3214
B. R. WAKEFORD AND G. R. FREEMAN
Radiolysis of Cyclohexene.
111.
Cyclohexene-d,,l
by B. R. Wakeford and G. R. Freeman Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada
(Received J u n e 17, 1964)
The yradiolysis of liquid cyclohexene-dlo gave the following product yields. The corresponding yields from light cyclohexene are given in parentheses: hydrogen, 0.65 (1.28); cyclohexane, 0.66 (0.95); 2,2'-dicyclohexenyl, 1.36 (1.90) ; 3-cyclohexylcyclohexene, 0.66 (0.58) ; dicyclohexyl, 0.47 (0.21) ; unidentified dimer, 0.32 (0.22). The over-all rate of decomposition of cyclohexene-dlo is quite similar to that of light cyclohexene, although the product distribution is quite different. Moderately large isotope effects (-3.6) are evident in the over-all radiolytic reaction. Considering the activated species that produce hydrogen, activation transfer occurs from the c-CaDlospecies to c-CsHlo, as well as to 1,3and 1,4-cyclohexadiene and to benzene.
Introduction The radiolysis of liquid cyclohexene-dlo, in the presence and absence of inhibitors and of light cyclohexene, has been studied in an attempt to elucidate the mechanism of radiolysis of cyclohexene.2
Experimental The cyclohexene-dlo, obtained from Merck Sharp and Dohme of Canada, Ltd., contained up to 12% CGDSH,depending on the sample, -0.3y0 cyclohexane and -1% benzene. The cyclohexane and benzene were removed from the cyclohexene by vapor phase chromatography (v.P.c.). 1,3-Cyclohexadiene (1,3-D) and 1,Qcyclohexadiene (1,4-D) were obtained from hldrich Chemical Co. and were purified by vacuum distillation followed by v.p.c. Research grade benzene from Phillips Petroleum Co. was used as supplied. The total hydrogen yield was determined by lowtemperature distillation and pressure-volume measurement. The Hz, HD, and Dz contents of the gases were measured using a Metropolitan-Vickers 315-2 mass spectrometer that was calibrated with Hz, Dz, and a mixture of Hz,Dz,and HD. Other pertinent experimental details have been reported earliera2
Results The product yields from cyclohexene-dlo are given in column 1 of Table I, with the corresponding results T h e J o u r n a l of Physical Chemistry
from light cyclohexene2&given in parentheses. The yields of heavy hydrogen, cyclohexane, and 2,2'dicyclohexenyl from deuterated cyclohexene were less than the yields of their light counterparts from protonated cyclohexene, while the yields of heavy dicyclohexyl, 3-cyclol~exylcyclohexene, and the unidentified dimer D-12a were greater than those of the corresponding light compounds (see column 2, Table I). The yields of products from a dilute solution of light 1,3-D (electron fraction of 1,3-D, t l & D = 0.0013) in heavy cyclohexene are given in column 3 of Table I. The results for a corresponding light cyclohexene solution, obtained by interpolation of earlier results,2b are given in parentheses. The relative efficiencies of interfering with the formation of the various products in the heavy and light cyclohexene systems are indicated in column 4. The relative amounts of inhibition in the heavy and light systems are essentially the same, with two exceptions. Cyclohexane formation is much more readily inhibited in the heavy than in light cyclohexene, while the unidentified dimer D-1 is somewhat less readily inhibited in the deuterated system. The rate of 1,3-D consumption was higher in the cyclohexene-dlo solution than in the light cyclohexene solution (column 3 , Table I). The yield of dicyclohexadiene was higher in the cyclohexene-dlo solution, (1) This work was supported in part by T h e National Research Council of Canada. (2) B. R. Wakeford and G. R.Freeman: (a) part I, J . P h y s . Chem., 6 8 , 2635 (1964); (b) part 11, ibid., 68, 2992 (1964).
RADIOLYSIS OF CYCLOHEXENE
3215
Table I : Product Yields from Cyclohexene-dlo, in the Presence and Absence of a Trace of Inhibitor" (2) heavy yield/ light yield (uninhibited)
(1)
G €1,~-D =
Product
Hydrogen Cyclohexane 2,2 '-Dicyclohexenyl 3-C yclohexylcyclohexene Dicyclohexyl Dimer D-1 Dicyclohexadiene Total Clz hydrocarbon* 1,3-D
0 . 0000
0 . 6 5 (1.28) 0.66 (0.95) 1.36 (1.90) 0.66 (0.58) 0.47 (0.21) 0.32 (0.22)
0.51 0.70 0.72 1.14 2.24 1.46
2.7(3.2)
0.84
(3)
(4)
G € I , ~ - D =0.0013
column 3/ column 1
0 62(1 23) 0 43(0 88) 1 15(1 64) 0 56(0 49) 0 38(0 18) 0 30(0 18) 1 5 ( 0 7) 3 8 ( 3 7) -4 3 ( - 1 6)
0 0 0 0 0 0
95(0 65(0 85(0 85(0 81 ( 0 94(0
96) 93) 86) 85) 86) 82)
1 4 ( 1 2)
Dose = 1 X lom e.v./g. Dose rate = 8 X a Values in parentheses are those for the corresponding light cyclohexene system. 10l8e.v./g. hr. E ~ , ~ .=D electron fraction of 1,3-D in solution. * Average of two values obtained using different V.P.C.columns.
Table 11: Hz, Dz, and H D Yields from Solutions of Cyclohexene, 1,3-D, 1,4-D, and Benzene in Cyclohexene-dlo"
_____----C-CBDIO
0.880 0.813 0.359 0,790 0.347 0.789 0.343 0.440 0.396 0.174
Electron fractions of components in solutions---------C - C ~ D ~ H ~ c-CeHio 1,3-c-CsHs 1,4-c-CeHs
0.120 0.086 0.038 0.108 0.047 0.108 0.047 0.060 0.054 0.024
0.003 0.018
__________ G values_________CsHa
0.098 0.585 0.102 0.606 0.103 0.610
0,500 0.453 0.216
a Dose rate = 7.65 x 1018 e.v./g. hr. 1) and 9.6% e-CsDgH (batch 2).
0.098 0.585 Dose range = 1.02-1.54 X lozoe.v./g.
but the increase in the yield of this product was much too small to explain the much greater rate of disappearance of 1,3-D. This probably means that the polymer yield was greater in the heavy than in the light solution under these comditions. The hydrogen yields were measured for binary solutions of benzene, 1,4-D, and 1,3-D in cyclohexene-dlo. Also, a 50-50 solution of heavy and light cyclohexene was irradiated with and without added 1,3-D. The total hydrogen yields from these solutions are given in Fig. 1. The dash-dot lines represent the cor-. responding hydrogen yields for the light cyclohexene system. The Hz,HD, and Dz contents of the hydrogen products are givcn in Table 11.
Discussion The over-all consumption of cyclohexene, based on the measured C g and ClZ hydrocarbon product yields, was reduced by only about 10% by deuteration. How-
H2
-0.008 0.030 0.142 0.087 0.690 -0,007 0.025 0.463 0.292 0.227
HD
Da
Total
0.039 0.092 0.073 0,229 0.242 0.063 0.052 0.378 0.206 0.055
0.604 0.238 0.025 0.374 0.039 0,440 0.083 0.129 0.062 0,008
0.65 0.36 0.24 0.69 0.97 0.51 0.16 0.97 0.56 0.29
* Cyclohexene-dlo contained 12.0% c - C ~ D ~(batch H
ever, the distribution of the product yields was considerably altered, which indicates that moderately large kinetic isotope effects occur in this system. It appears that the 2,2'-dicyclohexenyl, 3-cyclohexylcyclohexene, and dicyclohexyl are formed by the combination of free radicals in the cyclohexene radiolysis system. 2a It is possible to write several mechanisms for the formation of the free radicals and for the formation of hydrogen. Some of the possible mechanisms involve atoms and radicals, others involve ions, and others involve excited molecules. The present work shows that, whatever the mechanism is, it is subject to nioderately large kinetic isotope effects. For the sake of siinplicity, only an atomradical inechanisni mill be used to illustrate the effects. The hydrogen yield was reduced and the yields of dicyclohexyl and 3-cyclohexylcyclohexene were increased by deuteration of the cyclohexene. These Volume 68,Number 11
November, 1964
3216
B. R. WAKEFORD AND G. R. FREEMAN
stituted for H). However, based upon the yields of hydrogen, dicyclohexyl, and 3-cyclohexylcyclohexene, k3/k4 is reduced about 3.6-fold by substitution of C-C~DIOfor C-CBHIO.hccording to this mechanism, k3 a G(Hd and k4 a G(c-CeH11) a G(C12H22 (312H20) because the cyclohexyl radicals generated by reaction 4 react according to steps 5-9.
+
2C-CeHii
CizHz2
(5)
C-CeHiz f C-CeHio
(6)
-
--f
---f
C-CeHii f C-CeHg
---)
CizHzo
(7) ~-CsHi2 C-CeHs (8)
+
2c-CaHio (9) In liquid cyclohexane, ke/ks = 1.33 and it will be assumed to have the same value in liquid cyclohexene. In liquid cyclohexene, < 0.22a and, considering the relative exothermicities of reactions 8 and 9, it is probable that Ic$/k.~< 0.2. If it is assumed that (kp f k ~ ) / k 7= 0.2, then k t / k 4 = G(H~)/G(C-CBHII) = 1.28/1.67 = 0.77. If it is assumed that (ks IC$)/ k7 = 0.0, then ka/kr = 0.83. Using the same assumptions for the cyclohexene-dlosystem, k 3 / k 4 = 0.22 and 0.23, respectively. The respective values of the isotope effect in k3/k4 are 3.5 and 3.6. This assumes that the disproportionation to combination ratios are the same for c-CeH11 and c-CaDll radicals. Even if the disproportionation to combination ratio were zero for c-CsD11 radicals, the apparent reduction in k 3 / k 4 would be 2.0-fold, so a moderately large isotope effect is evident. The smaller yield of 2,2'-dicyclohexenyl in the deuterated system is consistent with the smaller expected yield of cyclohexenyl radicals (reactions 1 and 3). Other isotope effects are evident from Table 11. The cyclohexene-dlocontains 1.2 atomic % H, yet the product hydrogen contains 4.2 atomic % II. This constitutes an over-all isotope effect of 3.5. More detailed isotope effects could be calculated from the hydrogen results, but the reaction mechanism is not sufficiently well known to make this worthwhile. The preceding discussion clearly illustrates that the reactants that produce cyclohexyl radicals and hydrogen are of sufficiently low energy that they suffer moderately large kinetic isotope effects. It was concluded in part I,2& that the reaction ----f
+
0
2
4
6 €0
a
i
o
* 10
Figure 1. Hydrogen yields from liquid cyclohexene-dlo solutions; dose rate = 7.6 X 1018 e.v./g. hr.; dose = 1.01.5 X 1020 e.v./g.; E, = electron fraction of additive: 0 , hydrogen yield from pure light cyclohexene; , yields from corresponding light cyclohexene solutions. A : 1,4-D solutions. B: 0, 1,3-D solutions; 0, 1,3-D added to a 50-50 solution of cyclohexene and cyclohexene-dio. C: benzene solutions. -.-.-e
facts are consistent with the isotope effects that would be expected in the reactions c-CeHlo* c-CeHio*
--+ c-CeHg f
+ hf
H f C-CeHio
----f
---f
H
C-CeRlo f YI Hz f C-CsHg
--+ C-CeHii
(1) (2)
(3)
(4)
Substitution of D for H in cyclohexene would be expected to decrease the values of the ratios kl/kz and k3/k4. The results in Fig. 1 indicate that k l / k z is reduced by about 14Oj, when 14 in reaction 2 is 1,3-D and by about 4% when IVI is benzene. The initial slopes of the curves were calculated using the points at ea = 0.0 and 0.1. These effects are relatively small (recall also that, based on the cg and Cl2 product yields in pure cyclohexene, the over-all consumption of cyclohexene was reduced by roughly 10% when D was subThe Journal of Physical Chemistry
c-CeH11
+ C-CeH10
+ C-CeHlz
+ C-CeHg
(10)
does not occur to an appreciable extent under the present conditions of irradiation. This is consistent (3) C. E. Klots and
R.H.Johnson, Can. J . Chem., 41,
2702 (1963).
RADIOLYSIS OF CYCLOHEXENE
3217
Table I11 : Relative Abundance5 of Ions in Mass Spectra of c-C&o and c-CeD10' CmHn+/ CmDn'
6 6 6 6 6 6 5 4
10 9 8 7 6 5 7 6
1.00 0.30 0.03 0.17 0.05 0.15 2.67 2.09
1.00 0.20 0.02 0.10 0.05 0.11 2.69 2.38
1.o 1.5 1.5 1.7 1.0 1.4 1.0 0.9
ii:
5
11.48
11,71
0.98
n
n
a The height of a peak a t a given mass to charge ratio is reported relative t o that of the parent peak. The total sensitivity of each compound was obtained by summing the heights of all the peaks in its mass spectrum and dividing the sum by the pressure of the compound in the reservoir. The ratio of total sensitivities was c-C6Hlo/c-C6Dl~= 1.06. The ratio of the parent peak heights, per micron of pressure in the reservoir, was C6H10+/(C6DIO+CoD9Hi-)= 1.08. * Includes C,D,-lH+ from c-CsDoH impurity.
+
1.7. For the production of ions by C-C bond cleavage, the effect of substituting D for H was much smaller (C,Hn+/C,Dn+ = 1.0-1.3). Similar effects are observed in the mass spectra of cyclohexene and cyclohexene-dlo (see Table 111). All the mass spectra in the present work were obtained using 70-v. electrons. The total sensitivity of a compound was obtained by summing the heights of all the peaks in its mass spectrum and dividing the sum by the pressure of the compound in the reservoir. The total sensitivity of cyclohexene was 6% greater than that of cyclohexene-dlo. The difference was probably mostly due to a secondary type of mass discrimination in the mass spectrometer because the value of the ratio of the gas phase diffusion coefficients of the two compounds is 1.06. This conclusion was confirmed by showing that the values of the ratios of the total sensitivities of C2H4/C2D4 and CzH6/CzD6were nearly the same as the ratios of their diffusion coefficients (Table IV). Jesse,j using a simple ion chamber,
with the present results because, although the yield of cyclohexyl radicals iiicreased, the yield of cyclohexane decreased when cyclohexene-dlo was substituted for cyclohexene. These facts are not! explained by a competition between reactions 10 and 11 because a c-CsH11
+ c-(z6H1, + c-CtjII~i-C-C&io
(11) self-consistent mechanism that included reaction 11 could not be devised which would explain the observed product distribution. Cyclohexane is formed by the disproportionation of radicals and by some other mechanism.** When cC6D10 was substituted for c-C6Hlo, the cyclohexane yield decreased while the dicyclohexyl yield increased. These changes cannot be due entirely to isotope effects in the disproportionation to combination ratios of the radicals because, in the cyclohexene-dlo system, cyclohexane formation is more readily inhibited than is the formation of dicyclohLexy1 (column 4, Table I). The increased facility of inhibition of cyclohexane formation is consistent with its lower yield in the deuterated system because both facts indicate that the cyclohexane formation reaction occurs with greater difficulty in the deuterated than in the nondeuterated system. The present work points out a great need for information about isotope effects in disproportionationcombination reactions and in ion-molecule reactions. Isotope effects are evident in mass spectra. Gordon and Burton4 compared the mass spectra of benzene and benzene-da and found the ratio of the abundances of the ions, ce,H,+/c~lD,+ (n = 3, 4, or 5 ) , to be about
-
0w 2 4 1 20
(3
0
2
4
6
8
1
0
e,x IO Figure 2. H and D content of the hydrogen from the C-C&o solutions; ea = electron fraction of additive: A, C-C~H~O; 0 , 1,4-D; 0, benzene; 0,1,3-D. A filled point is common to a set of curves. A: 2G(D*) G(HD). B : 2G(Hz) G(HD).
+
+
(4) 9. Gordon and M. Burton, Discussions Faraday SOC., 12, 88 (1952). ( 5 ) W. P. Jesse, J . Chem. Phys., 38, 2774 (1963).
V o l u m e 68, Number 11
November, 1964.
B. R. WAKEFORD AND G. R. FREEMAN
3218
found that the total ionization produced by @-particles of several kev. energy in CzH4 or CzH6 was 1% less than that produced in CzD4 or C2D8, respectively. The measurement of total ionization using a mass spectrometer gives a false picture because the instrument evidently discriminates against molecules (or perhaps their ions) according to their diffusion coefficients. It is not clear a t what stage of the mass spectrometric process this discrimination occurs, but Table IV : Mass Spectrometric Total Sensitivities TSa
c-CaHio C-CeDio C2H4 C2D4 CZH6 CZD6
78.8 74.5 37.3 35.6 45.2 40.9
TS-h/ TS-db
D-h/
1.06
1.06
1.05
1.07
1 11
1.10
D-do
Total sensitivity, mm. peak height per micron pressure in the reservoir. * Ratio of the T S value of the light isomer to that of the heavy isomer. Ratio of the gas phase diffusion coefficient of the light isomer to that of the heavy isomer.
The Journal of Physical Chemistry
measurements with our instrument indicate that it occurs before the ions reach the analyzing magnetic field. The D and H content of the hydrogen from the cyclohexene-dlo solutions are shown in Fig. 2. The cC6Dlo-c-CeHloplots are linear. Attempts to explain this behavior by a simple atomic hydrogen mechanism, using isotope effects of various assumed magnitudes, failed. If the end points of the H and D curves in Fig. 2 are fixed, the atomic hydrogen mechanism requires a value of the H : D ratio, in the hydrogen produced from the solutions, that is lower than is observed experimentally. The results indicate that, for the activated species that produce hydrogen, activation transfer occurs ‘from the c-C~DIO species to c-CeHl0. However, so little is known about the ion-molecule reactions of olefins that it would not be useful to speculate further about the mechanism of production of hydrogen at this time. For reasons similar to those mentioned previously, the curves in Fig. 2 indicate that, for the activated species which produce hydrogen, activation transfer occurs from cyclohexene to 1,4-D, to benzene, and to 1,3-D in their respective solutions.
