3168
J. Phys. Chem. 1995,99, 3168-3172
Radical Reactions in the Radiolysis of Cyclopentane Laszlo Wojnarovitst and Jay A. Laverne" Radiation Laboratory, University of Notre, Dame Notre Dame, Indiana 46556 Received: July 14, 1994; In Final Form: September 29, 1994@
The end products produced in the y-radiolysis of cyclopentane have been measured at very low total doses (25-50 krad). Iodine scavenging techniques in solutions of 0.1-30 mh4 were used to elucidate radical yields and reaction mechanisms. The yields of the main radical species were found to be as follows: cyclopentyl, 4.9; 1-pentyl, 0.2; 3-cyclopentenyl, 0.07; H atom, 1.3 radicaV100 eV. The change in yields from neat cyclopentane to 0.1 mh4 iodine solution suggests that about 79% of the cyclopentyl radicals escape the spur and react in the bulk medium with a disproportionation to combination ratio of 0.97. Radical precursors account for about 50% of the total end product yield, which is much smaller than found in the radiolysis of cyclohexane or cyclooctane. The radiolysis mechanism for cyclopentane is discussed and compared to those for cyclohexane and cyclooctane.
Introduction
mine the reaction mechanism in cyclopentane. There is a great need for better measurements of products at low dose. Neat cyclopentane and solutions of 0.1-30 mM iodine were irradiated with y-rays to doses of 25-1000 krads. The yields of the major products (cyclopentene, bicyclopentyl, and cyclopentyl iodide) which are produced mainly by the reactions of cyclopentyl radicals were determined. The yields of 1-pentene, n-pentane, n-pentylcyclopentane, cyclopentylcyclopentene, and 1-pentyl iodide also were measured. A mechanism consistent with the production of these products is presented and compared to those for cyclohexane and cyclooctane. The similarities and differences between the cycloakanes are discussed.
In recent studies the radical reactions in the radiolyses of cyclohexane' and cyclooctane* have been examined at doses in the range of 25-50 krads. The use of a low dose is very important in the radiolysis of hydrocarbons since it is generally found that at a high dose the yields of products are dependent on doseG3In addition, the use of scavenger techniques to help elucidate radical yields and reaction mechanisms is usually complicated by the need for high scavenger concentrations to avoid depletion of the scavenger during the course of the irradiation. An analytical technique, based on selected ion monitoring in a gas chromatograph-mass spectrometer system, now makes it possible to determine products at radiation dose Experimental Section levels of 25-50 krads, Le. at 1 or 2 orders of magnitude lower dose than usually applied. At this dose level, scavenger Because of the low doses used in the present experiments, concentrations as low as 0.1-0.2 mM can be used, and in product concentrations were on the order of 10 pM and extreme particular, iodine can be used as a radical scavenger in a care had to be used in the purification of the medium. concentration range of several hundred-fold (0.1-30 mM in Cyclopentane from Fluka (299%) was first distilled using a 70 cyclopentane). This work presents the results of the y-radiolysis cm glass column. This process was followed by distillation of cyclopentane using iodine as a radical scavenger. In addition, using a grease-free Model 36 100A spinning band still made a summary of the low-dose studies on the radical reactions in by B/R Instrument Corp. All distillations were performed at the y-radiolyses of cyclopentane, cyclohexane, and cyclooctane atmospheric pressure. Finally, the last traces of unsaturated is given. These studies enable a comparison of the mechanisms compounds were removed by passing the middle fraction from to be made for cycloalkyl radical and hydrogen atom formation. the latter distillation through a 40 cm column (i.d. 2 cm) The radiolysis of cyclopentane has been performed by several containing silver nitrate on alumina prepared by the method of research groups,'-'2 and iodine scavenging techniques also have Murray and Keller.I9 Cyclopentane thus purified contained no been applied.5,9,'3-'6 Only one radical species, the cyclopentyl detectable impurities that could have disturbed the analysis of radical, was found in the liquid phase ESR studies on cyclothe radiolysis products. pentane." However, radical scavenging techniques in liquid Irradiations were made at room temperature using a T o y have revealed the formation of several other radicals, albeit with source with a dose rate of 150 rads. The sample cell was made low yield^.^,'^ Holroyd and Klein used a 14CzH5' radical from a quartz cuvette and contained about 3 mL of solution. scavenging technique and found that the most abundant of the Nitrogen was used to purge the sample before irradiation, and radicals are cyclopentyl, 3-cyclopentenyl, and 1-pentyl radicals the cell was sealed with a rubber septum. The dosimetry was (G = 4.0, 0.2, and 0.12 molecules/100 eV, r e s p e c t i ~ e l y ) . ' ~ ~ ' ~ made in the same sample cell by using the Fricke dosimeter The iodine scavenging e x p e r i m e n t ~ ~ ~found ~.I~~ the' ~cyclo~'~ and allowing for the differences in the electron densities. pentyl iodide yields to be about 2-3 molecules/100 eV of Product analyses were carried out with an EXTREL ELQenergy absorbed, but they generally used iodine concentrations 400- 1 gas chromatograph-mass spectrometer operated in ion of several millimolars and doses of a few hundred kilorads to selective monitoring mode. The products 1-pentene and nmegarads. The large differences in values for the yields of pentane were detected at mass 42 while all the other products cyclopentyl radicals make it very difficult to accurately deterwere detected at masses 67 and 69. Chromatographic separations were made with a 30 m Crompack CP-Sil-5-CB capillary ' Permanent address: Institute of Isotopes of the Hungarian Academy column by applying splitless mode injection of a 0.2 p L of Sciences, P.O. Box 7 7 , Budapest H-1525, Hungary. Abstract published in Advance ACS Abstracts, February 15, 1995. solution. The initial column temperature was maintained at 38 @
0022-365419512099-3168$09.0010
0 1995 American Chemical Society
Radical Reactions in Radiolysis of Cyclopentane
TABLE 1: Radiation Chemical Yields of Products (molecules/100 eV) in the Radiolysis of Cyclopentane dose,krad 25 50 100 1000 25" 506 c d nm nm nm nm hydrogen nme nm 5.35 5.20 cyclopentene 3.25 3.25 3.15 3.02 2.30 2.67 2.97 3.10 bicyclopentyl 1.25 1.24 1.23 1.10 0.26 0.85 1.29 1.23 cyclopentyl0.06 0.06 0.05 0.05 0.02 0.07 0.11 0.05 cyclopentene n-pentylcyclo0.07 0.07 0.06 0.06 0.04 nm 0.06 0.05 pentane 1-pentene 0.90 0.90 0.88 0.75 0.85 0.66 0.74 0.62 n-pentane 0.17 0.16 0.17 0.16 0.10 0.15 0.14 0.16 cyclopentyl iodide 4.0 n-pentyl iodide 0.05 a 0.1 mM 12 added. 0.07 M COZadded. Reference 7, extrapolated to zero dose. Reference 8, extrapolated to zero dose. e Not measured. "C for 7 min, during which 1-pentene,n-pentane, cyclopentene, and cyclopentane were eluted in this order. The temperature was then raised to 138 "C at 30 "Clmin and 1-pentyl iodide, cyclopentyl iodide, n-pentylcyclopentane, cyclopentylcyclopentene, and bicyclopentyl were eluted. A typical chromatogram took about 20 min. Reference compounds for quantitative analysis included 1-pentene, n-pentane, and cyclopentenefrom Fluka, cyclopentyl iodide and 1-pentyl iodide from Pfaltz and Bauer, and bicyclopentyl from Wiley. Cyclopentylcyclopentene and n-pentylcyclopentane were qualitatively identified by their mass spectra. Their quantitative yields were first determined from samples irradiated with higher doses and analyzed using a flame ionization detector, assuming the same response factor for these compounds as for bicyclopentyl. Analysis of the same highdose sample using the mass spectrometer detector in the selected ion monitoring mode gave the relative response factors at mass 67 which were then used for the analysis of low-dose samples.
J. Phys. Chem., Vol. 99, No. 10, 1995 3169
cyclopentene
v
v
0
10.4
v
7
10.3
-V
Y
10.2
bicyclopentyl 10'
Figure 1. y-Radiolysis of cyclopentane as a function of iodine concentration: (0, 0 ) cyclopentene, (V, V) bicyclopentyl, (0, W) 1-pentene, (A, A) cyclopentyl iodide, (0, +) ACs, twice the net change in yields of cyclopentene and bicyclopentyl, (*) cyclopentyl radical yield calculated from kdk,, and the change in yield of bicyclopentyl. The closed symbols are at 25 krad and the open symbols at 50 had.
Cyclopentane with Iodine: Major Products. Iodine at a concentration of 0.1 mM in cyclopentane is not completely depleted during a 25 krad irradiation. The addition of 0.1 mM iodine to cyclopentane reduces the yields of cyclopentene and bicyclopentyl from 3.25 and 1.25 to 2.3 and 0.26, respectively (Table 1 and Figure 1). With increasing iodine concentration these yields remain essentially constant until about 1 mM, where the bicyclopentyl yield slowly decreases and cyclopentene shows a rapid decline in yield. The rate constant of the reaction of the cyclopentyl radical with iodine has been reported to be2I 1.9 x 1O'O M-' s-l so that the average lifetime of cyclopentyl radicals at 0.1 mM iodine is about 0.5 ps. This time is much longer than the estimated lifetime of spur processes. Therefore, with the addition of 0.1 mM iodine, the radicals which would normally have reacted in the bulk medium are scavenged. From Results and Discussion the decrease in the yields of cyclopentene and bicyclopentyl Neat Cyclopentane. The yields of liquid products found in the ratio of the disproportionationto combination reaction rates the y-radiolysis of neat cyclopentane are collected in Table 1 for cyclopentyl radicals is calculated to be 0.97. This value for doses of 25-1000 krads. The small yields of the light agrees well with the former liquid phase9 (1.O) and gas hydrocarbon products ( I C 4 that are formed mostly in unimo(1.0 and 0.73) determinations. lecular ring fragmentation processes could not be detected with Figure 1 shows that the yield of bicyclopentyl decreases the present irradiation technique. At 25 krads cyclopentene and steadily with increasing iodine concentration above 1 mM. It bicyclopentyl yields were determined to be 3.25 and 1.25, can be assumed that all of the Clo hydrocarbons are formed respectively. These values are nearly constant at doses of 25only by radical combination reactions involving cyclopentyl 100 krads, and they are in good agreement with the yields radicals: bicyclopentyl in the reaction of two cyclopentyl obtained by the extrapolation of high-dose studies to zero dose. radicals, cyclopentyl cyclopentene in the reaction of a cycloAbove 100 krads the yields of cyclopentene and bicyclopentyl pentyl radical and a 3-cyclopentenyl radical, and n-pentylcydecrease slightly with increasing dose. clopentane in the reaction of a cyclopentyl radical and a 1-pentyl radical. A comparison of these yields in pure cyclopentane and An attempt was made to determine the yields of minor in solutions with 0.1 mM iodine suggests that 21% of the total products in order to better understand the complete reaction scheme of cyclopentane. The formation of cyclopentylcyclocyclopentyl radical reactions occur in the spur. This value is pentene as a primary product was formerly reported by comparable to the 17% and 30% obtained for parent alkyl Freeman,7 and now it is verified by the present measurements radicals produced in the y-radiolyses of cyclohexane and at low dose. The low-dose yields of 1-pentene, n-pentane, cyclooctane. The much smaller percentage of decrease in the n-pentylcyclopentane, and cyclopentylcyclopentene are 0.90, yield of cyclopentene (30%) compared to that of bicyclopentyl 0.16, 0.07, and 0.06, respectively. All of these products show (79%) upon addition of 0.1 mM iodine agrees with previous very little dependence on the total dose from 25 to 1000 krads. suggestion^^^^^^^'^ that in addition to the radical pathway for its The difference between Freeman's value for the yields of H2 formation there is also a non-radical mechanism. (5.35) and that of the hydrogen-deficient productsz0measured The decrease in the yields of cyclopentene, bicyclopentyl, here is ca. 0.85 units. It was previously shown that ring scission and other higher molecular weight saturated hydrocarbons with products have virtually no influence on the H2 b a l a n ~ e . ~ , ~ . ' ~the addition of 0.1 mM iodine to neat cyclopentane gives the Freeman reported the production of polymer-type compounds, yield of scavengeable cyclopentyl radicals which escape the spur and their inclusion considerably improved the HZbalance in at about 4.08. This value agrees well with the measured yield that study.7 of 4.0 for cyclopentyl iodide and further suggests that at this
Wojnarovits and LaVerne
3170 J. Phys. Chem., Vol. 99, No. IO, 1995 concentration iodine has hardly any effect other than the scavenging of radicals. The present value for the yield of scavengeable cyclopentyl radicals agrees with that measured by Holroyd and KleinI5 by the I4C2H5* radical sampling technique. The low values of cyclopentyl iodide measured in other s t ~ d i e s ~ . ~are . ' ~probably -'~ due to reactions of iodine other than radical scavenging and also to the effect of secondary reactions at the higher doses used in those works. The total initial cyclopentyl radical yield can be estimated from the yield of bicyclopentyl in neat cyclopentane and the kdlk, ratio. After applying a small correction for cyclopentyl radicals that end up as n-pentylcyclopentane and cyclopentylcyclopentane (-0.1) the yield is found to be 4.9. This value is considerably smaller than the estimated yields of 6.8 for cyclohexyl radicals in cyclohexane radiolysis and 6.6 for cyclooctyl radicals in cyclooctane radiolysis. Clearly, there is a smaller contribution by radicals to the formation of end products in the radiolysis of cyclopentane than for the other two cycloalkanes. As shown in Figure 1, increasing the iodine concentration from 1 to 30 mh4 results in a sharp drop in the yield of cyclopentyl iodide. The yields of cyclopentene and bicyclopentyl also decrease, and twice the difference between the measured yields of these products at a given iodine concentration and those found in neat solution is shown in Figure 1 as AC5. Large differences are observed between the calculated drop in cyclopentene and bicyclopentyl yields and the cyclopentyl iodide yields at high iodine concentrations. These differences can be attributed to the scavenging of thermal hydrogen atoms and to the interference of iodine with intermediates other than radicals. The decrease in cyclopentene from 1 to 30 mh4 iodine is much greater than observed for bicyclopentyl, which suggests that iodine is scavenging another precursor to this product. Previous studies with cyclohexane have suggested that iodine may interfere with cationic species.24 The cyclopentyl radical yield can be calculated from the kdlk, ratio measured above and the observed decrease in the bicyclopentyl yield with increasing iodine concentration. The difference between the so-calculated yields of cyclopentyl radicals and the measured cyclopentyl iodide yields is due only to the scavenging of hydrogen atoms. At the highest iodine concentration used here the scavenged H atom yield is found to be 1.2. In the radiolysis of cyclohexanez4 the ratio of the rate constant for H atom scavenging by iodine to that for abstraction of an H atom from cyclohexane was found to be 4.7 x lo3. From the kinetic data related to abstraction by chlorine atom complexes25the same ratio in cyclopentane is estimated to be 3.9 x lo3. At 30 mM iodine concentration about 94% of H atoms should be scavenged so that the total H atom yield is about 1.3. Previous studies have reported H atom yields in cyclopentane which range from 1.O to 2.0.5,9,26,27 Cyclopentane with Iodine: Minor Products. The yield of 1-pentene shows only a slight decrease from neat cyclopentane to 0.1 mM iodine solution, which suggests that it is mainly formed in non-radical processes. There is a further, gradual decrease in its yield with increasing iodine concentration (Figure 1). Isomerization of excited cyclopentane molecules, which is observed in photolysis, may be a source of 1-pentene format i ~ n ; ~however, ," l-pentyl radicals must also be responsible for some of its formation. The production of n-pentylcyclopentane is probably due to the combination reaction of 1-pentyl radicals with the more abundant cyclopentyl radicals. Disproportionation reactions of these radicals lead to the production of 1-pentene and cyclopentane or n-pentane and cyclopentene. The similar decreases in the yields of n-pentylcyclopentane, 1-pentene, and n-pentane (Figures 1 and 2 ) with increasing iodine concentration support the mechanism suggested.
n-pentyi iodide n-pentane np cyclopentane cp cyclopentene
0.00
1
1
0
10.2
10.3
[I,]
10''
(MI
Figure 2. y-Radiolysis of cyclopentane as a function of iodine
concentration: (0)n-pentane, (A)n-pentyl iodide, (U) cyclopentylcyclopentene, (v)n-pentylcyclopentane at a dose of 50 h a d . The disproportionation to combination ratio of the 1-pentyl radical reaction with cyclopentyl radical probably is less than 1. Since the yields of 1-pentene and n-pentane (0.90 and 0.16, respectively) are much greater than that of n-pentylcyclopentane (0.07), the major part of their production must be due to reactions other than disproportionation. Ausloos et a1.lz suggested that an ion-molecule reaction between cyclopentyl cation and a cyclopentane molecule may lead to n-pentane. However, the iodine scavenging experiments indicate that the hydrogen abstraction reaction of 1-pentyl radical from cyclopentane and the disproportionation reaction of 1-pentyl radical with cyclopentyl radical also contribute to the production of n-pentane. Certainly not all of the n-pentane is formed by reactions of I-pentyl radicals. Figure 2 shows that with increasing iodine concentration the yield of n-pentane decreases more than the n-pentyl iodide yield increases. The total yield of 1-pentyl radicals is estimated to be 0.2, which is considerably lower than that of the more abundant cyclopentyl radicals (4.9). Suggested sources of 1-pentyl radicals are hydrogen abstraction by the pentamethylene biradical (a proposed intermediate of the cyclopentane to 1-pentene isomerization), ion-molecule reactions between a cyclopentane radical cation and a cyclopentane molecule, and ring rupture induced by hot hydrogen atoms (an analogous process to tritium recoil reaction^).^^^^^.^^ The large decrease in the yield of cyclopentylcyclopentene with increasing iodine concentration (Figure 2) suggests that it originates totally from the combination of 3-cyclopentenyl radicals with cyclopentyl radicals. As a source of 3-cyclopentenyl radicals Holroyd and Klein proposedI5 a decomposition mode of cyclopentane molecules in which an HZmolecule and an H atom are eliminated from a single cyclopentane molecule. Such a process should occur consecutively, e.g., first an HZ molecule is eliminated from a (probably vibrationally excited) cyclopentyl radical cation, and then, after neutralization, an H atom is detached from the excited alkene molecule. This mechanism is in agreement with the slight increase in the cyclopentylcyclopentene yield in the presence of C02, which is an efficient electron scavenger (Table 1). The elimination of H2 from alkane radical cations has been observed in freon matrices.28 According to the gas phase thermochemical data such a reaction is exothermic in cyclopentane, endothermic in cyclohexane, and thermoneutral in c y c l o ~ c t a n e .In ~ ~the liquid phase this process is probably of minor importance due to the fast relaxation of the excited cations and also due to the fast neutralization process. Elimination of an H atom from an excited alkene molecule is known to be one of its basic
Radical Reactions in Radiolysis of Cyclopentane
J. Phys. Chem., Vol. 99, No. 10, 1995 3171
TABLE 2: Radiation Chemical Yields of Products (moleculedl00 eV) in the Radiolysis of Cycloalkanes product cyclopentane cyclohexanea cyclooctaneb G(H2, 5.35 5.6 5.9 G(c-CnH~n-2) 3.25 3.1 3.85 G(c-CnH2,- I 12 1.25 1.75 1.9 G(H2 deficit) 0.85 0.75 0.15 G(c-CnH2n-1.) 4.9 6.8 6.6 GW) 1.3 1.65 2.3 a Reference 1. Reference 2.
I
C.CaHis
'&- 1 e
6.05
1 .a
0.65
-
H.
H2 + C-CsHio + c G H e
H2
+ (C5Hg)2 1.25
12
Figure 3. Mechanism of hydrogen and radical formation in the radiolysis of cyclopentane. eC6HI2
e-
1.45
I
+ c-CgH12+
-
H2 + Products
/
1.65 H2 +
H2 +
c - C ~ H +, ~c-CsHio 1.65
(CeHid2 1.9
Figure 5. Mechanism of hydrogen and radical formation in the radiolysis of cyclooctane.
+ Products
and not the above hydrogen-deficit products. These reactions are denoted in the schemes in Figures 3-5 as ec-C,Hz,+ H2 products. The radiation chemical yields of the cycloalkyl radicals in all three cycloalkanes are much higher than twice the scavengeable hydrogen atom yields. In each of the cycloalkanes there must exist processes in which the cycloalkyl radical is produced with concurrent formation of Hz.Gaumann and c o - ~ o r k e r s ~ ~ have shown that there is no isotope effect for this process in n-heptane. They suggested "hot" hydrogen atoms or some ionic species as intermediates of the reaction. Trifunac and cow o r k e r ~claim ~ ~ that the intermediate is the c-CnH2,+l+ cation that forms in an ion-molecule reaction. Since a distinction between these pathways is beyond the scope of the present work they are drawn as dotted lines in Figures 3-5. Regardless of the mechanism the measured kdk, values at low doses can be used to estimate the contribution of the cycloalkyl radicals to the formation of the cycloalkene. This calculation is performed by assuming that all of the bicycloalkyl is produced by cycloalkyl radical precursors. By following the technique described above the H atom yields for cyclohexane and cyclooctane can be The difference between onehalf of the total cycloalkyl radical yield and the H atom yield must give the amount of reaction in which two cycloalkyl radicals and H2 are produced together. The SI excited molecules in the radiolysis of cycloalkanes form mainly in geminate recombination reactions. Fluorescence decay and absorption measurements in cyclohexane give the lifetime of this state at about 1 ns while quenching measurements in cyclopentane and cyclooctane suggest much shorter lifetimes of about 0.1 and 0.3 ns, re~pectively.~~ In cyclohexane the yield of this state is reported to be around 1.5.36,37It can be assumed that the unimolecular elimination of H:! occurs exclusively from the S I excited molecules. The quantum yield for this reaction in cyclopentane and cyclooctane is 0.90 while in cyclohexane it is 0.80.'2,37 The resulting yields for the SIexcited molecules are 2.25, 1.75, and 2.7 for cyclopentane, cyclohexane, and cyclooctane, respectively. In addition to H2 elimination (SI S, type transition) the SIexcited states decay in fluorescence (found in cyclohexane only) or in an intersystem crossing process (SI T, type transition) to give the cycloalkane triplet states.37 This latter process makes a very small contribution to the yield of triplet excited state formation. The triplet excited molecules are formed mainly in electronion recombination reactions in spurs containing multiple electron-cation pairs, with loss of spin correlation, and probably also in direct excitation. Their characteristic decomposition mode mainly gives radicals by carbon-hydrogen or carboncarbon bond rupture.38 Of course, in cycloalkanes, triplet excited states can also decompose by carbon-carbon bond
-
5.6
0.15
1.4
+ c-CaH,.+
0.7
H2 + Products
'\\
