THE JOURNAL
OF
PHYSICAL CHEMISTRY
R e g i s h e d in U.S. Patent O s e e @ Copyight, 1967, by the American Chemical Society
VOLUME 71, NUMBER 5 APRIL 14, 1967
Radical Reactions in Liquid Cyclohexane. I. The Photolysis of Solutions of Diphenylmercury in Cyclohexane
by
W.A. Cramer
Reactm Inathut, Delft, The Netherhnd8 Accepted and Transmitted by The Faraday Society
(September 6, 1966)
Solutions of diphenylmercury in cyclohexane have been irradiated with light of 2537 A in the presence and absence of oxygen. Phenyl radicals formed from the solute react with cyclohexane to produce benzene and cyclohexyl radicals. I n the absence of oxygen, reactions between cyclohexyl radicals give rise to cyclohexene and dicyclohexyl formation. The results indicate that the ratio of rate constants for disproportionation and combination reactions between these radicals has a value of 1.1. In the presence of oxygen, cyclohexyl radicals react with this additive, giving rise to the formation of approximately equal amounts of cyclohexanol and cyclohexanone.
Recently, reactions between cyclohexyl radicals in liquid cyclohexane have attracted much interest, since they contribute considerably to product formation in the radiolysis of cyclohexane. Dicyclohexyl, for example, which is one of the major products in irradiated cyclohexane, is to a very large extent produced by the combination of two cyclohexyl radicals. Cyclohexene, on the other hand, is not only formed by disproportionation reactions between two cyclohexyl radicals, but also by unimolecular decomposition of cyclohexane molecules into cyclohexene and a hydrogen molecule or two hydrogen a t o r n ~ . ~ JHowever, the relative importance of these processes is uncertain. It would .therefore be of interest to know the yield of cyclohexene that can be attributed to disproportionation reactions between cyclohexyl radicals. This can be calculated if the ratio of rate constants
for disproportionation and combination reactions between cyclohexyl radicals is known. Values for this ratio in the liquid phase of 1.31 and 1.47 have recently been reported by Klots and Johnsen4 and by Falconer and Burton,b respectively. These values, calculated from experimental results obtained in the mercuryphotosensitized decomposition of cyclohexane, are based on the assumption that only reactions between cyclohexyl radicals contribute to the formation of cy(1) R.Blackburn and A. Charlesby, Proc. Roy. SOC.(London), A293, 51 (1966). (2) P. J. Dyne and W. M. Jenkinson, Can. J . Chem., 39, 2163 (1961). (3) 8. 2. Toma and W. H. Hamill, J . Am. Chem. Soc., 86, 1478 (1964). (4) C. E. Klots and R. H. Johnsen, Can. J . Chem., 4 1 , 2702 (1963). (5) J. W.Falconer and M. Burton, J . Phya. Chem., 67, 1743 (1963).
1171
W. A. CRAMER
1172
clohexene and dicyclohexyl. It has recently been shown that cyclohexene is also produced in this system by reactions between a cyclohexyl radical and its sibling hydrogen As a consequence, the reported values, which were supposed to be equal to the ratio of the yields of cyclohexene and dicyclohexyl, must be corrected. However, since the contribution of this reaction has not yet been well established, such a correction cannot be applied with sufficient accuracy. As an alternative method to study reactions of cyelohexyl radicals in cyclohexane, the photolysis of solutions of diphenylmercury in cyclohexane was investigated. Decomposition of the solute results in the formation of phenyl radicals. They react with the solvent by H-atom abstraction to produce benzene and cyclohexyl radicals. Experiments were carried out both in the absence and presence of oxygen.
Experimental Section Cyclohexane was prepared from pure benzene (mp 5.46-5.50") by catalytic hydrogenation at 150°, using nickel on diatomaceous earth as a catalyst. After hydrogenation, the liquid was treated with sulfuric acid to remove traces of unsaturated hydrocarbons, then washed, dried, and finally distilled from sodium in a 17-plate column. Diphenylmercury, Fluka purum, was recrystallized from cyclohexane. Irradiations were carried out in Vycor cells after degassing the liquid by the conventional freeze-thaw technique. For experiments in the presence of oxygen, a stream of oxygen gas was passed through the solution for 2 min prior to irradiation. The solutions were irradiated by ultraviolet light of 2537 A from a lowpressure mercury lamp at about 23". Irradiation times were varied from 15 sec to 2 min. During the irradiation, the vapor phase above the liquid was masked. Hydrogen was collected a t - 196" and measured volumetrically. Other products were measured with a gas chromatograph, using a flame-ionization detector. Cyclohexene and benzene were analyzed at 40" on a 10-m column containing 10% P,P'-oxydipropionitrile and 1% AgNOa on 60-70 mesh kieselguhr (Embacel, May, and Baker Ltd.). Dicyclohexyl, cyclohexanol, and cyclohexanone were separated at 150" on a 4-m column containing 20% polyglycol4OOO Ltd') On 70-100 mesh Embacel* Standard (E* solutions were used for quantitative measurement of all the liquid products. Results and Discussion Irradiation of the solutions results in decomposition of the solute according to the over-all reactions.Q The J m m l of Physical Chemi&y
Hg(CaHd2 -%- Hg
+ ~CSHE,
(1)
It has been shown that phenyl radicals react very efficiently with cyclohexane.8-lO This results in the formation of benzene and cyclohexyl radicals. CeHti.
+ CeHi2
CeHs -k CeHii.
+
(2)
In the absence of oxygen, the latter radicals react with each other according to reactions 3 and 4 2C8Hll'
-
----)
2csHii
C6HlO
+ C6H12
C12H22
(3) (4)
Experiments were carried out at low conversions in order to minimize the contribution of the mercuryphotosensitized decomposition of cyclohexane by mercury atoms formed in reaction 1. The contribution of this process was evaluated by measuring the amount of hydrogen which is produced according to reactions 5,6, and 7. Hg(68Pi)
+ CaHiz Hg(6lSo) + CeHii. + Ha H * + CeHn Hz + CsHn H. + C8Hll. Ht + C8HlO --j
+
+
(5) (6) (7)
The experimental results are listed in Table I. In addition to the products listed in Table I, very minor amounts of diphenyl, phenylcyclohexane, and cyclohexylcyclohexene could be detected at the highest conversions. The ooncentrations of these M. products were always smaller than The last two columns in Table I show that the yield of benzene plus twice the yield of hydrogen is approximately equal to twice the yields of cyclohexene and dicyclohexyl, as would be expected from material balance considerations. Based on the proposed mechanism, it is possible from the experimental results to calculate the ratio of the rate constants of reactions 3 and 4 if the contribution of reaction 7 is known. It has been shown that the fraction of the hydrogen, formed according to reaction 7 in the mercury-photosensitized decomposition of cyclohexane, amounts to ca. 23% or less.' This figure, together with the observed hydrogen yields as listed in Table I, indicates that the contribution of re(6) R.R. Hentz, J. Y. Chang, and M. Burton, J. Phys. Chem., 69, 2027 (1965). (7) W.A. Cramer, ibid., 71, 1112 (1967). (8)G. A. Rasuvaev, G. G. Petukhof, Y. A. Kaplin, and L. F. Kudryavtsev, DoM. A h d . Nauk SSSR, 141, 371 (1961). (9) G. A. Rasuvaev, Zh. Vses. Khim. Obshchestva im. D. I . Mendelema, 7, 325 (1962). (10) W. A. Cramer, Thesis, University of Amsterdam, 1961.
RADICAL REACTIONS IN LIQUID CYCLOHEXANE
1173
Table I: Products Formed in the Photolysis of Solutions of Diphenylmercury in Cyclohexane
4.3 4.8 4.8 4.8 4.8 4.8 7.1 7.1 7.1 7.1 11.8 13.1 12.8 11.8 12.9 12.8
11.8
4.22 7.21 9.28 11.06 14.49 17.11 6.39 10.02 15.77
0.41 0.43 0.80 0.76 0.98 0.95 0.73 0.51 0.58 0.88 0.17 0.15 0.31 0.31 0.41 0.56 1.07
1.45 2.24 2.72 3.56 4.83 5.84 2.01 2.89 4.69 5.17 1.40 1.69 2.13 3.22 4.84 5.18 10.84
18.11 4.91 6.46 7.89 10.43 17.81 18.96 33.47
action 7 to cyclohexene formation is small. This has been confirmed by experiments which were carried out in the presence of oxygen. Such a study of the effect of oxygen in the mercury-photosensitized decomposition of cyclohexane has shown that, whereas the yield of dicyclohexyl can be reduced to zero, there always remains a residual yield of cyclohexene.' Oxygen, which is known to be an efficient radical scavenger, can interfere with reactions 3 and 4 between radicals in the bulk of the solution. However, if present at moderate concentrations, it cannot prevent the occurrence of reaction 7 between sibling hydrogen atoms and cyclohexyl radicals within the liquid cage immediately after their formation, which was proposed to occur in the mercury-photosensitized decomposition of cyclohexane. The residual yield of cyclohexene from this sensitized decomposition in the presence of oxygen may be taken as evidence for the occurrence of reaction 7. In the photolysis of diphenylmercury in the presence of oxygen, it was observed that the yields of both cyclohexene and dicyclohexyl are reduced to zero if prior to the irradiation, which lasted for 2 min, a stream of oxygen gas was passed through the solution for 2 min. Cyclohexanol and cyclohexanone were found as new products in approximately equal amounts, as shown by the results listed in Table 11. These compounds are generally assumed to be produced according t o the over-all reaction 8.11 Benzene is still produced C6H1102'
+ CSH1102'
-
1.01 1.91 2.62 3.23 3.97 4.93 1.75 3.17 4.68 5.70
1.4 1.2 1.0 1.1 1.2 1.2 1.2 0.9 1.0 0.9 1.2 1.0 1.1 1.2 0.9 1.0 1.2
1.18 1.66 1.98 2.75 5.36 5.27 9.36
a x (CtHlo CizHn)
5.0 8.1 10.9 12.6 16.5 19.0 7.9 11.0 16.9 19.9 5.3 6.8 8.5 11.1 18.6 20.1 35.6
4.9
+
+
02
(8)
in these experiments in the presence of oxygen. This
+
8.3 10.7 13.6 17.6 21.5 7.5 12.1 18.7 21.7 5.2 6.7 8.2 11.9 20.4 20.9 40.4
is accompanied by the formation of cyclohexyl radicals according to reaction 2. Hence the disappearance of both cyclohexene and dicyclohexyl indicates that Table 11: Product Formation in the Presence of Oxygen [HdCaHdrl,
[CsHoI,
[CsHioOl,
[CsHiiOHl,
lo-* M
10-4 M
10-4 M
10-4 M
2.25 2.25 5.63 5.70 7.53
2.87 2.05 3.58 2.89 2.80
2.31 1.78 2.99 2.18 2.28
2.52 1.85 3.19 2.41
2.64
cyclohexyl radicals are scavenged by oxygen before they react with each other and that reaction 7 does not contribute to cyclohexene formation. It is now possible to calculate the ratio of rate constants of reactions 3 and 4, using the results listed in Table I. The contribution of reaction 7 being small, this ratio is equal to the ratio of the yields of cyclohexene and dicyclohexyl. From the experimental results a value of 1.1 is obtained with an error of 0.14. The observation that no cyclohexene is formed in the presence of oxygen also shows that reaction 9 which was proposed by Ho and Freeman,12 does not occur to an appreciable extent at room temperature. C6H11'
C~HIOO CSHIIOH
+
COHO HI)
(2 X
+
0 2 ----t
C6HlO
+ HOZ.
(9)
(11) R. Blackburn and A. Charlesby, Trans. Fafaday Soc., 62,1159 (1900). (12) 9. K. Ho and G. R. Freeman, J. Phys. Chem., 68, 2189 (1964).
Volume 71, Number 6 April 1967
SERGIO PETRUCCI
1174
The importance of this observation with respect to the mechanism of the mercury-photosensitized decomposition of cyclohexane will be discussed elsewhere?
Acknozuledgmmt. The author wishes to express his appreciation to Miss L. E. W. van den Do01 for her assistance in the measurements.
Ionic Association. I. Viscosity Effect on the Ultrasonic Relaxation of Magnesium Sulfate in Water-Ethylene Glycol Mixtures at 25"
by Sergio Petrucci Departmeni of Chemistry, Polytechnic Institute of Brooklyn, Brooklm, New York (Received April 18, 1966)
Measurements of ultrasonic absorption in the frequency range 50-170 Mc/sec for M@Or in water-ethylene glycol mixtures at 25' are presented. The relaxation frequencies decrease by increasing the viscosity of the solvent. This dependence is interpreted in terms of the effect of the viscosity on a diffusion-controlled reaction between solvtated Mg2+ and to give a solvated ion pair.
Recent developments of relaxation kinetics' and in particular the interpretation of the sound absorption spectra by Eigen and his school2 have brought new insight into the association process. According to Eigen and Tamm,2 the over-all process can be schematized for 2-2 salts in water as a three-step mechanism as Mea+(aq)
kir
kn
For the present investigation, MgS04has been chosen as the electrolyte because of recent investigations of ultrasonic absorption of its aqueous ~olutions.~ In order to increase the viscosity of the medium, the solvent mixture water-glycol has been chosen. In the frequency region investigated (50-170 Mclsec), no relaxation effect of the solvent mixture has been observed at 25.0".
kn
kn
Experhenhi Section
+ Lig2-(aq) E Me2+(H20)zLig2-E Step b
Step a kU
Me2+(HzO)Lig2-
MeLig (I) k4s
Step c Relatively few ultrasonic investigations have been made in mixed solvent s y ~ t e m s . ~ The main purpose WBS to study the effect of the dielectric constant on the relaxation frequencies of the electrolytic solutions. To this author's knowledge, no study has been made on the effect of the solvent viscosity on the upper relaxation frequency of 2-2 electrolytes. The J o u W of P h y W Chem&trU
Materials. MgSO, (Baker and Adamson), anhydrous reagent grade, was dried at 210" for a period of 72 hr and weighed up to constant weight (within *O.l mg). Analysis of weighed MgS04 samples by cation exchange and titration of the resulting acid gave re(1) M. Eigen and L. DeMaeyer, "Techniques of Organic Chemistry," Vol. VIII, Part 2. A. Weissberger Ed., John Wiley and Sons, Inc., New York, N. Y., 1963, pp 895-1055. (2) M. Eigen and K. Tamm, 2. Elektrochem., 66,93,107 (1962). (3) K. Tamm and G. Kurtze, Acustica, 4, 380 (1954); D.A. Bies, J. Chem. Phys., 23, 428 (1955); J. R. Smithson and T. A. Litovitz, J. Acowrt. SOC.Am,, 28, 462 (1956); S. K. Kor and G. S. Vema,
J. Chem. Phys., 29, 9 (1958). (4) G. Atkinson and S. Petrucoi, J. Phys. Chem., 70, 3122 (1966).
