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Radiolysis of Cyclohexene.
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R.WAKEFORD A K D G. It. E‘KEEMAN
Effects of Additives]
by B. R. Wakeford and G. R. Freeman Departmerd of Chemistry, University of Alberta, Edmonton, Alberta
(Received M a y 18, 19643
The addition of benzene and 1,3-cyclohexadiene inhibited the formation of all of the inajor products of the cyclohexene radiolytic system, the latter additive having the inore inarlted effects. The addition of 1,4-cyclohexadiene enhanced the cyclohexane yield slightly, but inhibited the formation of the remaining “cyclohexene” products. Of the three additives, 1,3-cyclohexadienc gave a relatively iiiuch greater yield of polymer and had the highest rate of disappearance. Benzene apparently gave “sponge type” protection to cyclohexene. Activation transfer froin cyclohexene to 1,3-cyclohexadiene was clearly denionstrated by the formation of dicyclohexadienyl in good yield (G = 6) a t low diene concentrations (about 2 mole yG). This diene probably also acted as a radical scavenger. The effects of 1,4-cyclohexadiene appeared to be best explained by a combination of activation transfer and free radical reactions.
Introduction I n the first paper of this series, the radiolysis of pure cyclohexeiic was discussed.2 This paper is concerned with the effects, on the liquid cyclohexerie radiolytic system, of the addition of each of benzene (R), 1,3cyclohexadiene ( 4 3 - 4 , and 1,4-~yclohexadiene(1,4-1)).
Experimental
the extent to which the formation of each product is inhibited or sensitized. The basis for this comparison is taken as the “expected yield” of a product P, G,,(P), which is defined by G,,(P)
=
G,O(P)ec
+ G,O(I’)e,
where G,O(P) and G,O(P) are the yields of the product P from pure cyclohexene and pure additive, respectively, and E~ and E, are their respective electron fractions in the solution. The dashed lines in the figures indicate
The techniques described previously2 apply also to this work. G,., . The saniples were irradiated a t 22 f 2’ a t a dose Hydrogen. The yields of hydrogen (G(H2)) from rate of 8 X 10lx e.v./g. hr. The dose given iiiost of the pure compounds at a dose of 1.4 X lozo e.v./g. the saiiiples was 1.5 X lo2”e.v./g. were: cyclohexene, 1.28; benzene, 0.038; 1,3-cyclo1,3-u (Columbia Organic Cheiiiicals and Aldrich hexadienc, 0.24 ; and 1,4-cyclohexadiene, 1.19. The Chemical Co.) and 1 , 4 - ~(Aldrich Cheiiiical Co.) yield of hydrogen froin cyclohexene is independent of were purified by vacuuni distillatioil and vapor phase dose (the region 2.5 X 1 O l X to 2.2 X lozoe.v./g. was chroniatography (v.P.c.). Iiosearch grade benzeiie studied), so the yield from each of the other coinpounds (Phillips I’etroleuiii) was used as supplied. listed can also be assumed to be independent of dosc A diiiier of 1,3 D (tricyclo [4,4,27~Ln,0]dodeca-2,8in this region. diene) was prepared by heating 1 , 3 - a~t 200’ for 20 hr. in Z ~ U C U O with ~ ~ , ~ about 0.05 niole yGof added 2,2diphenyl-1-picrylhydrazyl to inhibit frcc: radical proc(1) The authors are griLteful to the Natiotid 1tese:irch Council of Cnnndn for partinl support of this work. B. R. W. is indebted esses. The product was purified by v.p.c. The niass to Consolidated Mining and Smelting Co. for the :Lw.iird of ti Cominco arid n.1n.r. spectra of this iiiaterial were consistent’with Fellowship in 1960. the cxpccted ~ t r u c t u r e . ~ (2) B. R. Wakeford and G . R. Freeman, J . I’hys. Chem., 68, 2635
Results It is coriveiiicnt to compare the effccts of different additives on thc cyclohexenc product yields in ternis of The Journal o j Physical Chemistry
(1964). (3) (a) K . Alder and G. Stein, Ann. Chem., 496, 197 (1932); (b) B. A. Kaznnskii and 1’. F. Svirskaya, J . Gen. Chem. liSSIZ, 29, 2550 (1960). (4) R . R . W:ikeford, 1’h.D. Thesis, University of Alberta, 1964.
RADIOLYSIS OF CYCLOHEXENE
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The hydrogen yields froni the binary solutions of 1,31 , 4 - ~ ,and B in cyclohexene are given in Fig. 1. All of the additives appeared to give some inhibition of the hydrogen yield, the greatest effect being observed for 1 , 3 - ~ . Cyclohexane. I3 inhibited the formation of cyclohexane to about the saine degree as it had inhibited hydrogen forination (l’ig. 1 and 2). 1 , 4 - ~caused a slight sensitization of cyclohexane formation. With added 1 , 3 - ~ the , yield of cyclohexane fell sharply to G < 0.2 as ~ 1 , 3 - was ~ increased to 0.1. The residual yield of cyclohexane was inhibited only slightly or not at all by 1 , 3 - (Fig. ~ 2). “Cyclohexene Type” Dimers.2 Each of the three additives inhibited all of the “cyclohexene type” dimers, namely, 2,2’-dicyclohexenyl (Fig. 3), 3-cyclohexylcyclohexene (Fig. 4), dicyclohexyl (Fig. 5 ) , and the unidentified dimer, designated D-1 (Fig. 6). The most effective inhibitor was 1,3-1)in each case. “New” Dimers. The yields of new dimers formed in the solutions of B in cyclohexene mere negligible (G< 0.1). A dinier of 1 , 3 - ~(dicyclohexadiene) was formed in good yield even at small coricentrations of 1 , 3 - ~ D,
14 I
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0 6 4 2
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Figure 1. Hydrogen yields: 0 , pure cyclohexene; 0, 1 , 4 - ~solutions; F) B solutions; 0,1,3-u solutions.
0
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Ea ~ 1 0 Figure 2. Cyclohexane yields: 0 , pure cyclohexene; 0, 1 , 4 - ~solutions; 0,B solutions; 0,1 , 3 - ~solutions.
10
“0
2
4 €a x
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Figure 3. 2,2’-1~icyclohexenyl yields: 0 , pure cyclohexene; 0, 1,4-u solutions; 0,B solutions; 0,1,3-u solutions.
Volume 68,A’umber IO
Octoher. 1.9fi4
B. R.WAKEFOKU A N D G . 13. L;REEMAX
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0 15
2 0
0 7 X
1
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n 0
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Ea x 10 Figure 4. 3-Cyclohexylcyclohexene yields: 0 , pure cyclohexene; 0, 1,4-11 solutions; 0,B solutions; 0,1,3-0 sulutions.
(Fig. 7‘4). This product mas separated from all of the “cyclohexene type” dimers on two v.p.c. columns and had the same reteiition time as tlie authentic dicyclohcxadierie (z.e., tricyclo [1,4,2710,0]dodeca-2,8dime) on both coluiiins. Irradiation of a 6 X niole fraction solution of 1 , 3 - in ~ cyclohexane also gave rise to dicyclohexadiene. The mass spectra of the dicyclohexadierie formed in the irradated cyclohexane solution arid of the authentic dicyclohexadielie were siniilar, but the discrepancies indicated that soirie different isonieric forms of the dimer were probably present in the two sa~iiples.~The yield of dicyclohe>xadicric from the cyclohexene solutions increased rapidly with increasing 1 , 3 - concentration ~ up to G = G 3 a t el j.l) = 0.05. =\s the 1,3-1) concentration was further increased, the yield of dicyelohexadiene decreased. I’igure 713 gives the yields of dinieric products (excluding dicyclohexadiene) froni the 1-3-11 solutions. The initial decrease with increasing el ].D is due to the rapid dccrcase of the “cyclohcxene type” dimers. T h c yield begins to increase a t about €1 3-11 = 0.2, prcsuiriably due to dimeric products involving 1,3-n.
-0
~ 1 0
7-7I
0
-
1 Ea
2
4
\
6
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E , x 10 Figure 5 . D i c y h h e x y l yields: 0 , pure cyclohexene; 0, 1,4-11 solutions; 0,B solutions; 0,1 , 3 - ~solutions.
When 1,4-1) was added to cyclohexene, no dicyclohexadiene formation was observed. However, two dimeric products, designated D-2 and ~ - 3were ) fornied (Fig. 813). These products were tentatively identified as cyclohexenylcyclohexadiene and dicyclohexadienyl, respectively, 011 the basis of their retention times on two v.p.c. c01unins.~ The yields of these products increased rapidly as el 4 - ~was increased to -0.05, but becanie relatively independent of €1 1-D a t higher conceritratinns of the diene. Polymer. The yields of polymer (excluding diiners) from the three types of solution are given in E‘ig. 9. The 1 , 3 - ~solutions produced polymer in relatively much higher yields than did the 1 , 4 - or ~ B solutions. Cs-Hyd? ocaybon Products. N o new C6-hydrocarbon products were detezted when B was added to cyclohexene. 1,3-1) was fornied in the solutions of 1.4-u in cyclohcxene (I’ig. 8A), the yield increasing rapidly n-ith increasing el 4 - a~ t low concentrations. S o 1,4I) formation could be detected in the 1J-u solutions The yield of B from the 1 , 3 - ~solutions (l’ig. 711) increased more rapidly a t low tha.1 a t high 1 , 3 - ~ concentrations. H was appare-itly fornied in the 1 , 4 1 1 solutions, but nieaningful nieasurenicnts were not ohtained due to analytical difficulty.
RADIOLYSIS OF CYCLOHEXEKE
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0 4
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(3
1
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€a
n
"0
2
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Figure 7 . Yields of products from 1 , 3 - ~ solutions. A: 0 , total dimer yield from pure cyclohexene; 0, total CIS products; 0,dicyclohexadiene; 0, benzene B: 0 , total dimer yield from pure cyclohexene; 0, dimer yield, excluding dicyclohexadiene.
Ea x 10 Figure 6. Unidentified dimer (I>-1)yields: 0 , pure cyclohexene; 0, 1 , 4 - ~solutions; 0,B solutions; 0,1,3-n solutions.
Table I : Appearance Potentials
~ 1,4-1) Additive Consumption. The rates of 1 , 3 -and disappearance could be measured in the dilute solutions (Fig. 10). 1 , 3 -had ~ a high rate of disappearance that increased rapidly with increasing concentration. 1,4D reacted much less extensively than did 1,3-~.The rate of R consumption was so small that it could not be measured with the analytical technique used.
(Cyclohexene)+ (Cyclohexene-&) (Benzene) + ( 1,3-Cyclohexadiene) ( 1,4-Cyclohexadiene)
--.\ppearance This worka
Ion
Discussion Benzene Addition. The yields of all of the major cyclohexene products were reduced to a similar extent by the addition of B. This general inhibition is consistent with "sponge type" protection.j 1,S-Cyclohexadiene Addition. 1,3-~was a more effective inhibitor than either R or 1,4-ufor all of the cyclohexene type products. This may be due to a higher probability of activation transfer from excited or ionized cyclohexene molecules to the additive when thc additive is lJ-1). Values for the appearance potentials of singly ionized molecular species of the conipounds used in this work6 are given in Table I. Only
+
+
+
9.2 f0 2 9.2 f0.2 9 9 f 0.3
potential, e.v.Other
Ionization potential, e.v. (spectroscopic )
9 24 XI= 0.07h'c 9 . 2 f O . O j d 9.52"."
9.24'
8 . 7 zk 0 . 3 9.2 f0 2
Xenon was used as the internal standard. * J. I). Morrison and A. J. L. Kicholson, J . Chem. Phys., 20,1021 (1952). Values, obtained by electron impact, have been reported in the range 9.2-9.7 e.v. for (cyclohexene)+ and 9.2-9.9 e.v. for (benzene)+. See F. H. Field and J. L. Franklin, "Electron Impact Phenomena," Academic Press, New York, S . Y., 1957. W. C. Price and W. T. T u t t e , Proc. Roy. Soc. (London), A174, 207 (1940). e J. I:). Morrison, J . Chem. Phys., 19, 1305 (1963). W. C. Price, Chem. Rev., 41, 257 (1947).
'
(1,3-n)+ had an appearance potential lower than that of (cyclohexene) +. The appearance potentials ( 5 ) >I. Burton and S.Lipsky, J . Phus. Chem , 61, 1461 (1957) (6) The radiolysis of oyclohexene-dio will be reported i n n h t e r communication.
Volume 68, .Vumher I O
October, 1964
B. R. WAKEFORD AXD G. R. FREEMAN
2996
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40
6 4
30 2
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x o
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a 8
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Ea x 10 Figure 8. Yields of products from 1 , 4 - ~solutions. A: 0, 1,3-cyclohexadiene; B: 0, D-2 (cyclohexenglcyclohexadiene?); 0 , 0-3 (dicyclohexadienyl?).
were measured using a Sletropolitan-Vickers AIS2 mass spectrometer. The results were calculated by the method described by Lossing, et al.? The high yield of dicyclohexadiene fornied a t low 1,3-~concentrations clearly indicates that some form of activation transfer from the cyclohexene system to the added 1,3-~was occurring. Discussion of the nature of such a process would be very speculative a t this stage. Although the yields of the cyclohexene type dimers could not be measured for e1,3-D > 0.002, due to analytical difficulty, it is clear that their yield was decreasing more rapidly than was the hydrogen yield with increasing 1 , 3 - ~concentration. A possible explanation is that the decreases in the dimer yields were not due to activation transfer alone, but also to radical scavenging. Although hydrocarbon radicals do not add to cyclohexene at an appreciable rate under the present cond i t i o q 2 1 , 3 - ~is a much better radical scavenger The Journal of Phvsical Chemistry
"0
2
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Ea x 10 Figure 9. Polymer yields (C, units): 0 , pure cyclohexene; 0, 1,4-n solutions; 0,B solutions; 0,1 , 3 - ~solutions; +, 1,3-0 solutions, with oxygen correction. Analysis of the residue of the 89% 1,3-cyclohexadiene, 11% cyclohexene solution gave a composition of ( C6H80)n with a n average value of n = 8.7. If the oxygen addition took place before the polymer Kas weighed,% and i t is assumed that oxygen addition took place in all of the 1,3-0 solutions to the same extent, then the values for G(po1ymer) Kill be those given by
+.
than is cyclohexene. Methyl radicals add about 700 times more readily to 1 , 3 -than ~ to cyclohexene.8 Of the c6 ring olefins studied in this work, only 1,3D can readily undergo long chain polymerization. I n the cases of both cyclohexene and 1 , 4 - ~formation , of a trimer by successive addition of c6 ring units is sterically hindered and formation of a tetramer is extremely difficult without some ring cleavage. The observed high yield of polymer in the 1,3-~ solutions may presumably be initiated either by a free radical or an ion. (7) F. P. Lossing, A. W. Tickner, and W. A. Bryce, J . Chem. Phys., 19, 1254 (1951).
(8) J. Gresser, A. Rajbenbach, and M. Szwarc, J . Am. Chem. SOC., 83, 3006 (1961).
2997
RADIOLYSIS OF CYCLOHEXENE
R
E , x 1001 Figure 10. Additive diciappearance from ryclohexene solutions: 0, 1 , 4 - ~solut,ions; 0,1 , 3 - ~ solutions.
1,4-CyclohezadieneAddition. It has been suggestec12 that, under the conclitions of the present experiments, essentially all of the dicyclohexyl, 3-cyclohexylcyclohexene, and 2,2’-dicyclohexenyl and part of the cyclohexane arise from radical conibination and disproportionation reactions. Addition of 1 , 4 - ~inhibits the formation of the above-mentioned “diniers” but slightly enhances the forma tion of cyclohlexane. Activation transfer from cyclohexene to 1 , 4 - ~cannot alone explain these results. Cyclohexyl and 2-cyclohexenyl radicals can react with 1 , 4 - by ~ reaction 1 to yield the resonance stabilized
+ c-CsHs + R H + c-CsH.i
(1)
cyclohexadienyl radical. The reaction will be exothermic in both cases.9 However, reaction 1 will be 15-20 kcal./mole niore exothermic when R is a cyclohexyl than when it is a 2-cyclohexenyl radical.l0 Thus the inhibition of the cyclohexene-type diiners cannot be due to reaction 1 alone, since in such a case the formation of dicyclohexyl should be much more effectively inhibited than is the formation of 2,2’dicyclohexenyl, and the cyclohexane yield would be markedly enhanced. The rate of addition of hydrocarbon radicals to the isolated double bonds in 1 , 4 - ~is probably very siniilar to that of addition to cyclohexene, and therefore negligible under the present conditions. It appears, therefore, that both activation transfer and the radical reaction 1 are occurring simultaneously. Both niechanisnis can be expected to give rise to cyclohexadienyl radicals, since this radical species has been observed in irradiated 1 , 4 - ~ and ~ ~is also the radical product of reaction 1. The similarity in the dependence of the yields of 1 , 3 - ~D-2, , and D-3 on €1,4-D suggests that the forniation of the three products might involve a coninion precursor, presuniably the cyclohexadienyl radical. This is support for the tentative identification of D-2 and D-3 as cyclohexenylcyclohexadiene and dicyclohexadienyl, respectively. (9) A. Rajbenbach and M. Szwarc, Proc. Chem. SOC.,347 (1958). (10) R. Klein and M. D. Scheer, J . Phys. Chem., 67, 1876 (1983). (11) R. W. Fessenden and R. H. Schuler, J . Chem. Phys., 39, 2147 (1963).
Volume 08, Sumber 10 October. 1964
