RADIATION CHEMISTRY OF CYCLOPENTANE
2707
The Radiation Chemistry of Cyclopentane
by B. Mason Hughes and Robert J. Hanrahan Chemistry Department, University of Florida, Gainesville, Florida
(Received March 8,1965)
The products of the radiolysis of pure liquid cyclopentane were found to be hydrogen, methane, ethane, propylene, propane, cyclopropane, Cis, n-pentene, n-pentane, cyclopentene, and bicyclopentyl. The G value of the major products are: hydrogen, 5.20; ethylene, 0.40; n-pentene, 0.62; cyclopentene, 3.10; bicyclopentyl, 1.22. The total G value for radical production was determined to be 6.40 using H I as a scavenger. Hydrogen production was shown to be temperature dependent when scavengers were present at concentrations between 1 and 10 mM. The reaction scheme is similar to that of cyclohexane except for the occurrence of greater C-C bond rupture and ring cleavage. The latter effects can be attributed to strain in the C5ring.
Introduction There has been considerable interest in the radiation chemistry of cyclohexane for a number of years. Recently there have been studies of the radiolysis of mixtures of the analogous CScompound, cyclopentane, with cyclohexane as well as other substances.lZ2 However, to date, little effort has been devoted to studies of the radiation chemistry of pure cyclopentane.a Because of its low freezing point (- 93') there are a number of questions which may be investigated using this compound, such as temperature dependence of various processes and possible measurement of activation energies. This paper includes a report of major and minor product' yields in the radiolysis of pure liquid cyclopentane and a discussion comparing its radiation chemistry with that of cyclohexane. A preliminary report of experiments showing temperature dependence of the hydrogen production with scavengers present is also included.
Experimental Phillips Petroleum Co. pure grade cyclopentane was stirred with fuming sulfuric acid for 2 to 3 hr., washed, and distilled. The f i s t fraction was collected, and the cyclopentane peak was isolated on an Aerograph Autoprep preparative scale gas chromatograph. The first fraction was used since the lower boiling impurities could be separated chromatographically from the cyclopentane fraction more easily than could the higher boiling impurities that would be present in the later fractions. Flame ionization gas chromatography
showed the cyclopentane purified in this manner to be purer than Phillips research grade, with no impurity of cyclopentene. Standards of methane, ethane, propane, propylene, and cyclopropane were obtained at 99% purity from Matheson Co.; ethylene and pentene-1 was obtained at 99% purity and cyclopentene was obtained at 99.89% purity from Phillips Petroleum Co. Iodine was Merck resublimed, methyl iodide was obtained from Eastman Kodak Co., and hydriodic acid was Baker Analyzed Reagent. Bicyclopentyl was prepared by a Wurtz-type reaction of sodium with cyclopentyl bromide. The fraction that was collected a t 184' was further purified by gas chromatography to a purity better than 99%. The collected peak was confirmed to be bicyclopentyl by mass spectral analysis. Hydrogen iodide was produced by dehydrating hydriodic acid. The hydriodic acid was frozen to liquid nitrogen temperature in a round-bottom flask, and PzOs was added on top of it. The frozen acid was then attached to the vacuum line and degassed. The hydriodic acid was allowed to melt and interact with the PzOs,and the HI gas released was collected in another portion of the vacuum system. This collected hydrogen iodide was degassed again and stored at liquid (1) J. A. Stone, Can. J. Chem., 42, 2872 (1964). (2) G. A. Muccjni and R. H. Schuler, J. Phys. Chem., 64, 1436 (1960).
(3) (a) A. R. Lepley, Anal. Chem., 34, 322 (1962) ; (b) A. R. Lepley, Dissertation, University of Chicago, 1957.
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B. MASON HUGHES AND ROBERT J. HANRAHAN
2708
nitrogen temperature until used. The amount of hydrogen iodide added to the samples was determined by gas measurements. Samples were dried over Pz05,degassed and melted three times, and distilled under vacuum to the radiation vessels. For determination of the liquid products, 4-ml. vessels were used. For the hydrocarbon products which were gases at room temperature, 178-pl. ampoules were prepared using a vacuum-line buret.4 Irradiations were performed using a cobalt-60 y-irradiator which has been described previ~usly.~The dose rate absorbed by cyclopentane in the 4-ml. vessel was determined by the Fricke dosimeter [G(Fe3+) = 15.61 to be 4.42 X lo1' e.v./ml. min., and the absorbed dose rate in the 178-pI. sample was found to be 1.04 X lo1* e.v./ml. min. by using G(12) = 2.11 for the radiolysis of ethyl iodide. (This value of G(12) is the value obtained when ethyl iodide was irradiated in the 4-ml. vessel at the above dose rate.) The radiolyses of cyclopentane with added iodine or hydrogen iodide were done in 4-ml. vessels with attached optical cells. The optical densities were measured at various time intervals of irradiation with a Beckman DU spectrophotometer obtaining either rates of production or consumption of iodine. The extinction coefficient of iodine in cyclopentane was measured to be 906 l./mole cm. at the absorption maximum of 524 mp. The identification of products was done in two independent ways: comparison of retention times of known standards with the irradiation products and direct determination of mass spectroscopic fragmentation patterns of the individual radiolysis products. The mass spectroscopic techniques have previously been described by Fallgatter and Hanrahan.6 I n their procedure a gas chromatograph is connected with a Bendix time-of-flight mass spectrometer so that a spectrum of each component of the radiation mixture can rapidly be recorded. These spectra are then compared with standard spectra to determine their identity. For quantitative determinations, liquid standards were diluted with the g.1.c. purified cyclopentane and injected into a laboratory-made gas chromatograph with an Aerograph flame ionization detector. These injections varied from 5 to 35 pl. Gas standards were injected into the chromatograph using 35 p l . injections of the pure gas with gas-tight Hamilton syringes. To determine the yields of the liquid products, the 4-ml. vessels were broken open, and samples of the standard and the irradiated cyclopentane were alternately injected. The peak areas were compared with the standard peak areas using a Photovolt electronic integrator. The yields of the gaseous products The Journal of Physical Chemistry
were determined slightly differently. First, an ampoule was placed in the chromatograph within a breaking device; then the gas standards were injected with a gas-tight Hamilton syringe and analyzed. After three or four injections of the various standards, the ampoule was finally broken, and the peak areas of the irradiation products were compared with the average peak area of the standard injections. Using a McLeod gauge, the hydrogen gas yields were determined as the gas volatile at - 196'. Table I : Yields in Cyclopentane Radiolysis ( G Values) Product
Hydrogen Methane Ethylene Ethane Propylene Propane Cyclopropane C4
n-Pentene n-Pentane Cyclopentene Bicyclopentyl G( - 12) G( 12)(added HI)
Tbia work
Leplep
Other workers
5.20 3.79 5.2b 5.25' 0.04 1.50 0.08 0.16 0.08 0.10 Trace 0.62 0.16 3.10 1.22 3.0f 3.20
4.97d 5.78"
1.40 0.55 0.16 0.13 0.59 0.15 1.50 3.4b 2.94' 3.0d 1.62 1.06b 1.30d 2.53g 3.6h
Lepley's data3 were taken a t several doses to 100 Mrads and beyond. Values given here are approximate extrapolations of his data to zero dose. Holroyd.lo Toma and Hamill.ll Stone.' e Hardwick.lZ Approximate initial value; after m. 5 X 104 rads, G( -12) is 2.47, in agreement with the value of Dauphin.l7 Dauphin.'? * Weber, Forsyth, and Schuler.16
*.O
3.0
t
I 0.50 1.00 1.50
2.00 2.50 3.00 3.50 Dose, e.v./ml. X lo-'*.
4.00 4.50
Figure 1. Dependence of hydrogen yield on total dose absorbed. (4) R. J. Hanrahan, J. Chem. Educ., 41, 623 (1964). (5) R. J. Hanrahan, Intern. J. Appl. Radiation Isotopes, 13, 254 (1962). (6) M. B. Fallgatter and R. J. Hanrahan, J. Phys. Chem., 69, 2059 (1965).
2709
RADIATION CHEMISTRY OF CYCLOPENTANE
50
25
75
Dose, e.v./ml. X 10-19.
Figure 2. Production of ethylene (a), propylene ( 8 ), and cyclopropane (9) us. dose.
Results Pure Cyclopentane. The main products of the radiolysis of cyclopentane are hydrogen, cyclopentene, and bicyclopentyl with lesser amounts of n-pentene and ethylene. A resume of all of the G values is given in Table I. All of our reported values are extrapolated to zero dose. The G value for production of H2 was found to be 5.20. As can be seen from Figure 1, it is quite dependent upon dose. Of the CrC4 products, by far the largest product is ethylene with a yield of 0.40. The other low boiling products include all of the possible alkane and alkene hydrocarbons between C1 and CB,including cyclopropane. The rates of production of ethylene, propylene, and cyclopropane are shown in Figure 2. The linearity at these low doses suggests that they are being produced in primary processes. Although the precision of the ethylene determination is not especially good, the absolute error in the G value is not more than f0.05. The Csyields are 3.10 for cyclopentene, 0.62 for npentene, and 0.16 for n-pentane, and the yield of bicyclopentyl is 1.22. Figure 3 shows that there is only slight dependence of the yields on dose a t these low doses.
5
10 15 Dose, e.v./ml. X 10-19.
20
Figure 3. Production of cyclopentene (a), bicyclopentyl ( 0 ), n-pentene (9), and n-pentane (a) us. dose.
Cyclopentane with Scavengers. The initial G value for IZproduction in systems in which HI was added to cyclopentane was determined to be 3.20. As can be seen from Figure 4 the initial slope of a graph of IZ concentration vs. dose for all of the runs was the same, regardless of the initial concentration of HI. The graphs for more dilute solutions began their curvature earlier, indicating competitive scavenging by the Volume 69,Number 8 Auguat $966
B. MASONH U G ~AND S ROBERT J. HANRAHAN
2710
1.0
2.0
3.0 Radiolysis time, hr.
4.0
5.0
6.0
Figure 4. Iodine production in the radiolysis of pure cyclopentane with added HI. The initial molar concentration of HI for each experiment: 0 , 10.1 X lo-+, @, 7.66 X 0 , 4.52 X 0, 1.92 X I 2 produced E n the reaction. An interesting regularity is found if the approximation is made that, at the maximum of the experimental graphs, the residual H I concentration can be calculated by subtracting from the initial HI concentration the number of equivalents of iodine atoms present as 1 2 . With this assumption, it is found that the ratio of the concentration of H I to I2 a t the maximum is a constant, 0.365, with a variation of only 3% in a set of three experiments in which the initial HI concentration varied by a factor of 5. Results of several experiments with iodine added as a scavenger are shown in Figure 5. It can be seen that these graphs of I2 concentration vs. dose are curved. After doses of the order of 50,000 rads the G value for iodine consumption in all of the experiments reached a nearly constant value of 2.47, decreasing somewha,t at greater doses. Some difficulty was encountered in extrapolating iodine disappearance plots to zero1 dose because of initial curvature. Our best current value for G(-12) is 3.0, which is probably within experimental error of the value of 3.20 for iodine production with added HI. Variation of Temperature. Temperature dependence was noted in the G value for the production of H2 with scavengers present. The G value for hydrogen production was found to have the same value, 3.24, a t 25 and -78’ for solutions in which [MeI] = 0.02 M and [ I 2 1 = 0.002 M; presumably, the value obtained at
The Journal of Physical Chemistry
both temperatures is just the “plateau” value for complete scavenging of the bimolecular hydrogen yield.7~8 However, in radiolyses of cyclopentane with 0.001 M Me1 and I p , there is approximately a 10% decrease in G(H2) a t the lower temperature. Although these measurements are somewhat qualitative, they do show that the production of hydrogen with scavengers present is temperature dependent when the concentration of scavengers is in the region in which only a part of the bimolecular hydrogen yield is being scavenged.
Discussion Qualitative IdentiJicatim of Products. Our qualitative results agree, for the most part, with those reported by L e ~ l e y . ~The only disagreements are the trace amount of Cq compounds detected here with the aid of a flame ionization detector but not reported by Lepley and the intermediate products between cyclopentene and bicyclopentyl reported by Lepley but not found in the present work. We believe that these higher boiling substances are secondary radiolysis products which might be expected at the very high doses used in Lepley’s investigation. Hydrogen Yield. As in the radiolysis of cyclohexane,g the hydrogen yield decreases with dose, presumably (7) R. H. Schuler, J . Phys. Chem., 61. 1472 (1957). (8) M.Burton, et al., Radiation Res., 8, 203 (1958). (9) W. S. Guentner, T. J. Hardwick, and R. P. Nejak, J . Chem. Phys., 30, 601 (1959).
RADIATIONCHEMISTRY OF CYCLOPENTANE
2.0
2711
-
Radiolysis time, min.
Figure 5. Iodine consumption in the radiolysis of pure cyclopentane with added IO.
owing to scavenging by unsaturated products. Our value for the hydrogen yield extrapolated to zero dose is 5.20, which agrees well with results reported earlier by Holroydlo and by Toma and Hamill.ll Stone’s value1 of 4.97 is also consistent with our result if a correction is made for the fact that his measurement does not refer t,o zero dose. The value of 3.79 reported by Lepley was taken a t a dose of 30 Mrads, and his data are not sufficient to allow extrapolation of this figure to lower doses. The G value for hydrogen production of 5.78 reported by Hardwick12appears to be inconsistent with other values reported to date. Other Gaseous Products. The only data available for products in the range C1 through C4 are those of Lepley and the results reported here. The G values reported by Lepley for methane, Cz hydrocarbons, and propylene seem unusually large, presumably owing to his very high doses. His value for propane is a t least comparable to ours, and his value for cyclopropane agrees with the present work within experimental error. We estimate that our G values for these products are accurate to within approximately 20770. Since the G values for all of these compounds are small, errors of this magnitude have only minor effect on the material balance. Products Liquid at Room Temperature. The yields of n-pentane, .n-pentene, cyclopentene, and bicyclo-
pentyl determined from the yield curves in Figure 4 are estimated to be accurate to approximately 1 5 % . Since these are the main radiolysis products, the mass balance depends primarily upon these measurements. I n addition to the Cs products reported, there were also four other chromatograph peaks in the Cs region which were not identified but corresponded to a combined G value of less than 0.1. Lepley’s yields for the liquid products, corrected approximately to zero dose, are also recorded in Table I. His data for n-pentene and n-pentane agree well with ours, and his value for bicyclopentyl is high but reasonable. However, Lepley’s value of 1.5 for the yield of cyclopentene cannot be compared with our results because the yield of this compound is strongly dose dependent; an extrapolation from his high doses to zero dose is not possible. Stonel and Toma and Hamillll obtained values near 3.0 for the cyclopentene yield, but their values are probably (according to our results) lower than the initial yields by 0.1 or 0.2 G unit, owing to the dose used. The initial cyclopentene yield must lie in the range 3.1 to 3.4,the latter being the result obtained by Holroyd.lo (10) R. A. Holroyd, J . Phys. C h a . , 66,730 (1962). (11) S. Z. Toma and W. H. Hamill, J . Am. Chem. Soc., 86, 1478 (1964). (12) T.J. Hardwick, J. Phys. Chem., 65, 101 (1961).
Volume 69,Number 8 August 1966
B. MASONHUGHESAND ROBERTJ. HANRAHAN
2712
It can be seen from Table I that values of the bicyclopentyl yield varying from 1.06 to 1.62 have been reported by four groups; we obtain an intermediate value of 1.22. The average of the reported yields is 1.30. Material Balance. For the hydrogen yield in the cyclopentane system the material balance expression can be written
+
G(H2) = G(cyc1opentene) G(bicyclopenty1) (?(%-pentane) - G(methane) G(ethane) - G(propane)
If our G values are inserted in the above equation it is found that 1.30 moles of hydrogen are not accounted for. Use of average values from Table I does not improve the situation appreciably. However, Toma and Hamillll have recently shown that the ultraviolet spectrum of irradiated cyclopentane cannot be accounted for by the known products, particularly cyclopentene. They suggest that cyclopentadiene is produced; a G value for this compound of 0.65 would account for the previously mentioned discrepancy in the hydrogen yield. It seems unlikely that the discrepancy could be due to errors in the experimental results since it corresponds to 25% of the total hydrogen yield. An examination of the data reveals that rupture of the C5ring to give C1 and C4fragments is a very minor process (G 2 0.04), while rupture to give Cz and C3 fragments is relatively important (G 2 0.4). Radiolysis Mechanism. The high yields of hydrogen gas, of olefin derived from the parent cycloalkane, and of dimer suggest that the major portion of the reaction in the radiolysis of cyclopentane is analogous to the case of cyclohexane and that similar mechanistic steps apply c-C~H~O --+ c-C5Hlo+ e- +C-C~HIO*(la)
+
c-C~H~O -# ~-CbHlo*
+ ~-CbHlo* c-CbHg* + Ha €3. + C-C~H~O +C - C ~ H+ ~ .Hz c-CgHg* + C - C ~ H ~ . (0C5Hg)z C - C ~ H+ ~ .c-CgHg** ~-C5H10+ c-C~H~ ~-C5Hlo*+c - C ~ H ~ Hz
(1b) (2) (3)
(4)
(5)
(6) The same difficulty with respect to the actual nature of the bimolecular hydrogen yield, namely, the question whether it is really a simple hydrogen abstraction reaction as shown in step 4 or a more complex energytransfer process, applies here as in the case of cyclohexane.l 1 * l 3 The Journal of Physical Chemistry
The analogy with cyclohexane cannot be extended to the low molecular weight products because there is considerably more fragmentation in the present case than in the case of cyc10hexane.l~ It is reasonable to postulate that this difference is due to greater ring strain in cyclopentane, leading to a greater probability of ring opening and fragmentation. It is known that ions of mass 70, 55, 42, and 41 are the most intense in the mass spectral fragmentation pattern of cyclopentane.15 These correspond to the formulas C5H10+, C4H7+, C3H6+, and C3H5+, respectively. n-Pentane and n-pentene could result from ring rupture either of the parent ion or of the excited species produced on neutralization. However, in the case of n-pentane at least, it seems preferable to assume that ring opening and hydride ion transfer from substrate (followed by hydrogen atom abstraction) occur rather than neutralization since the latter process would necessitate double hydrogen atom abstraction from substrate in order to produce the saturated straight-chain compound. Production of n-pentene could follow from ring rupture of either the ion or the excited molecule. Production of lower molecular weight fragment ions in the mass spectrometer can be formulated as C5H10+ +C4H7+
+ CH3.
(7)
C~HIO+ +
4- CzH4
(8)
CbH10+ +C3H5+
+ CzH5.
(9)
It seems likely that similar reactions occur under radiolysis. Reaction 7 is quite important in the mass spectrometer since the ion C4H7+has a high abundance but apparently is a rather minor process in the liquid phase radiolysis. Reaction 8 must be very important in the mass spectrometer since the ion C3H6+is the most abundant in the fragmentation pattern and is the base peak. This reaction appears to occur fairly efficiently in the liquid phase radiolysis as well since G(C2H4)is 0.40. The total G value of C3hydrocarbons was found to be 0.32, and the precursors are probably both C3H6+ and C3H5f; the latter is also prominent in the mass spectral results. Although it is possible to write a reasonable set of reactions leading to the various observed products from the ions indicated and involving various ion-molecule reactions, neutralization steps, and free-radical reactions, we have chosen not to do so because such a scheme would be speculative and could not be defended in any detail on the basis of present results. (13) P. J. Dyne, J. Phys. Chem., 66, 767 (1962). (14) S. Sato, et al., J. Chem. Phys., 41, 2216 (1964). (15) American Petroleum Institute Research Project 44, Serial No. 116.
RADIATION CHEMISTRY OF CYCLOPENTANE
2713
Scavenger Experiments. Previously reported scavenger measurements of the free-radical yield in cyclopentane include G values for iodine disappearance of 3.6 reported by Weber, Forsyth, and Schulerl8 and 2.53 reported by Dauphin.17 Our initial G value of 3.0 for iodine disappearance is probably a minimum, owing to marked initial curvature of the yield vs. dose curves. The rate of iodine production with added H I can also be taken as an indication of the free-radical yield18; our value of 3.20 is in reasonable agreement with the results of the iodine experiments. Although H I and Iz at concentrations in the region to M may interfere with the radiolysis of a hydrocarbon solvent by electron capture or energy transfer,l 9 they are certainly capable of participating efficiently in free-radical reactions
+ H. + R-
I.
12 12
+ I. +HI + I. +R I
+ 1.
--3 I 2
(10)
(11) (12)
and for hydrogen iodide
+ H I +RH + 1. H * + H I +Hz + 1.
R.
(13) (14)
The nonlinear decrease of iodine concentration in radiolysis mixtures to which iodine has been added can be accounted for by H I formed in reaction 11 and competing with Iz for radicals as in steps 13 and 14. At very small doses, the ratio of H I to iodine changes very rapidly, changing the rate of iodine consumption. After several minutes of radiolysis the HI/I2 ratio becomes approximately constant, and the yield curves are then nearly linear. In experiments with added hydrogen iodide, during the early stages of the radiolysis, the only efficient free-radical scavenger present is HI, and each thermal free radical should release an iodine atom by reaction 13 or 14. In this case, 1 mole of radicals causes the production of 0.5 mole of Iz,while with added Iz1 mole of radicals causes the disappearance of 0.5 mole of Iz. Thus, G(12) with added H I should be equal to G(-Iz) with added Iz; this is observed to be true, at least approximately. During the experiments with added HI, as the radiolysis proceeds, 1 2 accumulates and begins to compete with H I for free radicals. When the net rate of (10) plus (11) equals the net rate of (13) plus (14), a maximum should be reached in the concentration of Iz. If a single rate constant can represent reactions Of these scavengers with both and hydrogen atoms, it is possible to write for the condition a t the maximum
+
+
h,@* H.108)t ~HI(R*H*)(HI)
(15)
or
{(I~)/'(HI)]rnsx= kEI/h = conshat (16) As WM noted above, such mnoentration ratios (calculated approximately) were fovnd to be constant experimentally. Further work related to this interpretation is in progress in our It should be noted that the above considerations are not directly affected by the possibility that varying proportions of the& hydrogen ohms may be scavenged a8 the total concentration of scavenger variec3. A s indicated in aq. 4, when a hydrogen atom abstracts from mbs%rate, it i8 replaced by an alkyl radical, and the total conoentretion of aIkyl radicals plus hydrogen ahms is unaft'eoted. A more important quesltion concerns the possibility that substances such as HI and I* may have important effects on the radiolysis of hydrocarbons aside from their action as free-radid scavengers. Nash and H.amiUlS have shown that El can interfere with the radiolysis of c-CSI)lz, presumably by electron capture, at concentrations tt8 low ~ E I5 X lom3M, and that the effect increases with increeusing RI concentration. If the resultant yield of Hn noted by Nash and Hamill is matched by a corresponding yield of I2from this process, then solutions of H I in cyclopentane at concentrations from 5 X to lo-$ M might be expected to show a concentration-dependent exeeas yietd of Iz compared to the yield for I2 disappearanme using added I2. Although we have noted that a($,)with added H I is somewhat greater than G(-Iz), the difference is only about 0.2 molecule/100 e.v., and it is not concentration dependent (Q. initid slope%, FfSure 4). The reason for this disagreement with the implications of the work of Wash and Hamilll@ is not apparent. In spite of the problem dimmed in the preceding paragraph, we believe that the f&tures of graphs of iodine concentration with added HI and IZ can be accounted for by free-radical compbtition reactions as indicated in eq. 10-14. Although a given total quantity of electronegative solute may modify radical and molecular yields somewhat as compared with the pure hydrocarbon substrate, this effect should be relatively small when the additive concentration is not much greater than lowaM. IWthermore, the inter(16) E.N. Weber, P. F. Forsyth, and R. H. Schuler, Radiation Res., 3, 68 (1955). (1,) J. Dsuphin, J. chim, phys., 59, 1207 (1962). (18) R, H. Sohuler, J. Phys. Chem., 62, 37 (1958). (19) J. R. Nash and W. H. Hamill, %bid.,66, 10$7 (1962). (20) I. Mani, this laboratory, unpuMiPhed results.
Volunzs Be, Number 8 August 1966
2714
conversion of small amounts of 1 2 , HI, and alkyl iodides should have only a small effect on the electron capture yield. Temperature Egects. In general, temperature changes are expected to have little effect on primary processes in radiation chemical reactions since the fundamental cause of reaction is deposition of energy from a radiation source, rather than collisions of molecules. Temperature changes can affect secondary processes provided that alternate routes exist with differing activation energies. In hydrocarbon radiolysis such a competitive situation may exist with respect to the hydrogen yield if it is assumed that part of the observed yield is due to thermalized hydrogen atoms. On the assumption that a thermalized hydrogen yield does exist, the fully scavenged Hz yield observed with ca. M Iz would not be temperature dependent, nor would the unscavenged yield (at least near room temperature). However, the hydrogen yield with intermediate concentrations of scavengers might be expected t o decrease with temperature. Unfortunately, investigation of temperature dependence of scavenging by I2 is prevented by the decrease in its solubility a t low temperature. We chose to use methyl iodide as an alternate scavenger21since it should react efficiently with hydrogen atoms according to the equation
It is necessary that iodine be present as well to prevent a chain reaction yielding methane.22 It has been established that the effects of CH3I and 1 2 on the hydrogen yields from cyclohexane a t room temperature are very similar.21s22 Using solutions of Me1 and I2 in cyclopentane, we have confirmed that the fully scavenged or “plateau value” of the Hz yield in cyclopentane is not temperature dependent. The unscavenged hydrogen yield found here is 5.20, and the plateau value is 3.24. A solution which was 1.0 mM in both methyl iodide and iodine gave G(H2) of 4.17 a t room temperature and 3.70 a t -78”. The difference between 4.17 and 3.24 represents the residual, unscavenged thermal hydrogen atom yield in the solutions studied, at room temperature; the difference between 3.70 and 3.24 represents the corresponding residual unscavenged thermal hydrogen atom yield a t -78’. The decrease of 0.47
The Journal of Physical Chemistry
B. MASONH U G ~AND S ROBERT J. HANRAHAN
G unit is 50% of the thermal hydrogeL atom yield of 0.93 unit (that is, 4.17 - 3.24) which was unscavenged in these solutions at room temperature. This decrease of 50% in the residual hydrogen atom yield for a drop of 103’ corresponds to only about 2 kcal./mole for the difference in activation energy between hydrogen atom abstraction and reaction with scavenger. This is much lower than the value of 6 to 8 kcal./mole expected from conventional kinetics12and suggests that the hydrogen atoms in question may be ((hot’’or that other more complex effects such as energy transfer or electron capture are involved. (This conclusion is in agreement with results of several other lines of investigation. 1 1 ~ 3 ~ 2 3 - 215 There appears to be only one previously reported value for the fully scavenged hydrogen yield in cyclopentane, a value of 2.72 published by Hardwick.12 Although this is in fair agreement with our value of 3.24, Hardwick also found a much higher unscavenged hydrogen yield of 5.78 molecules/100 e.v. Hence, Hardwick’s value for the “thermal hydrogen atom yield” is 3.06, approximately 50% greater than ours. Although we cannot fully resolve this discrepancy, it does not affect the qualitative conclusionobtained above concerning the temperature effect. The temperature effect is even smaller on a percentage basis using Hardwick’s data, making the apparent activation energy even less than 2 kcal./mole. The present experimental results do not bear directly on the nature of the unimolecular yield of hydrogen from irradiated hydrocarbons. Although formal recognition of a scavenger-insensitive component of the hydrogen yield was made in steps 1 and 2 of the reaction sequence given above, there is evidence that this portion of the reaction mechanism is more complex and is, in fact, related in some way to the bimolecular hydrogen yield.23-25
Acknowledgment. This work was supported by A.E.C. Contract AT-(40-1)-3106 and by the University of Florida Nuclear Science Program. (21) L. C.Forrestal and W. H. Hamill, J . Am. Chem. Soc., 83, 1535 (1961). (22) R.H. Schuler, J . Phys. Chem., 61, 1472 (1957). (23) P. Dyne and W. Jenkinson, Can. J . Chem., 39, 2163 (1961). (24) P. Dyne, J. Denhartog, and D. R. Smith, Discussions Faraday SOC.,36, 521 (1964). (25) S. Z. Toma and W. H. Hamill, J . Am. Chem. SOC.,86, 4761 (1964).
