E'. J. JOHNSTON, T.-H. CHENAND K. Y. WONG
728
Vol. 65
EFFECTS OF TEMPERATURE AND ADDED HEXACHLOROETHANE O N THE RADIOLYSES OF CARBON TETRACHLORIDE AND CHLOROFORM BY F. J. JOHN ST ON,^ TUNG-HO CHENAND K. Y. WONG Department of Chemistry, University of Louisville, Louisville 8, Kentucky Received Julg 89, 1960
HC1 yields from irradiated chloroform are, for doses a t least as high as 1.3 X 1021 e.v. per gram, greater a t 70 i 4" than at 20". The initial G-value at the higher temperature is 31 compared wit! an average of 11.9 at 20'. Chlorine yields from irradiated carbon tetrachloride are slightly smaller a t i 0 f 4" than at 20 . The corresponding G-values are 0.58 and 0.65. The addition of hexachloroethane to carbon tetrachloride prior to irradiation causes a nearly linear decrease in G(cin)with per cent. hexachloroethane. The addition of hexachloroethane to chloroform rior to irradiation causes, a t low doses, an ) that in pure chloroform. G(nci, in chloroform-hexachEroethane mixtures decreases with dose and increase in G ( H c ~above above approximately 1 x 1021e.v. per gram, becomes less than in pure chloroform. These results are discussed in terms of free radical mechanisms.
Schulte,2 Miller3 and Chen, Wong and Johnston4 have studied the radiolyses with Co-60 gammas of chloroform and carbon tetrachloride. The latter have studied radiolyses of mixtures of the two. The results from irradiation of thoroughly degassed chloroform were consistent with the mechanism CHC1.q CHClz CHC1.q -+cCl3 h*5-
These reactions are followed by
+ +
+ C1
+ I€
+ +
C1 CHCL +HC1 CClr H CHCL +HC1 CHClz
(1) (2)
(3) (4)
Since no H2or C12was observed, the reactions C1 H
and
+ CHC1.q +CHCll + Clz + CHC1, +Cc13 + Hz
must not compete with 3 and 4 in this system. The tri- and dichloromethyl radicals produced in 1, 2, 3 and 4 combine to form a higher boiling residue and hexachloroethane. Miller3 reports the formation of CH2C12and CC1, in smaller yields indicating that these radicals may undergo abstractions such 11s and or and
+ CHC4 --++CHzClz + Cls + CHCla +CC1, + CHClz CHClz + HC1 CHzC12 + C1 CCls + HC1+ CC1a + H
CHClz Cc13
ic
Experimental Reagents.-"Raker Analyzed" reagent grade chloroform and carbon tetrachloride were dried with anhydrous calcium c'Lloride and distilled through a Widmer type fractionating column. Middle fractions comprising roughly 1/3 of the starting materials were used for the sample preparations. Eastrnan hexachloroethane was doubly sublimed in vacuo and stored in the dark prior to use. Sample Preparation.-Pyrex irradiation cells of approximately 20-cc. volume containing weighed samples of chloroform, carbon tetrachloride or hexachloroethane solutions were thoroughly degassed by several cycles of freezing, evacuating and thawing on a vacuum line and sealed off. Radiation Source.-Irradiations were carried out in a cobalt-60 source of the type described by Burton, Ghormley and Hochanadel.6 Dose rates in these experiments were 1.70-2.06 X 10'9 e.v. per ml. per hour on the basis of Fe++F e + + +dosimetry. For irradiations a t the higher temperature, the irradiation cell was placed inside an auxiliary tube which was surrounded with a heating coil and asbestos and filled with alumina. The temperature was measured by a thermocouple-potentiometerys stem. Temperature control at 70" was only t; within Z+Z 4'. Control experiments were performed at 20 in the same system. Analysis of Reaction Mixtures.-HC1 and Cl, were determined as described in our previous paper . 4
Results Effect of Increase in Temperature.-Table I summarizes Clz yields from carbon tetrachloride as a function of energy absorbed a t 20 and 70 f 4". A slight but definite decrease was observed at the higher temperature. The corresponding G-values are 0.65 and 0.58. (We have previously reported G(CI,)= 0.66 at the lower temperature.)
We have confirmed the formation of CHzClz and CC1, but have not made quantitative measurements TABLE I on their yields. EFFECT OF INCREASING TEMPERATURE O K C',cln) Results from the irradiation of thoroughly deCC1, gassed carbon tetrachloride were consistent with E abs., Clr vield. the reactions (e.v./g.) (molecules/g.) CCla MI-3 CC13
+ C1
(5)
Temp., '(2.
20
followed by 2C1+ Clz 2CCls ---f CzCle
(6)
(7)
The G-value for Clzproduction has been reported as 0.80,' 0.872and 0.66.3 We have studied the effects of an increase in temperature and addition of hexachloroethane on G(HC1) from carbon tetrachloride and on G(Hc~,from chloroform. (1) Department of Chemistry, University of Georgia, Athens. (2) S. TV. Schulte, J . A n . Chem. Soc., 79, 4643 (1957). (3) W. Miller, Ph.D. thesis, University of Edinburgh, 1958. (4) T. E. Chen, K. Y.Wong a n d F. J. Johnston, J. P h y s . Chem., 64, 1023 (1960).
70 f 4
x
10-30
4.41 7.42 8.25
x
10-18
GGIZ
90 29
0 G58 ,654 ,641
15
,645
11.10
2 4 6 7
11.75
7 80
4.30 8 00 12.58
FROM
85
Av.
,603 ,652
Av.
,579 ,587 ,578 ,581
2 49 1 70 7 15
In Table I1 are shown HC1 yields from chloroM. Burton, J. H. Ghormley a n d C. J. Hochanadel, N z r ~ 1 ~ o n i c s .
(5)
13, No. 10,74 (1955).
RADIOLYSES OF CARBON TETRACHLORIDE AND CHLOROFORM
May, 1961
form at the same two temperatures. In contrast to the carbon tetrachloride system a marked increase was observed a t the higher temperature. A deviation from the linearity of HC1 yield with adsorbed energy, which is characteristic of the results a t 20", was evident at 70". The initial value for G(HC1) at 70" was 31 compared to an average of 11.9 at 20". (We have previously reported 11.4 for ( ~ ( H C I )a t the lower temperature.) This temperature coefficient corresponds to an apparent activation. energy of 3.4 kcal. per mole.
TULEIV HC1 PRODUCTION FROM C2Cl&HC& SOLUTIONS E abs., (e.v./g.)
x 10-90 8.11 9.04 9.55 7.45 7.71 8.29
Wt. % CzCla
2.46 4.75 8.84 12.67 20.71 26.15
Temp., OC.
20
70 f 4
E abs., (e.v./g.)
x lo-% 1.61 2.60 5.18 7.72 12.88 1.78 4.27 6.04 8.54 12.84
HCl yield, (molecules/g., x 10-10
5.49 13.37 16.60 18.97 24.30
30.8 31.3 27.5 22.2 18.9
12.4 13.0 13.0 14.0 13.4 12.6
3.63 5.17 9.09 12.17 16.45 20.04
15.8 14.0 10.5 11.5 11.0 9.5
8.19
1.91 3.63 5.60 8.35 10.88 13.17 14.92 16.35 20.05
3.13 5.28 7.10 9.29 12.72 14.12 15.07 16.50 18.17
16.4 14.6 12.7 11.1 11.8 10.7 10.1 10.1 9.0
18.02
2.30 3.95 9.39 10.99 15.06 21.25
3.76 6.03 11.52 12.53 18.18 19.99
16.4 15.3 12.3 11.4 10.1 9.4
O(HC1)
12.4 12.3 11.7 11.9 11.5 Av. 11.9
G(FICU
10.09 11.78 12.40 10.42 10.36 10.46
2.30 3.69 8.64 10.56 15.51 21.02
FROM
2.00 3.20 6.05 9.21 14.85
HC1 yield (molecules/g.) x 10-19
6.54
TABLEI1 EFFECTOF INCREASINQ TEMPERATURE ON Gtaol, CHClS
729
Effect of Added Hexachloroethane.-In Table I11 are listed Clz yields at 20" from carbon tetrachloride solutions containing varying concentrations of hexachloroethane. An approximately linear decrease in G{Cl2)was observed with percentage hexachloroethane for a series of mixtures subjected to a dose of 1.07 X 1O2I e.v. per gram. The data in the last 4 rows of Table I11 illustrate the dependence of G(C1,) from solutions containing 20% hexachloroethane upon dose. Solubility limitations prevented carrying out experiments at hexachloroethane concentrations greater than approximately 28%.
Radiolysis of Pure Hexachloroethane.-One sample of doubly sublimed, degassed hexachloroethane was subjected to a dose of 9.77 X lozoe.v. per gram and analyzed for Cl2. None was detected. The solid sample, however, appeared moist following irradiation and a second sample similarly treated was irradiated in a break seal tube. Following a dose of 2.53 X 1021e.v. per gram, this tube was opened to an infrared cell. This spectrum showed only the presence of a peak at 12.55 I.L which was characteristic of carbon tetrachloride. Our experiTABLE I11 ment did not permit the evaluation of a G-value C1, PRODUCTION FROM CzClo-CCL SOLUTIONS for the formation of carbon tetrachloride. E abs., Clz yield (e.v./g.) (molecules/g.) Discussion x 10-33 x 10-1s G ~ l s Wt. % CnCla 6.41 0 602 10.65 4.91 A rate expression for the formation of ClZfrom 4.76 .447 carbon tetrachloride may be obtained readily in 10.65 9.84 4.68 .440 terms of reactions 5-7 and the reaction 10.65 13.18 3.40 .319 10.65 18.88 CCb + Cl +CCli (5') 1.85 .174 10.65 27.20 Writing aJ, as the rate of 5 and assuming steady 0.75 .333 2.25 20.0 .319 state conditions 5.16 1.65 20.0 I
20.0 20.0
11.oo 18.30
3.30 5.10
.300 .279
In Table TV are shown HC1 yields in chloroformhexachloroethane solutions for several concentrations of hexachloroethane and as a function of absorbed energy. In contrast to the Clz yields in the carbon tetrachloride-hexachloroethane system, G(Hc~)at low doses was greater than in pure chloroform. With increasing doses G(HCI) values decreased and beyond approximately 10z1 e.v. per gram fell below that for pure chloroform.
d o dt
= aI.-
kg' (CCla)(C1)- k,(Cl)Z
=
0
(1)
Subtracting I1 from I we obtain CCla = (k6/k7)'/?(C1)
(111)
And by substitution into I (C1)* = c J ~ / [ ? c ~ ' ( k~ ~) ~ ] '*+ (IV)
So that
F. J. JOHSSTOX, T.-H. CHEX-4iYD K. I-. WOXG
730 dt
=
+ 1 ~ ~ ' / ( k ~ k ~ )(v) 14
X - ~ ( C=I O~ ~J J ~
Equation V represents the rate of Clz production in systems in which reactions involving product Cln and hexachloroethane do not significantly contribute. The linearity of our observed Clz yields with absorbed energy indicates that this is true to a t least 12 X lozoe.v. per gram. The numerator on the right hand side of V should increase slightly with temperature if the products of reaction 5 are formed with a near normal energy distribution. If highly energetic radicals are formed, it should be essentially independent of temperature. Kinetic data are not available to allow speculation concerning reactions 5', 6 and 7 with respect to activation energies and frequency factors. It would be expected, however, that rates of such radical combinations in solution would be largely diffusion controlled and, therefore, have temperature coefficients corresponding to apparent activation energies of approximately 3 kcal. per mole. Changes in the rate k6'/(k&7)1/z with temperature would certainly be small and it is not improbable that any such change would be positive. On the basis of these considerations, and the form of equation V, it is not surprising that the change in G(cl,, with temperature is small and in a negative direction. The observed decrease in G(c1,) in carbon tetrachloride-hexachloroethane solutions with increasing hexachloroethane concentration is much greater than would result from a dilution by an inert substance. At a dose of 1.07 X 1021e.v. per gram, G(cl2)in a solution containing 27.2% hexachloroethane is approximately one-fourth the value in pure carbon tetrachloride. In the one series of solutions for which Clz yields were measured as a function of dose, a consistent decrease in G(c1,)with absorbed energy was observed. This result suggested that a contributing factor to the lowered G(c1,) values involved a reaction between radicals produced from hexachloroethane and molecular chlorine. While we may not extrapolate directly our observations concerning the radiolysis of solid hexachloroethane to the liquid phase, the absence of C1, suggests that the reaction CZClS M_, c2c15
+ c1
(8)
is less probable than C&le
+2cc13
(9)
Carbon tetrachloride might then be formed as a wsult of the abstraction reaction CC13
+ CzCls
-3 CCl,
f CzCls
(10)
ITe cannot>,however, rule out the possibility that carbon tetrachloride is formed directly from hexachloroethane, C2Cl.6 -+cc1,
+ CCll
Assuming that reaction 9 is the most probable result of energy absorption by hexachloroethane, lowered G[cl9, values in carbon tetrachloride-
Yol, 65
hexachloroethane mixtures may then be explained in terms of several mechanisms any or all of which may contribute. (1) The increased concentration of CCls radicals will result in an increased frequency of reaction 5'. (2) Occurrence of the reaction Cc13 f Clz ---f CClr f c1
(11)
(3) Occurrence of the reaction 2cc13
+ c1, +2CCl4
(1'4
Reaction 12 is that proposed by Schumacher6 as the chain breaking step in the photochlorination of chloroform. We have felt that such a termolecular process involving two free radicals would occur with a very low frequency. The observed decrease of G(c12)with dose suggests, however, that either or both of reactions 11 and 12 are important. The rate of HC1 formation from chloroform cannot be simply expressed in terms of energy absorption and the rate constants of equations 1-4 and of corresponding radical recombination reactions. Qualitatively, however, an increase in temperature would favor the abstraction reactions 3 and 4 more strongly than radical recombination reactions such as the reverse of 1 and 2, the latter reactions involving small activation energies. The observed increase in G(Hc~)with temperature is in agreement with these considerations. The effect of added hexachloroethane on G~HCI) from chloroform can best be explained, within the framework of free radical mechanisms, in terms of a significantly greater rate for the reaction H
+ CsC16 +HC1 + CzCls
(13)
than for reaction 4. H atoms produced in reaction 2, which in pure chloroform would recombine with the sibling radical, may react by means of 13 with hexachloroethane molecules forming part of the cage wall. G(HCI)values as a result are greater, a t low doses, in the mixtures than in pure chloroform. As more HC1 is formed reaction 14 (and possibly 15) becomes important and G(HCI)decreases with dose.
+
CCl3 HC1+ CI-IC13 2CC13 HC1+ CHC13 CC14
+
+
(14) (15)
It is interesting to note that initial G(HcI)-values do not change significantly above 6.54% hexachloroethane. In terms of the above mechanism, this suggests that a t least above this concentration all available H radicals are undergoing reactions 4 and 13. Measurements of G(Hc~) as a function of absorbed energy were not made below this concentration. The above discussions in terms of free radical reactions are consistent with an observed results. Additional or alternative reactions involving ionic species might well be involved. This work has been supported by iitomic Energy Commission Contract AT-(40-1)-2055. (6) H.
(1934).
K. Schumacher and K. Wolff, Z. p h y s i k Chem., 25B, 161
