VAPORPHASERADIOLYSIS OF ETHANOL
1699
since it is to both ODMol and Dsl with "& is a Measan accuracy of Several tenths Of 1%. .. of the &flerence between the forces experienced by limiting amounts of Components 0 and 1, respectively, in excess quantities of component 1.
tracer diffusion coefficient of benzene in solutions of biphenyl in benzene and Sandauist and Lyons have measured the corresponding mutual diffusion- coefficient; If we designate biphenyl as the solvent, 0, tagged benzene the tracer component, 2, and pure benzene the other solute, 1, then from the data given in the above papers
Acknowledgment. The author wishes to thank professorL. J , ti^^ for many stimulating discussions and suggestions.
is a measure of the diference between the forces experienced by limiting amounts of biphenyl and benzene, respectively, in excess quantities of benzene. The above figure for zDD~zshould be accurate i o better than 0.5%.
*4012 = (1.558
- 2.247) X
10" = -0.689 X 10-6
The Vapor Phase Radiolysis of Ethanol1
by L. W. Sieck and R. H. Johnsen Department of Chemistry, Florida State University, Tallahassee, Florida
(Received December 10, 1964)
Initial and nonscavengable yields for the radiolysis of ethanol vapor at -45 mm. (25") have been measured. The contribution made by ionic species to the reaction products is discussed.
Introduction Although extensive radiolytic investigations of ethanol have been carried out in condensed phases, little information is available concerning the vapor phase decomposition. Ramaradhya and Freemanz8 have determined product distributions at high pressures and temperatures (108"), using a-particles. An interpretation of the chemistry was proposed in terms of a free-radical mechanism. A second investigation by these same workerszbconsidered the effect of added cyclohexene and benzene on the CO, CH,, and Hz yields. More recently, Myron and Freeman have studied the ethanol system using a high intensity ysource employing pentadiene as a scavenger.3 In the present work, initial yields have been determined, and nitric oxide has been used as a scavenger as well as oxygen and propylene. These studies were carried out at 25"' where reactions with significant activation energies would be relatively unimportant. The use of NO was prompted by Hoare's observation4 that complete scavenging occurred in the photolysis
of acetone-nitric oxide admixtures, suggesting no peculiar effects in oxygenated systems.
Experimental Apparatus. The irradiation vessel was fashioned from a brass cylinder equipped with 0-ring-sealed, 0.0127-cm. aluminum foil windows, through which the electron beam passed along an axis perpendicular to the longitudinal axis of the cell. Filling and evacuation was achieved through Whitey Teflonseated needle valves fitted with metal tapered joints for connection to a vacuum and sample handling system. The cell contained a volume of 2040 cc. and could be evacuated to mm. with no detectable leakage after a 24-hr. period. The entire apparatus (1) This work was supported in part by the U. S. Atomic Energy Commission under Contract AT-(40-1)-2001. (2) (a) J. M. Ramaradhya and G . R. Freeman, Can. J. Chem., 39, 1836 (1961); (b) ibid., 39, 1843 (1961). (3) J. J. J. Myron and G . R. Freeman, ibid., 43, 381 (1965). (4) D. E. Hoare, ibid., 40, 2012 (1962).
Volume 69,Number 6 Mag 1966
1700
was cooled during radiolysis by passing water through copper tubing wound around the exterior of the vessel. Sample Puri$cation and Handling. The ethanol used in these experiments was U. S. Industrial Chemicals Analyzed reagent refluxed over AgzO, dinitrophenylhydrazine, and BaO in that order for 72-hr. periods each. After distillation on a 50-theoreticalplate column and retaining only the middle fraction, the ethanol was found to contain 0.00037% acetaldehyde and 0.01% water. Before admission to the radiolysis vessel, the samples were subjected to bulb distillation under vacuum. The nitric oxide used was Matheson C.P. grade and was found to contain about 1% SzO. Matheson propylene and oxygen, as well as the nitric oxide, were used without further purification. Sample Recovery and Analysis. Prepurified and outgassed ethanol was allowed to diffuse into the evacuated irradiation vessel by volatilization from a side-arm trap maintained a t 20.0" (v.p. = 42.4 mm.). After equilibration was achieved, the cell contents were irradiated a t an ambient temperature of 24 to 25". Any additive was always admitted after equilibration of the ethanol. Gaseous radiolysis products consisting of Hz, CO, CH,, Cz,CB, and trace amounts of C4 hydrocarbons were collected in a modified McLeod-Toepler apparatus and subsequently anaIyzed by gas chromatography. Hz, CH4, and CO yields were determined on a 2.5molecular sieve column, while the C2-C4 m., 5-Ai. hydrocarbon group was analyzed on silica gel. Liquid products were removed from the irradiation vessel under vacuum after the admission of a 10-cc. flush aliquot of previously outgassed ethanol. The temperature of the cell was raised slightly, and, after agitation, the contents were frozen out in a removal side-arm trap a t -196" and stored in the dark a t -76" prior to analysis. In some cases methyl and isopropyl alcohols were employed as the diluent materials. The recovery of the cell contents by this method was quantitative within the limits of analytical accuracy. In order to establish that the more volatile liquid products were not lost from the diluted samples during the syringe injections into the chromatograph, some experiments were analyzed by freezing the entire condensible cell contents into small ampoules attached to a side arm without the addition of any diluent alcohol. These ampoules were then sealed under vacuum and the contents injected directly into the chromatograph carrier gas stream using a samplecrushing apparatus. This procedure afforded results identical with the sample-flushing technique described The Journal of Physical Chemistry
L. W. SIECKAND R. H. JOHNSEN
above. Liquid products including ethers, aldehydes, and synthesis alcohols were determined on either a 2.5-m. 1,2,3-tris(2-cyanoethoxy)propane or a 1.5-m. Perkin-Elmer "F" column. Glycols were analyzed on both Carbowax 1500 (1.5 m.) and polypropylene glycol. Flame ionization detection was used in most cases owing to the dilute nature of the samples. Radiation Xource and Dosimetry. The irradiation source was a High Voltage Engineering Corp. 3-Mev. Van de Graaff electron accelerator delivering a collimated beam of 2.0-Mev. electrons a t a current of 10 pa. Dosimetry was determined by the radiolysis of acetylene [G(-C2H2) = 71.91. The absolute G values for the products reported are probabIy accurate to within 401, (expected reliability in the dosimeter G value).
Results The major products (G > 0.3) obtained from the radiolysis of gaseous ethanol were hydrogen, acetaldehyde, formaldehyde, methane, ethane, ethylene, acetylene, carbon monoxide, and 2,3-butanediol. The dose dependence of the yields in the absence of any additive gases (plotted as differential yields with respect to ethanol and therefore corrected for consumed ethanol) are shown in Figures 1 and 2. The yields of CH2O were difficult to obtain accurately and displayed a high degree of scatter in the various experiments. An average G(CH20) = 0.9 f 0.3 was obtained, but it was impossible to determine even qualitatively the conversion dependence of this particular product. The yields obtained in the presence of sufficient nitric oxide to prevent thermal free-radical reactions are compared with the unscavenged yields for all products determined in Table I. In addition to the products tabulated, isopropyl and sec-butyl alcohols were also observed in some cases, but no quantitative estimate of their G values was obtained in the present work. Although the yield of CHICHO was reduced to negligible amounts in the nitric oxide-ethanol system, there is some question as to the reason for this effect. Analysis of the liquid products resulting from the NO-scavenged radiolysis indicated a large amount of acetal formation (G S 4.0). It was subsequently determined that mixing NO with a prepared gaseous mixture of CH3CHO and ethanol resulted in the almost complete conversion of the CH,CHO to acetal. Although it is not possible from our experiments to ascertain whether or not the NO was reacting with CHzCHO which was already produced during the radiolysis (a postirradiation effect), such a conclusion seems reasonable in view of the results from the propylene-scavenged system listed in Table I. Assuming quantitative
VAPORPHASE RADIOLYSIS OF ETHANOL
1701
Table I : Comparison of G Values for Various Products in the Presence and Absence of Additive Gases
Hz
Additive
None" Nitricoxide, 1 to 3 mm. Propylene, 1.60- 1 . 9 mm. 0 2 , 0.45 mm. 0 2 , 4 . 5 mm. Ref. 2d
CH4
CeH4
CO
CzHe
11.0 0.90 1.60 0.65 1.24 3.50 0.32 1.60 0.01 0.71 3.70 c c C C C
C
C
C
C
C
C
C
C
c
" Initial yield whenever possible. using a PozlO a-source.
CHsCHO
4.20 O.OOb 3.2 =!= 0.3 6.85 10.02
' See discussion in Results section.
Diethyl ether
Butanediol
0.07 0.08 c c
1.20 0.9-1.2 c 1.03
c
0.41
Propanediol
0.16 0.16
CZHZ COz
0.30 0.10 0.30 c
CHsOH
ce + C,
0.12 0.00
0.05 0.35
C
C
C
C
C
C
C
C
C
C
C
c
c
C
C
Total dose = 6 X l o m e.v./g. a t 108"
Not determined.
conversion of the scavenged CH3CH0 to acetal following radiolysis, G(CH3CHO) obtained with nitric oxide is 2.44 h 0.25.
Discussion of Results The unscavenged yields obtained in the present work are in reasonably good agreement with those reported by Freeman's laboratory. Some differences which can be specifically related to variations in radiolysis conditions will be discussed in another section. In contrast to the work of Ramaradhya and Freeman, we find a strong dose dependence for G(Hz), G(CZH4), and G(CZH~), and little, if any, effect on G(CH4). Owing to the complexity of the product distribution, no attempt will be made to interpret the observed dose dependence in terms of specific reaction schemes. It is interesting to point out qualitatively, however, that the concurrent depletion of the Hz and CzH4 yields with increasing irradiation time parallels similar variations observed in the study of saturated hydrocarbon systems. Of much more significance are the yields obtained in the presence of nitric oxide or propylene. Hydrogen (G = 3.5), C€L (G = 0.32), CO (G = 0.71), and CzHe (G = 0.01) are drastically reduced from the lower conversion yields, while G(CzH4) equals the initial yield within experimental error. In spite of the unexpected experimental difficulties encountered in the determination of G(CH,CHO) in the NO-CzHaOH system, the value observed in the presence of C3H6 is indicative of a high "molecular" yield for this product. A similarly high yield is obtained assuming that the NO additive converted CH3CH0 already produced in the radiolysis to acetal. The scavenged values for CH4, C Z H ~CZH4, , and CzHz are in essentially quantitative agreement with those reported by Myron and Freeman using pentadiene, indicating that these molecular yields are independent of temperature effects and the nature of the scavenger.
8 '
7.
t e l b a
-h
5.
CHSCHO
W
4'
A 0
0
H2 (wlth NO) 0
32. Butanrdiol
I.
*--
The glycolic products (2,3-butanediol, 1,2-propanediol) were generally unaffected by the addition of sufficient NO to prevent the reactions of thermal free radicals although the addition of 10% 0 2 t'o the system reduced G(butanedio1) by a factor of 3. The apparent disagreement between the two sets of data cannot be easily reconciled although the possibility of heteroelimination from an adduct such as (CH3CHOH)zN0 to produce butanediol cannot be ruled out. However, it is still difficult to imagine how the yields in the presence of the two scavengers could be so high. If one considers that the obvious precursors for glycol formaVolume 69, Number 6 M a y 1965
L. W. SIECKAND R. H. JOHNSEN
1702
smaller yields of the fragments HCO+, C2H6+, C2H3+, C2H2+, and CH3+. It is difficult to assign any numerical values to the various primary yields owing to the time scale shift between mass spectrometric analysis times sec.) and pressures corresponding to collision frequencies of to 10-lo see. It is apparent, however, that those ions produced by secondary decompositions of primary fragments will be depleted a t higher pressures, therefore establishing CzH60H+,CzH40H+,and CH20H+as the major ionic species present. The subsequent ion-molecule reactions of these intermediates with ethanol include proton transfer and hydride transfer. Owing to the exothermicity of proton-transfer reactions in ethanol, ion is possible the further dissociation of the CzHSOH2+ as pointed out by Wilmenius and Lindholm.s In a recent study,6it was shown that CzH60Hz+ was stable only when the over-all reaction was
l.6[
0.6.
CzH4OH+
A2
.04
.06 .08
.IO
.IS
.20
ENERGY ABSORBED (rv/molrculd
Interpretation In view of our findings, it is appropriate to speculate briefly on those reactions which would explain the magnitude of the observed yields. Ion-Molecule Reactions. The importance of ionmolecule reactions in gas-phase radiation chemistry cannot be overemphasized. In the ethanol system a t moderate pressures, the most abundant primary ions will be CzHsOH+, CzH40H+, and CHzOH+, with The Journal of Physical Chemistry
4CzH50Hz+
+ C2H40
(1) Although hydride transfer to CHzOH+,which should be the major ionic species present, is exothermic, the very small yield of CH30H suggests that the over-all reaction is CHzOH+
Figure 2. Product yield as a function of total dose for the minor products.
tion are CHZCHOH radicals produced primarily by H . abstraction processes, the yields of precursors should be drastically reduced because both NO and O2 are excellent scavengers for hydrogen atoms. This particular aspect of the problem should certainly be subjected Do further investigation since the results suggest a significant “molecular” contribution to glycol formation in the presence of presumably efficient free-radical interceptors. Myron and Freeman, on the other hand, have shown that the additsionof 0.2 electron fraction of pentadiene completely suppresses glycol formation. Whether this is due to a scavenging process for free radicals or some other mechanism is not completely clear.
+ C2HsOH
+ CzH50H
4
CHzO
+ C2H4 + H30’
(2)
which is thermoneutral. A similar process is exothermic for the reaction of the parent molecular ion; ViZ.
CzHBOH+
+ C2HsOH + H30+ + CzH4 + CH3CHOH
(3)7 Reaction 1 produces unscavengable CH3CH0, while (2) and (3) result in the formation of molecular CH20 and CZH4. HsO+ can react further with ethanol by proton transfer, which may explain in part the yield of H 2 0 observed by Myron and Freeman.8 Hydride transfer to some of the other fragment ions listed previously serves to increase the importance of reaction 1 in the ethanol system, thus increasing G(CH3CHO) by a molecular process. It seems possible, therefore, to account in whole or in part for the yields of CH3CH0, CHZO, CzH4, and HzO by assuming ion-molecule reactions of the type described (proton transfer). Free-Radical Reactions. The magnitude of the difference between the scavenged and unscavenged Hz ( 5 ) P. Wilmenius and E. Lindholm, Arkiv Fy9ik, 21, 97 (1961).
(6) K.Ryan, L. W. Sieck, and J. H. Futrell, J. Chem. Phys., 41, 111 (1964).
(7) I(.R. Ryan, private communication, (8) J. J. J. Myron and G. R. Freeman, private Communication.
VAPORPHASE RADIOLYSIS OF ETHANOL
1703
yields indicates a very high G(H.) in the ethanol system. Assuming subsequent abstraction to produce CH&HOH, it is evident that the disproportioncombination ratio for the reaction, 2CH3CHOH+products, must be very high owing to the relatively small yield of glycolic products. Although it is not possible to arrive at any quantitative G values for the free radical intermediates, we believe that the ethane produced (which is completely scavengable) results from combination reactions of methyl radicals produced in the ionic fragmentation process CH&H20H+* CHzOH+ CH3.. This decomposition accounts for almost half of the ionic cracking pattern a t lower pressures in the mass spectrometer, and it is reasonable to assign a similar contribution under the radiolysis conditions employed in the present study The work of Myron and Freeman, which was carried out a t elevated temperatures, resulted in a much higher yield of methane (G = 2.3) and a lower ethane yield (G = 0.2) when compared with our results although the scavenged values of G(CH4) = 0.28 and G(C2He) = 0.01 are in excellent agreement with our values. After subtracting the “molecular” yields from the unscavenged values and assuming that,
+
-
to a first approximation, the ethane results from methyl combination and the scavengable methane from hydrogen abstraction by methyl radicals, G(CH3.) becomes 2.4 for the work of Myron and Freeman and about 1.9 for the present work. Although such a calculation does not, of course, consider other possible intercombination reactions of methyl radicals with other radical intermediates, the values arrived a t for the two investigations are certainly in reasonable agreement and suggest that abstraction predominates a t higher temperatures while combination is favored in our work. Unfortunately, sufficient mechanistic uncertainties exist to preclude the calculation of an activation energy for the reaction CH3 CH3CH20H + CH4 products; however, a value of 8.7 kcal. has been reported.
+
-+
Acknowledgments. The authors wish to thank Messrs. D. Pritchett and M. Riggenbach for technical assistance. The authors are also grateful to Professor G. Freeman and Dr. J. J. J. Myron for access to their data prior to publication. (9) A. F. Trotman-Diokenson, J. Chem. Phys., 19, 329 (1951).
Volume 69,Number 6 M a y 1066
