Julie, 1959
RADIATION CHEMISTRY OF CYCLOHEXANE
813
RADIATION CHEMISTRY OF ORGANIC COMPOUNDS. IV. CYCLOHEXANE BY HAROLD A. DEWHURST General Electric Research Laboratory, Schenectady, N e w York Received November 6 6 , 1968
The major products formed in the 800 kvp, electron radiolysis of cyclohesane are hydrogen, cyclohexene and dicyclohex 1. Minor amounts of products result from fragmentation and isomerization of the ring system. The initial yields are, G ( C d 0 ) = 2.5 and G(dicyc1ohesyl) = 2.0 and were unchanged by irradiation either in 10 atmospheres of hydrogen or a t liquid nitrogen temperature. I n the presence of solutes, oxygen, iodine and benzene, the cyclohexene yield decreased to a limiting value of G = 0.7. The results are discussed in terms of a mechanism which involves the decomposition of excited cyclohexane molecules. The results show that of the excited molecules that decompose approximately 15y0 give products directly by an unspecified molecular process.
A specially designed high-pressure cell8 was used for irThe gaseous products from the radiolysis of radiations in high pressure (10 atmospheres) of oxygen and cyclohexane liquid are mainly hydrogen and small hydrogen. amounts of Cz hydrocarbons.lm2 Recently it has Product Analysis.-Conventional high-vacuum techbeen shown that the liquid products are cyclo- niques were used for the preparation and analysis of samples.8 hexeiie, dicyclohexyl and small amounts of prod- Infrared spectra were obtained with a Perltin-Elmer (model 21) spectrometer. The cyclohexene concentration was deucts which result from ring f r a g m e n t a t i ~ n . ~termined a t 718 cm.-' using a molar extinction coefficient of Because of the relative simplicity of product for- 45.4 determined in separate experiments. The concenmation, cyclohexane is a particularly interesting tration of cyclohexanone was determined a t 1717 cm.+ molecule for radiation chemical investigation. using the molar extinction coefficient of 304 reported by Cross and Rolfe.'O A Perltin-Elmer vapor fractometcr was Previous studies with cyclohexane have been used for the gas-liquid partition chromatography. Most concerned mainly with the gaseous products. of the analyses were performed with either a 1- or 2-meter For example, the hydrogen yield has been shown to didecyl phthalate column. The fractometer was calibrated be independent of linear energy transfer4 and with known solutions of cyclohexene and dicyclohexyl in cyclohexane. I n most cases the cyclohexene concentratemperatures6 However, in the presence of solutes, tion was determined by both I R spectrophotometry and notably benzene2 and i ~ d i n e , the ~ J hydrogen yield gas chromatography, and the agreement was always good. decreased. The formation of liquid products under these conditions has not been investigated and is Results and Discussion essential for a complete understanding of the Gas Products.-The gaseous products volatile a t radiation chemistry. The present report is pri- -120" consisted largely of HP with small amounts marily concerned with the effect of solutes and of methane and CShydrocarbons, mainly ethylene. temperature on liquid product formation. The volatile gas had the following composition: H2 = 96%) CH, < 0.4%, CzHs < 0.3% and C2H4 Experimental = 3.1y0iu fair agreement with the data of Manion Materials.-The following samples of cyclohexane were used as received: (a) Eastman Kodak, spectro grade; ( b ) and Burton.2 Phillips, research grade; and (c) API grade, which conLiquid Products.-The gas chromatograms tained 0.010 + 0.006 mole % impurity. Gas chromato- showed t,his product distribution
grams of the spectro grade material showed a large impurity peak (-0.8 mole %) which was probably methylcyclopentane. Chromatograms of the Phillips and A P I material did not show any impurity peaks. The same product yields were obtained with all three samples of cyclohexane. The following solutes were used: cyclohexene (Phillips pure grade); benzene (Phillips research grade); IP (Mallinckrodt); 02 (Matheson); and hydrogen (Matheson electrolytic grade). Irradiation.-The samples contained in a 2" diameter aluminum dish were irradiated a t room temperature with the 800 kvp. electron beam from a resonant transformer unit.s Unless otherwise specified all samples were prepared for irradiation in a Nz.dry box. The electron beam dose rate determined by ionization chamber mzasurements and based on 93 ergs/g./r. was 8.7 X 1020e.v./g./min. This value was in agreement with energy absorption calculations based on the hydrogen yield from cyclohesane liquid utilizing G(H2) = 5.5.
linear Ca hydrocarbon (hcsene) methylcyclopentane cyclohexene intermediate cyclohexanes dicyclohexyl cyclohexylcyclohexene (?)
The cyclohexeiie and dicyclohexyl accounted for approximately 90% of the liquid products. The formation of cyclohexene as a function of the energy absorbed is shown in Fig. 1. Each point represents the average of a t least two separate determinations. The initial slope of the curve was used to calculate the initial yield of cyclohexene, G(CGHIO)i = 2.5. The cyclohexene yield gradually decreased with increase in energy absorbed toward a limiting concentration. ( 1 ) C . 8. Schoepfle and C . H. Fellows, I v d . Brig. Chem., 23, 1396 The formation of dicyclohexyl as a function of ( 1 93 1). (2) J. P. Manion a n d M . Burton, THISJOURNAL, 66, 5GO (1952). the energy absorbed is shown in Fig. 2, where each (3) H. A . Dewhurst, J. Chem. P h y s . , 24, 1254 (1956). point represents the average of a t least tv70 sepa(4) R . H. Schuler a n d A . 0. Allen, J. Am. Chem. Soc., 77, 507 rate determinations. From the initial slope the (1955). (5) M . Hamashima, &I. P. Reddy a n d M. Buyton, THISJOURX'AL, initial yield was calculated, G(dicyclohexy1)i = 62, 246 (1958). 2.0. The reproducibility of the cyclohexene and ( 6 ) R . H. Schuler, ibid., 61, 1172 (1957). (7) M. Burton, J. Chang, 9. Lipsky a n d M . P. Reddy, R a d . Research, 8 , 203 (1958). (8) H. A. Dewhurst, THIS JOURNAL 61, , 1460 (1957).
(9) E. J. Lawton a n d J. S. Balwit, private communication. (10) L. H. Cross and A. C . Rolfe, T r a n s . Faraday Soc., 47, 354
(1951).
14.4ROLD
A. DEWHURST
Vol. 63
initial yield ratio G(COIIIO)/G(CI~H~~) = 1.25 is more than double the ratio of rate constants for disproportionation and combination of cyclohexyl radicals (kdisp/kcomt, = 0.5) measured by Gunning, et al.,ll in the gas phase. This discrepancy suggests that if Gunning's ratio can be accepted for the liquid phase, then a simple interpretation of product formation based entirely on dispro4 OIportionation and combination of cyclohexyl radicals is not valid. The formation of the other product groups (Fig. 3) was found to be a linear function of the energy absorbed. For the purpose of estimating G-values it was assumed that area per cent. of the peaks on the chromatogram was directly proportional to mole per cent. On this basis the yield of hexene product was G(hexene) Ei 0.2, G(methylcyc1opentane) E 0.3 and G(intermediate cyclohexanes) 0.3. The latter products were not well characterized but had retention times similar Lo/ to the n-alkyl (ethyl to hexyl) substituted cyclohexanes. Infrared examination of the irradiated liquid showed, in addition to a strong cy0 clohexene band a t 718 cm. -l, weak absorption bands a t 972 and 1379 cm.-' characteristic of trans-vinylene unsaturation and methyl groups, respectively. The transvinylene band has been assigned to the hex0 5 10 15 20 25 ene product and the methyl band to the meENERGY ABSORBED, ev/g x 10-2' thylcyclopentane product. Fig. l.-Format,ion of cyclohexene a t room temperatui-e ( 0 ) To determine whether any significant and liquid nitrogen temperature (X). Filled circles, added amount of products higher than dimer were cyclohexene = 5.2 x 10-4 mole/g. formed a residue determination was done. The irradiated sample (2.9 X loz1e.v./g.), evaporated to constant weight in a vacuum desiccator, did not contain any measurable amount of product higher than dimer. The infrared spectrum (0.025 mm. micro cell) of the residue, essentially dimer, was found to contain a negligible amount of unsaturation. The above results are a t variance with the product yields reported by Nixon and Thorpe. The differences have been attributed to the effect of dose rate. Similar differences, attributed to dose rate dependent reactions, have been found with cobalt-60 y-rays.13 Effects of Products.-The effect of adding 5.2 X 10-4 mole/g. cyclohexene is shown in Fig. 1 (filled circles). The cyclohexene concentration decreased rapidly toward the limiting concentration. Under these conditions the initial yield for cyclohexene disappearance was G(- C~HID)= 2.3, the hydrogen yield was G(H2) = 2.9 and the dicyclohexyl yield mas unchanged. The decrease in the hydrogen yield, AG(H2) = 2.4, is in good agreement with the yield for cyclohexene disapY I I I I I pearance and suggests that cyclohexene is reacting 0 5 K) 15 20 25 ENERGY ABSORBED e v / p xIO-*l. either with H-atoms or their precursor. These Fig. 2.-Formation of dicyclohexyl a t room temperature ( 0 ) results are consistent with the reactions.
i"/
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and liquid nitrogen temperature (X).
dicyclohexyl results is indicated by the vertical lines in Figs. 1and 2. It is of considerable interest to note that the
(11) P. W.Beck, D. V. ICniebcs and H. E. Cunning, J . Ckem. P h y s . , 22, 672 (1954).
(12) A. C. Nixon and R. E. Thorpe, ibid., 28, 1004 (1958). (13) H. A. Dewhurst and R. H. Schuler, J . Am. Chem. Soc., 81, in press (1959).
'c
b
RADIATION CHEMISTRY OF CYCLOHEXANE
June, 1959
+
C6Hiz ---+ ' C ~ H I I H C6HIa H +'cSH11 HZ CeHio H +'C6Hii r+ C B H ~ OCaHiz 2.C6Hl1-j CiaHz2
+
+
+
+
(1) (2) (3)
(4) (5)
This reaction sequence predicts iio change in the number of cyclohexyl radicals formed and, therefore, no change in dimer yield in agreement with experiment. The irradiation of cyclohexane a t room temperature in the presence of 10 atmospheres of added hydrogen (concentration approximately mole/ g.) did not change the yield of any of the reaction products measured by infrared arid gas chromatography. Under these conditions, therefore, hydrogen does not enter into any significant back reaction at room temperature. mole/ Effect of Solutes.-In the presence of g. of benzene, both the cyclohexene and dicyclohexyl yields were markedly decreased. The methylcyclopentane and Cshydrocarbon products were apparently unchanged. For a total dose of 5.8 X loz1 e.v./g. the benzene disappearance corresponded to a yield of G( - CeHe) = 1.2 as measured by gas chromatography and by the change in infrared absorption a t 672 em.-'. Under these conditions the cyclohexeiie yield was G(CeHlo) = 1.2, and the dicyclohexyl yield was G(C12H22) = 0.8. The infrared absorption spectrum showed a new band a t 724 cm. -1 which is in the region for monosubstituted benzene absorption. These results show that the disappearance of one benzene molecule results in a corresponding decrease in the yields of cyclohexene and dicyclohexyl. If these products are formed by disproportionation and combination of cyclohexyl radicals (reactions 4 and 5), then each benzene must remove either two cyclohexyl radicals or their precursor. Oxygen was found to have a profound effect on the radiolysis of cyclohexane. The effects observed were essentially independent of oxygen pressure mole/g.) between one and 10 atmospheres and directly proportional to the total energy absorbed up to 5.8 X loz1e.v./g. The gas chromatograms showed the formation of two major oxidation products in approximately equal amounts which were identified as cyclohexanol and cyclohexanone. Infrared examination of radio-oxidized cyclohexane showed the presence of both hydroxyl and carbonyl groups and served to confirm the gas chromatography assignments. The cyclohexene yield in the presence of oxygen was G = 0.7, less than one-third the value in the absence of oxygen. Under the same conditions the yield of dicyclohexyl product was negligible (G< 0.1). The combined yield of oxidation product determined by gas chromatography was G(cyc1ocyclohexanone) = 7.2. The carbonyl hexanol yield determined by infrared absorption was G(cyclohexanone) = 3.5 and by difference the cyclohexanol yield was G(cyclohexano1) = 3.7. The carbonyl yield determined by infrared and the total yield of cyclohexanone and cyclohexanol determined by gas chromatography was found to 1)c indcpendent of dose rate over a 10-fold rnngc.
+
815
'
L
7
a
E
1 0.6
a
04 r
X
0 C g Hydrocarbon
X A l k y l Cyclohexane
3 '0
0
I
I
I
I
I
5
10
15
20
25
ENERGY
ABSORBED. e v / @ x 1 0 - 2 1 ~
Fig. 3.-Formation of lirieltr c6 hydrocarbons (hexene) and alkyl cyclohexanes a t room temperature.
Treatment of the irradiated solution with sodium sulfite did not change the amount of cyclohexanol formed. l 4 This suggested that cyclohexyl hydroperoxide, known to be stable above room temperature,l5 was not a major product. Bakhle has reported that hydroperoxide is the major oxidation product (G = 1.2) along with carbonyl (G = 0.6) and a small amount of acid (G = 0.2). The total yield of oxidized product reported by Bakh is considerably smaller (factor of three) than the present results. The oxygen results show that the residual cyclohexene yield is independent of the oxygen concentration which suggests that it may be formed by a molecular process CSHlZ -+
+
C ~ H I O HZ
(1-a)
The major oxidation products can be accounted for by the sequence which was proposed recently
+
CaHii 0 2 --j C6HiiO:' 2Cfifl1102+CsHiOO iCsHiiOH
+
(6) 0 2
(7)
by RusselL17 I n the absence of any complicating chain reactions the product yields lead to G(cyc10hexyl radicals) = 7.2. In the absence of oxygen a similar calculation based on product yields gave G(cyclohexy1 radicals) = 7.6. Oxygen appears, therefore, to be a useful solute for radical counting. In view of the good agreement for the yield of cyclohexyl radicals obtained in the presence and absence of oxygen, it is proposed that the H-atoms formed in reaction 1 either abstract as in reaction 2 or, more likely in the presence of oxygen, they forin HO2 radicals which react as 1302 C6Hiz +HzOz Ci"i (8) Cyclohexane samples saturated with iodine (-0.04 nil) were irradiated in a special water-cooled cell equipped with a magnetic stirrer to ensure that
+
+
(14) The author is indebted t o G. A. Russell for this suggestion. (15) A. Farkas and E. Passaglia, J . A m . Chem. Soc., 711, 3333 (19.50). (10) N. Bakh, Inter. Conf. Peace. Use A ! . Enevgy, 7 , 638 (1D5F), United Nations, N. Y . (17) G . A . RIISSCII, J. A m . Chrm. ROC.,79, 3871 .(l!X7).
816
HAROLD A. DEWHURST
Vol. 63
saturation with iodine was maintained. The acts to suppress the molecular formation of cyclochromatograms showed the presence of the fol- hexene. lowing products : cyclohexene, hexyl iodide, cyEffect of Temperature.-The effect of temperaclohexyl iodide and a small amount of dicyclohexyl. ture on the radiolysis of cyclohexane was examined The cyclohexene yield under these conditions was a t room temperature and a t liquid nitrogen temG(C6H10) = 0.8. The yield of hexyl iodide was perature. There was little effect of temperature G = 0.3, while the cyclohexyl iodide yield was G = on the cyclohexene formation (Fig. 1, crosses) and 4.0 and the dicyclohexyl yield G = 0.3. Fessenden only a small effect, if any, on the formation of and Schulerls have reported that a t low-iodine dicyclohexyl (Fig. 2, crosses). The formation of concentrations the yield for formation of alkyl methylcyclopentane and the Cg hydrocarbon prodiodide was G = 5.6 which increased to about G = uct also appeared to be independent of tempera7.5 in saturated iodine solutions. I n the present ture. These results are consistent with the recent work the yield of observable alkyl iodide was G = results of Burton, et d , 5 showing that the hydrogen 4.3, which is much lower than the value reported yield is independent of temperature. by Fessenden and Schuler. However, since the latter authors did not specify the nature of the alkyl Conclusion iodides formed, a rigorous comparison is not posThe major products formed in the radiolysis of sible a t present. liquid cyclohexane result from C-H bond cleavage The yield of cyclohexyl radicals deduced from the without rupture of the ring. A minor amount of iodine experiments would be G(cyclohexy1radicals) product results from fragmentation and isomeriza= 4.0, a value appreciably smaller than that detion of the ring system. These results are qualiduced from the oxygen and vacuum experiments. tatively consistent with mass spectrometric studies These results are consistent with the above niech- of cyclohexane which high stability for the anism, if it is assumed that the H-atoms react with parent molecule-ion. show The role of moleculeiodine, thereby eliminating reaction 2 and ef- ion reactions in the condensed phase is, at the fectively decreasing the yield of cyclohexyl radi- present time, largely a matter of speculation. The cals by one-half. Schulerlg has cautioned against present results are consistent with a mechanism the use of iodine as a radical counter a t concentra- which involves the formation and decomposition tions greater than lod3ill. I n the liquid butane of excited cyclohexane molecules. Reaction l b is system, however, the yields of C1to Ch radicals included to represent the small amounts of products were independent of iodine concentrations from which result from ring fragmentation and isomerizaIO+ Ad to above 10-2.20 It is concluded there- tion reactions. The present results show that of the fore that the yield of cyclohexyl iodide is a measure excited molecules that decompose approximately of the cyclohexyl radical yield from reaction 1. 15% give products directly (molecular process). Since it is postulated that reaction 2 is eliminated rC-CeHii H (1) by iodine, then the total yield of cyclohexyl radicals would be 8 (in absence of iodine) in approxic-C~HI? c-CBH~~-+ C-CsHio H, (la) mate agreement with the results obtained in the RI & ( R H ) (1b) presence and absence of oxygen. The residual yield of cyclohexene in the presence Acknowledgments.-The author is indebted to of iodine is consistent with the suggested molecular J. S. Balwit for the electron irradiations and to reaction (la). This conclusion is contrary to that E. H. VC'inslow for considerable assistance with the of Burton, et al.,' who have suggested that iodine gas chromatography. The author is indebted to (18) R. W. Fessenden and R. H. Schuler, THISJOURNAL, 79, 273 the referee for calling attention to the paper by (1987). Burton and Meshitsuka, Radiation Research, 9, (19) R. H. Schuler, THISJOURKAL,68, 37 (1958). 152 (1958), to be cited in support of the production (20) C. E. McCauley and R. H. Schuler, J. A m . Chem. Soc., 79, and scavenging of H-atoms. 4008 (1957). -n*y+
L
+ +
+
f
1*
