H. F. BARZYNSKI, R. R. HENTZ,AND AI. BURTON
2034
Radiolysis of Benzene and Benzene-Cyclohexane Mixtures in the Presence of Nickel Tetracarbonyl
by Helmut F. Barzynski, Robert R. Hentz, and Milton Burton Department of Chemistry and the Radiation Laboratory,l University of Notre Dame, Notre Dame, Indiana (Received January 9, 1965)
46666
G values of hydrogen and biphenyl formation in y-irradiated mixtures of benzene and nickel tetracarbonyl show that some excited benzene species are protected against decomposition by nickel tetracarbonyl. It is argued that two kinds of primary process contribute to the hydrogen yield: (1) a rapid unimolecular elimination of molecular hydrogen that is not quenched by carbonyl; (2) a process giving a second-order hydrogen yield that is associated with biphenyl formation and is quenched by carbonyl. Yields were determined for the major radiolysis products from benzene-cyclohexane mixtures containing nickel tetracarbonyl at fixed electron fraction ( E 0.68) a t different electron fractions of each hydrocarbon in the range e 0 to 0.32. At t (nickel tetracarbonyl) 0.68 most of the cyclohexane decomposition is inhibited; the residual decomposition of the cyclohexane is to a major extent by rearrangement to ultimate molecules and to a minor extent by rupture into free radicals and atoms. These residual decomposition processes are not appreciably quenched by further addition of either protective agent. The suppression of products by benzene, in this case, represents actual scavenging of free radicals and atoms.
Introduction Nickel tetracarbonyl seems to possess remarkable resistance to decomposition by ultraviolet light2 and y-radiation. Flash photolysis studies indicate that this ,appearance of resistivity is the result of a rapid back reaction between the primary decomposition products, nickel tricarbonyl and carbon monoxide. The fact that nickel tetracarbonyl has an electronimpact ionization potential of 8.64 e . ~ .below , ~ that of either benzene or cyclohexane, suggests that it can act as a chemically inert energy sink and thus, by its presence, protect other substances from radiolysis. A study of the radiolysis of mixtures of cyclohexane and nickel tetracarbonyl has indeed shown that the latter protects cyclohexane very effectively.6 The possibility that it might likewise truly protect a protective agent like benzene is intriguing. Accordingly, this paper reports a limited study of the radiolysis process in solutions of nickel tetracarbonyl containing benzene and benzene plus cyclohexane. The J o u r d of Physical Chemistry
Experimental Phillips Spectrograde cyclohexane was purified by successive steps of distillation through a spinningband column and by preparative vapor-phase chromatography with a silicone grease column (Aerograph Autoprep Model 700). Benzene was purified by crystallization. K and K nickel tetracarbonyl was used without further purification. Five-milliliter samples, in glass ampoules fitted with breakseals, were degassed by repeated freeze-thaw cycles on a vacuum line prior to seal-off and then (1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the Atomic Energy Commission. This is A.E.C. document no. COO-38-368. (2) A. P. Garrstt and H. W. Thompson, J. Chem. SOC.,1817 (1934). (3) H. Barzynski and D. Hummel, 2. physik. Chem. (Frankfurt), 38, 103 (1963). (4) A. B. Callear, Proc. Roy. SOC.(London), A265, 71 (1961). (5) R. E.Winters and R. W. Kiser, Inorg. Chem., 3 , 699 (1964). (6) H. Barzynski and D. Hummel, 2.physik. Chec. (Frankfurt), 39, 148 (1963).
RADIOLYSIS OF BENZENE AND BENZENE-CYCLOHEXANE MIXTURES
irradiated in a 10-kc. ‘j0Co source at a dose rate of -2 X 10’8 e.v. g.-l inin.-’ depending on the electron density of the solution. For solutions of nickel tetracarbonyl in benzene doses up to 3.6 X lozoe.v. g.-l were used, and for solutions in benzene plus cyclohexane doses up to -2 X loz1e.v. g.-l were used. Dosage was established by Fricke dosimetry using G(Fef3) = 15.6 and a correction factor equal to the electron density of the particular solution relative to that of the dosimeter. A small amount of carbon monoxide is formed in unirradiated samples as the result of thermal decomposition; it was found that such an amount was without effect on the radiolysis of the pure hydrocarbons. The amount of carbon monoxide in irradiated samples was not measurably different from that in unirradiated samples and was independent of dose. Mass spectrometric analysis of gas noncondensable a t -196” gave the hydrogen yields. Liquid products were determined by liquid-vapor chromatography. N o products related to nickel tetracarbonyl could be detected.
2035
B’ -+ uHZ
B’ B”
+D
--f
+ br#~Z B +D
(5)
(6)
B” +c H ~
(7)
+ D -+ B + D
(8)
Reaction 0 represents energy deposition in the system
S to give some unspecified state (or compendium of states) S*. Reaction 1 represents all internal conversion processes of S*which do not yield product. Reaction 2 represents quenching of the excited system (i.e., unproductive dissipation of the energy of the system) I
’
,
I
06
0.04
Results Figure 1 shows plots of G values (100-e.v. yields) of hydrogen and biphenyl as a function of electron fraction,’ e, of Ni(CO)* dissolved in benzene. Chromatographic peaks corresponding to hydrogenated biphenyls were observed, but positive identifications were not made. Ni(CO), reduced the hydrogenated biphenyl areas much more than the biphenyl areas. At e[Ni(C0)4] < -0.20, irradiation caused the samples to become yellow. Addition of irradiated benzene to unirradiated Ni(C0)4 did not yield a yellow color. Figures 2 and 3 show G values for the major radiolysis products in solutions of Ni(CO), in benzene plus cyclohexane. In this portion of the work the Ni(C0)h electron fraction was k e d at -0.68; the electron fraction of each hydrocarbon was varied from zero to -0.32. The abscissas show the electron fraction of benzene relative to that of total hydrocarbon in the solution.
Discussion
0.02
0
0.2
0
s --+- s* s*+s S* + D - + S
+D*+S+D
(0) (1)
(2)
S*+B+S+B’
(3)
S* + B -+ S + B”
(4)
0.6
1 .o
0.8
(nickel tetracarbonyl).
Figure 1. Hydrogen and biphenyl yields in mixtures of benzene and nickel tetracarbonyl. I
0
Nickel Tetracarbonyl and Benzene. With the objective of generality, we present the equations
0.4 6
0.2 e
0.4
0.6
I
1 .o
0.8
(benzene) ,elative t o total hydrocarbo
Figure 2. Hydrogen and cyclohexene yields in mixtures of benzene, cyclohexane, and nickel tetracarbonyl with carbonyl electron fraction equal to 0.68 in all mixtures. ~~
~~
(7) No special significance is attached to the use of electron fraction as a measure of relative concentration. It is selected entirely as a convenience because so much other work has been reported on this basis. Certainly, in this case, its use appears without special t h e e retical significance. Cf.A. G. Maddock, Discussions Faraday SOC., 12, 118 (1952); M. Burton, ibid., 36, 7 (1963).
Volume 69, Number 6 June 1966
H. F. BARZYNSKI, R. R. HENTZ,AND M. BURTON
2036
The effect of suppression of such reactions by Ni(CO)* would be corresponding reductions of hydrogen and biphenyl yields. The actual amounts of reduction would depend on the competing processes by which the free radicals and atoms might otherwise disappear. Further, it seems reasonable that GI would correspond to a process not associated with biphenyl formation (e.g., CsHs* + C$-14 Hz). A logical inference is that f( [D]) $ fe([DJ) and that to some degree, after localization of energy on benzene, nickel carbonyl suppresses a second-order hydrogen yield associated with biphenyl formation. Thus, excited benzene may decompose in two processes to yield both HZand (Pzin one reaction (5) or just H2 in the other (7). The processes 6 and 8 are quenching ( i e . , protective) processes; it is assumed consistently with the small effect of Ni(CO)4 on Hz yield that the rate r8 = 0. In benzene, neither radiosensitized decomposition of metal perphenyls'O nor radiosensitized luminescence of scintillatordl affects product yields from the benzene itself. Such results tend to confirm the notion that decomposition in irradiated benzene occurs very rapidly either before neutralization of the parent ion or from a highly excited molecule. Thus, interference with such decomposition processes requires the presence of a sufficiently high concentration of additive either to compete with localization of energy on benzene and thus to prevent formation of the labile species or to compete by energy transfer with the rapid decomposition of the labile species after localization. The states involved in decomposition of benzene are clearly not those involved in luminescence studies; for the latter, a long lifetime exceeding 15 nsec. has been measured.1°-12 Appearance of a yellow color at low concentrations of carbonyl suggests some reaction of carbonyl with a benzene species, formation of which is suppressed at higher carbonyl concentrations. Nickel Tetracarbonyl and Benzene Plus Cyclohexane. In cyclohexane at 0.01 electron fraction of carbonyl, the value of G(C6Hl0)/G(C1zH22) remains essentially the same as in pure cyclohexane, 1.7, but the G values are reduced by 750j0.6 Such a result suggests a com-
+
0 E
0.2 0.4 0.6 0.8 (benzene) plative t o total hydrocarbon.
1.o
Figure 3. Bicyclohexyl, biphenyl, and phenylcyclohexane yields in mixtures of benzene, cyclohexane, and nickel tetracarbonyl with carbonyl eIectron fraction equal to 0.68 in a11 mixtures.
by localization of energy on molecules of the protective agent D, in this case Ni(C0)k. Reactions 3 and 4 are processes in which energy of the excited system is localized on molecules of B ( i e . , benzene) to yield two excited states of the latter. The fraction of deposited energy localized on B is determined by some unspecified function of Ni(C0)4 concentration, fe([D]). Figure 1 shows that biphenyl yields are reduced by added carbonyl to a considerably greater extent than are hydrogen yields. Consider that some function of Ni(CO), concentration, f( [D]), other than electron fraction, exists and may be chosen as abscissa in Figure 1 so that each point on the hydrogen curve would be translated horizontally to the dashed hydrogen line. Then each Corresponding point on the biphenyl curve would be translated an equal number of units along the abscissa. Cursory examination of Figure 1 shows that the biphenyl curve would still fall well below the dashed biphenyl line. Thus, even if f( [D]) were identical with fe( [D the conclusion would be that there must be a process involving localization of excitation in benzene molecules which thereupon yield biphenyl and that such excited benzene niolecules are quenched by Ni(C0)4. This reaction (6) is not to be confused with reaction 2. According t o Dyne and Jenkin~on,~ G(H2) = 0.038 from benzene can be separated into a first-order GI = 0.02 and a second-order Gz = 0.018. I t seems likely that any second-order process which gives hydrogen would be associated with biphenyl formation, for example CB&+ $. C6Hs
----f
(CeH5)2+
2C6H6* + (C6H5)2 CsH,* +C6H5 The Journal of Physical Chemistry
+ Hz
+ HZ
+H
(8) Note that f([D]) would then correspond to a relative cross section for localization of deposited energy on benzene less than that corresponding to the use of electron fraction; use of electron fraction corresponds to a relative cross section that is onehalf of that corresponding to the use of mole fraction. (9) P. J. Dyne and W. *M. Jenkinson, Can. J . Chem., 40, 1746 (1962). (10) D. B. Peterson, T. Arakawa, D. A. G. Wdmsley, and M. Burton, to be published. For sensitized decomposition of metal perphenyls, G = 1 at concentrations of -1 mole %. (11) M.Burton and W. N. Patrick, J . C h a . Phys., 22, 1150 (1954); C.Reid, ibid., 22, 1947 (1954). (12) Recent results of M. A. Dillon from this laboratory.
RADIOLYSIS OF BENZENE AND BENZENE-CYCLOHEXANE MIXTURES
mon precursor for most of these products.13 However, a t 0.68 electron fraction of Ni(C0)4, G(CBHIO)/ G(C12H22) becomes 6.4 (cf. Figures 2 and 3). It appears that processes of energy deposition and localization, as well as the nature of the species involved in decomposition and energy-transfer processes, may be quite different a t very high carbonyl concentrations. Figures 2 and 3 show that reduction of bicyclohexyl yield relative to that of cyclohexene is accentuated by the addition of benzene. Such results suggest the existence, a t 0.68 electron fraction of Ni(C0)4 in cyclohexane, of two residual cyclohexane decomposition processes not readily quenched by further addition of any protective agent
+ H2 CBH~I +H
CaHi2 ---+ CsHio C6H12 --“+
The “molecular” reaction is apparently predominant.
2037
The suppression of all yields and formation of C6HIlC6H5 is then understandable in terms of scavenging of H and CaHll by benzene; the small reduction of cyclohexene yield is consistent with a disproportionation to combination ratio of 1.3-1.5.14 The curves of dimer yields are similar to those in benzene-cyclohexane mixtures without additive.15 However, the maximum G(CBH~IC~H~) occurs a t 0.25 electron fraction of benzene in excess carbonyl as compared to 0.12 in the system with no additive.15 This result is consistent with considerably greater excitation localization on benzene in the mixtures free of carbonyl as compared to mixtures in which carbonyl completely quenches states of cyclohexane responsible for most of the product yields. (13) 8. Z. Toma and W. H. Hamill, J . Am. Chem. SOC.,86, 1478 (1964). (14) J. W.Falconer and M. Burton, J . Phys. Chem., 67, 1743 (1963); C.E.Klots and R. H. Johnsen, Can. J . Chem., 41,2702 (1963). (15) T.Gaeumann, Helv. Chim. Acta, 44, 1337 (1961).
Volume 69, Number 6 June 1966
