M. G. ROBINSON AND G. R. FREEMAN
1780
The Radiolysis of Cyclohexane in the Presence of Deuterated Olefins. The Involvement of the Olefins in Hydrogen Formation' by M. G . Robinson and G . R. Freeman Department of Chemistry, University of Alberta, Edmonton, Canada
(Received November 28, 1967)
The hydrogen yields from the radiolysis of cyclohexane solutions containing low concentrations of deuterated olefins (C,-C7) have been measured. The presence of significant amounts of HD and Dz in the products confirmsan earlier suggestion that the olefins participate in hydrogen formation as well as acting as inhibitors, However, it is mainly the alkyl groups of the fully deuterated olefins that are involved in HD formation. A small amount of molecular Dz elimination occurs from the olefinic carbons. The addition of ethanol to a solution of CaD6in cyclohexane reduced the yields of HD and Dz by the same proportionate amounts. The results are explained by ionic reactions. There is no clear-cut evidence for the occurrence of hydrogen atom scavenging.
Introduction Hydrogen atoms have frequently been proposed2-0 as being the reactive precursors of the molecular hydrogen formed in the radiolysis of liquid alkanes. I n an investigations of the effect of structurally different olefins on the radiolysis of n-hexane, the decrease in yield of hydrogen was attributed to scavenging of hydrogen atoms by the olefin. From a kinetic treatment of the results, it was concluded that hydrogen atom abstraction from the olefin RzCCHz occurred, reaction 1, as well as hydrogen atom addition, reaction 2, The ratio of hydrogen abstraction to hydrogen H
+ RZCCHZ +Ha + RgCCH. H + RzCCHz-+RzCCH3
(1)
(2)
addition, iil/kz, was found to be -0.3 for olefins having the structures RCHCHz and RzCCH2. Furthermore, the evidence purported to show that the hydrogen abstraction occurred mainly from the olefinic carbons (e.q.,reaction 1). These conclusions seemed surprising in view of the fact that the dissociation energies of the C-H bonds on the olefinic carbons are considerably greater than those of C-H bonds in alkyl groups in the olefins.'O Also, there is no clear-cut evidence for the interaction of hydrogdn atoms with olefins in the condensed-phase radiolysis of alkanes. The evidence for such interactions involves kinetic plots based on a mechanism that has been shown not to be unique." It was, therefore, decided to check the above conclusions by using olefins suitably labeled with deuterium.
Experimental Section Fisher Spectroanalyzed cyclohexane was thoroughly degassed, taking care to remove carbon dioxide. Phillip@ research grade propylene was purified by distillation and freeze-pump-thaw cycles in a vacuum The Journal of Physical Chemistry
system. The deuterated olefins, from Merck Sharp and Dohme, were similarly deaerated before use. Benzene-free ethanol (Reliance Chemical Co.) was purified as reported elsewhere.lZ The nmr spectrum of CD3CHCHz indicated less than 0.1% of isotopic scrambling. That of hept-lene-1,1,2-d3 showed 10% scrambling of -CD=CD2 and -CH=CD2, while C8Ds was shown to contain 99% D. The samples were prepared by standard vacuum techniques. Most samples consisted of 2.0 ml of liquid in 2.5-ml cells. They were irradiated in a 'j0Co Gammacell-220 a t 23". The dose rate was 5 x 10'7 eV/ml min and the doses given were in the range 2 X lo'* to 2 X 1019 eV/ml. For hept-1-ene-1,1,2-d3, 4.0-ml aliquots of standard solutions (0.05-1 mol %) in cyclohexane were thoroughly degassed and distilled into 4.5-ml cells. These samples were given doses of 9 X lo1*eV/ml. Gaseous products that were volatile at 77°K were collected by vacuum distillation and measured in a McLeod-Toepler apparatus. The isotopic composition of the mixture was determined by using a CEC(21-614) (1) The work was partly supported by The Defense Research Board of Canada. (2) G. E. Adams, J. H . Baxendale, and R. D. Sedgewick, J . Phys. Chem., 63, 854 (1959). (3) C. R. Freeman, Can. J. Chern., 38, 1043 (1960). (4) T. J. Hardwick, J . Phys. Chem., 64, 1623 (1960). (6) T . J. Hardwick, ibid., 66,291 (1962). (6) T. J. Hardwick, ibid., 66, 1611 (1962). (7) S. 8. Toma and W. H . Hamill, J . Amer. Chem. Soc., 86, 1478 (1964). (8) J. Y. Yang and I. Marcus, J . Chem. Phys., 42, 3315 (1965). (9) R. A. Holroyd, J. Phys. Chem., 70, 1341 (1966). (10) J. A. Kerr, Chem. Rev., 66,465 (1966). (11) M. G. Robinson and G. R. Freeman, J . Chem. Phys., in press. (12) J. C. Russell and G. R. Freeman, J . Phvs. Chem., 71, 765 (1967).
THERADIOLYSIS OF CYCLOHEXANE
1781 I
mass spectrometer. The instrument was calibrated with standard mixtures of Hz, HD, and Dz, the relative proportions of which were in the ranges found experimentally in the irradiated samples.
I
I
I
0.8
10
I
0.16
0.12
Results The deuterated olefins used were: CHsCDCDz, CD3CHCHz, CsD6, (CD3),CCHz, (CD3)&CD2, and CHa(CH2)4CDCDz. The yields of HD and Dz from solutions of these olefins in cyclohexane are shown in Figure 1. The H D yield depends greatly on the position of deuterium substitution on the olefin. Very small yields of Dz were produced from solutions containing C3D6, CH3CDCDz, (CD&CCDz, and CH3(CHz)4CDCD2, and the G(Dz) us. olefin concentration curves were identical (Figure 1); G(Dz) = 0.005 at 1 mol Yo olefin. A somewhat smaller amount of Dz was formed from (CDs)zCCHz(Figure 1) and no Dz could be observed (G(D2) < 0.001) from solutions of CDaCHCHz. It appears that the Dz is formed mainly by ejection from the olefinic carbons of excited olefin molecules or ions. The decrease in hydrogen yield, AG(Hz), caused by a given concentration of olefin varied somewhat from olefin to olefin, but the G(HJ vs. olefin concentration curves were all similar to that reported earlier for the cyclohexane solutions of CsH6 and C3D6.11 What is more important, the value of G(HD)/AG(Hz) was independent of concentration for a given olefin, but varied greatly from one olefin to another (Table I).
n 0.08 (3
0.04
0 0
-n0
0
samples
CH 8 CD CD 2 CDaCHCHz C3H6 CHa(CH2)aCDCDz (CDa)zCCHz (CDa)zCCD$
8 10 4 7 8 6
0.038-1.09 0.072-1.20 0.20-1.86 0.11-1.05 0.055-1.16 0.046-1.14
0.4
0.6
1.2
Mole X
Figure 1. Yields of HD and D2 from cyclohexane solutions containing various deuterated olefins (A, HD; B, Dz): 0, (CDs)&CDz; 0, (CDa)zCCHz; m, CaD6; A, CDsCHCHz; 0, CHa(CH2)aCDCDz; A, CHsCDCDz.
t
X (3
Table I: Ratio of the HD Yield to the Decrease in the H z Yield
Olefin
0.2
[Olefin]
l4
Conon range, mol %
0.005 0
+
No. of
. B
0.01
N
10
O(HD)/
0.010 i 0.002“ 0 . 0 5 1 i 0.003 0,067iO.003 0.018 zk 0.003 0.062i0.004 0.081 i 0.004
AU(H2)
6
4
The deviations were random and no trend was noticeable.
The value of this ratio varied from 0.01 for CH3CDCDz to 0.08 for (CD,)2CCDz. Examination of the table leads to the conclusion that most of the HD is formed by the reaction of a hydrogen precursor from cyclohexane with the alkyl group of the olefin. I n cyclohexane that contained 1.86 mol % CsDe, the addition of ethanol suppressed the yields of both HD and D2 by equal proportionate amounts, while the total hydrogen yield remained virtually unchanged (Figure 2).
2 ’ 0
I
2
I 4
I
I
1
6
6
10
[Ethanol], Mole %
Figure 2. Yields of Ha ( O ) , H D (0),and De (A) from cyclohexane solutions containing 1.86 mol % propylene-&, as a function of added ethanol concentration. f = 5 for Hz, 100 for HD, and 1000 for Dz.
Discussion The formation of HD and D2 from the present solutions supports the earlier conclusion6 that olefins enter into hydrogen-forming reactions. However, the results in Table I contradict the suggestion that “hydrogen abstraction” occurs mainly from the olefinic carbons.6 Comparison of the values of G(HD)/ AG(H2) for the solutions containing CHsCDCDz, Volume 7 9 , Number 6
M a y 1068
M. G. ROBINSON AND G. R. FREEMAN
1782 CDaCHCHz, and C3D6 shows that about '/e of the abstraction occurs from the olefinic carbons and that The contributions 5 / 6 occurs from the methyl group. of the olefinic groups in the larger olefins to hydrogen formation are likewise small (Table I). The Dz is formed mainly by molecular ejection from the olefinic carbons of excited olefin molecules or ions. If HD were formed mainly by hydrogen atom abstraction, for example H
+ CDaCDCDz +H D + CDzCDCDz
(3)
then the addition of ethanol to the solution should have had little effect on the HD yield. However, not only did ethanol reduce the HD yield but it also reduced the Dz yield by the same proportionate amount (Figure 2). Ethanol reacts with positive ions in cyclohexane under irradiation,13 so it appears that both HD and Dz have positive-ion precursors in the present systems. The donation of H and Hz by alkane ions to olefin molecules, e.g., reactions 4 and 5, has been shown to occur in both the gasl4!l6 and liquid16 phases. Although these reactions help to explain the decrease in
+ CaD6 +C-C~HI~' C3D6H c-CGH~~+ f C& +C-C~HIQ'+ C3D6Hz C-C~HIZ'
(4) (5)
the hydrogen yield, they cannot explain the appearance of HD and Dz in the products. The yields of HD from all of the deuterated olefin solutions and the yields of DZfrom the >C=Dz olefins are too great to be explained by a direct radiolysis effect. Some other reaction, such as electron abstraction or proton donation, must also occur in these solutions
+ C-CsHlz + CaDa' c-CGHiz+ + C3Da +C-Ct&i + C3D6H' c-CGHtp+
--3
(6)
(7)
Neutralization of ions such as those formed in reactions 6 and 7 could explain the formation of both H D and Dz. The formation of HD and of Dz each has several possible routes and speculation about them is not worthwhile here. It has been demonstrated by kinetic analysis that the decrease in hydrogen yield that occurs when propylene is added to cyclohexane can be attributed mainly to either hydrogen atom scavenging or to electron scavenging, but the latter process offers a somewhat better explanation of the over-all results.l1 The fact that monoolefins do not have positive electron affinities in the gas phase does not mean that olefins will
The Journal of Physical Chemistry
not "associate" with electrons in alkane solutions." Positive-ion reactions such as reactions 6 and 7 doubtless occur in the same solutions. When scavengers that react either with positive ions or with electrons, but not with both, are used, suppression of the hydrogen yield is almost an order of magnitude more sensitive to electron-scavenger con~ e n t r a t i o n ' ~ Jthan ~ to positive-ion-scavenger conc e n t r a t i ~ n . ' ~ JThis ~ is probably due, at least in part, t o greater mobilities of solvated electrons than of positive ions in these liquids.l0 However, the scavenging of the electrons reduces the mobility of the negative species and increases the lifetimes of the positive ions, thereby increasing their probability of being s c a ~ e n g e d . The ~ ~ ~correlation ~~ of G(HD) with AG(Hz) in the present systems can, therefore, be explained if the olefins react with both positive ions and electrons. This would also explain why, within experimental error, G(HJ was independent of ethanol concentration while G(HD) and G(D2) decreased in the experiments recorded in Figure 2. The neutralization of an alcohol oxonium ion by an olefin anion apparently does not yield more hydrogen than does the corresponding neutralization of a hydrocarbon cation. Hardwick's5 ratios, L(abstraction)/k(addition), are equivalent to the present ratios G(HD)/AG(H,) for the fully deuterated olefins. The value of 0.08 for (CD3)zCCDzis much smaller than Hardwick's value of 0.3 for undeuterated olefins of this structural type.6 This difference is probably due to an isotope effect in the formation of hydrogen from the olefins. Isotope effects of this magnitude have previously been observed in systems containing ~ l e f i n s . ~ ~ - ~ ~ (13) J. W. Buchanan and F. Williams, J . Chem. Phys., 44, 4377 (1966). (14) P. Ausloos and S. G. Lias, ibid., 43, 127 (1965). (15) F. P. Abramson and J. H . Futrell, J. Phys. Chem., 71, 1233 (1967). (16) (a) P. Ausloos, A. A. Scala, and S. G. Lias, J. Amer. Chem. Soc., 88, 1583 (1966); (b) P. Ausloss, A. A. Scala, and S. G. Lias, ibid., 89, 3677 (1967). (17) G. Scholee and M. Simic, Nature, 202, 895 (1964). (18) S. Sato, R. Yugeta, K. Shinsaka, and T. Terao, Bull. Chem. Soc. Jap., 39, 156 (1966). (19) G. R. Freeman, J . Chem. Phys., 46, 2822 (1967). (20) E. G. Spittler, S. J. P. Jordan, L. M. Dorfman, and M. C . Sauer, Jr., J. Phys. Chem., 67, 2235 (1963). (21) B. R. Wakeford and G. R. Freeman, ibid., 68, 3214 (1964). (22) J. Y. Yang and I. Marcus, J . Chem. Phys., 43, 1585 (1965). (23) The value G(H2) = 0.80 was measured for each of the pure liquids CaHs and 1-CdH8 irradiated a t O o , Pure CaDe gave G(Dd = 0.42. The over-all isotope effect G(Hn)/G(Dz) = 1.9 for propylene agrees with that observed in cyclohexene.21
