W. N. PATRICK AND MILTON BURTON
424
Vol. 55
RADIATION CHEMISTRY OF MIXTURES : PROPIONALDEHYDE ,4ND BENZENE-d,1*2 BY W. N. PATRICK AND MILTON BURTON Department of Chemistry, University of Notre Dame, Notre Dam& Indiana Received Januarz! 16, 1964
In liquid propionaldehyde bombarded with 1.5 Mv. electrons 100 e.v. yields of gaseous products are Hz, 1.26; CO 1.60; CzHe, 1.!2; CH4, 0.114; C2H4, 0.34; C&, traces. In liquid benzene-d6 100 e.v. yields are C2D2, 0.014; Dz, 0.0llk. In liquid mixtures protection of CzHbCHO by C6D6 is not expected (and not found) because the lowest exc/ted state (presumably triplet) of the former lies lower than that of the latter. Yields of CH, and CzH4 are linear functions of electron fraction of propionaldehyde. Yields of HZand HD are used for a roximate calculation of relative rates of H CzHsCHO -.+ H z residue (8), H C& -c H D residue (9) rqnd, H + polymer (10): klo/ks :I> 7.3; ks/k9 N 2.7 a t room temperature. The k1o/ks data are consistent with an interpretation involving "hot" H atom reactions. The CzH6 yields are interpreted as evidence of a rearrangement decomposition of CzHbCHOsensitized by excited benzene-de.
+
+
,+
1. Introduction I n radiolysis of liquid mixtures of benzene and aliphatic hydrocarbons benzene appears to afford a protective effect, at least so far as the appearance of gaseous product is ~ o n c e r n e d . ~It has been shown that this protection is consistent with a mechanism which involves energy transfer to the benzene molecules and energy dissipation in the benzene itself." The latter process may involve a rapid degradation of energy into thermal energy in successive transfers between adjacent benzene molecules. The high efficiency of such successive "down-hill" transfers is attributed to the relatively small distance between successive vibrational states of excited benzene.b I n spite of the formally very attractive features of this picturele it has been apparent from the very beginning that other processes, perhaps entirely chemical in their nature14may be responsible for the seeming protection in these cases.' A great difficulty in consideration of the previous cases studied has been an essential inability to establish unequivocally the sources of the gaseous products observed. I n the work here reported on mixtures of propionaldehyde and benzeneda, the source of any H atoms 0 1 H2molecules must be propionaldehyde. Furthermore, it appears (as was expected on theoretical grounds) that H atom production in radiolysis of such a mixture is a linear function of the electron fraction" of propionalHD and Dzin the dehyde. Thus, the origins of Hz, gaseous mixtures may be examined and their relative yields can be subjected to reasonable interpretation. 2. Experimental The methods employed were similar to those described in previous studies of the radiation chemistry of m i ~ t u r e s . ~ J (1) A contribution from the Radiation Project operated by the University of Notre Dame and supported in part by the Atomic Energy Commission under Contract AT(l1-1)-38. (2) Abstract from a thesis presented to the Department of Chemistry of the University of Notre Dame by W.' N. Patrick in partial fulfillment of requirements for the degree of Doctor of Philosophy. (3) C/. C. S. Schoepfle and C. H. Fellows. Ind. Eng. Chem., 28, 1396 (1931); M. Burton, PTOC. Con/. Nuclear Chem., Chem. Inst. Canada, 179 (1947). (4) J. P. Manion and M. Burton, THIE JOURNAL, 66, 560 (1952). (5) S. Gordon and M. Burton, Discs. Faraday Soc., No. 12, 88 (1952). (6) Cf.J. L. Magee. THIE JOURNAL, 66, 555 (1952). (7) At thii time, one of us (M. B.) wishes to acknowledge stimulating discussions on this point, with Dr. E. J. Y. Scott, with Dr. J. C. Devins, and most recently with Dr. M. Magat at the Faraday Society Radiation Chemistry Conference at Leeds, England, in April, 1952.
+
+ 658
2.1. Chemicals.-Two liters of Merck C.P. benzene, thiophene-free, was purified in three successive recrystallizations, with rejection of approximately one-quarter a t each freezing. The residue, distilled at atmospheric pressure in a 50-theoretical-plate column, yielded a middle third with b.p. 80.0' at 74.88 cm., retained for subsequent use; d 6 D 1.4976, d2s 0.8733; Soffer and De Vries,8 n Z 51.49748, ~ dz5 0.8734 f 0.0002. Benzene so urified and subsequently kept dry over sodium wm emproyed in synthesis of benzene-da according to the method of Ingold, Raisin and Wilson9 using five successive equilibrations in 51 mole per cent. in heavy water.1° The product had a deuterium content of 98.5% corresponding to 8.4 mole per cent. C6D5Himpurity. A 500-g. sample of Eastman Kodak propionaldehyde, boiling range 47.&50.5', distilled in a 50-theoretical plate column in an atmosphere of nitrogen, yielded a middle fraction with b.p. 48.1' a t -769 mm. pressure. 2.2. Cell Filling.-Reagents were carefully dried before introduction into irradiation cells. Benzene-de was dried over barium oxide overnight and then vacuum distilled into a flask containing phosphorus pentoxide. There it was shaken and permitted to stand 15 minutes and then vacuum distilled and sealed into small storage flasks of 1.5-3 ml. Propionaldehyde collected under a nitrogen capacit atmospKere was thoroughly degassed on a vacuum line, distilled into a tube containing Drierite, and sealed off. Since propionaldehyde is oxidized on exposure to air, it cannot be transferred into cells by syringe-operated pipets, as reviously described.' &stead, it was vacuum distilled and sealed in a variety of amounts into small vessels made of 2, 4 and 6 mm. tubing, equipped with break-off seals. The level of the liquid in each tube was marked with a file, so that when emptied the exact volume of the liquid previously contained in the tube could be determined by simple mercury-weighing technique. A single such tube was sealed onto the vacuum line together with an irradiation cell. Benzene-d6 was measured out with a micropipet into a tube joined to the vacuum line by means of a ground glass joint4 and vacuum-distilled iqto the irradiation cell through a plug of phosphorus pentoxide, which was then sealed off. The benzene-de was thoroughly degassed. Then, the break-off seal on the propionaldehyde tube was broken and the pro ionaldehyde was distilled into the cell. Thus, mixtures of Enown composition were introduced into irradiation cells without atmospheric contamination. 2.3. Electron Bombardments ,-All irradiations were made a t 1.5 Mv. and currents ranging from 1.0 to 2.1 P amp. on a type A, model S HVEC Van de Graaff generator. The cells were the identical ones used by Gordon and Burton.6 The windows were ground glass, 5 mils thick and 0.8 cm. in diameter. Techniques of irradiation, current measurement and calculation of energy expenditure in the window have been described by Hentr and Burton." At the volt-
,
.
(8) H. Soffer and T. De Vries, J . Am. Chem. Soc., 78, 5817 (1951). (9) C. K. Ingold, C. G. Raisin and C. L. Wilson, J . Chem. doc., 915 (1936); A. P. Best and C. L. WiLon, ibid., 242 (1946). (10) The heavy water, furnished by the Atomic Energy Commission, had a deuterium content of 98.5%. (11) R. R. Hentz and M. Burton, J . Am, Chem. Sac., 7 8 , 532 (1951).
-
.
May, 1054
RADIATION CHEMISTRP
OF
MIXTURES : PROPIONALDEHYDE
age em loyed the correction for energy loss in the window was 3#, The accuracy of determination of the number of coulombs flowing into the cell is estimated a t &2%. During irradiation, cells were kept a t approximately room temperature by an air stream directed so that both window and walls were cooled. Data on yield are reported in the usual G units; i.e., number of molecules produced or converted per 100 e.v. of energy expended in the irradiated material. 2.4. Product Analyses.-After irradiation, the gaseous contents of each cell were divided into fractions by conventional methods: a first fraction non-condensable at -196" (hydrogen, methane, carbon monoxide) and a second depending on the liquid examined. For pure benzene-&, the second fraction included non-condensables at - 120' (ethylene, acetylene); if propionaldehyde were present, the second fraction was taken a t -95" and also included ethane and CS hydrocarbons, of the latter of which there was a small amount. The pressure of each fraction was determined in a calibrated volume on a Saunders-Taylor apparatus.12 The first fraction was collected in a bulb with sto cock closure, the second in a tube equipped with a break-offseal. Both fractions were analyzed on a Consolidated Model 21103A Mass Spectrometer.1s 2.5. Reliability of Data.-Determinations of G for CO, CHI, Hz and HD, as well as gases collected at -95", were accurat.e within 5%. Because of the large volume of gases produced in the irradiation of propionaldehyde and pf its mixtures, irradiations were only for short periods, 75 to 181 sec. I n such intervals, the amount of DZproduction is less than 0.3% of the gas collected over liquid nitrogen. Therefore, values of G(D2) in the mixtures have low accuracy. However, this fact does not decrease the significance of the other results. The mass spectrometer indicated the presence of COhydrocarbons in the -95" fraction from irradiated mixtures, by peaks 0.5-10 divisions in height in the 39 to 43 mass range. Such peak heights are to be compared with a 2,176 division height for mass 28. A reported yield of CI hydrocarbons represents the. difference between a calculated value for Cn hydrocarbons and the total gas collected at -95". Strictly speaking, it merely represents ~1 trace of CI hydrocarbons in the fraction; the values have little absolute reliability.
3. Results and Discussion 3.1. Theory of Radiolysis of Liquids.-In previous discussions of the behavior of mixtures,*J the mechanism of the decomposition has included consideration of the chemical consequences of two separate primary steps. M - e M * M-Mf+e
(1) (11)
Thus, it has been necessary to discuss excitation and ionization transfer as well as possible chemical decomp~sition'~of ionic species. Recently, however, consideration of the phenomena involved in the radiolysis of water has led Samuel and Mageels to the conclusion that in that case the back reaction HzO+ e + H2Ot takes place in a time short in comparison with sec. The excited species H20 so produced is in a higher state than the excited species produced as in reaction I and may consequently enter into characteristically different reactions. It is an important feature of the Samuel-Magee picture that their model is more likely to be correct
+
(12) K. W. Saunders and H. A. Taylor, J . Chum. Phys., 9, 616 (1941).
(13) The authora express their grateful appreciation to Prof. R. R. Williams and Mr. H. L. Weisbacker for their aooperation in these analyses. (14) Cf.M. Burton, Ann. Reu. Phye. Chem., 1, 113 (1950); J . Chem. Educ., 28, 404 (1951). (15) A. H. Samuel and J. L. Mapee, J . Chem. Phya., 91, 1080 (1953).
AND
BENZENE-&
425
if the liquid has low dielectric constant.16 Thus, for organic liquids especially, instead of reaction 11, one should write the operationally more correct equation M -+
Mi
(11')
where Mt represents a highly excited state which may differ in chemical activity from M*. Arguments against such theory have been presented particularly by P1at~rnan.l~However, it is unlikely that there will be any definitive settlement of the question for some time to come. As a matter of convenience we shall use the 1-11' (rather than the 1-11) picture. It will be seen to contribute features of simplicity to interpretation of the results. 3.2. Propionaldehyde.-Table I summarizes some results of experiments on bombardment of pure liquid propionaldehyde. It was not convenient to separate either such gaseous products as were non-volatile above -95" or possible liquid products. However, one sample of the irradiated liquid, distilled from the exposure vessel under vacuum, left an extremely small residue of white needle-like crystals inadequate for identification. TABLE I
-
GAS YIELDSIN RADIOLYSIS OF LIQUIDPROPIONALDEHYDE WITH 1.5 Mv. ELECTRONS AT 1 AMP. Energy input, 1020 e.v. 4.79 7.41 G(tota1 gas) 4.53 4.61 G(Hz) 1.24 1.28 G(CH4) 0.113 0.110 G(C0) 1.61 1.58 G(CzH4) 0.330 0.356 G(C2Hs) 1.08 1.15 G(C8's) approx. 0.052 0.051 G( Con) 0.110 0.077
The 100 e.v. yield of carbon monoxide is greater than those either of ethane or of hydrogen, which are not greatly different. It is much less than the sum of the two. Evidently, some decomposition products do not involve carbon monoxide as an ultimate product. For understanding of the radiolytic behavior of liquid propionaldehyde, we write CzHsCHO
CzHsCHO*
(1)
where, generally, C2HsCHO*refers to a whoIe spectrum of excited species which may react differently and which may proceed by internal conversion from high excited states to lower ones. Production of ethylene and methane may be by reactions of the type
+ + + +
CzHsCHO* +C ~ H I HzCO CzH&HO* +C I H ~ Hz CO CiHbCHO* +CHI CH&O
(2) (3) (41'8
Some of these reactions (specifically indicated by reference) and others not here included have been considered by Blacet and Pitts18in their study of (16) Cf. M. Burton, J. L. Magee and A. H. Samuel, {bid., 20, 760 (1952). (17) R. L. Platzman, Report of Highland Park Conference on Basic Mechanisms in Radiobiology, National Research Council, 1953, $ublication 305. p. 1. (18) F. E. Blacet and J. N. Pitts, Jr., J . Am. Chem. Soc., 74, 3382 (1952).
-
W. N. PATRICK AND 1MILTON BURTON
426
the photolysis of propionaldehyde vapor. Reaction 3 is of a type ordinarily discounted on the basis of the principle of microscopic reversibility. However, in this connection we may extend an idea employed by Blacet and Pitts in their consideration of the behavior of the free HCO radical formed from propionaldehyde. If the HzCO of reaction 2 is formed with sufficient excess energy (a very probable event in radiation chemistry) it may dissociate immediately subsequent to its formation by the reaction H&O’
+He + CO
(24
substantially as if reaction 3 had occurred in a single elementary process. If all the H2C0 were to decompose this way, the appearance would be as if reaction 2 had not occurred a t all and as if reaction 3 were the only process for production of ethylene. Evidence for just such a situation is given in section 3.5.2. 1.60
d
Vol. 58
recent results of DoddZ0on the effect of temperature in the photolysis (in addition to those of Blacet and PjttsI8) indicate that there are two simultaneous modes of decomposition of approximately the same kinetics and magnitude. Thus, reactions 4a and 4b are not ruled out. However, choice between ultimate-molecule and free-radical mechanisms for radiolytic production of methane is not an objective of this paper. Reaction 5 is offered as a feature of the radiolysis because it is very probably the precursor of H atoms via the reaction HCO+H+CO
(6) l8
I n sections 3.5.1. and 3.5.2 it is concluded that there are two sources of hydrogen production : reaction 3 and a reaction in which atomic H seems to be produced in a primary chemical act. It will be seen that ethylene is produced exclusively by reaction 3 or an equivalent sequence and that, in mixtures a t least, ethane is probably also produced by a lowenergy rearrangement process , CZHbCHO* +CzHa CO (7)18
+
0.8
0 0
1
~(ceD6). Fig. 1.-Variation of 100 e.v. yields of some gaseous products with electron fraction of C& in a mixture with propionaldehyde.
Reaction 4 may also have its free radical counterpart which can be written partially as C2H5CHO*+CH3
+ CH?CHO
(4a)Ig
followed by CHI
+ CzH&HO
--3
CHd
+ residue
(4b)Ig
Contrary to the analytical results of Garrison and Burtonlg which indicate that in photolysis of the vapor the only significant free-radical decomposition of the excited species is by the reaction CzH,CHO* +CzHs HCO (51’8
+
(19) W.M. Garrison and M . Burton, J . ChPm. P h y s . , 10, 730 (1942).
In this case the CzHsCHO*is probably in an optically unattainable state too low for free-radical decomposition.21 Decomposition of aldehyde into alkane plus carbon monoxide is almost thermoneutral. Carbon dioxide production was unexpected. It has not been reported in photolysis. The small yield here reported and also found in the mixtures, may represent either some complicated reaction, which is doubtful, or a trace of COS impurity present in the propionaldehyde originally. Mass spectrometric analysis of the original sample showed a barely discernible 44 peak. No analysis was performed on the vapor above the original sample. 3.3. Benzene-de.-Radiolysis of liquid benzened s a t room temperature gave results substantially the same as those hitherto found.5 The earlier values for benzene-& of slightly greater deuterium content were G(Hz) = 0, G(HD) = 0.0004, G(D2) = 0.0113; in this work the values were G(H2) = 0.0001, G(HD) = 0.0006, G(D2) = 0.0114. 3.4. Mixtures of Propionaldehyde and Benzene-d6.-All the results of the investigation of mixtures are shown in Figs. 1-3 inclusive. While the yields of methane and of ethylene are very nearly a linear function of the electron-fraction4 of propionaldehyde, the situation is cliff erent for the other products. Yields of ethane and of hydrogen (Fig. 1) depart regularly from a linear relationship and in opposite directions. Yield of carbon monoxide is not a simple function of propionaldehyde concentration. Attempts to obtain a material balance on the basis of gas analyses alone (which is all that was studied) proved futile. Among the several conclusions, the most obvious speculation is that non-volatile products, undetected by our methods, have been formed. On the other hand, the linear nature of the methane and ethylene yields (Fig. 2) suggests that they (20) R. E. Dodd, J. Chem. S O L , 1878 (1952). (21) For discussion of the contribution of low-lying states in electric discharge processes cf. H. Wiener and M. Burton, J. Am. Chem. ~ o c . , 75, 5815 (1953), and also J. C. Devins and M. Burton, ibid., 7 6 , 2618 (1954).
RADIATION CHEMISTRY
May, 1954
1
O F M I X T U R E S : P R O P I O N A L D E H Y D E AND
0.06
I
1
I
427
BENZENE-& I
I
0
d
0.2 0.04
6
1
0
0.02
e(CaD6). Fig. 2.-Variation of 100 e.v. yields of methane and ethylene with electron fraction of benzene-de in mixture with propionaldehyde.
truly reveal a rather simple aspect of these results, namely that the probability of electronic excitation 0 and of consequent primary chemical processes in 1 the . propionaldehyde . is proportiona1 to its electron fraction. Fig. 3.-Variation of 100 e.v. yields of DZ and HD with system is electron fraction of benzene-de in a mixture with propionalde3.5. Mechanism in Mixtures.-This too complicated for real analysis of mechanism. hyde. Nevertheless, interpretation of the HD yields in relation to Dz, Hz and polymer f o r r n a t i ~ nre~ ~ ~and ~ 10. The fate of the -C6D6H radical may be quires consideration of the following (purely for- written variously as mal) reaction scheme, its conaequences and some (12-1) of its details. (12-2)
+
C2H6CHO H residue (5) H CZH~CHO +Hz residue H CsDa +HD residue H CODE +polymer formation CeDe D, Dz, etc.
+ + +
+
+
+ (6)
(8) (9) (10) (11)
(polymer formation
(12-31 (12-4)
According to Schiff and Steaciez5 the exchange reaction of D atoms (produced in a Wood-Bonhoeffer tube) with benzene occurs with collision yield somewhat greater than the disappearance of Reaction 10 is introduced in a formal sense to H atoms in the analogous reaction between H and represent the class of reaction in which free H atom C6D6(cf. Melville and RobbZ8). We must accorddisappears without formation of gaseous product. ingly consider the possibility that HD may result There is evidence in the case of liquid b e n ~ e n e ~ not . ~ ~only from reaction 9 but also from the reaction that the disappearance is accompanied by polymer D f C2HsCH0+HD + residue (Sa) formation. It is not necessary to the argument In sections 3.5.1-3.5.3 we shall examine the extreme that the same polymer be formed in a propionaldehyde mixture but it is a reasonable presumption possibilities that (i) all H D is formed via reaction 9 that the first step in the disappearance reaction is and (ii) all HD is formed via reaction 8a. Assumption ii of the previous paragraph is equivthe same in the mixture as in pure benzene. According to Melville and Robb123reaction of H alent to acceptance of the statement that reaction atom with benzene in the vapor state has low activa- 12-2 occurs with high probability compared with retion energy. Their method ( L e , , test for survival of action 12-1. The work of Schiff and Steacie26does H atoms with a molybdenum oxide mirror) did not not inherently support such an interpretation. Exdistinguish between addition and H-atom extrac- change of D with CsH6via -C6H6D is more probable tion as the modes of reaction. However, earlier than exchange of H with C6D6via -CeDsH, for the work of Forbes and Clinez4supports the interpre- former process involves a split of a C-H bond, which tation that addition occurs as a first step. One in- has a higher zero-point energy than the C-D bond, whose split is involved in the second process. Thus, terpretation of our results is that the reaction it is altogether possible, even in view of the results H CeDa +-csDeH (12) of Schiff and Steacie, that reaction 12-2 does not ocis a common step in the path of over-all reactions 9 cur to an important extent in this work. I n such event practically exclusive occurrence of (12-l), as (22) For polymer formation in radiolysis of benzene, cf. W. N. '
+
Patrick and M. Burton, J . Am. Chem. Soc., 7 6 , 2620 (19.54). (23) H. W. Melville and J , C. Robb, Proc. R o y . Soc, (London), A202, 181 (1950). (24) G. S. Forbes and J. E. Cline, J . A m . Chrm. Roc., 65, 1713 (1941).
(25) H. I. Schi8 and E. W. R. Steacie, Can. J . Chem., 29, 1 (1961). The exchange reaction is measured in this work in t e r m of D atoms introduced into the beneene. The mechanism ia not indicated and may even involve nn cytmction rcnction (rJ. reaction (12-3)) as an interiiiediat? stet,.
428
W. N. PATRICK AND MILTON BURTON
compared with (12-2), would have the appearance of mere scattering of H atoms by CaDe. The other fates left open to the -CeDeH radicals are represented by reactions 12-3 and 12-4. Reaction 9 is thus the combination of processes 12 and 12-3 while reaction 10 includes (12) and (12-4). The fact that production of HZdeparts negatively from a linear relationship to electron-fraction of propionaldehyde is consistent with such reactions. Most of the previous discussion in this section is without bearing on this work if H atoms are not formed in radiolysis of propionaldehyde and if the important free radical reactions, similar in their consequences to (8),(9) and (lo), are
+ + +
+ +
HCO CzH6CH0----t He residue C6De+H D residue HCO HCO CBD6+polymer formation
(8b) (9b) (lob)
However, the results of Blacet and Pittsl* on photolysis of propionaldehyde vapor support the interpretation that reaction 6 is of increasing importance with increase of energy supplied to the propionaldehyde (i.e., with decreasing wave length in their case) and that reaction 8 occurs. If reaction 6 occurs, as suggested by Blacet and Pitts, it must be as a "hot" radical process before the first collision; ie., sec. in their work. They offer no evidence for reaction 8b; Le., according to their work in the gaseous state r6 > r8b, where the r's are rates. Under the more energetic conditions of radiation chemistry, it consequently appears proper t o write CZH&HO
H'
+ residue
garding the relative importance of reactions 8 and 8a. , 3 5 1 . Ratio klo/ks.-The ratio lclo/kg is calculated according to the following argument, in which r has the general significance of rate and Y is a yield. kl_o = 20= Y (polymer from (10)) 1 ko
rg
Y_ (HD from (9))
The term Y(po1ymer from (10)) is vague and is bebter equated to the H atoms which react according to (IO). As for Y(HD from (9)), we shall adopt assumption (1) in section 3.5, namely, that 12-2 does not occur to any important extent. Furthermore, the D atoms coming directly from radiolysis of benzene itself, via reaction 11, can make only a negligible contribution to H D yield in reaction 8a because G(Dz), which includes D2 formed both via free-atom and ultimate-molecule processes, in pure benzene-& totals only 0.0114. We write klo -= k9
G(H2 unaccounted for) GWD)
,
I
where G(Hzunaccounted for) is simply the decrease in H2 yield below that expected on the law of averages after provision is made for such H atoms as may have reacted to give HD. Thus h o G'(Hzlpure X 4prop) - G(HJ - G(HD) 8 GWD)
k0
where G(Hz)pure is the 100 e.v. yield of H2from pure propionaldehyde. The values for the three concentrations studied are e(CeHs) kio/ke
(13)
as an important process, which may be primary or may include reactions 5 and 6, with the H atom actually formed with excess energy. Reaction 8b would not occur in the presence of benzene if it does not occur in its absence. According to (9b) and (lob) the function of the benzene is to remove HCO before it decomposes by (6) ; Le., in the liquid state r9b and ?lob > re. However, a necessary corollary assumption would be that under such circumstances HCO could react with propionaldehyde by (8b); i.e., in the liquid state r8b > re. This conclusion is not necessarily inconsistent with the contrary one regarding the relative rates in gases. However, it may be emphasized that reaction 13 probably occurs with such rapidity that reaction 6 need not be considered a t all as an intermediate. Thus, while the possibility of reactions 8b, 9b and 10b must be considered, the probability is that they are not responsible for the effects observed. However, the possibility that they do occur in the liquid state does require experimental test. Other assumptions of reaction mechanisms of increasing complexity may be considered. I n general, they depart more and more from our knowledge of these cases. I n sections 3.5.1 and 3.5.2 it will appear that the facts are consistently and adequately interpreted without inclusion of (Sa), (8b), (9b), (lob) or more complicated mechanisms. This study is primarily concerned with the establishment of the rate of reaction 10 relative t o the rates of other easily measurable reactions of H atom. I n particular, we should like to determine the ratio klo/ke and come to some conclusions re-
Vol. 58
0.222 4.8
0.477 3.6
0.762 3.0
The calculated curve taken from the H2 and H D curves of Figs. 1 and 3, respectively, extrapolates to Iclo/kg 7.3 at ~(CSDB)= 0. A simple interpretation of this result is that the smaller values of Iclo/ks a t higher concentrations of benzene-de reflect a small effect of hot hydrogen atoms favorable to reaction 12-3 as contrasted with 12-4 and thus favorable t o the over-all reaction 9. 3.5.2. Ratio k8/lcg.-We continue for the moment with the assumption that (8a) makes no contribution to the H D yield. Furthermore, we make the reasonable assumption that the ratio r2/r13 remains constant a t all concentrations of the mixture. As written, reactions 2 and 13 imply that the propionaldehyde receives its energy in a primary excitation process. It is very unlikely that primarily excited CsDa* retains a large amount of energy a long time; rather, the evidence6 is that internal conversion and collisional deactivation cause rapid energy degradation in C6Ds*. Thus, if C6D6*, after such energy depletion, transmits energy to CzH6CH0 in a radio-sensitization process, it is improbable that the CzH5CHO* produced contains sufficient energy for decomposition by bond rupture t o give H atoms in a process analogous to (13). Consequently, it is required by the assumption of the constancy of the ratio r2/r19that the atomic hydrogen yield is a linear function of €(prop); ie., G(H2) a 4prop). For the calculation, we note that the statements of the previous paragraph entail the relationships G(Hz, r e m ) = €(prop) X G(Hz, 4
:I>
.
May, 1954
RADIATION CHEMISTRY OF MIXTURES : PROPIONALDEHYDE AND BENZENE-&
where the two G values refer t o Hz production by rearrangement in mixture and in pure propionaldehyde, respectively, and G(H2, 8) = G(H,)
- G(H2, rearr)
6
429
range. The conclusion must consequently be that a t least some of the D atoms disappear in a reaction other than D
+ CeDe+Da + residue
(94
The obvious reaction for such disappearance is (8a). Furthermore, the nature of the G(D2) data suggests that there are approximately twice as many D atoms available for reaction 9a at e(C6D6) where N(pr0p) and N ( C ~ D ~are ) respective mole = 0.477 as a t e(C6Ds) = 0.762. Thus, it would fractions. appear that reaction 12-2 makes a real, if perhaps Table I1 lists values of k 8 / h calculated on the small, contribution to the various possible fates of basis of various possible assumptions, for the three -C6D6H. On the other hand, this contribution mixtures studied. None of the assumptions gives cannot be large a t e(C6D6) = 0.222, for a t that perfect concordance for all the mixtures; indeed, value G(D2)goes precipitously almost to zero. errors of measurement introduce so much inaccuracy We conclude that, although reactions 12-2 and in the low-propionaldehyde-concentration mixtures 8a are not to be ignored, they do not enter importantly into the over-all effect. Consequently, the TABLE I1 conclusion klo/k9 > 7.3 is probably very closely corTESTSOF POSSIBLE MECHANISM OF HYDROGEN PRODUCTION 2.7 is probably rect. Similarly, the value kg//ce AS REFLECTED I N CALCULATED RATIOOF also rather reliable. . RATECONSTANTS ks/ks The best justification for the omission of considAssumption : G(Hn, rearr.) = e prop) X G(CzHdobsa eration of (12-2) and (sa) from the calculations of sections 3.5.1 and 3.5.2 is that the values thus oh0 0.3 0.34 0.4 4CsDs) N(CODO) ka/k~-----tained are nevertheless internally consistent. 0.222 0.186 4.3 2 . 8 2 . 8 2 . 5 2.7 3.6. Protection and Radiosensitization.-The .477 .410 5.0 4.3 2 . 8 2.4 3.1 results signify that benzene-de exerts no protective .762 .708 6 . 9 3 . 0 2.4 1.7 2.2 effect in radiolysis of propionaldehyde. I n order a G ( C ~ H 4 ) ois~the a yield actually observed (see Fig. 2). to understand the mechanism of protection4J4 Because of experimental error of analysis, which becomes we can fasten our attention on the excited molecule important a t small yields, it is not exactly the same as the (cf. section 3.1). The substance B can effectively calculated value e(prop) X G ( C I H ~ ) ~ ~ ~ ~ . protect A that perfect concordance is not to be expected. A* + B +A + B* (14) However, the assumption that the frequency of occurrence of the primary process in propionaldehyde only when EB < EA,where the references are to is a linear function of its electron fraction and that respective lowest excitation potentials of the two G(Hz, rearr)pureis -0.34 gives a reasonably small species. These are probably not optically observspread of values. Thus, k10/k9 :I> 7.3, and ks/kg = able excited states. Benzene is known to have a ' triplet 2.7 f 0.1 (at room temperature). Correspondingly, low-lying triplet state at -3.6 e . ~ . ~The states of propionaldehyde have not been described Ics/(kg kio)
