Sept., 1962
THERADIOI~YSIS OF SATURATED HYDROCARBONS q =
2.801 X 108/~1~21q = (dp(1 l/i)
+
8. = 14.28, Bjerrum critical distance in Angstroms for a 2:2 electrolyte in water a t 25”. This
c
=
m =
varies as a function of pressure concentration in moles/l. concentration in moles of solute/kg. of solvent
for symmetrical electrolytes, z1 = z2 = 2 for MgS04 I = 4ca, ionic strength of 2-2 salt T = absolute temperature (y = degree of dissociation E = dielectric constant q = viscosity pr = relative density of water
1611 X Z ; XI=
THE RADIOLYSIS OF SATURATED HYDROCARBONS BY T. J. HARDWICK Gulf Research & Development Conzpany, Pittsburgh 30, Pennsgluania Received February 81,1968
The yieldij of primary products in the radiolysis of some twenty saturated hydrocarbons ( C&~O) have been measured. For all paraffins investigated, the yield of “moleGu1ar” hydrogen gas is approximately a constant fraction of the t o t d hydrogen gas (0.40) despite changes in hydrocarbon structure and total hydrogen gas yield, Evidence is preseqted which indicates that this “molecular” hydrogen arises by a “hot” hydrogen atom mechanism rather than by a molecular detachment process. The balance of the hydrogen gas forfied arises from the reaction of radiolytically produced chemical hydrogen atoms with the solvent. Congruent ‘Lmolecular’’ unsaturation (olefins of the same carbon number produced in the presence of alkyl radical scavengers) ranges from 5040% of the “molecular” hydrogen yield. Radicals produced by C-C bond scission interact within the cage of formation by disproportionation or recombination, or may escape from the cage to react with other radicals or solutes. Yields of such freely diffusing radicals have been measured, and a definite relation to chemical structure has been found. The results for all hydrocarbons studied are in complete agreement with the mechanism previously postulated for saturated hydrocarbon radiolysis. Determinations of free radical yields by e.p.r. give results in good agreement with other methods.
Introduction From previous studies on the mechanism of the radiolysis of saturated hydrocarbons (Hereinafter called paraffins, to include both alkanes and naphthenes), it appears that two types of processes occur, often giving identical products. The first forms stable, identifiable products by a mechanism which is uninfluenced by small concentrations (< 2%) of solute. The second forms hydrogen atom and/or alkyl radical intermediates, the sobsequent reactions of which are controlled by diffusion processes and the types of added solute, if any. These radical intermediates may react with themselves, with irradiation products, or with added solutes, eventually giving stable products. It is one purpose of this paper to distinguish between these two types of processes. For the first process, experiments are designed to investigate the mechanism. For the second process, measurements of the radical yields and some indication of their subsequent reactions will be given. This has been done for some twenty paraffins, normally liquid a t room temperatures. I n a previous paper, a sequence of reactions was postulated for the radiolysis of n-hexane, and as will be shown later, this reaction scheme is generally applicable to all liquid paraffins. Three primary processes account for most of the initial radiolytic reactions2 Gi
RH -4H2
+ products
(1)
(1) T. J. Hardaick, .I. Phus. Chem., 64, 1623 (1060). (2) The nonienclature used in this paper is that employed previously.1 Where necessary, new definitions of quantities have been made.
Gz
RH - - + - R + R
(2)
GR
RIJ R, (R) Reaction 1 produces hydrogen gas, plus other products, in yield GI by reactions unaffected by the presence of scavengers. Reaction 2, with yield G2, is a simple C-H bond scission giving a freely diffusing hydrogen atom and an alkyl radical. I n reaction R, C-C bond splitting gives t3woalkyl radicals. The interaction of these radicals with themselves and with solutes gives rise to stable products. It has been shown that the hydrogen atom, produced by reaction 2, reacts competitively with solvent (RH) by hydrogen abstraction (3) or with solutes by addition (4) or hydrogen abstraction (5). -+
ka
H
+ RH +HZ + R
(3)
kr
H+S-+HS
(4
ks
Ir
+ s +11%+ R’
(5) From the kinetics it is possible to determine the yields of reactions 1 and 2 (GI and Gz) from a measurement of the decrease in radiolytic hydrogen gas yield as a function of solute c~ncentration.’.~ Experimental Materials.-All paraffins were Phillips pure grade. They were further purified by prolonged stirring with con(3) T. J. Hardwick, J . P h g r . Chem., 6 6 , 101 (1961).
1612
T. J. HARDWICK
centrated sulfuric acid until the unsaturation content was sufficiently low for use. In all cases the unsaturation of the purified stock material was measured by bromination. An indication of the purity is found in the first column, Table 11. Aromatics were pure grade; olefins present in them were removed by silica gel. Other reagents used in solution make-up were the best grade available. For unsaturation analysis all materials were reagent grade. Diphenylpicrylhydrazil (DPPH) was obtained from K and K Laboratories. Coppinger’s radical ( C o p y was kindly prepared by Dr. H . 0. Strange of the Product Development Division. The e.p.r. rating agreed well with previous batches and measured 98-102y0 radical. Hydrogen Yields.-The method for determining hydrogen gas yields has been described in detail previously.1~3 Briefly, 100 ml. of deaerated solution +vas irradiated with X-rays to about 20 krads, and the hydrogen gas was removed, isolated, and measured on a McLeod age. The energy absorbed by the samples was concurrentfy monitored using a Fricke dosimeter. Unsaturation.-Irradiations were made in an all-stainless steel circulating loop of 400 ml. capacity, using a spot beam of e1ectrons.l Samples were irradiated in an atmosphere of nitrogen. Unsaturation was measured by the uptake of bromine in acetic acid medium. Conversion of the excess bromiue to iodine before titration enabled the excess to be measured by the more predse iodometric method. “Initial” Unsaturation.-Measurements were made to determine the initial unsaturatioa yield arising from congruent olefins.6 It was necessary, however, to remove lower boiling olefins resulting from C-C bond scission; otherwise, a spuriously high value would have been obtained. This was accomplidhed by fractional distillation, wherein a paraffin of appropriate boiling point was added t o improve the separation of the two clasfies of olefin. The amount of unsaturation in the residue, measured by bromination, was taken as the yield of congruent olefins. Distillations were made in duplicate or triplicate, giving a reproducibility of measurement of &2%. As an illustration of the efficiency of separation, nu measurable olefin was found in the forerun of the following mixtures: ( a ) pentane-hexene2-n-hexane, (b) hexane-2-methylhexene-l-2,4-dimethylpentane; furthermore, in both cases the measured olefin content of the residue corresponded to that present initially within zk170. Molecular Unsaturation.-In order to measure the molecular unsaturation, a scavenger was needed to remove all radical intermediates. Solutions of paraffin containing 1 . 2 5 3 methacrylic acid were irradiated in the circulating loop in the same manner as the pure paraffin. .4 small amount of fluffy preci itate was formed, but the inclusion of this in the sample $id not alter results. The irradiated sample was fractionated in the same manner as before, but after removal of the light hydrocarbons, the contents of the still pot were added to a steam distillation unit, made alkaline with 4 N PiaOH, and steam distilled. The unsaturation of the organic distillate layer, measured by bromination, was taken as a measure of the molecular unsaturation. Methane Yields.-Samples of pure paraffin and paraffin containing 1% methyl methacrylate (MlllA) were prepared and irradiated in a manner identical to that for hydrogen gas measurements. The evolved gas was pumped by a Toepler pump through a trap a t - 100’ and collected in a sample flask. Mass spectrometric measurements were made on the CH,/H%ratio, and from the measured hydrogen yields, known from previous experiments, the methane yields were calculated. At least two runs were made for each hydrocarbon. E.p.r. Measurements.-All e.p.r. measurements were made with a Varian Associates X-Band spectrometer, with a &in. magnet and rectangular resonant cavity. Relative intensities of COP were measured by taking derivative peak heights and comparing such numbers with the intensity (4) G. Coppinger, J . A m . Chem. floc., 79, 501 (1057). ( 5 ) W. 5. Guentner a n d T. J. Hardwick, Intern. J . A p p l . Radiatzon Isotopes, 13, 98 (1962). (6) The term “congruent” is used when referring to olefins or radicals having the same number of carbon atoms as the solvent: e . ~ . hexene , a n d hexyl radicals are the congruent olefins and radicals in hexane radiolysis.
Vol. 66
obtained from a standard solution of D P P H (I m M ) in benzene. The ratio of COP intensity to that of D P P H should represent the relative concentrations of unpaired electrons. The e.p.r. signal intensity of standard solutions of COP in benzene, measured under standard conditions, was linear with the molar concentration of the radical at least up to concentrations of 25 mM. Stringent removal of oxygen was necessary to obtain reproducible results. It was found difficult to prepare a standard solution of COP in many paraffin solvents, presumably due to traces of reactive impurities. Where marked deterioration in a solvent was evident, further purification steps were taken. S s a further test, all solutions were stored for at least 5 days after preparation and before e.p.r. measurements. If less than 5% decay of the radical was observed in the succeeding 5 days, the samples were used in the radiolysis experiments. Samples were prepared in the following manner. About 17 mg. of COP was placed in tubes, shown in Fig. la. A 5-ml. volume of purified solvent was pipetted into these tubes, filling the lower bulb about three-quarters full. The solution was degassed and sealed from the atmosphere at 3,giving a tube suitable for irradiations (with the solution in the bulb) and e.p.r. measurements (with the solution in the narrow tube) as shown in Fig. Ib. The outside diameter of the glass in the narrow portion of the tube was close to 4.8 mm., the diameter of a circular hole in the resonanre cavity. Irradiations were made using X-rays from a Van de Graaff accelerator. The rate of energy absorption, about 200 krads/lir., was measured by the Fricke dosimeter solution. The necessary corrections for energy absorption were made by comparing the electron densities of the paraffin and the dosimeter solution. Once the zero-order kinetics were confirmed, the usual irradiations were such as to remove about 70% of the initial radical signal. The time between the final pre-irradiation e .p.r. measurement and the corresponding post irradiation measurement was always less than 2 days. Separate experiments showed that the low exposurc received by the upper end of the irradiated tubes had no noticeable effect on the e.p.r. signal in the region of measurement. To overcome any drifts in the instrument sensitivit,y, all e.p.r. results were normalized using a standard sample of D P P H which tests had shown to be invariant over several months. After radiolysis, the tubes were opened at the bottom, cleaned, and the narrow part filled with a standard solution of DPPH. Variations in the diameter of the tubes, with corresponding effects in the signal intensity, thus were measured directly, and corrections were applied. Net changes were obtained in units of spins/cc. X constant/Mrad absorbed. The method for subsequent treatment of the data is found in the Results.
Results Hydrogen Gas Yields.-The yields of hydrogen gas from the purified compounds (GHZJ are found in Table I. The low olefin content of the stock solutions made unnecessary any corrections to the hydrogen yield. The values of G2 were determined by measuring the yield of radiolytic hydrogen gas (GHZ(s))for paraffins containing a range (0.10 . 7 ~ / v o l . )of methyl methacrylate (MMA) scav(~) enger. For all paraffins the kinetic plot ~ / ( G H ~G H 2 ( s ) ) (= l/AG,,) vs. (RH)/(S) gave a straight line, from whose intercept Gz was obtained. Values of Gz found in this manner are given in column 3, Table I; values of GI, obtained by difference ( G I I ~-~G~z ,) , are listed in the preceding column. Unsaturation.-In measuring the initial congruent unsaturation, it is necessary to account for olefin disappearing by reactions 4 and 5; otherwise, low values will be obtained. In practice, a compromise must be found between producing sufficient olefin for analysis and minimizing the amount of olefin reacting with hydrogen atoms. Fortunately, data are available to make reliable corrections in the latter case.
THERADIOLYSIS OF SATURATED HYDROCARBONS
Sept., 1962
1613
TABLE I HYDROGEN YIELDSFBOM IRRADIATED PARAFFINS GI Gz G H ~ ( ~ Moleoule/100 I e.v. GL/GH~(o)
n-Pentane n-Hexane n-Heptane n-Octane n-Nonane %-Decane
6.35 5.28 6.06 6.18 6.05 4.90
2.10 2.12 2.36 2.85 2.52 1.70
4.25 3.16 3.70 3.33 3.53 3.20
0.33
2-Methylbiitane 2-Methylpentane 3-Methylpentane
4.24 4.47 4.56 5.78 5.56 4.62 4.76 3.78
1.66 1.61 1.60
.39 .36 .35
2.72 2.43 1.99 2.05 1.68
2.59 2.86 2.96 3.06 3.13 2.63 2.71 2.10
1.69 1.59 1.21 1.16 1.24 1,16
2.33 2.60 1.75 1.96 1.67 1.54
.42 .38 .41 .37 .43 .43
Cyclopentane Cyclohexane Methylcyclopentane Methylcyclohexane 1,2-Dimethylcyclohexane 2,3-Dimethylbutane 2,4-l)imethylpentane 2,3,4-Trimethylpentane 2,2-Dimethylbutane 2,2,4-Trimethylpentane 2,2,5-Trimethylhexane
4.02 4.19 2.96 3.12 2.91 2.70
.40
.39 .46 .42 .35
-
.47
-- 3
.44
0
.43 .43
E N
.44
As part of the determination of Gz, data are obtained from which the rate of reaction of 13 atoms with the solvent hydrocarbon (ka) can be expressed in absolut,e terms.3 In other work reported elsewhere17the reactivity of hydrogen atoms with olefins in paraffin solution has been determined. From such results it has been shbwn that for olefins produced by radiolysis 1c4 = 6.0 X loll cc. mole-’ see:-’, kg 1.2 X lo1’ cc. mole-’sec.-l. Knowing k3, k4, and l c ~ one , can determine for a given olefin concentration the fraction of the hydrogen atoms reacting with paraffin and olefin (fi,f4, fs for reactions 3, 4, and 5). Reaction 4 will remove the olefin to form an alkyl radical: reaction 5 will produce an alkenyl radical. Since alkenyl radicals react mostly by addition to alkyl radicals,l an olefin is the stable product, and no olefin is lost from the system. Alkyl radicals are produced to the same extent regardless of whether reaction 3 or 4 takes place. Summing up the possible reactions, it can be shown that, except for the negligible effect of reaction 5 to reduce the alkyl radical concentration, the net result of reaction 4 is to remove one olefin molecule. A summation of the value of f4 for a series of olefin concentrations (1, 3, 5 mM, etc.) was made, using the appropriate length of sum to correct for the total olefin produced. Final olefin concentrations ranged from 7-14 mM for ca. 50 ,j./g. of energy absorbed. The corrections resulting from reaction 4 varied from 1.5% for 2,3-dimethylbutane to 11% for n-pentane. Values of the initial congruent olefin yield (G,) corrected for this olefin loss and for initial olefin content arc listed in column 3, Table 11. I n the separation preceding analysis, olefins were found in the forerun of the distillation in yields of 0.14.7 molecule/100 e.v., indicating that olefins of lower carbon number are formed.
0
Fig. 1.-Cell for measuring the total radicaI yield by e.p.F.: (a) unfilled cell; (b) cell filled and sealed for radiolysis and measurement.
TABLE I1 IN~TIAL AND “MOLECULAR” UNSATURATIOX IN IRRADIATED PARAFFINS “Moleoular” yield of conjruent olefin
::1
(7) T. J. Hardwick, J . Phys. Chem., 66, 291 (1962).
Initial yield of congruent olefin
Initial olefin (Gid (Gd oontent, -Molecule/mM 100 e.v.
n-Pentane n-Hexane %-Heptane n-Octane n-Nonane
5’ GI
(Cu G d Gz
0.11 .10 .12 .11 .15
1.42 1.47 1.51 1.47 1.40
4.07 0.68 0.62 3.30 .69 .58 3.43 .65 .52 3.05 52 .48 2.93 .55 .43
Isopentane 2-Methylpentane 3-Methylpentane 2,4-Dimethylpentane
.16 .09 .18 .31
1.27 1.12 0.87 1.24
3.35 2.78 2 11 2.62
.77 .70 .52 .78
.80 .58 .42 .53
2,3-Dimethylbutane 2,2,4-Trimethylpentane
.23 31
1.09 2 52 1 00 1 92
.65 .80
.61 55
64 84 72 56
82 84 60 56
Cy clopent ane Methylcyclopentane Cyclohexane Methylcyclohexane
07 21 09 18
1 73 1 67 1 75 1 15
4 3 3 3
24 89 61 09
Part of this congruent unsaturation is the socalled “molecularJ’ unsaturation, the yield of which ( G a ) is unaffected by the presence of scavengers. The presence of a polymerizable monomer during paraffin radiolysis removes freely diffusing alkyl radicals which are normally precursors of olefins through disproportionation. Using acidic monomers, a steam distillation from alkaline solution separates the olefin from the monomer and polymer.
1614
T. J. HARDWICK
Preliminary experiments were made to determine the effect of type and concentration of monomer scavenger. Solutions of n-hexane containing 0.5. 1, 2, and 3%/vol. methacrylic acid (MAA) were irradiated to 100 j./g. After correcting for direct absorption of energy in the solute, Glc was found to be 1.48 f 0.03. Acrylic acid (Ah) solutions in n-hexane, prepared and treated in the same may, gave Glc = 1.45 f 0.04. These results are in good agreement with those previously published using methyl methacrylate,' where Glc = 1.47 f 0.02. Methacrylic acid is preferred over the other monomers: separation is easier with an acid group, and less precipitate forms with MAA than with AA. Irradiations mere made with n-hexane-JIAA solutions a t doses ranging from 50-150 j./g. 3-0 change in Glc was found. The low olefin concentration relative to that of the monomer makes a correction for olefin disappearance unnecessary. As standard procedure, 1.2570/vol. solutions of MAA in paraffin were irradiated. In the preanalytical fractional distillations, olefin was found in the forerun in all cases, showing that olefins having fewer carbon atoms than the congruent one are formed by some process which is not associated with scavengeable free radicals. The yields of congruent unscavengeable olefins, Glc, measured in this fashion are given in column 3, Table 11. Total Free Radical Yield (GTR) .--Preliminary tests made with four concentrations of COP in nhexane showed that the free radical disappearance was truly zero order. Our confidence in directly measuring the absolute concentration of COP in solution was not better than f5%. Accordingly, an indirect method of measuring absolute yields (G- cop) was required. Solutions of DPPH and COP in benzene and toluene were prepared and irradiated in the same manner as for paraffins. In the case of the DPPH solutions, it was feasible to measure the change in DPPH concentration with good accuracy, and this result could be related to the absolute yield of the free radical disappearance (G-DppII). I t mas assumed that G - D P ~ H is a true measure of the total radical yield. A comparison is made in Table 111 of the total radical yields (GTR)in benzene and toluene determined by various experimental techniques. As can be seen, the values obtained in the present study agree well with previous results. Accordingly, considerable confidence has been placed in the e. p. r. method. A further reasonable assumption was made: the disappearance of COP likewise measured GTR. Experimentally one determines by e.p.r. some value proportional to the concentration of COP ( K X [COP]). The changc in concwitration per Ptlrad A(K x [COP])/Mrad is related to the yield of COP disappeared (G- cop). I n comparing the change in COP concentration per Itfrad for henBene and toluene, calibration factors cancel, and the ratio G- cop(benxene)/G- cop(to1uene) (0.84) is in good agreement with the corresponding ratio for DPPH (0.626). From such results the e.p.r. response for COP solutions could be related directly to the free radi-
Vol. 66
T A B LI11 ~ TOTAL RADICAL YIELD (GTR)I N BENZENE A N D TOLUENE RADIOLYSIS GTR Method of measurement
Polymerization Disappearance of D P P H (colorimetric) Reduction of ferric ion Disappearance of DPPH (e.p.r.)
molecule/ 100 e.v. Benzene
0.74 .78 .74 .78 .77
Ref.
8 9 10 11 This work
Toluene
Polymerization Disappearance of IIPPII: (colorimetric) Disappearance of DPPH (e,p.r,)
1.1 1.2 1 1 1.23
8 12 13 This work
cal concentration. Accordingly, GTR could be determined directly for alkane solutions using COP as solute. Although hydrogen atoms react rapidly with COP (IQ = 1.0 X 1013 mole-1 sec.-l) a considerable fraction of them will react with the solvent. This, however, produces another radical, which in turn reacts with COP. This secondary formation of radicals has no effect on the net change of COP concentration as measured by the e.p.r. method. Total radical yields (GTR) for a series of paraffins are shown in Table IV. One striking result is that for the alkanes, the total number of radicals formed lies in a small range, 7.8-9.5, and is reasonably independent of structure. Cycloalkanes, as a group, show similar yields (6.7-7.7). Scission of a C-H bond produces atomic hydrogen, and the yield of this intermediate has been accurately measured (Gz), The tota! yield of radicals from a C-H bond split is therefore 2Gz, and values of this are shown in the second column of Table IV. The yield of radicals formed from a ) GTR - 2Gz, and such C-C bond scission ( ~ G R is values are shown in column 3, Table IV. The paraffins listed in Table IV have been grouped according to general chemical structurc, n-alkanes, singly branched methylalkanes, etc. The extent of radigal formation from C-C bond scission is clearly related to the chemical structure. As has been suggested before,14 the greater the branching, the. greater the number of radicals formed by C-C bond scission. The ratio ~ G R / G T R(column 4, Table IV) is the fraction of the total radicals which are formed by C-C bond scission. The similarity of this ratio within each structural, type is quite marked, and permits a more sensible grouping according to structure; e.g., I,%dimethylcyclohexane is classed with 2,3-dimethylalkanes rather than with methylcycloa!kanes. The agreement within each struc. (8) A. Cliapiro, J . chzrn. phl/s.. 47, 747, 764 (1950). (0) C. Cousin, Thesis, Univerulty of Par-. 1953. (10) I,. I b u b y and A. Chapiro, .I. chzrn. p h g s , 62, 645 (1955). C'liermak, E, Collinqon, F. S. Dttinton, and G. &I. N o a burn, Proc. Chem. Soc., 54 (1068). (12) W.11. Seltzer and A. V. Tobolsky, J . .4m. Chem. Soc., 7 7 , 2687 (1955). (13) N. Oilson, reported by A. Cliapiro, J . Phys. Chem.. 68, 801
(1059). (14) €1. A. Dcwhuist, J . Am. Chem. Soc., 80, 5607 (lY5Y).
THERADIOLYSIS OF S A T U R A T E D
Sept., 1962
HPDROCARBONS
1613
clohexylhexene are found among the products of TABLE IV TOTALRADICAL YIELDS (GTR) AS MEASUREDBY E.P.R. pure cyclohexane radiolysis. The yield is suffiGTR
2G2
26’~
cient to account for about 80% of the observed radicals, if the intermediate entities are scavenged by COP. Methane Yields.-The radiolytic methane yields for several pure Cg paraffins are shown in the first column, Table V. The second column gives the methane yield when the solution contained initially 1.25%/vol. (117 mM) methyl methacrylate monomer. Preliminary experiments showed no variation in methane yield with 0.5-3.0%/vol. monomer, although both the hydrogen gas yield and the methane/hydrogen ratio varied over the concentration range. The methane formed with monomer present, therefore, is considered to originate in some process which does not involve freely diffusing methyl radicals. Methane formed from scavengeable free radicals would therefore be the difference of these two measurements, and such values are given in column 3, Table V.
~GR/GTR
Molecule/100 e.v.
n-Pentane n-Hexane n-Heptane %-Octane n-Nonane n-Decane
8.7 7.8 9.1 8.3 9.4 8.9
8.50 6.32 7.40 6.66 7.06 6.40
2-Methylbutane 2-Methylpentane 3-Methylpentane
7 . 9 5.18 2 . 7 8 . 5 5.72 2 . 8 8 . 7 5.92 2 . 8
::;/0.33 .32
Methylcyclopentanc Methylcyclohexane
6 . 9 5.26 1 . 6 6.9 5.42 1 . 5
.23/o,23 .22
2,4-Dimethylpentane 2,3-Dimethylbutane 1,2-Dimethylcyclohexane 2,3,4-Trimethylpentane
9.3 9.2 6.7 8.1
.44 .49 .51 .55
2,2-Dimethylbutane 2,2,4-Trimethylpentane 2,2,5-Trimethylhexane
9 . 5 3.92 5 . 6 8 . 9 3.34 5 . 6 8 . 9 3 . 08 5 . 8
Cyclopentane Cyclohexane
6 . 5 6.12 0 . 4 7 . 7 6.26 1 . 4
5.20 4.66 3.34 3.50
0.2 1 . 5 0.19 1 . 7 .19 1 . 6 .19 2 . 3 .23 2 . 5 .28
4.1 4.5 3.4 4.5
:.65ii 10.63
tural group is quite striking, and provides a measure of the relative number of radicals arising from C-H and C-C bond ruptures which is independent of the total radical yield. It should be remembered that the ratio ~ G R G T R refers to free radicals only, and not to the total number of bonds broken. The ratio offers assistance in establishing that paraffins of similar structure will break down in similar fashion during radiolysis. Although the number of C-C bond ruptures are in the miin consistent with structure, ‘there are three cases which merit further comment. The value of 2GR = 0.2 for n-pentane is obviously wrong, but measurements of both the total radical yield and the hydrogen yields give consistent results. However, a recent measurement of the radical production in n-pentane, using a novel scavenging technique,lh gives a total radical yield of 10.13/ 100 e.v.; ~ G R / G T Ris therefore 0.16 for n-pentane, in good agreement with the values for other nalkanes. Cyclopentane produces radicals by a C-C bond split (GR = 0.2). Such a value would be consistent with a very low probability of ring opening on cyclopentane radiolysis. Cyclohexane, on the other hand, produces 1.6 radicals by C-C bond rupture. Evidence is accumulating18 that hexene and other straight chain products are formed on radiolysis, presumably through an intermediate biradical. As such a hiradical is the normal consequence of ring opening, it may be that each event in the net result accounts for the disappearance of two COP radicals. Both hexene-1 and cy(15) R Holroyd a n d G . W. Klein Presented a t the Thirteenth Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittaburgh, Pennsylvania. March, 1062. ( 1 6 ) (a) A. C. Nixon and K. E. Thorpe. J . Client. Phye., 28, 1004 (19.58); (b) C. R. Freeman. z h d . , 88, 71 (1B60); (c) P. J. Dyne, private coin miinic&tion.
TABLE V METHANE YIELDS IN
cg
PARAFPIXS, WITH AND WITHOUT
RADICAL SCAVENGER Methane yield, molecule/100 e v. Methyl Decreasz radical Paraffin in Gcxa yield,‘ Pure 11.25% due to molecule/ paiaffin MAA scavenger 100 e.v.
/
