RADIATION CHEMISTRY OF MIXTURES : CYCLOHEXANE A N D BENZENE-&
May, 19*54
421
s by the first solvent AI as A v ~ and A ~ the Lewis at- benzene (whose van der Waals attraction for traction as ALBA,and similarly for the second sol- anthracene is presumably quite high) to a larger extent for Al than for A2 to produce the maximum vent A2, we can define the affinity ratio
+ +
solubility, This is the explanation of why the R = AvZA~ AAVZA~ (1) maximum occurs a t a higher proportion of benzene LSA~ ALBA, for iodoethane than for iodobenzene. so that R must have a definite value for any solute The other observations on cosolvency recorded which when reached for any mixture of A1 and A1 in this paper are in general accord with the ideas dewould produce the maximum solubility under the veloped above. Thus, the cosolvent power deconditions and the more the actual R departs from creases in the order: benzene > cyclohexene > cythe ideal value (either higher or lower) the further clohexane, is due to the fact as is well known from the mixture will be from maximum solubilip,at’.ion. the color of iodine solutions that they form comFor iodobenzene, van der Waals attraction A V ~ isA plexes with iodine in the above order, and that the weaker in comparison with Lewis attraction A L ~ A whereas for benzene it is just the other way round van der Waals attraction for anthracene is exand so, a suitable mixture makes R attain the opti- pected t o be in the above order. But the point mum value for maximum solubility. It is realized we want to make is that though the &values may that this rough formulation is not much helpful ex- have their place in understanding the phenomenon cept for qualitative understanding of cosolvency of solubility in pure solvents, their applicability but has the merit of making it possible to express to mixtures will be on quite a limited field only, because in most cases solvation phenomena arising our idens briefly. It is clear from equation 1 that if for two solvents out of donor-acceptor interaction between solute and solvent would introduce complications not AI and A2, Al has a higher value for R1 = Av& ALBA,,i.e., R1 > R2 we have to add the cosolvent envisaged in the regular solution theory.
RADIATION CHEMISTRY OF MIXTURES : CYCLOHEXANE AND BENZENE-d6112 BY MILTONBURTON AND W. N. PATRICK Department of Chemistry, University of Notre Dame, Notre Dame, Indiana Received January 16, 1964
Radio1 sis of a mixture of cyclohexane and benzene& by 1.5 Mv. electrons ields among other gaseous products Hz, The yield of HD is used to estimate the number of H atoms whicz disappear by a reaction associated with formation of benzene-& polymer. It is shown that the results are consistent with an interpretation that benzene-& actually protects excited cyclohexane from decomposition, the mechanism of protection being energy transfer. Some slight decomposition of cyclohexane by a rearrangement mechanism to yield Hzin an elementary process appears to be sensitized by ben2ene-d~which has been excited t o a low-lying state.
HD and $2.
Introduction G(H2) a(A)‘G(H2p 1 + bNB A) + s(B).G(Hz, B) 1 I n radiolysis of mixtures of benzene and aliphatic hydrocarbons under electron bombardment, de- where G(H2) is 100 e.v. yield of Ha in a soh. of an composition crease of gas yield below that indicated by a law of G(Hz,A) and G(H2, B) are 100 e.v. yields oPHI in pure A averages has been attributed to a protective effect3 and pure B, resp. of the “sponge” type4 by the benzene. Since only E (A) and E(B)are the corresponding electron fractions gases were measured there is a possibility, considNE is the mole fraction of B, and b is a constant resultant from the scheme ered also by Manion and Burtonla that what may occur in the mixture is a preferential reaction of a A -v*-f A* (1) free atom or radical with the benzene: The funcB -*B* (21 tional relationship which expresses the yield G(H2) A* +H2 ultimately (3) A* + B +reactions which do not produce Hn (4) in a mixture in terms of electron fractions and mole fractions of the components may be transcribed B* +H2 ultimately (5) from the form of Manion and Burton into the For this scheme b = h / k 3 and reaction 4 should be equally correct but more general expression (over-all) one order higher in B than reaction 3. Reaction 4 includes the possibility that H atoms (1) A contribution from the Radiation Project operated by the University of Notre Dame and supported in part by the Atomic produced, for example, from the reaction 1.
+ residue
Energy Commission under Contract AT (11-1)-38. (2) Abstracted from a thesis presented t o 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) J. P. Manion and M. Burton, THIS JOURNAL,56, 560
react according to
(1952). (4) M. Burton, 8. Gordon and R. R. Hentz, J . ohim. phys., 48, 190 (1951). (5) One of us (M. B.) is partioularly indebted t o Doctors E. J. Y. Scott, J. C. Devins and M. Magat for discussions of this point.
as well as the possibility of the simple energy transfer reaction
A* +H
H
+ B -+
(6)
disappearance of H (e.g., to form polymer) (7) A*
+ B +A + B*
(8)
MILTON BURTON AND W. K. PATRICK
422
I
I
\
Vol. 58
I
4
HD
_-
6 2
-De
0
I
I
1 e(C6De). Fig. 1.-Variation of 100 e.v. yields of H D and Dz with electron fraction of benzene-& in a mixture with cyclohexane. 0
0
0
1
a(C6Ds). Fig. 2.-Variation of 100 e.v. yield of hydrogen with For reaction 8 to occur it is required according to electron fraction of benzene-dsin mixtures with cyclohexane: the simple excited-molecule theory of liquids3s6-7 0,. Hz yield actually observed; 0 , calculated maximum that EA > E B where the reference is to the lowest primary yield of hydrogen.
excitation levels (not necessarily optically attainable) of A and B, respectively. Although Manion and Burton6 established by logical argument that reaction 8 was in reality responsible for the apparent protection of aliphatic hydrocarbons by benzene, there has been no direct experimental verification of the conclusion. Recently, however, studies of the radiolysis of a liquid mixture of propionaldehyde and benzene-de have indicated that for the reactions H
+ CeD6
----f
HD
+ residue
H 3. CsDa + disappearance of H (e.g., to form polymer)
(9) (10)
the ratio klo/ks :I> 7 . X 8 In this paper the value of kl&9 is employed in a study of the 1.5 Mv. electron-induced radiolysis of a mixture of cyclohexane and benzene-deto estimate the portion of the general Hz disappearance process 4 which is in reality a contribution of reaction 7 (the equivalent of 10) to reaction 4 and, thus, further to demonstrate the reality of protection in radiolysis of liquid mixtures. Such a test was not possible in the previous work,3 where hydrogen contribution from the benzene was unlabeled. 2.
Experimental
Chemicals.-Benzene-& employed in this work was a portion of the same sample used in the propionaldehyde study.* I t had a deuterium content of 98.5Oj, correspondin t o 8.4 mole per cent. of C6DLHimpurity. A 508-g. sample of Fisher Scientific Co. cyclohexane, C.P., was distilled in a 50-theoretical-plate column and the middle one-third boiling a t 80.4" at a,,pressure of 74.54 cm. D was retained; d% 1.4231; "T.C.T., T L ~ ~1.4266. 2.2. Cells and Cell Filling.-The irradiation cells were the same as those used in previous work.* The windows were ground glass, 5 mils thick and 0.9 cm. in diameter. Reagents were well dried before introduction into the cells. Cyclohexane was stored over sodium wire. Ben2.1.
( 6 ) M. Burton, J. L. Magee and A. W. Samuel, J . Chem. P h y s . , 20, 760 (1952). ( 7 ) A. H. Samuel and J. L. Magee, ibid., a l , 1080 (1953). (8) W. N. Patrick and M. Burton, THISJ o U R N A r . , 68, 424 (1954).
zene-de was treated as described .for the propionaldehyde mixture^.^ Individual components of the mixt8ures were introduced into the cells by syringe-operated pipets. Micropipets were used for the accurate measurements required with mixtures. A check, made by filling the cells under vacuum and distilling the contents through a plug of P205 to remove all water, showed no noticeable change in the results upon irradiation. 2.3. Electron Bombardments.-Electron bombardments were made with 1.5 Mv. electrons from an HVEC Van de Graaff generator precisely as described for the work with propionaldehyde mixtures.8 Corrections and errors were as previously described. 2.4. Product Analyses.-Analyses of the product gases were also as previously described except that the separation was into gases non-condensable a t -196' and -120" (the latter instead of -95"). The former consisted of hydrogen and methane, the latter of ethane, ethylene and acetylene. Analysis was by mass ~pectrometry.~ 2.5. Reliability of Data.-For reasons already given,8 determinations of G(H2) and G(HD) were accurate to better than 5% while measurements of G(D2) had much lower accuracy. The hydrocarbons including ethane, ethylene and acetylene were detepmined with accuracy less than 5 % and, in some cases, with accuracy, no better then the Dz values. Individual values for the hydrocarbons are not reported since they are not germane to the purposes of the present work. Results were consistent with the findings of Manion and Burton.3
3. Results and Discussion Results on Hz, H D and Dz are summarized in Figs. 1 and 2. Figure 1 shows that as the concentration of CeD6 is increased the 100 e.v. yield of HD first increases and then falls. Free H atoms result from the reaction C-CeHlz* -3 H
+ residue
(11)
the contribution of which decreases with increasing electron fraction of CeD6. Yield of H D comes essentially from reaction 9. Since G(Dz) from pure C6D6is only 0.0113, lo contribution of a reaction of D atoms on cyclohexane can be neglected. Consump(9) The authors are indebted to Prof. R. R. Williams and Mr. H. S. Weisbeoker for their cooperation in these analyses. (10) S. Gordon and M. Biirton, Discs. Faradav SOC.,No. 12. 88 (1952).
.
May, 1954
R4DI4TION
CHEMISTRY
OF
MIXTURES : CYCLOHEXANE
tion of H atoms in reaction 10 has been shown by previous work not to exceed 7.3 times HD production. Thus, if 8.3 X G(HD) is added to G(H2) as in Fig. 2, we obtain a curve which would appear to represent the maximum possible 100 e.v. yield of primary reaction giving either ‘Hz or H atom. Obviously, this curve departs markedly from a law of averages result (Le., a straight line) in a manner clearly explicable on the basis of a sponge-type protection mechanism4 in which benzene-& protects cyclohexane by an energy transfer process such as reaction 8. The equivalent in this case of the general reaction 3 is the over-all reaction c-CeH12 +HZultimately
(12)
A portion of the Hz yield so shown may be produced via the reaction H
+ c-CaHlz +HZ$. residue
(13)
The remainder of the Hz may be formed by a rearrangement mechanism in a single elementary process
AND
BENZENE-&
423
There is a suggestion in the regular increase of p with be1~ene-d~ concentration that at such higher concentrations the yield of H2 is favored by a reaction not involving H atoms for it would be unlikely that reaction 13 is catalyzed by C&. The simplest alternative explanation is that reaction 14 becomes increasingly important a t high C ~ DconcenB trations, This unexpected, but not astonishing, conclusion may be understood in terms of energy transfer from C6D6to c-C~HIZ. According to the views heretofore expressed on the resistance of benzene to high-energy radiation, stabilization occurs by a series of internal-conversion and collisional-deactivation processes until the benzene is finally a t an energy level too low for decomposition. While this level can be a t 4.8 e.v., it appears likely that excited benzene will degrade to an even lower state, e.g., at 3.6 e.v.ll Excited C6D6*in such a low triplet state is very persistent. It may transfer its energy to cyclohexane CeDs*
C-CaHiz +CsD6
+ c-CeHiz*
(15)
and produce a (triplet) state which possesses insufficient energy for a free-radical split but may dec-C6H12* +Hs + residue (14) compose by rearrangement into Hz plus a triplet The total G(Hz) may be represented as G(Hz, 12), residue, as by reaction 14. This radiosensitization Le., G(H2) = G(H1, 12) = G(H2, 13) G(H2, 14). reaction is similar to one offered in explanation of If the ratio G(H2, 13)/G(Hz, 14) is constant one the variation of G(CzHs)in propionaldehyde-benwould expect that, with N ( C - C ~ Hand ~ ~N ) ( C ~ Das ~ ) zene-d6mixtures.8 The linear relationship between G(D2) and the mole fractions, the ratio DB) in Fig. 1 would tempt speculation as to the mechanism of DZformation. This is quite unlike 2 the results of Gordon and Burton on mixtures of would be a constant since it seems to have the form benzene and ben2ene-d6.l0 In the latter case, unof a ratio of rate constants. The values of p for like the present work, the absolute reliability of D2 various mole fractions of C6D6 are shown in Table determinations mas of the same order as that of HD and Hz determinations. In spite of the clearly 11. shown straight line we hesitate to draw conclusions TABLEI1 regarding the mechanism of Dz formation in radiolVALUES OF p (DEFINEDAS IN EQUATION 2 AS A FUNCTION ysis of benzene-&: if the reality of the straight line O F MOLE FRACTION O F BENZENE-de I N MIXTURES WITH were accepted it would be consistent with an exclusive rearrangement mechanism for production of CYCLOHEXANE) D2 but inconsistent with other work.lo N(CaD6) 0.051 0.234 0.492 0.711 0.978
+
P
2.6
4.1
5.0
6.3
39
(11) Cf.H.Shull, J . Chem. PhUs., 17, 295 (1949).
