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JEANH. FUTRELL
VOl. 64
GAS PHASE RADIOLYSIS OF %-PENTANE BY JEANH. FUTRELL Aeronautical Research Laboratories, Air Force Research Division., Wright-PattersonAir Force Base, Ohio Received April Y, 1960
Pyrex ampoules of n-pentane vapor were exposed to cobalt-60 *prays and hundred electron volt yields of the lower molecular weight roducts were determined. I n decreasing order of importance these included hydrogen, propane, ethylene, ethane, metlane, propylene acetylene and butane. The magnitude of the yields and the product distribution are consistent with the assumption that the reaction sequence is ionization of the molecule, fragmentation, reaction of the fragment ions with pentane molecules, and neutralization of the product ions, followed by intercombination of free radicals from these processes. Certain implications of this mechanism are suggested.
Introduction A recent paper discussed the radiation chemistry of n-hexane and demonstrated a remarkably quantitative correlation between the mass spectral fragmentation pattern and the yields from gas phase radio1ysis.l Shortly after this paper was submitted, Back and Miller reported a detailed investigation of the gas phase irradiation of npentane with y-rays from PoZl0and Cm242and with X-rays.2 Calculations based on the same reaction scheme proposed for n-hexane revealed essential agreement with the yield data of these authors for hydrogen and the C2hydrocarbons, but disagreed in that the calculations predicted that propane was a major product, while experimentally a low value was found. The present research was undertaken in an attempt to resolve this discrepancy. Experimental Procedures
using prepurified nitrogen carrier. B t the lowest conversion levels the methane-hydrogen fraction was analyzed with a mass spectrometer. Analyses for methane may have been in error for some samples because of non-linearity in response of the detector using nitrogen carrier.6
Discussion of Results The experimental results are present'ed in Table I as G-values, the yield of products expressed as molecules per hundred electron volt's of energy dissipated in the system. I n the absence of elaborate dosimetry studies for the source employed, the yields have been normalized arbit'rarily to the value G(CzH6) = 1.1, the value determined by Back and Miller2 for a-radiolysis. Extrapolation of yield data to zero dose required irradiations in which as little as 0.03% of the pentane was decomposed, as this research confirmed recent observations2,6 that product distributions change markedly a t rather low conversions. This point will be the subject of a future publication.
Irradiation Procedures .-Phillips research grade nTABLE I pentane was degassed by repeated freeze-pump-melt cycles and trap-to-trap distillation under vacuum and was dis- HUNDRED ELECTRON VOLTYIELDSOF PRODUCTS FROM GAS tilled into Pyrex ampoules 21 X 2.5 em. diameter fitted PHASE RADIOLYSIS O F n-PENTANE with seal-off and break-seal tubes. These were sealed Back and Millera This research b under vacuum and subsequently irradiated for periods of one hour t o one month in a 1500 curie C060 source of Brookhaven Hz 7.3 >6 design.3 Essential agreement between the lowest molecular CHI 0.8 0.7 weight product distribution and that of Back and Miller2 CzHz 0.5 0.5 was obtained, and our results were arbitrarily normalized CzH4 1.6 1.5 t o the hundred electron volt yield of ethane reported by C2He 1.1 1 .l b these authors. The irradiated samples were sealed into a vacuum system C3H6 0.04 0.6 for separation of products in a manner analogous to that CsHs 0.3 2.0 described by Newton4 for liquid phase studies. Fractions CaHio 0.04 0.4 volatile a t -190" (H, and CH,) and at -125" (predominately n-pentane but also containing the Cz-Cd hydroa Reference 2. Polonium 210 and curium 242 a-sources. carbons) were separated and the number of pmoles in each Yields normalized t o G(CP&) = 1.1. See discussion in were determined by means of pressure and volume meas- text. urements a t a given temperature. These fractions were Factors involved in dosimetry for gas phase pumped into bulbs for subsequent analysis by gas chromatography and mass spectrometry. The residual fraction irradiations with Cow sources have been discussed was retained and examined routinely but was found in all recently by Back6 and by Armstrong and Spinks.7 cases t o contain only pentane in detectable quantities. Analysis.-The vacuum separation techniques served Cross sections for interaction of Co6O y-rays are so merely to concentrate the products sufficiently that gas small t'hat only t'he order of 0.5% of the dose rechromatography using thermal conductivity detection ceived by the hydrocarbon is accounted for by could be used for analysis. The total gas samples sepa- primary interaction. The major process is the rated in this fashion were injected into the carrier gas stream of the gas chromatography apparatus by means of a by- ejection of secondary electrons from the walls of pass system incorporated into the flow system. The CZ-CS the container; hence the gas is exposed t o a flux products were determined on a 10 meter column packed of high energy electrons varying from ea. 1 Mev. with a hexadecane adsorbed on 30-60 mesh firebrick using down. Their number per unit dose and actual helium carrier, and the methane-hydrogen fraction was determined on a 1.5 meter molecular sieve 5A column energy distribution is determined by the Pyrex walls (1) J. H. Futrell, J . Ana. Chem. Soc., 81, 5921 (1959). (2) R. A. Back and N. Miller, Trans. F o r a d a ~Soc., 56, 911 (1959). (3) M . C. Atkins, WADC Technical Note 55-302, "Accessory Equipment and Procedures for Use of a 1500 Curie Cobalt-60 Gamma Ray Source," April 1956, available from Office of Technical Services, U. S. Department of Commerce, Washington 2 5 , D. C. (4) A. S.Yeaton, Anal. Chem., as, 1214 (195fi).
of the irradiation cells and is independent of t.he gas in the cell. (5) L. J . Schmauoh and R. A. Dinerstein, Anal. Ckem., 32, 343 (1960). (6) R. A. Back, T m s .JOURNAL, 64, 124 (1960). (7) D . A. Armstrong and J . W. T. Spinks, Can. J . Chem., 3'7, 1210 (1959).
Nov., 1960
GASPHASE RADIOLYSIS OF n-PENTANE
These experimental conditions are clearly quite different from those in a-particle irradiations, and the normalization of yield data may be questioned.8 This is especially true in view of recent mass spectrometer studies which indicate that ionization processes for electron and a-particle impact differ ~ignificantly.~However, earlier work by de Vries and Allen,l0 Schuler,” and Davison12 shows that, a t least for many liquid hydrocarbons, changing radiation quality produces only a minor effect on G values of the ultimate products. The similarity of the prodnot distributions in Table I for products through C2 support this assumption for n-pentane vapor.8 The G-values determined in this fashion appear reasonable, and we attribute the differing Ca and C4 yields to differing analytical procedures rather than to an effect of radiation quality. In both sets of experiments the significant measurements were performed at conversions of less than 0.1% to avoid secondary reactions, and analytical problems are rather difficult in this regime. Only by combining separation by vacuum techniques with analysis by gas chromatography were we able to measure these products with meaningful accuracy. Mechanism.-In calculating the G-values for pentane radiolysis the same simple assumptions previously applied to n-hexane‘ are made. These assumptions correspond to a logical sequence of events following interaction of a pentane molecule with ionizing radiation: (1)Ionization occurs with fragmentation of the parent molecule-ion, and it is assumed that the extent of fragmentation corresponds to the mass spectrum of n-pentane.13 (2) Ion-molecule reactions occur a t every collision for these fragment ions (ca. 10-lo sec.), and it is assumed that so-called hydride-ion transfer reactions14 dominate this phase of the reaction. (3) Ions surviving these processes combine with thermalized electrons yielding various neutral species. Details of this step are purely speculative and the calculation is based on the assumption that the highly excited species formed on neutralization decays by fragmentation into a hydrogen atom and the corresponding entity. (4) Free radical fragments formed in these various processes react by coupling and disproportionation. (5) The Gvalues of the various products from radiolysis are determined by summing the molecules produced in steps 1 4 and relating the totals to the energy absorbed by the system. It is assumed that the mass spectral fragmentation pattern for n-pentane with 70 v. electrons gives the proper distribution of ions for radiolysis also, (8) Data obtained recently from 1 Mev. electron irradiations using thin target current measurement dosimetry indicates C(CzHs) = 1.1 i 0.2. These fragmentary results show produot distribution and yielddose relationship very similar to those in the Cog0 study reported here. (9) C.E.Melton and P. S. Rudolph, J . Chem. Phys., SO, 847 (1959); P. S. Rudolph and C. E. Melton. THISJOURNAL,68, 916 (1959). (10) A. E.de Vries and A. 0. Allen, ibid., 6S, 879 (1959). (11) R. H. Schuler, ibid., 63, 925 (1959). (12) W. H. T. Davison, “The Chemical Society (London) Special Publication,” No. 9, 151 (1958). (13) “Catalog of Mass Spectral Data,” API Project 44, Carnegie Institute of Technology, Pittsburgh, F. D. Rossini, Editor, Serial 6, Contributed by National Bureau of Standards, October 31, 1947. (14) F. H. Field and F. W. Lampe, .I. Am. Chem. SOC.,SO, 5587 (1958)
1635
and that the proper distribution of neutral fragments is given by the ionization mechanisms postulated to correspond to appearance potential processes by Franklin. and Field.16 This assumption may be illustrated by considering the prominent ion of m/e 27, for which the mechanism C5&
+ e +GH3+ + HP + C ~ H T+ 2e
(1)
is given. The endothermicity of this reaction, obtained from the tabulated heats of formation of reactant and product radicals, ions and molecules equals approximately 10.9 e.v., the appearance potential for this reaction. Indeed, this is the primary basis for suggesting this as the ionization mechanism. These authors offer mechanisms for the principal ions produced from n-pentane under electron impact. To the C2H3+ ion from reaction 1 is applied the criterion of Field and LampeI4that the hydride-ion transfer reaction. R+
+ C5Hn +R H + CjHii+
(2)
will occur for all ions R+ for which the reaction is approximately thermoneutral or is exothermic. For R + = C2H3+ reaction 2 is exothermic by ca. 80 kcal./mole, which exceeds the energy of the decomposition reaction CzHc +CzHz
+ Hz
(3)
even when this excess energy is partitioned between the products of reaction 2 by the requirement of conservation of momentum. From the tabulation of ions, molecules and free radicals initiallv formed, the ions are considered as possible r e a c t a h in reaction 2. It appears likely that CH3+, CzH3+ (C2H2 H2), C2H4+, C2H6+ H d , C3H3+, C3H5+, C ~ H B +C3H7+ , and (C2H4 CAHO+ will react. Products in Darentheses indicatethat further decomposition oi the product R H is allowed thermodynamically as illustrated above for R = C&+ and is assumed to occur. The completion of the calculation requires certain assumptions regarding the neutralization of surviving ions and reactions of free radical species. Essentially the same procedures previously applied to n-hexane were followed’ with appropriate modifications to account for the presence of two ionic species and four types of free radicals in significant quantities. Results of these calculations16 are summarized in Table I1 in which the molecules resulting from each process are presented on the basis of 100 ions initially formed. These quantities are converted to G-values by dividing by W , the energy expended in forming an ion pair in the system. Columns 5 and 6 compare the experimentally measured G-values with calculations, based on an estimated W = 26 e.v.17 The agreement is satisfactory. I n the original paper applying this calculation method to gas phase radiation chemistry, certain serious objections were indicated. Briefly these included the statement that the mass spectrum of a
+
+
(15) J. L.Franklin and F. H. Field, “Electron Impact Phenomena,” Academic Press, Inc., New York, N. Y., 1957. (16) Details of the calculation may be obtained from the author on request. (17) G. J. Hine and G. L. Brownell, “Radiation Dosimetry,” Academic Press, Inc., New York, N. Y.. 1958,p. 38.
K. U. INGOLD
1636
tribution of ions which should be considered in ab
TABLE I1 COMPARISONOF CALCULATED A N D EXPERIMENTAL GVALUES Basis of 100 ions formed From From initial From ion radical fragmolecnle intermentation reactions actions
Hz 18.1 CHI CzHz CzH4 8.0 CzH6 32.4 CsHe 2.0 CaHs CJXo a Assuming W
19.4 1.5 11.4 8.0
120 4.2 8.1 6.4 6.2 9.3 5.8
13.3 33.0 4.1 = 26 e.v./ion pair.
Vol. 64
Total
157.5 5.7 11.4 24.1 38.8 21.5 42.3 9.9
G-Values Ca1cd.a Obsd.
6.1 0.2 0.4 0.9 1.5 0.8 1.6 0.4
>6
0.7 0.5 1.5 1.1 0.6 2.0 0.4
compound represents the extent of unimolecular decomposition of the molecule ion in a microsecond, while the time interval appropriate to radiation chemistry is ca. 10-lo sec.18; that only the hydride ion transfer reaction was considered, while diverse types of ion-molecule reactions have been observed; and that non-ionizing excitation events have been ignored. While the latter point has not been explored further, cert,ain general remarks pertinent to the first two objections can now be made. In a recent paper, StevensonlS applied the methods of the quasi-equilibrium theory of mass spectralg to the question of the competition between reaction and fragmentation of hydrocarbon molecule ions. He concluded that for the more complex alkanes the rates of dissociation are comparable with the rates of bimolecular ion-molecule reactions, so that reactions of the fragment ions must be considered; for the simpler paraffins reaction presumably will occur before extensive fragmentation results. Hence we may infer that the mass spectrum represents a reasonable first approximation to the dis(18) D. P. Stevenson, Rad. Reseavch, 10, 610 (1959). (19) H. 11. Rosenstock, M. B . Wallenstein, A. L. Wahrhaftig and H. Eyring. Proc. Nut. Acad. Sei., U.S., 38, 667 (1952).
initio calculations. The best justification for considering only the socalled hydride ion transfer reaction is found in the mass spectrometer study of this reaction.I4 I n parallel experiments these workers studied the intensity of the ion product of the transfer reaction as a function of pressure for a series of paraffin hydrocarbons and also studied the ion intensity for constant pressure of hydrocarbon and varying pressure of methane. Cross sections which were calculated from the assumed occurrence of all thermoneutral or exothermic hydride-ion transfers in the pure hydrocarbon experiments agreed with the cross sections for the mixture experiments in which the reactant ions were known with greater certainty. This fact not only supports the basic assumption that all thermodynamically allowed reactions occur but also implies that, for the paraffin hydrocarbons studied, this reaction competes effectively with other possible ion-molecule reactions. Field and Lampe14 also noted that, contrary to theoretical expectations, the measured cross section decreased relatively sharply with increasing molecular weight. Accepting this result permits us to suggest that calculations of the type we have employed can be significant for only a limited number of molecules. For large molecules the cross section for the hydride-ion transfer reaction becomes so small that it probably does not dominate the reaction mechanism for the fragment ions. As discussed above, for the simplest paraffin hydrocarbons mass spectra theory indicates that the mass spectrum cannot be used as a measure of the ion distribution. Hence the range of usefulness of this calculation method is sharply bounded, and its principal value may be the demonstration that the radiation chemistry of certain hydrocarbons can be rationalized on the basis of a consistent set of ion-molecule reactions.
INHIBITION OF OIL OXIDATION BY ~,6-DI-t-BUrl'YL-4-SUBSTITUTED PHENOLS BY K. U. INGOLD Division of Applied Chemistry, National Research Council, Ottawa, Canada. Issued as N.R.C. h'o.
8011
Receised A p r i l 11, 1060
A number of 2,6-di-t-butyl-4-substituted phenols have been prepared and their efficiencies in inhibitin the autoxidation of a saturated white mineral oil have been compared. Electron releasing Psubstituents increase the ekciency of the inhibitors and, provided the substituents are small, the results can be represented by a Hammett p u plot. For bulky alkyl substituents in the 4-position, the relative inhibiting efficiencies can be related t o the Taft .steric substituent constants E.. Replacement of the phenolic hydrogen by deuterium does not affect the efficiency of the inhibitors. It is concluded that the rate controlling step of inhibition involves an addition reaction, perhaps by a charge transfer process, in which the attacking peroxy radical becomes conjugated with the aromatic ring of the inhibitor, probably via the ?r electrons on the phenolic oxygen atom.
Introduction We have shown recently that the inhibition of autoxidation of a saturated white mineral oil by certain very weak inhibitors (such as phenol and aniline) exhibits a kinetic isotope effect,' replace-
ment of the active hydrogen by deuterium resulting in a less efficient inhibitor. This result was assumed to favor an inhibition mechanism in which the (1) K. (1959).
U. Ingold and I. E. Puddington, Ind. Eng. Chem.,
61, 1319
