678
NOTES k=co
1.5-
1.0-
e 0.5-
0
'I
I
0
I
I
I
0.75
0.50
0.25
1.00
PIP,
Figure 1. Adsorption isotherms for the equation
P / P ~=
ee-ke
+ e-110'.
behavior is encountered in the k = 0.5 isotherm of this figure. Because of the nature of this loop, it is not possible to find two phases of equal spreading pressure. Indeed, a calculation of spreading pressure, shown in Figure 3, indicates that the spreading pressure, although it follows the ideal gas law at low coverages, may ultimately pass through zero and become negative. The thick film cannot be in equilibrium with the thin film; this situation corresponds to a liquid not wetting a surface. At saturation the surface would be covered with a rarified film, given by the lowest intersection of the isotherm with the saturation line. We are indebted to Dr. J. P. Hobson, who sent us a preprint that contains an isotherm of xenon on silver quite similar t o the upper part of the IC = 1.4 isotherm in Figure 1. However, there is a strongly held first layer and a resultant knee in the isotherm that our equation in its simple form cannot fit.
Radiation-Induced Chain Decomposition of Hexachloroethane in Cyclohexane Solutions. Reactions of the Pentachloroethyl Radical by A. Horowitz and L. A. Rajbenbach 0.25
0.50
Soreq Nuclear Research Centre, Yavne, Israel (Received March 9, 1969)
0.75
P/P,
Figure 2. Adsorption isotherms of argon on xenon and eq 1 in the form PIP0 = (1.0) Oe-2.16 e
+
Investigation of the kinetics of chlorine atom elimination from chloro-substituted alkyl radicals CJ&n+1-,C1,
CnHzn+l-mClm-1
+ C1
(1)
has been so far limited to several studies in the gas phase.1-6 Such reactions can appropriately be studied in the gas phase since they require relatively high temperatures, mainly because the activation energy of the forward step of reaction 1 is quite high2J (-20 kcal mol-') while that of the reverse step is negligible.6 The purpose of this study was to investigate the kinetics of the chlorine elimination reaction from ,
1
-
2
4
,
6
r
a
I
10
Figure 3. Spreading pressure curves based on the isotherm
P / P ~=
ee-ke
+ e-"".
initial slope and ultimate cube-law regions. The agreement in the intermediate region is reasonable. It is clear from the shape of the examples of the isotherm that unstable regions are a possibility. Aside from the obvious two-dimensional phase change in the IC = 2.1, 1.4 isotherms in Figure 1, another type of The Journal of Physical Chemistry
(1) (a) P.B.Ayscough, A. J. Cocker, and F. S. Dainton, Trans. Faraday SOC.,58, 1128 (1962); (b) P. B. Ayscough, A. J. Cocker, F. S. Dainton, and S. Hirst, ibid., 58,295 (1962); (c) ibid., 58, 318 (1962); (d) F. S. Dainton, D. A. Lomax, and M. Weston, ibid., 58, 308 (1962). (2) (a) S. Dusoleil, P. Goldfinger, A. M. Van der Auwera, G. Martens, and D. Van der Auwera, ibid., 57, 2917 (1961); (b) P. Goldfinger, G. Huybrechts, and G. Martens, ibzd., 57, 2200 (1961); (c) P.Goldfinger and G. Martens, ibid., 57,2210(1961). (3) 6. Huybrechts, L. Meyers, and G. Verbeke, ibad., 58, 1128 (1962). (4) J. H. Knox and J. Riddiok, ibid., 6 2 , 1190 (1986). (6) P.B.Ayscough, F. S. Dainton, and B. E. Fleischfreser, ibid., 62, 1838 (1966). (6) C. Cillien, P. Goldfinger, G. Huybrechts, and G. Martens, ibid., 63, 1631 (1967).
NOTES
679
Table I: The Radiolytic Yields of the Main Products of the Radiolysis of C&la Solutions in Cyclohexane"
[CnClsI,
mM
100 25 50 75 100 100 25 50 75 100
Temp, OC
U(CnC1)
U(HC1)
U(CzC1aH)
U(c-CsHuC1)
24 50 50 50 50 75 100 100 100 100
102 342 416 550 587 1600 1200 2375 3400 4100
100 368 461 578 632 1510 1270 2470 3410 4050
180 206 240 323 352 490 192 366 510 595
255 546 644 795 844 1990 1450 2575 3580 4480
~
Total dose: 3.3 X 1018, 1.6 X 1Ol8,6.6 X 1017,and 2.2 X erage of four determinations.
... ... ...
... ...
1900 1495 2700 3850 4050
l O I 7 eV m1-I for experiments at
pentachloroethyl radicals (reaction 1) in the liquid phase. Chlorine atoms react readily with alkanes and therefore the use of alkane solvent would prevent the back reaction and thus destroy the equilibrium. Such conditions would favor the study of the above elimination reaction in the liquid phase. The reaction between alkyl radicals and hexachloroethane was found' to lead to the formation of pentachloroethyl radicals. The irradiation of dilute solutions of hexachloroethane in cyclohexane would thus seem to offer a convenient way to generate pentachloroethyl radicals through the interaction of radiolytically generated cyclohexyl radicals with the solute. Experimental Section Materials. Hexachloroethane (Fluka technical grade) was purified by sublimation. The purified material was more than 99.9% pure as determined by analytical gas chromatography and did not contain free chlorine. Phillips Research grade cyclohexane, stated purity 99.99%, was used as received after it was found that treatment with sulfuric acids did not affect, within experimental error ( * 5 % ) , the yield of products. Procedure. Sample preparation and irradiation techniques were identical with those described by us previously.*tg The irradiations were carried out a t a dose rate of 2.1 X 10'' eV ml-I min-'. The compounds C2C16, C2C16H,C2C14,and c-CeHllC1 were determined gas chromatographically, using a 12-ft column of 20% silicon oil DC-200 on Diatoport S, 60-80 mesh. They were identified by comparison of retention times with those of corresponding pure samples on both the silicon and on a 12-ft, 20% Ucon column. To determine the yield of hydrogen chloride, the irradiated samples were kept a t liquid air temperature and opened under a layer of water. The aqueous layer was separated, and its HC1 content was determined coulometrically using the Aminco-Cotlove chloride titrator.
U( -CzCle)*
U(CzCl4) U(CzC1aH)
0.57 1.66 1.74 1.70 1.67 3.24 6.25 6.50 6.65 6.90
____ U(C-CsH1iCl)
1.10 1.00 1.02 1.10 1.12 1.02 0.96 1.06 1.09 1.04
24, 50, 75, and looo, respectively.
Av-
Results and Discussion The main radiolytic products at 24, 50, 75, and 100" were found to be pentachloroethane, chlorocyclohexane, tetrachloroethylene, and HC1. The yields of these products are given in Table I. They are expressed in terms of G values and refer to the total number of molecules formed or disappearing per 100 eV of energy deposited into the solution. Mechanism of Product Formation The radiolysis of alkanes results in the formation of alkyl radicals via carbon-carbon and carbon-hydrogen bond fission. I n the case of pure cyclohexane, the C-C bond breakage is relatively small, and the radiolysis of cyclohexane results primarily in the formation of cyclohexyl radicals and hydrogen atoms. Subsequently, the reaction between the hydrogen atoms and the solvent leads to the formation of hydrogen molecules and cyclohexyl radicals. Therefore, per one hydrogen molecule formed through this mechanism, two cyclohexyl radicals are formed. The radiolytic generation of cyclohexyl radicals can thus be represented by 2C-CgH12 - A2c-CsHll
+ Hz
(2)
It is possible in the radiolysis of dilute solutions of C2C16 in cyclohexane that a part of the energy initially deposited in the solvent may be transferred to the solute, leading to its enhanced decomposition. Such energy transfer processes would not be expected to decrease the over-all rate of formation of radicals. The high yields of CzClbH, CzC14, HC1, and especially of c-CeH&l (the G value of the latter product approaching 4500 at 100" at 0.1 M solute concentration, (7) J. P. West and L. Schmerling, J. Amer. Chem. SOC.,7 2 , 3525 (1950). (8) L. A. Rajbenbach and U. Kaldor, J. Chem. Phys., 47, 242 (1967). (9) A. Horowite and L. A. Rajbenbach, J . Amer. Chem. Soc., 91,4626 (1969).
Volume 74, Number S February 6, 1970
680
NOTES
as compared with approximately 5 for cyclohexyl radicals in the radiolysis of cyclohexane) indicate that all these products are formed by a long chain reaction mechanism. The free-radical nature of the chain mechanism is further supported by the finding that in the radiolysis at 100" of 0.025 M solutions of hexachloroethane in cyclohexane the addition of cyclohexene (0.1 M ) depressed the yield of all major products by approximately 90%. The length of the chain and the fact that the radicals involved in the chain mechanism are the same as those that are expected to be formed mainly as result of the radiolytic decomposition of CzC16show that no significant error is introduced when reaction 2 is considered as the initiation step of the chain sequence. The fact that within experimental error, the following equalities are observed G(c-CaHiiC1) = G(C2C15H)
+ G(CzCl4)
G( - C2C16)= G(c-CBHllC1)
Thus the yields of CzClbH and C&14 should be proportional to the C-C~HI~CI yield. The fact that expressions I11 and I V are obeyed (see Figure 1) lends further support to the chain mechanism proposed.
c
i .) 1 0-
d /
4
0 - G(C,Cl,H)
a=5~10-~
d
G ( c i c i i ) a = I x 10-3
(I) (II)
3
suggests the following propagation steps of the chain reaction
+ czc15 c2c16 + C-CaHlz --+ CzC15H + C-CBHIl c&& c&14 + c 1 CI C-CeHlz +HC1 + C-CaHll
c-CeH11
CzC&---j c-CaH11Cl
(3)
2 (4) (5)
(6)
The chain termination can be expected to result lrom radical-radical interactions. It should be noted that according to our reaction scheme the only reaction in which chlorine atoms participate is the abstraction of hydrogen from the solvent (reaction 6). This assumption seems to be justified in view of the very high values of k6 and the large excess of cyclohexane over the reaction products and intermediates which could react with the chlorine atom. As far as the cyclohexyl radical is concerned we have not considered the possibility of its addition to tetrachloroethylene. We have recently reported9 that the addition of alkyl radical to CzC14 proceeds quite readily and leads to the formation of alkyltrichloroethylene, RC2C13. Cyclohexyltrichloroethylene, however, was not found among the products in the present system, its absence indicating that under the experimental conditions the addition of cyclohexyl radical to tetrachloroethylene can be discounted. Similarly, one would not expect the addition of the bulky C2Cls radicals to tetrachloroethylene. The fact that within experimental error G(HC1) = G(CzC14) is consistent with the assumption that C2C14 is not removed from the system by secondary reactions. The steady-state treatment of reactions 2-6 leads to the following expressions. The Journal of Physical Chemietry
aG
I
0
2 3 4 G(C-C~H,,CL)XIO-~
I
Figure 1. The yields of pentachloroethane and tetrachloroethylene a t 100' as a function of the yield of chlorocyclohexane, expressions I11 and IV.
Determination of the Relative Arrhenius Parameters of Reactions 4 and 5 and the Estimation of E6 Combining eq I11 and I V with the Arrhenius formulation for kb and Jcp, we obtain
log lc-CBHlOl
(V)
From the plot of log [G(C2ClJ/G(C&15H)] as a function of l / T (see Figure 2) we obtain A5/A4 = 106.9* s.2M and E5- E4= 7.0 0.2 kcal mol-'. The activation energy of the monomolecular elimination of chlorine atom from the pentachloroethyl radical has been determined in the gas phase2 and
*
681
I
I
I
1
I
I
The Wavelength Dependence of the Primary
I
Processes i n the Photolysis of Sulfur Dioxide by T. Navaneeth Rao and Jack G. Calvert Chemistry Department, The Ohio State University, Columbus,
Ohio 43,810 (Received July $2, 1969)
Recent studies of the photolysis of sulfur dioxide within the first allowed absorption band are in accord with the following reaction 'SO2
+ SO2
so2 + hv ---f
--f
'SO2
(1)
(2S02) + (ground-state SO2) (1)
+
+%02 SO2
'SO2
--t ----f
(2)
so2 + hvf so2
(3)
(4)
--+3S02 %02 -+SO2 hv,
(5)
+
I
I 2.8
I
I
I
I
3.2
3.0
---f
I
1
103/T "K
aS02
+ SO2
so2
-+(2502) +(probably SO3
(6)
+ SO)
(7) (8)
(7) (8)
The 'SO2 and 9 0 2 represent the first excited singlet (presumably 'BI) and triplet ( 3 B ~states, ) respectively, of sulfur dioxide. Very recent work from this laboratory suggests that the dominant ultimate product of reaction 1 is ground-state sulfur dioxide molecules, while that of reaction 8 is SO8 and SO.4 Although there is agreement on the major features of the mechanism, there remains considerable uncertainty as to the importance of reactions 4 and 5 relative to reaction 3. Rao, Collier, and Calvert concluded that the rate of singlet removal by internal conversion and intersystem crossing, (4)and ( 5 ) , outweighed that by radiative decay, reaction 3.'" On the other hand, Mettee concluded that reaction 3 was the only fate of 'SO2 at low pressures of sulfur dioxide.'" Strickler and Howell presented no evidence on this matter, but for theoretical reasons they accepted the dominance of (3) in their discussions.2 It is very important that further evidence bearing on this feature of the mechanism be obtained, since the significant contribution of first-order, nonradiative decay processes to the deactivation paths in a molecule as simple as sulfur dioxide is unexpected in terms of present
Assuming the activation energy of reaction 8 to be negligible, the over-all activation energy of 9.6 kcal mol-' for the formation of C-CBHI~CI was assigned to reaction 7. Using this value of E, as equal to E4, we get 16.9 kcal mol-' for Es, in close agreement with the results in the gas phase.
(1) (a) H. D. Mettee, J. Chem. Phzls., 49, 1784 (1968); (b) H. D. Mettee, J . Phys. Chem., 73, 1071 (1969). (2) 8.J. Strickler and D. B. Howell, J . Chem. Phys., 49, 1947 (1968). (3) (a) T. N. Rao, 8.8.Collier, and J. G. Calvert, J . Amer. Chem. Soc., 91, 1609 (1969); (b) T. N. Rao, S. 8. Collier, and J. G. Calvert, ibid.,
(10) A. Henglein, E. Heckel, Y. Ojima, and G. Meisner, Ber. Elmeenges. Phys. Chem., 67,988 (1963).
91,1616 (1969). (4) 9. Okuda, T. N. Rao, D. H.Slater, and J. G. Calvert, J . Phys. Chem., 73,4412 (1969).
Figure 2. Arrhenius plot for k&,
expression V.
was found to be 16.8 kcal mol-'. It would therefore be interesting to compare our results with those obtained in the previous study. The determination of E5 requires knowledge of the Arrhenius parameters for the abstraction of the hydrogen atom from cyclohexane by the C2C15 radical. Unfortunately, these parameters are not known. However, the Arrhenius parameters of the analogous reaction of the C c & radical, which may be expected to be fairly close to that of the C2C15 radical, have been reported. Henglein, et al.,10 arrived a t the Arrhenius parameters of the reaction of CCla radicals with cyclohexane by studying the temperature dependence of the chlorocyclohexane yield obtained by a chain mechanism in the radiolysis of CC14 solution in cyclohexane. The chain propagation reactions in the c-C6Hl2-CCl4 system were
+
+
CCls C-CBHIP--j CClaH C-CBHii cc& C-C&&1 ccl4 * d&HllCl
+
+
Volume 7.4, Number 3 February 6 , 1070
