W. A. CRAMER
1112
Radical Reactions in Liquid Cyclohexane. 11. The MercuryPhotosensitized Decomposition of Cyclohexane
by W. A. Cramer Reactor Institute, Delft, The Netherlands
(Received November $8, 1966)
Accepted and Transmitted by The Furaday Society
(September 6, 1066)
The mercury-photosensitized decomposition of liquid cyclohexane has been investigated in the presence and absence of oxygen. Dicyclohexyl and part of the cyclohexene are produced in reactions between cyclohexyl radicals. An appreciable fraction of the cyclohexyl radicals reacts with their sibling hydrogen atoms within the liquid cage, giving rise to unscavengeable cyclohexene formation. At higher conversions cyclohexyl radicals and hydrogen atoms react with cyclohexene. This results in the formation of cyclohexylcyclohexene and products with more than 12 carbon atoms.
Introduction The mercury-photosensitized decomposition of hydrocarbons has recently been investigated to study radical reactions in the liquid In the case of cyclohexane, the following reaction mechanism has generally been assumed to 0 c c u r , ~ ~and 2 ~ *experimental ~~ results have been interpreted accordingly. Hg(6lSo) Hg(6'Pi)
2537 A
Hg(G3Pi)
+ CeHlz +Hg(6'So) + CeHll. + He
+ CeHiz + CeHii. C8Hll. + C8Hll. C6HlO + %HI2 C1zH22 CeHii + CeHu €3.
+H2
*
*
--f
(1)
(2)
(3)
(4) (5)
However, it has recently been suggested that cyclohexene is not only formed in reaction 4 but also in reaction 6 between a cyclohexyl radical and its sibling hydrogen atom immediately after their formation within the cage of surrounding molecule^.^
He
+ CaHii. +H2 + CeHlo
(6)
This reaction may be favored by the formation of the intermediate HgH in reaction 2, as was suggested by Kuntz and Mains.' An isotope effect, observed in the quenching of excited mercury atoms by saturated hydrocarbons and their deuterated analogs, has been interpreted as to indicate that HgH and HgD are The Journal of Physical Chemistry
formed in the primary proce~s.'-~JEvidence for reaction 6 was obtained by Hentz, et who studied the mercury-photosensitized decomposition of mixtures of CBHIZ and C6D12. In the present work, we have tried to confirm the occurrence of reaction 6 in the mercury-photosensitized decomposition of cyclohexane. Furthermore, some experiments were carried out at high conversions to investigate reactions of cyclohexyl radicals with the product cyclohexene. Controversy still exists about these reactions. Addition of cyclohexyl radicals to cyclohexene has been proposed to account for polymer formation in the 7 radiolysis of cyclohexane.* On the other hand, it has also been suggested that cyclohexyl radicals do not react with cyclohexene, even in pure cyclohe~ene.~ (1) R. (1963).
R. Kuntr and G. J. Mains, J . A m . Chem. SOC.,85, 2219
(2) C.
E. Klots and R. H. Johnson, Can. J. Chem., 41, 2702 (1963).
J. Phya. Chem., 67, 1743 (1963). (4) R. R. Hentr, J. Y. Chang, and M. Burton, ibid., 69,2027 (1965).
(3) J. W. Falconer and M. Burton,
(5) P. W. Beck, D. V. Kniebes, and H. 22, 672 (1954).
E. Gunning, J . Chem. Phys.'
(6) E. G. Spittler, P. Jordan, L. M. Dorfman, and M. C. Sauer, J. Phys. Chem., 67, 2235 (1963). (7) M. G. Bellas, Y. Rousseau, 0. P. Strausr, and H. E. Gunning,
J . Chem. Phys., 41, 768 (1964). (8) R. Barker and M. R. H. Hill, Nature, 194, 277 (1962). (9) B. R. Wakeford and G. R. Freeman, J . Phys. Chem., 68, 2635 (1964).
1113
RADICAL REACTIONS IN LIQUIDCYCLOHEXANE
Experimental Section The preparation and purification of cyclohexane used in our experiments have been described elsewhere.lo Irradiations with light of 2537 A from a low-pressure mercury lamp were carried out at 23' in Vycor cells. Prior to irradiation, the liquid, containing a drop of mercury, was degassed by the conventional freezethaw technique. Solutions containing oxygen were prepared by passing a stream of oxygen gas through the liquid during 2 min prior to irradiation. During the irradiations, which lasted from 1min to 26 hr, the vapor phase above the liquid was masked. Hydrogen, the only product volatile at - 196O, was collected a t this temperature and measured volumetrically. Other products were separated by gasliquid partition chromatography and measured with a flame-ionization detector. Column materials and temperatures used in the analysis of cyclohexene, dicyclohexyl, cyclohexanol, and cyclohexanone, the latter two products being formed in the presence of oxygen, have been given in a previous publication.1° I n addition l o dicyclohexyl, two other Clz products were formed at high conversions in the absence of oxygen. They were separated at 150' on a 4-m column containing 2071, polyglycol 4000 (E. Merck Ltd) on 70-100 mesh Embacel. Both products could be converted to dicyclohexyl by catalytic hydrogenation at room temperature and were identified by their mass spectra. For this purpose, an appreciable amount of the' cyclohexane from an irradiated sample was evaporated. Part of the resulting concentrated solution was injected in a gas chromatograph. The two unsaturated Clz products were collected separately, using a stream splitter and introduced into a mass spectrometer. Both products showed parent peaks with a mass of 164. Since oniy C-H bond rupture occurs in the reaction between excited mercury atoms and saturated hydrocarbons, these parent peaks with a mass of 164 indicate that %he Clz products are two isomers of cyclohexylcyclohexene. Another part of the concentrated solution was introduced directly into a mass It was observed that, in addition spectrometer. to products with 12 carbon atoms, product molecules with 18, 24, and 30 carbon atoms were also present. Products containing 18 carbon atoms were also detected gas chromatographically, using a 2-m column containing 20% silicon oil on 70-100 mesh Embacel. Since no standard solutions were available for the quantitative measurement of the unsaturated Clz and the CISproducts, the detector response for these products was assumed to be equal to the response for dicyclohexyl and n-octadecane, respectively. Ultraviolet irradiation of oxygen containing solu-
0
1
2
3
-Conversion
(1o-Lt-1)
Figure 1. Product yields a t low conversions.
tions resulted in the formation of three new products, cyclohexanol, cyclohexanone, and a product with a retention volume slightly higher than that of dicyclohexyl on columns containing silicon oil or polyglycol. The polyglycol column was used for the separation of this unknown product. I n one experiment, it was isolated gas chromatographically from a concentrated solution and injected into a mass spectrometer. A parent peak with a mass of 182 was observed, suggesting that the product can be represented by the formula Cl2HZ20. The detector response for this product in the gas chromatographic analysis was again assumed to be equal to the response for dicyclohexyl. A product with similar retention volumes on both the silicon oil and the polyglycol columns could be prepared by catalytic hydrogenation of diphenyl ether at 150°, using platinum oxide as a catalyst. This synthesis strongly suggests that it consists of dicyclohexyl ether.
Results Experimental results, obtained with deaerated cyclohexane, are given in Table I and Figures 1 and 2. It was observed that the rate of decomposition of cyclohexane was not very reproducible and varied by as much as a factor of 2." The average rate for hydrogen formation was about 56 X loF4 mole of HZ(1. of cyclohexane)-' hr-l. Product formation in Figures 1 and 2 is therefore not represented as a function of exposure time but as a function of cyclohexane conversion, -C6H12. This conversion was approximated by the relation -C6H1z
=
C6Hm
+ 2C1zHzz +
2C12H20
+
3 c 1 8
(10) W. A. Cramer, part I of this series, to be published. (11) This may be due t o the differences in transparency of the Vycor cells used in these experiments. Moreover, not only was the vapor phase above the liquid masked during the irradiations, but part of the liquid was also.
Volume 71, Number 4 March 1967
W. A. CRAMER
1114
Table I : Product Yields (in lo-' mole/l.) in the Mercury-Photosensitized Decomposition of Cyclohexane Hz
CsHio
Ci:Ha
Ci:Hm (A)"
CuHm (B)"
CIS
1.11 1.50 2.54 2.94 32.6 50.8 69.5 97.4 103 150 261 323 49 1 1193 1587
0.61 0.88 1.72 1.72 20.4 33.6 41.8 54.8 54.7 74.2 103 110 136 145 159
0.34 0.50 0.97 1.09 13.0 24.2 28.8 38.5 39.7 57.8 104 139 218 608 531
N.d.* N.d. N.d. N.d. 0.39 0.85 1.8 2.7 2.5 4.1 8.7 13 19 34 32
N.d. N.d. N.d. N.d. Trace Trace 0.38 0.61 0.51 1 .o 2.3 4.1 7.3 28 25
N.d. N.d. N.d. Trace Trace 0.70 2.2 2.7 2.6 4.8 13 N.m." 30 123 N.m.d
a A and B correspond with two isomers of ryclohexylcyclohexene. pounds were present.
Not detectable.
A/B
(A
4.7 4.4 4.9 4.1 3.8 3.2 2.6 1.2 1.3
Not measured.
+ B)/Cis
1 .o 1.2 1.2 1.1 0.85 0.88 0.50
Conversion
1.3 1.9 3.7 3.9 47.2 85.8 110 147 148 214 372 715 1854
CI8,C2(, and Cgocom-
by the formation of two cyclohexylcyclohexene isomers and products with 18 carbon atoms. Formation of products with 24 and 30 carbon atoms was also observed at the highest conversions. The material balance is reasonably good at all conversions. Results of experiments carried out in the presence of oxygen are listed in Table 11. No attempt was made to measure the formation of hydrogen gas in these experiments.
Discussion Experiments at Low Conversions. From the experimental results at low conversions, shown in Table I
Figure 2. Yields of hydrogen, cyclohexene, and dicyclohexyl at high conversions.
Initial products consist of hydrogen, cyclohexene, and dicyclohexyl. From Figure 1 it can be seen that, at low conversions, the yields of these products increase linearly with increasing conversion. Figure 2 shows that at higher conversions a considerable decrease is observed in the yield of cyclohexene, relative to the yields of the other products. This is accompanied The Journal of Physical Chemistry
and Figure 1, a value of 1.73 f 0.09 is obtained for the ratio of the yields of cyclohexene and dicyclohexyl. The cause of the discrepancies between this value and the reported values of 1.31 f 0.042and 1.47* is not clear. It has been discussed previously that this ratio is not equal to h / k s if reaction 6 contributes.'* The experiments carried out in the presence of oxygen served to obtain more information about the occurrence of this reaction. Even at relatively low concentrations, this radical scavenger can interfere with reactions between randomly distributed radicals. However, only at much higher concentrations can this additive interfere with reactions between a cyclohexyl radical and its sibling hydrogen atom immediately after their formation. Hence, interference with reaotions 4 and 5 may be expected at moderate oxygen concentrations, but not with reaction 6 . The yield of dicyclohexyl may thus be reduced to zero, but if reaction 6 also contributes to product formation, the yield of cyclohexene will not be completely suppressed. The
RADICAL REACTIONS
IN
1115
LIQUIDCYCLOHEXANE
mole/l.) in the Mercury-Photosensitized Table I1 : Yields of Liquid Products (in Decomposition of Cyclohexane in the Presence of Oxygen CsHio CsHio
CnHm
CsHiiOH
CsHioO
CizHzzO
ZC6HlI
reaction 6
2.9 3.7 3.9 3.6 4.2 4.7 8.6 15.4
Trace Trace 0.1 0.2 0.3 0.2 0.3 0.7
5.3 8.7 11.1 10.4 11.9 14.0 24.0 42.6
8.9 17.3 17.7 19.7 22.5 22.6 42.1 86.7
0.8 1.3 1.3 1.8 1.7 1.8 3.4 6.7
18.7 32.3 35.6 37.9 42.9 45.5 82.4 160.3
2.9 3.7 3.8 3.4 3.9 4.5 8.3 14.6
experimental results, listed in Table 11, are in accordance with these expectations. However, the residual cyclohexene formation could also be due to reaction 7, which was proposed by Ho and Freeman12 CeHii.
+
CaHio
0 2
+ HO2.
(7)
It has been shown recently that this reaction does not occur at room temperature.1° In order to make a more quantitative estimate of the contribution of reaction 6, the number of cyclohexyl radicals formed in the presence of oxygen must be known. This number can be deduced from the products which are formed in these solutions. It can be seen in Table I1 that cyclohexanol, cyclohexanone, and dicyclohexyl ether are important new products.1x The formation of. cyclohexanol and cyclohexanone has been assumed to occur according to the over-all reaction14 2C~HiiO2'+C6HiiOH
+ CsHioO 4-
0 2
(8)
The cyclohexylperoxy radicals will be produced in reactions between cyclohexyl radicals and oxygen. Equal yields of the two products are to be expected if only reaction 8 occurs. This has been observed in the photolysis of solutions of diphenylmercury in cyclohexanei0 and in the radiolysis of cycl~hexane,~~ both in the presence of oxygen. The results listed in Table I1 show that in the present case cyclohexanone is formed in excess. We have no explanation for this observation, although a reaction similar to (8) but involving hydrogen peroxy radicals might contribute.
+
C ~ H I I O ~ . HO2.
-
+
C ~ H I ~ O H2O
+
0 2
(9)
Hydrogen peroxy radicals may be formed in reactions between hydrogen atoms and oxygen. Excess formation of cyclohexanone may also be partly due to a sensitized reaction of cyclohexanol. It was found that irradiation of a dilute solution (10-l M ) of cyclohexanol, with light of 2537 A and in the presence of oxygen and
mercury, results in a sensitized decomposition of cyclohexanol. Cyclohexanone and dicyclohexyl ether were observed as the main products. This process may also be involved in the formation of dicyclohexyl ether. I n calculating the total amount of cyclohexyl radicals produced, it was assumed that one cyclohexyl radical is involved in the formation of each cyclohexanol and cyclohexanone molecule, &s follows from the proposed mechanism, but that two cyclohexyl radicals are required for the formation of dicyclohexyl ether. To estimate the contribution of reaction 6, the amounts of residual cyclohexene, listed in the first column of Table 11, should be corrected for any contribution of reaction 4. This correction can be deduced from the figures in the second column of Table I1 by using an acceptable value for k4/k5, for which we have taken the value 1.1.l0 It will be clear that all these reactions must also be taken into account when calculating the amount of cyclohexyl radicals produced. The results of these calculations are given in the last two columns of Table 11. It can be seen from these figures that about 10% of all the cyclohexyl radicals react within the liquid cage with sibling hydrogen atoms to give rise to unscavengeable hydrogen formation. The fraction of cyclohexyl radicals formed in reaction 2 that react according to reaction 6 is of interest. If it is assumed that in the presence of oxygen all cyclohexyl radicals are formed in reaction 2, this fraction is equal to ca. 10% as was shown before. However, not all the cyclohexyl radicals are necessarily formed in reaction 2 only. I n the absence of oxygen, for ex(12) S. K. Ho and G. R. Freeman, J . Phys. Chem., 68,2189 (1964). (13) Some reaction products may not have been observed by our analytical methods. The formation of peroxides and hydroperoxides, for example, has been reported in similar systems, although the observed yields were lower than the yields of alcohols and ketones." (14) R. Blackburn and A. Charlesby, Trans. Faraday Soc., 62, 1159 (1966).
Volume 71, Number 4 March 1967
1116
ample, a considerable part of these radicals is produced according to reaction 3. I n the presence of oxygen this reaction will be less important because hydrogen atoms may be scavenged. The rate of reactions between hydrogen atoms and oxygen is about 5 X lo3 times the rate of abstraction reaction 3.15 However, whereas phenyl radicals react with oxygen about lo3 times as readily as with cyclohexane,16they were still found to react with cyclohexane in the presence of oxygen at concentrations similar to those in the experiments under discussion.lo This suggests that reaction 3 may contribute, even in the presence of oxygen. Assuming that all hydrogen atoms not reacting according to reaction 6 also form cyclohexyl radicals ultimately, even in the presence of oxygen, a maximum value of 19% is obtained for the fraction of cyclohexyl radicals formed in reaction 2 that react according to reaction 6. Similar calculations can also be made based on the results listed in Table I, which were obtained at low conversions, by making use of the measured value of 1.1 for k4/k5.10 Since dicyclohexyl is produced only according to reaction 5, the corresponding yield of cyclohexene formed in reaction 4 can be calculated. According to the proposed mechanism, the additional cyclohexene yield may be attributed to reaction 6. This calculation, taking into account reactions 1-6, shows that about 13% of all cyclohexyl radicals and 23y0 of the cyclohexyl radicals formed in reaction 2 react according to reaction 6 under the experimental conditions employed. The possibility that reaction 6 also occurs outside the liquid cage cannot be excluded in the absence of oxygen. In that case, the calculated values of 13 and 23Y0 represent the total contributions of reaction 6, occurring both within the liquid cage between a sibling radical pair as well as in the bulk of the solution. Experiments at High Conversions. At higher conversions, a considerable decrease in the yield of cyclohexene is observed relative to the yields of the other major products, as is shown in Figure 2. The results suggest that prolonged irradiation results in a "steadystate" concentration of cyclohexene. A similar situation occurs when cyclohexane is irradiated with ionizing radiation.l'~~~ This has been attributed to energy transfer from excited or ionized cyclohexane molecules to cyclohexene followed by a sensitized decomposition of cyclohexene, in addition to reactions of cyclohexyl radicals and hydrogen atoms with cyclohexene?j18-21 I n the mercury-photosensitized decomposition of cyclohexane, excited or ionized cyclohexane molecules are not produced. Therefore, energy transfer from cyciohexane to cyclohexene cannot occur and The Journal of Phgsical Chemistry
W. A. CRAMER
only reactions of hydrogen atoms and cyclohexy radicals with cyclohexene will be considered.22pZa H * f Ce"o
Hz f CsH9.
-
+
H * f C6HlO +C6Hil. C6Hll.
+ C6H10
C6H12
+ C6H9.
C6Hii f C6HlO --.f C6Hii-CsHio
(10) (11) (12) (13)
The radicals formed in reactions 10-13 may again react with cyclohexene or with other radicals present in the solution, most of them being cyclohexyl radicals. It has indeed been observed that new products are formed at higher conversions, notably two cyclohexylcyclohexene isomers and products with 18, 24, and 30 carbon compounds were present atoms. The C24 and C ~ O in trace amounts at the highest conversions. Cyclohexylcyclohexene may be produced according to the reactions CBHCI-k CeHii. CeHii-CsHio * f C6Hli.
--f
C&a-C6Hii
(14)
--f
C6HrC6Hii f CeHiz (15) Most of the cyclohexenyl radicals formed in reactions 10 and 12 will have the configuration H2
c-c /
H2C
\ c=c H
Hz
\ CH. / H
which is stabilized by resonance.24 As a consequence, reaction 14 will result in the formation of S-cyclohexyl1-cyclohexene,with the structure (15) M. Anbar and P. Neta, Intern. J . Appl. Radiation Isotopes, 16, 227 (1965). (16) G. A. Russell and R. F. Bridger, J . Am. Chem. Soc., 85, 3765 (1963). (17) H. A. Dewhurst, J. Phys. Chem., 63, 813 (1959). (18) P. J. Dyne and J. W. Fletcher, Can. J . Chem., 38, 851 (1960). (19) M. Burton and J. Y. Chang, Abstracts, 137th National Meeting of the American Chemical Society, Cleveland, Ohio, 1960, p RlO9. (20) S. Z. Toma and W. H. Hamill, J . Am. Chem. SOC.,86, 1478 (1964). (21) S. 2. Toma and W. H. Hamill, ibid., 8 6 , 4676 (1964). (22) Direct quenching of excited mercury atoms by cyclohexene may be neglected a t the prevailing cyclohexene concentrations, as follows from a consideration of the respective quenching cross sections of saturated and olefinic hydrocarbons.23 (23) R. J. Cvetanovih, Progr. Reaction Kinetics, 2 , 39 (1964). (24) R. F. Bridger and G. A. Russell, J . Am. Chem. SOC.,8 5 , 3754 (1963).
RADICAL REACTIONS IN LIQUID CYCLOHEXANE
HzC
C--4
\ / c=c H
H
1117
where A and B stand for 3-cyclohexyl-1-cyclohexene and 2-cyclohexyl-l-cyclohexene,respectively, and klbA and k15B are the rate constants for the formation of A and B according to reaction 15. From these relations, it follows that
CHz
\
/
C - 4 Hz Hz
+
d(A B)/dt - -ki5B _ dA/dt __ dCiddt kis dB/dt
Cyclohexylcyclohexene, formed in reaction 15, may have a similar structure, but may also consist of 2cyclohexyl-1-cyclohexene
+ he
klbB
(111)
-
It can be seen from the results listed in Table I that approximately constant ratios are found for [d(A B)/ dt ]/ [dC~s/dt] and (dA/dt)/(dB/dt) up to converHz H2 Hz Hz sions of 200 to 300, with values of ca. 1.1 and 4.5, rec--c spectively. Substitution into eq 111 leads to a value H/ \ of 0.2 for the ratio klSB/k16. Considering that in all H2C c--c CHz probability k15* > klSB and allowing for a contribution \ / \ / of reaction 14 to cyclohexylcyclohexene formation, C-C c--c it may be concluded from the values of klsB/k18 and Hz H Hz Hz [d(A B)/dt]/[dCls/dt], that 0.4 < kl5/kl6 < 1.1 or 0.2 < k15*/k16 < 0.9. It was observed, when comparing the mass spectra At still higher conversions, the ratios [d(A B)/dt]/ of the two ClzHzo isomers, that the contribution of the [dCls/dt] and (dA/dl)/(dB/dt) are found to demajor fragment peak with mass 82 differed considerably. crease. If this were the result of a change in the ratio The central C-C bond between the two rings is expected to be stronger in 2-cyclohexyl-1-cyclohexene. [H]/[CsHll], it follows from eq I and I1 that the experimental results would show the relationship Hence, it is suggested that the ClzHzo isomer with the highest contribution a t mass 82 consists of 3-cyclohexyl-1-cyclohcxene. Reaction 15 will be accompanied by the corresponding combination reaction 16, resulting in the formation The results listed in Table I clearly show that this is not of products with 18 carbon atoms. the case. The changes are observed when unsaturated Clz compounds are present in considerable concentraCaHii-CsHio. C8Hil' C1sHaz (16) tions and reactions of these molecules with cyclohexyl In addition to the reactions mentioned so far, reactions radicals and hydrogen atoms can no longer be neglected. 17 and 18 may also contribute to product formation H * C12H20 +Hz CizHi9. (19) CeHs. G H n * ---+ 2C6Hio (17)
+
/'--'\
+
+
+
CsHn-CeHio.
----f
+
+ + C6Hll' --+ ClzHn
With this reaction mechanism and using the steadystate approximation, the following relations can be derived
+ + + + + + -dA/dt hA ki2ki4(h5 + kie 4- kid dB/dt - IC16B -I- k13k15B(k14 + k17) + h e 4- hs) [HI kiaki~~(kir + k17) [CeHli] +
d(A B)/dt dCis/dt
kizki4(ki5 4h e ha) k13kl8(k14 kl7) kioki4(ki5 kl6 his) [HI (1) kl&l8(~14 kl7) [C8H11]
=-+ hi5
k16
+
kioki4(ki5
H*
CBH~O(18)
(11)
C&i*
+ CizHzo
+ CizHzo
+
+CizHzi *
+CsHiz
(20)
+ CizHig.
(21)
+ CizHzo +CisH3i (22) These reactions can account for the observed decrease in the ratio [d(A + B)/dtl/[dCl$dt]. The change CeHii *
in the ratio (dA/dt)/(dB/dt) may be due to a steric effect in addition reaction 22. It is to be expected that the addition of cyclohexyl radicals to 3-cyclohexyl-1-cyclohexene is favored relative to addition to 2-cy clohex yl-l-cyclohexene.
Acknowledgment. The author wishes to thank Miss L. E. W. van den Do01 for her assistance in performing the experimental work.
Volume 7 1 , Number 4
March 1967
