LESLIEM. THEARD
3292
Effects of Additives on the Radiolysis of Cyclohexane Vapor at 1000'
by Leslie M. Theard Sandia Laboratory, Albuquerque, New Mexico
(Recdved Februury 19, 1966)
Effects of benzene, ethylene, propylene, cyclohexene, nitric oxide, iodine, and hydrogen iodide on the y-radiolysis of cyclohexane vapor at 100' have been studied in an effort to determine some details of the radiation-induced energy degradation processes. Benzene, the olefins, and iodine markedly reduce the yield of hydrogen to a comparable degree, while nitric oxide reduces hydrogen considerably less, and hydrogen iodide increases hydrogen. Of the gaseous hydrocarbon products, determined in the presence of all the additives studied except ethylene and propylene, the unsaturated products (ethylene, propylene, andacetylene) , which comprise the major fraction, are practically unaffected by benzene, cyclohexene, or nitric oxide. Iodine and hydrogen iodide do not affect the yields of ethylene and acetylene, but they reduce the yield of propylene. Yields of ethane and propane are reduced by several additives but are unaffected by hydrogen iodide. The observed additive effects on the gaseous products appear to be unattributable to ionization transfer. The olefins and benzene apparently reduce hydrogen yield by two processes which are suggested to be scavenging of hydrogen atoms and scavenging of ionic precursors of hot hydrogen atoms, where the scavengeable ions are not also precursors of gaseous hydrocarbon products. The increase of hydrogen yield by hydrogen iodide is explained by an electron-capture mechanism from which it is estimated that ca. 36% of the hydrogen yield for pure cyclohexane arises from neutralization of positive ions. Depression of hydrogen yield by iodine is explained by a combination of electron capture and hydrogen atom scavenging. It is suggested that reduction of the yield of propylene by Izand H I is attributable to electron capture and that reduction of the yields of ethane and propane by several additives is effected by scavenging of ethyl and propyl radicals.
Introduction Studies of the radiolysis of cyclohexane vapor2-8 have been relatively limited, whereas studies of the radiolysis of cyclohexane liquidg and effects of additives have been extensive and have provided insight into the nature of some important radiation-induced elementary processes. Included among these processes are energy (excitation or ionization) transfer2a10p11 and ion neutralization. l2 I n a study of the a-radiolysis of gaseous mixtures of cyclohexane and benzene, Ramaradhya and Freeman5 suggested that the depression of G(H2)by benzene is attributable principally to the transfer of energy, probably in the form of ionization, from cyclohexane to benzene. To the contrary, in a study of the 7 radiolysis of vapor phase cyclohexane-benzene mixtures, Dyne, e2 aZ.,'j suggested that energy (excitation or The J m r d of P h y a i d Chemistry
ionization) transfer cannot account for depression of hydrogen by benzene. I n another study of the latter system, Blachford and Dyne' explained hydrogen ~
~~
~~
~~
~
~
~
(1) This work was supported by the United States Atomic Energy Commission. Reproduction in whole or in part is permitted for any purpose of the U. S. Government. (2) J. P. Manion and M. Burton, J . Phys. Chem., 5 6 , 560 (1952). (3) V. P. Henri, C. R. Maxwell, W. C. White, and D. C. Peterson, ibid., 56, 1953 (1952). (4) J. M. Ramaradhya and G. R. Freeman, J . Chem. Phys., 34, 1726 (1961). (5) J. M. Ramaradhya and G. R. Freeman, Can. J . Chem., 39, 1769 (1961). (6) P. J. Dyne, J. Denhartog, and D. R. Smith, Discussions Faraday SOC.,36, 135 (1963). (7) J. Blachford and P. J. Dyne, Can. J. Chem., 42, 1165 (1964). (8) R. D. Doepker and P. Ausloos, Abstracts, 148th National Meeting of the American Chemical Society, Chicago, Ill., 1964, p. 48V. (9) For a recent paper containing pertinent references see 5. 2.Toms 86, 1478 (1964). and W. H. Hamill, J . Am. Chem. SOC.,
EFFECTS OF ADDITIVES ON RADIOLYSIS OF CYCLOHEXANE VAPORAT 100"
depression as resulting from ion-molecule addition reactions of ion fragments (C,-C,> with benzene, as suggested by Borkowski and Ausloosla for the nbutane-benzene system. I n the present study, effects of a variety of additives on cyclohexane vapor radiolysis have been studied in an effort to understand better the nature of the processes responsible for the additive effects. Effects of benzene, ethylene, propylene, and cyclohexene suggest that reduction of the yield of hydrogen is unattributable to charge transfer and is partially attributable to hydrogen atom scavenging. The remaining reduction of hydrogen is suggested to be attributable to scavenging of ionic precursors of hydrogen as suggested by Blachford and Dyne, but it is shown that these ions are not precursors also of measured gaseous hydrocarbon products. The yield of ionic precursors of hydrogen is estimated with the aid of an electroncapture mechanism suggested by effects of hydrogen iodide and iodine.
Experimental Section Materials. Phillips Research grade cyclohexane was used from Lot N o . 435 and 1078 with stated purities of 99.94 and 99.99 mole %, respectively. Chromatographic analysis using a silver nitrate-@,@'-oxydipropionitrile column showed cyclohexene present a t concentrations of 0.09 mole "i; in Lot No. 435 and less than 0,001 mole %, the lower limit of detection, in Lot No. 1078. Samples used from both lots were successively passed through silica gel until the cyclohexene concentration was less than 0.001 mole yo. Two impurities, 2,4-dimethylpentane and methylcyclopentane, each present a t a concentration of ca. 0.005 mole %, were not removed by silica gel. Phillips Research grade cyclohexane and benzene, Eastman White Label cyclohexene, and Fisher Certified reagent iodine (in cyclohexane) were dried over P20sprior to use. Phillips Research grade ethylene and propylene and Matheson hydrogen iodide and nitric oxide, the latter containing 3% nitrogen, were used as received. Attempts to remove the nitrogen impurity from nitric oxide were unsuccessful. Apparatus and Sample Preparation. Cylindrical Pyrex radiolysis cells of ca. 15-cm. length, 7-cm. diameter, and 480-nil. volume were used. Two side arms were constructed 90" apart a t one end of each cell, one of which was used for sample introduction and the other, fitted with a break-seal, for gaseous product collection. Prior to introducing the samples, the cells were pumped overnight at a pressure of ca. 5 X mm. For 0.5 hr. prior to and for the duration of the irradiation, the samples were heated in a furnace
3293
constructed of nichrome wire (or heating tape) wrapped around formed copper sheet with an outer covering of asbestos. Cell temperature was controlled a t 100 f 2" by a thermoregulator. Cyclohexane samples and solutions were degassed by one of two methods. (1) Approximately 30-ml. lots of pure cyclohexane, after drying over P206under vacuum, were distilled into a storage bulb and degassed by the microstill-reflux meth0d.1~ Individual samples were vacuum distilled as required into a calibrated volume for measurement and subsequently into the radiolysis cells. (2) Individual pipetted samples, dried over P206under vacuum, were degassed by successive freeze-pump-thaw cycles at - 196 and -78" and distilled into the radiolysis cells. By adjustment of. the amounts of cyclohexane and additives, total pressure of the samples at 100" was maintained a t ca. 100 cm. Irradiation and Dosimetry. Samples were irradiated with y-rays from a 2000-curie Cow source. The dose rate (ca. 2 X 1019 e.v./g. hr. for cyclohexane vapor) determination was based on measuring Ht produced from ethylene (at 1-atm. pressure and 23") and assuming G(H2) = 1.28.15 Extrapolation to the samples under study was made on the basis that energy absorption is proportional to electron density. Total dose for most runs was ca. 3 X 1019e.v./g. I n order to determine zero-dose yields by extrapolation, several samples were irradiated at doses ranging from 1.7 X lo'* to 2.1 X lozoe.v./g. Analysis. Hydrogen in the product gases volatile at - 196" was determined from the pressure difference before and after passage through a heated palladium thimble (270"). Residual gases (ca. 5%) collected a t -196" were mixed with the measured gas fraction collected at - 110" by the microstill-reflux method.I4 This sample was then mixed with a measured comparable amount of cyclopropane which was used as a quantitative marker for chromatographic analysis.I6 The gaseous mixture was analyzed chromatographically by use of dibutyl maleate and silica gel columns. Separate duplicate samples were irradiated for chromatographic (10) P. J. Dyne and W. M. Jenkinson, Can. J . Chem., 39, 2163 (1961). (11) J. A. Stone and P. J. Dyne, Radiation Res., 3 , 353 (1962). (12) J. R. Nash and W. H. Hamill, J . Phys. Chem., 66, 1097 (1962). (13) R. P. Borkowski and P. J. Ausloos, J . Chem. Phys., 39, 818 (1963). (14)W.Van Dusen, Jr., and W. H. Hamill, J . Am. Chem. SOC.,84, 3648 (1962). (15)R.A. Back, T. W. Woodward, and K. A. hIcLauchlan, Can. J . Chem., 40, 1380 (1962). (16)K.H.Jones, W. Van Dusen, Jr., and L. M .Theard, Radiation Res., 23, 128 (1964).
Volume 69, Number 10 October 1966
LESLIEM. THEARD
3294
analysis of liquid products. Columns used and products determined were silver nitrate-@,@'-oxydipropionitrile for cyclohexene, Apeizon for dicyclohexyl, and di-n-decyl phthalate for ethylcyclohexane and propylcyclohexane.
Results Table I shows the effect of dose on the identified radiolysis products for pure cyclohexane vapor. The zerodoze yields were obtained by extrapolation. Also shown are product yields previously reported for pure cyclohexane vapor 7-radiolysis at 125" and a-radiolysis at 108°.4 Table I: Radiolysis Product Yields for Pure Cyclohexane Vapor Product
Hz CH4
CzHz C~HI CzHe Cab CaH, Ca hydrocarbons C y clohexene Dicyclohexyl Ethylcyclohexane Propylcyclohexane
Cb
Goa
4.8 0.36 0.35 1.7 1.4 0.55 0.44 0.6 1.2 1.0
4.7 0.36 0.32 1.35 0.71 0.35 0.30 0.59 1.0 0.8 0.3 0.2
f 0.2 f 0.03 f 0.04 f 0.08
f 0.14 f 0.04 f 0.03
B 5.3 0.6
...
2.8 1.9 0.43 2.2
f 0.10 2.2 1.0
QOd
8.0 0.13 0.65 2.48 0.28 0.78 0.95 0.34 0.77
0.5
5 Zero dose. Dose: ca. 3 X
[email protected]./g. Blachford and e.v./g. Ramaradhya and Dyne, -prays; dose: ca. 3 X lo1@ Freeman, a-rays; zero dose.
The hydrocarbon product yields reported for this work and that of Blachford and Dyne' differ markedly, for which there seems to be no readily apparent explanation. The slightly higher temperature (125') employed by the latter authors does not account for the difference since in the present study it was determined that an increase of temperature from 100 to 150" has no effect on gas yields. It is noteworthy that at the lower doses the yields reported for the present work would be considerably higher if they were computed on the basis of collected-gas pressure rather than by comparison with added cyclopropane, The difference of yields computed by the two methods increases with decreasing dose. For example, G(C2, cs, Cdtot press method - G(C2, C3, C4)cyclopropsne method is 2.6 and 0.8 at 0.3 X 1019 and 3 X 1019e.v./g., respectively. Two explanations appear plausible. (1) Small amounts of cyclohexane vapor are collected at - 110' as shown by chromatographic analysis. ConThe Journal of Physical Chemistry
sequently, because the amount of collected vapor should be independent of dose, the amount of collected vapor relative to products should increase with decreasing total products, i.e., with decreasing dose. (2) Alternatively, included in the gases collected may have been unidentified volatile products whose yields decrease with dose more markedly than the gas yields actually determined. I n this connection it is pertinent that Doepker and Ausloos* proposed a radiation-induced decomposition process for cyclohexane vapor leading to butyne formation. Butyne and propyne would have been difficult to determine by our analytical technique and consequently our lack of detection does not imply that they were not formed. Figure 1 displays plots of G(HZ)obsd, the observed molecules of hydrogen formed per 100 e.v. absorbed by the system, and g(H2), molecules of hydrogen formed from cyclohexane per 100 e.v. absorbed by cyclohexane, vs. electron per cent benzene. The dashed line in Figure 1 represents the expected yield of hydrogen, G(H2)expd, if the yields of hydrogen for each component of the mixture were unaffected by the presence of the other, and therefore it fits the relationship
where e is electron fraction, GO(H2) is the G value of hydrogen for the pure component, and the subscripts represent the pertinent component. The relationship ~ ~g(H2) ~ ~ is shown by the equation between G ( H P )and for cyclohexane-benzene mixtures G(Ha)obsd
+
ECEH,&(H~)C~H~~ €C~HQ(HZ)C~H~ (11)
It is assumed that the yield of hydrogen for benzene is unaffected by the presence of cyclohexane; i.e., ~(H~)= c ~G0(H2)cas H~ for all values of E C ~ H ~and , consequently the observed differences in values of G(H2),xpd
€7' BENZENE I N CYCLOHEXANE Figure 1. Effect of benzene on hydrogen yield.
3295
EFFECTS OF ADDITIVES ON RADIOLYSIS OF CYCLOHEXANE VAPORAT 100"
and G(HP)~M are attributed to a decrease of g(HZ)caH,l with increasing concentration of benzene. Similar assumptions are made for other systems for which are reported. values of g(H2),representing g(H2)CsHI2, Figure 2 presents plots of the G values of C2H4, CaHe,and C2H2us. electron per cent benzene. Straight lines drawn to fit the points extrapolate to values of G(C2H4)and G(C3H6)for pure cyclohexane which are higher than the values actually determined at a dose of 3 X 1019 e.v./g. (Table I). Table I1 includes effects Table I1 : Effects of Additives on Gaseous Radiolysis Product Yields for Cyclohexane Vapor
E
46 BENZENE IN CYCLOHEXANE
Figure 2. Effect of benzene on GHP,CaH6, and QHz. Additive
Concn., mole %
... 4 23 4 21 0.4 1.3 0.5 9
7
U
HI
CHI
CzHz
CZHI CzHs
CaHa
CIH~
4.7 3.9 2.4 2.5 2.2 1.7 7.3 4.3 2.9
0.36 0.34 0.33 0.29 0.44 0.07 0.33
0.32 0.30 0.30 0.28 0.33 0.27 0.36 0.36 0.32
1.4 1.6 1.6 1.5 1.7 1.3 1.6 1.7 1.7
0.30 0.10 0.04 0.12 0.05 0.12 0.34 0.07 0.08
0.35 0.50 0.50 0.44 0.53 0.06 0.05 0.50 0.43
...
...
0.71 0.10 0.10 0.11 0.12 0.05 0.86 0.07 0.05
of several additives which increase yields of C2H4 and CaH6. Similar effects have been observed previously for cyclohexane liquid" and n-hexane vapor.I8 The increase of yields can be explained by presuming that, in the absence of additives, products are partially consumed by reaction with intermediate species such as free radicals and ions. I n the presence of additives, the intermediate species may react preferentially with the additives rather than the products. Thus, the products are protected from consumption, and the net effect of the additives is an increase of product yield. Figure 3 gives plots of g(Hz) us. low concentrations of C2H4, and c-C6Hlo, and Figure 4 shows g(H2) over the full concentration range of these additives. I n Figure 4 a single curve representing all the points for the latter three additives has been drawn for simplicity. Perhaps as suggested by the data points there is a real difference of additive eff ect.s at high concentration, but the magnitude of the difference is small and is considered to be insignificant for over-all interpretation. Figure 4 also shows the effect of NO on g(Hz). The increase of g(H2) between 18 and 36 mole is qualitatively similar to an observation by Ausloos, et aZ.,19 on the effect of NO on G(H2) for CH4--C3Hgmixtures. Thus, attention is called to the fact that the curve for NO in Figure 4 may indeed increase as suggested by the
t
01
I
I
5
0
MOLE
I 20
I
IO
'5
70 A D D I T I V E
Figure 3. Effects of CZ&, C & 3 ,and c-C~HIO on g(H2).
't
01
0
I
20
I
I
1
40 60 80 MOLE Yo ADDITIVE
I IO
(17) 5. Sato, K. Kikuchi, and S. Shida, J. Chem. Phys., 41, 2216 (1984). (18) H.A. Dewhurst, J . Am. Chem. SOC.,83, 1050 (1961). (19) P. Ausloos, S. G . Lias, and R. Gorden, Jr., J . Chem. Phya., 39, 3341 (1963).
Volume 69, Number 10 October 1066
LESLIE M. THEARD
3296
data. However, in the absence of more data in the high concentration region, the shape of the curve is uncertain. The dashed line is an extrapolation of the low concentration data. Most of the points of Figures 1, 2, and 4 represent an average of at least two independent determinations. Figure 5 shows effects of H I and IZon g(Hz), and Table I1 includes effects of these additives on the principal gaseous hydrocarbon products. No Ht was produced in a 1.5% mixture of H I in cyclohexane prepared as were the irradiated samples and heated to 100" for 1 hr. Correction for Hz produced from direct irradiation of HI was made on the basis that G(H2) = 9.2, determined for pure H I at 1.4 cm. and 100". Table I11 reports some vapor phase radiolysis product yields for pure cyclohexene and pure benzene. For pure propylene G(H2) = 1.26.
Pure
G
?
Hz
CHI
CZHI
C~HI
CzHs
CaHs c-CsHio
0.084 1.2
... 0.27
0.61 0.45
0.02 1.4
0.04
...
CsH6 .
I
.
0.13
Discussion The zerodose yields reported in Table I show that products with an H : C ratio less than 2 and a total G value possibly as high as 5 have not been measured. The dose dependence of many of the products detected (Table I) suggests that secondary reactions account for the mass imbalance. Presumably, the principal unmeasured products are Since the curve for G(Hl)obsdin Figure 1lies below that for G(H2)expd, benzene inhibits the formation of hydrogen from the radiolysis of cyclohexane vapor. The extent of the effect of benzene is shown by the decrease of g(Hz) from 4.7 to 1.3, the latter value being the unaffected yield of hydrogen for cyclohexane. Table I1 shows that benzene also decreases g(C2Hs)and g(C3Hs) but at a rate faster than it decreases g(Hz). Thus, it can be concluded that the process responsible for the principal decrease of g(Hz) is different from that responsible for the decrease of g(CzH6) and g(C3H8). Further, since benzene does not affect CzH4, CZH~, C3H6 (Figure 2 ) , or CH, (Table II), it can be concluded that more than one precursor is involved in the formation of the gaseous products. It might be considered that part of the decrease of hydrogen, in the presence of benzene, is attributable to protection by benzene of a cyclohexane speciea The Journal of PhyaiOal Chemiatru
HI
2
OO
0.5
1.0
1.5
0
MOLE % ADDITIVE
Figure 5. Effects of HI and I) on g(H&
which, in the absence of benzene, yields hydrogen as its only gaseous product. For example, in pure cyclohexane vapor, neutralization of parent ions may lead to hydrogen production via the reactions
Table 111: Gaseous Radiolysis Product Yields for Pure Cyclohexene and Pure Benzene vapor
et
+ e- +c-C&.1z* C-CeHiz* H + c-CsH11 H + c-CsH12 +Hz + C-c~H11 c-CsHlz+
---t
(1) (2) (3)
where c-csHl2* represents highly excited cyclohexane. In the presence of benzene, hydrogen may be decreased by the following reactions. c-Cc&z+
+ CsHa +C-C~HI~+ c6Hs+ C G H ~4+-e- +no hydrogen
(4) (5)
Ramaradhya and Freeman5 considered ionization transfer to be the most plausible explanation for the decrease of hydrogen by benzene, cyclohexene, and propylene in the vapor phase a-radiolysis of binary mixtures containing cyclohexane. Effects of olefins, to be further discussed, determined in the present study are inconsistent with the above ionization-transfer mechanism. Blachford and Dyne found that benzene reduces bimolecularly produced hydrogen but does not affect unimolecularly produced hydrogen. Assuming that the decrease of hydrogen was attributable to hydrogen atom scavenging, they computed from their data the ratio of the rate constant for hydrogen atom addition to benzene to the rate constant for abstraction of H atoms from cyclohexane by H atoms. They argued that the computed ratio was unreasonably small and therefore concluded that the hydrogen atom scavenging mechanism was implausible. Alternatively,
3297
EFFECTS OF ADDITIVES ON RADIOLYSIS OF CYCLOHEXANE VAPORAT 100”
they suggested that benzene scavenges ionic precursors (probably CZ-c3 ion fragments) of hydrogen atoms. The present data show that, if CZ-c3 ion fragments are principal precursors of benzene-reduced hydrogen, these same fragments are not also precursors of measured c2-c3 products. This is concluded from the fact that the decrease of CzH6 and C3Hs with increasing benzene concentration is more rapid than is the major decrease of hydrogen, and CzH4, C2H2, and C3H6 are unaffected by benzene. The lack of effect of benzene on C Z H ~CZHZ, , and C3H6 suggests that they are not produced by free-radical or fragment-ion precursors because benzene reacts with both species.20 NO reacts with free radicals but apparently does not react readily with fragment ions. 2o Thus, the reduction of CZH6 and C3Hs by NO (Table II), and presumably also by benzene, cyclohexene, and iodine, very likely is attributable to scavenging of C2Hsand C3H1radicals. The observed dependence of g(H2) on the concentration of olefins suggests that they reduce g(Hz) by a t least two processes, one of which is more prominent a t low additive concentration. Figure 3 shows that CzH4, C3H6, and c-C6Hlo reduce g(H2) relatively rapidly a t lo(v concentration and more gradually, but persistently, a t higher concentration. Figure 4 shows that g(H2) continues to decrease over most of the additive concentration range. These results are inconsistent with a single process being responsible for the decrease of hydrogen over the full additive concentration range. If the low concentration effect were solely operative throughout, g(Hz) would be expected to reach a constant high concentration limiting value considerably higher than that observed in Figure 4. The similarity of the decrease of hydrogen as a function of C2Hs,C3H6, or c-CsHl0concentration appears unattributable to charge transfer from ground-state cyclohexane ions to the additives because the ionization potentials favor charge transfer only to C3H6 and c-C~HIO.That is, the ionization potential of cyclohexane is greater than those of C3H6 and C-CeHlo and less than that of CzH4.Z1 It appears also unlikely then thzt benzene reduces hydrogen by charge transfer although the ionization potential of cyclohexane is greater than that of benzene. It is suggested that the decrease of Hz by benzene and the olefins is attributable principally to scavenging of H atoms at low concentrabion and additional scavenging of fragment-ion precursors of hydrogen atoms a t high concentration. The latter mechanism, similar to that proposed by Blachford and Dyne to explain all the depression of hydrogen by benzene, is based on the suggestion by Lias and Ausloos20 of the occurrence
of radiation-induced fragment-ion addition reactions with olefins and benzene in propane. It should be noted that, if fragment-ion precursors of hydrogen atoms are scavenged principally a t high additive concentration, the hydrogen atoms normally produced by the unscavenged ions must be nonscavengeable and perhaps are hot H atoms. If the atoms were scavengeable, the scavenging of their ionic precursors would not decrease the concentration-dependent rate of hydrogen decrease, and the minimum over-all rate of decrease of hydrogen would be that expected for exclusive hydrogen atom scavenging. In other words, if in the presence of olefins the faster low concentration effect is attributed principally to hydrogen atom scavenging, the slower high concentration effect cannot be attributed to scavenging of precursors of the scavengeable hydrogen atoms. NO is not so effective in reducing g(Hz) as are the olefins (Figure 4). However, interpretation of the difference is complicated by the likelihood that some reaction involving NO produces hydrogen,lg which tends to increase the hydrogen yield at high NO concentrat ion. As components of radiolysis systems, 1 2 and H I may scavenge H atoms
+ +HI + I H + HI Hz + I H
12
(6)
(7)
and/or dissociatively capture thermal electrons.12t22 ee-
+ Iz I + I+ HI +H + I-
(8) (9)
Considering that the fate of H atoms in pure cyclohexane is formation of Hz via reaction 3, H atom scavenging by I2reduces g(Hz)while H atom scavenging by HI does not affect g(Hz). Electron capture by Iz, reaction 8, may reduce the yield of hydrogen if, for example, in pure cyclohexane radiolysis hydrogen is produced by reactions 1,2, and 3. I n the presence of Iz, neutralization of positive ions following reaction 8 probably occurs via 1-
+ C-CeHiz’
+1
+ C-CgHiz
(10)
where it is unlikely that the C-CeHlz species decomposes.12 Thus, the hydrogen yield may be depressed by quenching reaction 2. Of course, in cyclohexanevapor radiolysis, ions other than c-C6HI2+are formed, (20) S. G . Lias and P. Ausloos, J. Chem. Phys., 37, 877 (1962). (21) F. H. Field and J. L. Franklin, “Electron Impact Phenomena.”
Academic Press Inc., New York, N. P.,1957, pp. 108, 109. (22) D. C. Frost and C. A. McDowell, J. Chem. Phys., 2 9 , 5 0 3 (1958).
Volume 69, Number 10
Odober 1966
3298
and these ions also may produce hydrogen upon neutralization. Although electron capture by H I also can be expected to quench reaction 2, the loss of hydrogen may be equaled or exceeded by attendant hydrogen formation uia reactions 9 and 7. It is suggested that the increase of hydrogen shown in Figure 5 arises from such hydrogen formation via reactions 9 and 7 with greater efficiency than normally occurs via reactions 1 and 2 and other neutralization-induced decompositions, as suggested by Nash and Hamill12 for liquid cyclohexane. In other words, it is suggested that in pure cyclohexane all ion neutralizations do not produce hydrogen. Recent results on the effects of additives on gaseous hydrogen chloride23and propane24radiolyses have been explained on the basis of electron capture. Assuming the proposed electron-capture mechanism for the effect of HI, the yield of hydrogen resultant from neutralization reactions can be estimated by comparing the yield of electrons with the increase of hydrogen. The G value of electrons can be estimated by assuming that W , energy absorbed per ion pair produced, is ca. 24 e.v. a5 found for many hydrocarbons26: thus, G(e-) = 1OO/W 'v 4.2. Figure 5 shows that %heincrease of g(H2) by H I is 2.7, which is interpreted to equal the G value of electrons which do not yield hydrogen through neutralization in pure cyclohexane, assuming that each hydrogen-producing neutralization is responsible for formation of one molecule of hydrogen. Conversely, the G value of electrons which yield hydrogen is 4.2 - 2.7, or 1.5. The decrease of g(H2) by I2 via electron capture can be estimated to be equal to 1.5, the G value of electrons which yield hydrogen. Since the total decrease of g(H2) by I2is 3.0, an additional decrease of 1.5 is attributable to some other process, presumably hydrogen atom scavenging. It appears then that the decrease of hydrogen by iodine and, as suggested above, also by the olefins and benzene is attributable to two processes, one of which is hydrogen atom scavenging. The other process apparently involves suppression of ion-neutralization decomposition reactions with the distinction that iodine reacts with the hydrogen-producing ionic species by neutralization following electron capture and the olefins and benzene react by addition. The extent of hydrogen decrease by the two processes in the presence of the olefins and benzene may be approximately equal as suggested for iodine. However, Figures 3 and 4 only indicate that both processes are important, and the complexity of the decrease of g(H2) by benzene
The Journal of Phyeical C h a b t r y
LESLIEM. THEARD
is apparently masked by the fact that the processes responsible for the decrease occur with comparable rates. Table I1 shows that I2and H I reduce C3H6 markedly, while the other additives increase C3H6 slightly. The latter effect is presumed to be attributable to protection of CsH6 from consumption by a bimolecular dosedependent process. It appears that the decrease of C3H6 is attributable to electron capture by Iz and H I since this process is highly probable for these additives and improbable for the others. Thus, neutralization of cyclohexane molecule ions may produce C3HB C-CgHi2'
+ e-
42C3H6
(11)
and, following electron capture by I2and HI, substituted neutralization via reaction 10 presumably does not produce C3H6. Accordingly, the near lack of effect of Iz and H I on the yield of C2H4 supports the suggestion of Doepker and Ausloos8 that C2H4 is produced by decomposition of neutral excited cyclohexane. It appears that, in general, the determination of the effect of H I on the radiolysis of gaseous hydrocarbons may be quite helpful in elucidating the mechanism of formation of various products. The present study of the effect of H I suggests that the importance of neutralization processes in the formation of hydrogen and propylene can be estimated. In this regard, the approach may compare well with the electric field method.26 I n the latter case it is assumed that neutralization of positive ions at a negative electrode is nondissociative, and it appears that neutralization of positive ions by I- also is nondissociative. H I is interesting also in that it converts free radicals to stable products by addition of an H atom to the free radical. Therefore, free radicals that abstract hydrogen atoms in the pure hydrocarbon do likewise by reaction with HI. In this connection note that H I does not affect C2H6 and C3Hs(Table 11). Acknowledgments. The helpful assistance contributed by Drs. K. H. Jones and W. Van Dusen, Jr., through experimental work and discussions, is gratefully acknowledged. The experimental assistance of J. Schmidt, D. R. Begeal, and L. L. Stephenson is also gratefully acknowledged. (23)R.A. Lee, R. S. Davidow, and D . A. Armstrong, Can. J. Chem., 42, 1906 (1964). (24) G.R.A. Johnson and J. M. Warman, Nature, 203, 74 (1964). (25) G.G.Meisels, J . Chem. Phys., 41, 51 (1964). (26)H.Essex, J. Phys. Chem., 58, 42 (1954).
