,2-CzH4Brz, CH3Br, and HBr (at high dose). All other The Journal of Physical Chemistry, Vol. 80,No. 14, 1976
Flgure 7. Production of methyl bromide (pure, 0; 5 % bromide (pure, 0 )as a function of dose.
02, 0 )and vinyl
yields are decreased except for Ha, which is unaffected; C2H2 is essentially unaffected as well. The material balance in the radiolysis of C2HsBr is only moderately satisfactory; the ratio C/H/Br = 1.75/5.00/0.84 in the dose range (1.0-1.5) X lozoeV/g and 2.02/5.00/0.62 in the dose range (3.0-3.5) X lozo eV/g. It is clear that the major stoichiometric difficulty is a shortage of bromine, particularly at higher dose. It is interesting to make direct comparisons between the photolytic and radiolytic decomposition of ethyl bromide. For instance, it will be noted that our longest photolysis experiments ran for 10 min, corresponding to absorption of 8 X 1013 quanta/cm3 X 600 s X 91 cm3, or 4.37 X 10l8 quanta total. Taking into account the energy per photon at 253.7 nm, which is 4.89 eV, it is found that the actual energy input in a 10-min photolysis was 2.15 X 1019 eV. Similarly, the maximum dose in the radiolysis experiments was 6 X lozoeV/g in a sample weighing 1.76 X g, or 1 X l O I 9 eV total dose. Thus the termination of the curves in Figures 1-7 corresponds in energy input to about 5-min photolysis under the conditions stated. At that point, the actual product yields under photolysis were generally about twice the corresponding yields in the radiolysis. Thus, on the basis of total energy deposited, light of 253.7 nm is approximately twice as effective in decomposing ethyl
1535
Gas Phase Radiation Chemistry of Ethyl Bromide
TABLE 11: Relative Product Distribution of the Major and Semimajor Products from the Radiolysis and Photolysis of Ethyl Bromide Vapor at 100 Torr Pure radiolysis Pure photolysis Photolysis times, min Absorbed dose/g X Productsa Convex upward HBr CZH4 CzHz 1,1,2-CzH3Br3 Concave upward CH4 CH3Br CzH3Brd lJ-CzH4Brz 1,2-CzH4Brz
0-0.5
1.0-1.5
1.64 0.45 0.06 0.00
1.64 0.80 0.11 0.00
0.03 0.03 0.12 0.33 0.04
0.03 0.03 0.12 0.33 0.04
0.10 0.09 0.28 0.61 0.11
0.51 1.00
0.51 1.00
0.51 1.00
,
3.5-4.0
6.0-7.0
0-0.5
1.O-1.5
-0
-0
0.90 0.07
0.65
0.54
0.07
0.07
0.77 0.11 0.00
0.32 0.05
c c
0.00
0.10 0.09 0.28 (20.61) 0.11
0.00 C
0.00
6.0-7.0 -0 -0
c
c
0.01
0.01
0.01
0.00 0.00
0.01 0.01
0.01 0.01
0.02 0.26 0.02
0.04 0.26
0.51 1.00
3.5-4.0
c
c
1.00
1.00
c
-0.02
-0 0.70
0.06 c
1.00
0.70 0.07 c
1.00
a All values given in Table I1 were determined from the yields listed in Table I, this paper, and ref 4. Relative yields of all products not included in this table are less than 0.01. The y irradiated ethyl bromide was at 300 Torr. The compound was not detected. d CzH3Br is concave upward in the radiolysis but convex upward in the photolysis. e CzHs is linear in the radiolysis but convex upward in the photolysis.
bromide as 1.25 MeV y radiation (or, more correctly, the secondary electrons from the vessel walls). Discussion Based upon the marked similarity of the product yields from the radiolysis of ethyl bromide, shown in Table I, with the yields from the photolysis of this compound, listed in Table I of the companion it is evident that there must be considerable commonality of decomposition pathways under these two modes of activation. To facilitate further comparisons, we show in Table I1 the yields of major products from radiolysis and photolysis, normalized in each case to the basis that the ethane yield equals unity. It will be seen that in most cases the major products of the radiolysis are important products in photolysis as well. Furthermore, as mentioned in the Results section, it is found that the shapes of the yield-dose curves are similar in most instances. Additional similarities include the fact that 1,l-and 1,2-dibromoethane have comparable yields in both cases; C3 and C4 yields are very small under both photolysis and radiolysis; C1 yields are also small in both cases, although somewhat greater under radiolysis; and CzHe yields (used as a standard of comparison) are major in both cases. Also, in both the radiolysis and photolysis, net Bra formation is seen in the presence of 0 2 but not in its absence. As explained further below, however, we feel that the following differences are significant: hydrogen and acetylene are formed in the radiolysis, but not in the photolysis; ethylene is a much more significant product in the radiolysis, and vinyl bromide is somewhat more important in the radiolysis than the photolysis. The elementary processes which we propose for the photolysis of ethyl bromide4 are listed as neutral secondary processes in Table I11 of this paper; an examination of the table will indicate that the reactions in question would necessarily occur whenever ethyl radicals, bromine atoms, ethylene, and hydrogen bromide are present simultaneously in ethyl bromide vapor a t appropriate conditions of temperature and concentration. Accordingly, we postulate that the same set of secondary neutral reactions must contribute in the radiolysis as well as in the photolysis. However, there must be significant
differences regarding the relative importance of certain of the reactions, as will be discussed further below. It has long been recognized that high energy radiation causes both excitation and ionization in a chemical system. In Table I11 we list neutral primary processes, ionic primary processes, ionic secondary processes, as well as neutral secondary processes, which we consider important in the radiolysis of ethyl bromide vapor. The neutral primary processes, reactions 1,2, and 3, are the same as we will postulate in connection with the photolysis. Although higher energy excited states may be accessible under radiolytic conditions, it is reasonable to presume that the lowest nu* repulsive state plays a major role in radiolysis as in photolysis. Dissociation of the repulsive state into both CzHr plus BP (reaction 1)and CzH4 plus HBr (reaction 2) is indicated by our photolysis results. We found some evidence supporting reaction 3 in the photolysis; since there is a somewaht larger yield of CH4 and CH3Br in the radiolysis, it could be suggested that reaction 3 is more important under radiolysis. For convenience and simplicity, we formulate the mechanism in terms of the neutral process only, although ionic fragmentation of C2H5Br into CHs- CH2Br+ (1.8%)and into CH,+‘+ CH2Br. (0.6%)also must contribute somewhat to the radiolytic yields of CHI and CH3Br. (Percent abundances refer to mass spectral fragmentation.lOb) We make the reasonable simplifying assumption that the ionic primary processes which occur in the radiolysis of ethyl bromide are the same as the fragmentation processes seen in the mass spectrometer under low pressure conditions. (For a molecule as small as ethyl bromide, unimolecular fragmentation processes should be so rapid that any “time scale difference” between radiolysis and mass spectrometry should be slight.) Reactions 4-9 in Table I suffice to describe formation of major ions in the mass spectrum of ethyl bromide, under bombardment with 70-V electrons.1° Although only the ionic fragments are seen directly, a reasonable overall reaction can be formulated in each case. Essentially the only point of debate would arise in reaction 6, where the products could be listed either as Hz Br- or HBr H-. We have shown the former, since it is more favorable energetically.
+
+
+
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
1536
Arthur J. Frank and Robert J. Hanrahan
TABLE 111: Reactions in Ethyl Bromide Radiolysis Neutral Primary Processes C2H5Br
--
+ BP
C2Hy
_*M
_cn_+
+ HBr CHy + C H ~ B P CzH4
+ --.+ + + + + --.+ + + + + + + + - + Ionic Primary Processes
C2HsBr
C2H5Br+
C2H5+
e- (31.9%)
Br.
e- (25.3%)
C2H3+ H2 Br- e- (22.0%) CzH4+ HBr e- (5.0%) C2H2+ Hz HBr e- (7.3%) Br+ C2H5- e- (2.2%)
e- C2HsBr C2Hj- BrIonic Secondary Processes
+ CzHjBr C2H2+ + CzHjBr CzHjBr+ + CzHSBr C2H4+
+
----
+
+ CzH5Br + CzHsBr C2Hj+ + C2HsBr C2HjBr+H + CzHjBr Br+
C2H3+
+ CzHsBr+ CzH2 + CzHsBr+ C2H4
(CgHsBr)*+ C2HsBr+C2Hs Br.
+
CzHSBr+
+ Br.
CZHsBr+H + C2Hz
+ CzH4 CzH5Br+CzH5+ HBr
C2HjBr+H
-
Neutral Secondary Processes Bra
+ Bra
(M)
Brz
+ CzHsBr C2Hs + CH&HBr + + + C2Hj- + Br- C2HjBr Br- + C2Hy C2H4 + HBr Br. + C2H4 sCHzCH2Br CHzCH2Br BP + CzH4
C2Hy
-
C2H5’ HBr C ~ H G BP C2H5- Brz CzHsBr + Bra
--+ + + + + + + + + -+
CHSCHBr
CHzCH2Br
CH2CH2Br
HBr
Br.
+ CzHsBr
+
.CHzCH2Br Br2 BrCHzCHzBr Br. CH2CHzBr Br. BrCHzCHzBr Br-
CzHjBr
CH&HBr
HBr
Brz
CH&HBr
CHsCHBr BP
+ HBr
+ CzHsBr + Br.
CH3CHBrz
CH3CHBr Br. CH3CHBr2 Br. CH3CHBr C2H3Br + HBr Br. C2H3Br BrCHCH2Br BrCHCHzBr HBr Br. BrCHzCH2Br
+
+
+
Percent figures indicated for reactions 4-9 are the abundances of the corresponding ionic species as reported in the API mass spectral tables.loa We suggest that formation of the ethyl ion mostly proceeds by reaction 5 , giving bromine atom plus an electron. It is known that ion pair formation (C2H5+ The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
+
Br-) is important just above threshold energy;ll however, radiolytic processes generally occur at moderately higher energies. Replacement of reaction 5 with the ion pair process would not alter our interpretation in a major way, in any event. Secondary ionic processes in ethyl bromide have been investigated by several research groups. Hamill and co-workers12J3 as well as Sieck and Gordon14 have studied ion-molecule reactions in ethyl bromide using magnetic deflection techniques. Beauchamp et al.15 studied the system using ICR mass spectrometry. We have also briefly reinvestigated the subject, using a high-pressure time-of-flight mass spectrometer. Hami1112J3and Sieck and Gordon14 both observed the simple dimer (CzHbBr)z+ as well as the ether-type ion (CzHs)2Br+;they suggest formation reactions 13 and 14. Reactions 14-18 were postulated by Beauchamp15 to interpret the results of his ICR experiments; a number of the ionic pathways were observed by the other workers as well. We have also seen evidence for reactions consuming CzHsBr+, C2H5+, and CzH3+, as well as processes producing C2H5BrH+ and the two “dimeric” ions. Accordingly, our observations (which are described further elsewhere5) are in agreement with postulated reactions 13-18; it should be noted that each of the secondary ionic reactions 11-18 either produces the etherlike dimer or else produces another ion which, in turn, leads to it. Reaction 13 would appear to be an exception; no route for decomposition of (CzHsBr)z+is shown. Our own work indicates, however, that the relative yield of (CzHsBr)z+ decreases above 0.2 Torr pressure, while (CzH&Br+ continues to increase. We suggest that reaction 13 is reversible, and that at sufficiently high pressures all positive ions in the system are ultimately converted to the “ether” structure.16 Reactions 11and 12 are postulated to account for the fact that CzH4+ and CzHz+ are moderately important in the low pressure mass spectrum of ethyl bromideTo(abundances 5.0 and 7.3%, respectively) but are not significant under the conditions of high pressure mass spectrometry. Both C2H4+ and CzHz+ have ionization potentials which are close to, but slightly greater than, ethyl bromide. Accordingly, the charge transfer reactions 11and 12 should be quite efficient. We have not attempted to account for a number of even more minor species seen in the electron bombardment of ethyl bromide. We conclude that the primary ionic fragments formed in ethyl bromide radiolysis all ultimately evolve into (CzHs)zBr+, as shown in Table IV. In each case except the first, two successive ion-molecule reactions are involved in order to reach the ultimate etherlike species. The percentage yields shown in Table I11 were converted to approximate G values as follows: We assume for convenience a W value of 25 eV/ion pair for ethyl bromide;18this corresponds to a total G value for all ionic products of 4.0. This value was then multiplied by the fractional abundance of each ion to give a resulting G’ value: the prime (G’) indicates inferred yield of the ionic process. (The G’ value of the parent C2H5Br+ ion is not shown separately in Table IV; it would be 4.0 X 0.32, or 1.28). It is expected that all electrons formed in the system, also amounting to a G’ value of 4.0, undergo dissociative electron capture, reaction 10.19320 Since the electron affinity of ethyl bromide is 3.36 eV,21 the dissociative process is 9.5 kcal/mol exoergic. It is known to proceed through a compound negative ion state having a lifetime of less than s.19 Nondissociative electron capture leading to a negative ion with a lifetime much greater than bond vibration times is also known; however, this species can autoionize,lg giving a free electron back again, which could subsequently undergo reaction 14.
1537
Gas Phase Radiation Chemistry of Ethyl Bromide
TABLE IV: Evolution of Neutral SDecies from Primary Ions G
First generationu CzH5Brf CzH5+ Br C2H3+ + H2 + Br. C2H2+ + H2 + HBr C ~ H L++ HBr CzHr + Br+
+
Second generation CzHjBr+ C2H5BrH+ C2H4 Br C2H5BrH+ CzH2 H2 + BP CZHjBr+ CzHz Hz HBr CZHjBr+ CzH4 HBr CzHbBr+ Br CzHy
+ +
Neutralization
Third generation
+ +
+ + + + + + + Plus, (G = 4)e- + CzHjBr CzHr + Br-.
-
+ + + + +
C2Hj- + Br. HBr CzH4 HBr C2H2 Ha CzHz H2 + HBr CzH4 HBr C ~ H F Br.
(C2H&Br+ Br. (CzH&Br+ HBr CzH4 Br(CzH&Br+ + HBr + C2Hz H2 + Br. (CzH&Br+ BP CzHz Hz HBr (CzHj)zBr+ BP CzH4 HBr (CzH&Br+ Br. BP CzH5
+ +
+ + + + + + + + + +
+
+ + +
values 8.0
1.01 0.88 0.29 0.20 0.09
The first line would actually total G = 7.5 since the total intensity of the primary ions listed is only 93.7% rather than 100%. We set the total G value at CzHr and Br. equal to 8.0 on the assumption that most of the other minor ions would also ultimately react to give (CzH&Br+. Note that these predicted G values refer to ionic processes only. Additional yields of some species also result from neutral processes. Accordingly, we conclude that nondissociative electron capture, even if it occurs to an appreciable extent, has no net effect upon the system. The dissociative electron capture process, reaction 10, produces ethyl radical and Br- ion, with an assumed G’ value of 4.0. I t then follows that Br- undergoes a neutralization reaction with (C2H&Br+ (expected to be the major long-lived ion in the system),l6 with subsequent fragmentation of the compound structure. At this point, we make the arbitrary assumption that one ethyl bromide molecule survives the neutralization process intact. We defend this assumption on the basis of the obviously stable valence structure of the etherlike complex (implying a rather small neutralization energy, perhaps 7 or 8 eV), the necessity of furnishing energy corresponding to the electron affinity of bromine (3.4 eV),22and the large number of modes over which the remaining energy can be dissipated. If our assumption is too conservative, it would simply imply that the total yield of the radical pair C2H5. Br. would be somewhat greater than shown in Table IV. By our model, the total yield of these species is 8.09, including a small additional yield formed as shown in the last line of the table. It will be noted in Table IV that a variety of other small molecules are formed as the ion-molecule reactions progress, and in the original ionization steps. Summing up the yields of each product as shown at the right of Table 111, we find G’(HBr) = 2.38, G’(C2H4)= 1.21, G’(C2H2)= 1.17, and &(H,) = 1.17. The additional HBr and CzH4 yields are modified by participation in an extensive series of secondary reactions, as noted in our photolysis ~ o r k . 4 The 9 ~ postulated acetylene yield of 1.17 would also be modified by secondary reactions; much of it is probably converted into vinyl bromide:
+
Bra
+ C2H2
BrCHCH-
--
+ HBr
BrCHCH. C2H3Br
+ BP
(38) (39)
At this point, we find that it has been possible to account for every case shown in Table I1 in which the relative yields under radiolysis are greater than those during photolysis. (The two C1 species are an exception; as mentioned above, we assume that there is a somewhat larger yield of C-C bond rupture under radiolysis.) We suggest that CzHs, 1,1-C2H4Br2,and 1,2-C2H4Brz are formed predominantly in secondary radical processes, as shown in Table I11 and in the companion paper.4 It is also apparent that the net yields of HBr and CzH4 are modified by the occurrence of free radical processes. It may be significant that the actual hydrogen yield, G = 1.39, is rather close to the value of 1.17 predicted from Table IV. Furthermore, the sum of the experimentally observed yields of acetylene plus vinyl
bromide, approximately 1.06 during intermediate stages of the radiolysis experiments, is quite close to the prediction of 1.17 derived from Table IV. Since these are both labile species, it is to be expected that their yields would be diminished by further reactions. Thus far we have postulated no role for hydrogen atoms in the radiolysis of ethyl bromide. It seems likely that a t least a small yield of H. would be formed; those which react “hot” could produce an additional component of the Hz yield, which is entirely unscavengeable, according to our results. Thermal hydrogen atoms probably form HBr in preference to H2:
- +
CzHbBr
H. (hot) + CzHjBr
H. (thermal)
C2H4Br-
Hz
-
+ H-
C ~ H ~ B(+Psome HBr)
+ CzH5Br
HBr
+ CzHb.
(40)
(41) (42)
We can make a very rough estimate of the “hot atom” H2 yield from G(H2) = 1.39 (experimental) 1.17 (predicted, ionic) = 0.22, or 16%of the total. This result depends on the rather rough analysis presented in Table IV; furthermore, the residual nonionic hydrogen yield could be due to direct molecular Hz elimination as well as to hot hydrogen atoms. Nevertheless, a hot hydrogen atom yield of 16%would compare favorably with previous estimates that 15%of the H2 yield from ethyl iodide2 and 20% of the yield from ethyl chloride1 is formed from translationally hot hydrogen atoms, based on mass spectrometric isotope studies. As mentioned earlier, it is to be expected that the relative contributions of the various neutral secondary reactions (Table 111)are different under radiolysis than photolysis. In particular, the energy deposited in a 10-min photolysis experiment corresponds approximately to a 24-h radiolysis, that is, there is a difference of about lo2 in the rate of energy deposition, a corresponding difference in the rate of generation of free radical species, and consequently a difference of the order of lo4 in the rate of radical-radical reactions. Accordingly, we conclude that most of the radical-radical reactions shown in Table I11 are relatively less important in radiolysis than in photolysis. Clearly, however, one or more radicalradical reactions must persist during radiolysis as a termination step. We suggest that the bromine atom combination reaction may be the most important radical termination step during radiolysis. HBr is a fairly good radical scavenger, and is present in substantial concentration throughout the experiment; most other radicals would abstract hydrogen from HBr, yielding bromine atoms which would ultimately feed into the Brz steady stateU4 The effects of adding 5% 0 2 during the radiolysis of ethyl bromide parallel closely those seen in the photolysis. In both
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The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
Arthur J. Frank and Robert J. Hanrahan
1538
TABLE V: Comparison of G Values in Radiolysis of Pure Ethyl Chloride, Ethyl Bromide, and Ethyl Iodide Product
Ethyl chloride Ref l b Ref l a (0.37(?))c 4.5 d d 4.41 2.44 1.09 0.24 d 0.05 d 0.06 d
d
Ethyl bromide
Ethyl iodideb
3.89 1.39 -0.01 0.01 2.70 2.17 0.31 0.32
0.73 0.02 -0.02 -0.05 d 1.34 1.57 0.48
2.40 4.08 1.72 d
2.11 2.06 0.75 0.66
d d d
d d d 0.05
0.12 EO.01 0.01
0.10 0.05 0.49 0.76
