54 1
ABSTRACTION OF CHLORINE ATOMSBY METHYLRADICALS
-
Abstraction of Chlorine Atoms by Methyl Radicals
by D. M. Tomkinson, J. P. Gadvin, and H.0. Pritchard Chemistry Department, University of Manchester, hfanchester 13, England
(Receiaed September 26, 1963)
Methyl radicals have been shown to abstract chlorine atoms from carbon tetrachloride, hexachloroethane, and hexachloroacetone, --id the Arrhenius parameters of these reactions have been determined. In an appendix, w e make some cautionary comments on the abstraction reactions of CC13 radicals.
Introduction The kinetic experiments reported in this paper arose from some exploratory studies on CC13 radical reactions. CC14, C2C16,and CCl3C0CC13 were photolyzed a t 2537 and 3130 A. in the presence of aliphatic saturated hydrocarbons, the experiments with propane at 210' being typical. Carbon tetrachloride did not decompose a t 3130 A., biit a t 2537 PI., the products of reaction contained CzC16, CClBH, HCI, i-PrC1, and n-PrCl, with the ratio of i-PrC1:n-PrC1 being about 20. The principal products from hexachloroethane a t both wave lengths were CzC14, HC1, i-PrC1, and n-PrC1 with i-PrC1 :n-PrC1 50. Hexachloroacetone also photolyzed a t both wave lengths, giving CZCl6,CC13H[, CCl,, HC1 (in considerable quantities), and again, both propyl chlorides with i-PrC1:n-PrC1 c- 2 . The common factor in all these photolyses appears to be the formation of chlorine atoma. It is now well established from the work of Wijnei~I-~that CCL, photolyzes to CCl, C1 and our observations are consistent with this. Our experiments show that CZCl6 does not give CC13 radicals on photolysis, in contrast to the thermal decomposition which gives both C1 atoms and CC13 r a d i c a l ~ . ~ On # j the other hand, photolysis of CC13COCC13 does give CC13 radicals,6 but in addition, as is ithe case in the liquid phase,' a substantiad side reaction leads to chlorine atoms.* The interesting point, however, is that in none of these reactions is the ratio i-PrC1: n-PrC1 characteristic of the reaction of C1 a,toms, which a t 210' would give equa,l amounts of each isomer.$ Since neither propyl chloiride isomerizes under the experimental conditions, a possible source of this discrepancg is that the n-Pr and i-Pr radicals, produced by C1 atom and CC1, radical attack on propane, react differently with the
+-
three photolytic sources. We therefore undertook a series of kinetic experiments to determine the feasibility XC1, + RCl XCln-.l, of reactions of the type R and to avoid unnecessary complications R was chosen to be CH8 for the initial studies.
+
+
Experimental In view of the photochemical reactions of these three substances, it was decided to use di-t-butyl Known peroxide as a thermal source of CH, amounts of the peroxide and the perchlorg compound were sealed in a 300-ml. Pyrex vessel, which was then totally immersed in a thermostated oil bath and left there until about 10% of the peroxide had decomposed. The temperature range used was 90-140', with corresponding reaction times of I week down to 30 min. ; consumption of the chloro compound mas some 2-5%. (1) B. C. Rocquitte and M. €I. J. Wijnen, J . Chem. Phys., 38, 4 (1963). (2) J. E , Gregory and X I . H . J. Wijnen, ihid., 38, 2925 (1963). (3) B. C. Rocquitte and M. H. J. Wijnen, J . Am. Chem. Soc., Ei5, 2053 (1963). (4) F. 8. Dainton and K. J. Ivin, Trans. Faraday Soc., 46, 295 (1950). (5) L. A. Errede and J. P. Cassidy, J . Phus. ChPm., 67, 1358 (1963). (6) S. Hautecloque, Compt. rend., 254, 3671 (1962). (7) R. N. Hasseldine and F. Nyman, J . Cham. Soc., 3015 (1961). (8) I t is possible t h a t some HCl may be formed, particiilarly a t low pressures, by a disproportionation between c c h and R, e.g., CCla CHI + CClzCHz HC1. However, we failed to find CCl&FI? when acetone and hexacliloroacetone were photol3xed together; unfortunately, the analysis was not sufficiently definitive to prove t h a t such a reaction does not go, but it did estahlish that it is not very important in this system. We note, however. t h a t this tj.pe of disproportionation does occur with other halogenated radicals (G. 0 . Pritchard, unpuhlished). (9) J. H . Knox and R . L. Nelson, Trans. Favaday Soc., 5 5 , 937 (1959). (10) G. 0. Pritchard, H . 0. Pritchard, and A. F. Trotman-Dickenson, J . Chem. Soc., 1425 (1954).
+
+
Volume 68, A7zamber 9 March, 136.4
542
D. M. TOMKINSON, J. P. GALVIN, AND H. 0. PRITCHARD
The reaction vessel was reattached to the vacuum system and those products which were volatile a t -80' were trapped out a t -210O. The mixture consisted largely of ethane and methyl chloride, together with small amounts of methane and some acetone which is a product of the peroxide decomposition. A further distillation from - 100 to -210' removed the remaining acetone, and the amounts of ethane and methyl chloride collected were determined mass spectrometrically. We may then write CH3
+ XC1, CH3Cl + XCln-1 CH3 + CHB CzHe +
+
(1)
(2)
0.5
0.0
ir
+ h
3 -0.5
2
-
I
so that ki/k21/2
=
RcH~cI/Rc~H~'/~ [XCl, 1
and rate constant ratios calculated according to this equation are shown in Fig. 1. Results The Arrhenius parameters for the abstraction of chlorine atoms by CH3 radicals were calculated from the data given in Fig. 1 by least squares, and the results are shown in Table I. The absolute values of A1 are
- 1.0
- 1.5
2.4
2.5
2.6
2.1
2.6
lOJ/T.
Table I E~
Reactant
CCla CzCle
cc1,cocc1,
-
~/ZEQ
(kcal./mole)
13.4 f0 . 3 10.2 f 0 . 5
9 . 8 f0 . 4
log A ~ A ~ / Zlog A' (mi. 1/lmole-'/2 (ml. mole-' 8ec. - ' / 2 l sec. -1)
6.53 4.74 5.61
13.2 11.4 12.3
calculated from the value of kz = 2.3 X 1013ml. mole-' sec.-l a t 134' obtained recently by March and Polanyi." The frequency factors are rather higher than the analogous hydrogen abstraction reactions. They could in fact be high by as much as a factor of 2 because additional CH3C1might arise by disproportionation between XCl,-' and CHB, but this kind of disproportionation does not appear to occur between CC13 and CzH6.2 However, the possibility cannot be completely ruled out because in the case of the reaction with C2Cla, traces of CzC14 were formed without the production of an equivalent amount of HC1. However, a high frequency factor is not unreasonable because the C-C1-CH3 transition state will be looser and not so necessarily linear as the C-H-CH, transition state. The CH3-CC14 sys+-m has been studied previously. In the gas phase, CvetanoviE and Steacie12 photolyzed acetone in the presence of CCl,, but their results were The Journal of Physical Chemistry
Figure 1. Reaction of methyl radicals with chlorine compounds: HA, hexachloroacetone (z = 0); HE, hexachloroethane (z = f0.5); CT, carbon tetrachloride (z = 0).
complicated by the formation of C1 atoms from the CC1,. In solution, Edwards and l l a ~ o studied '~ the thermal decomposition of acetyl peroxide in the presence of a number of substances, including acetone and CCl,, and a t 100' they found a rate constant for C1 abstraction roughly comparable with our own. Acknowledgment.-We wish to thank Dr. G. 0. Pritchard for a gift of 100 g. of hexachloroacetone a t a time (1960) when it was only available commercially in the United Kingdom a t a cost of $7 per gram. Appendix The most straightforward way to study the abstraction of hydrogen from R H by cc13 radicals would be to measure the rates of formation of CC13H and C2Cle. However, this presents a formidable analysis problem, (11) R. E. March and J. C. Polanyi, Proc. Roy. soc. (London), A273, 360 (1963). (12) R. J. Cvetanovi6 and E . W. R. Steacie, Can. J . Chem., 31, 158, 171 (1953). (13) F. G. Edwards and F. R. Mayo, J . A m . Chem. soc., 7 2 , 1265 (1950).
543
ABSTRACTION OF CHLORINE ATOMSBY METHYLRADICALS
aggravated by the fact that some of the CChH will have arisen by disproportionation* of CCls and Bo so that it will also be necessary to analyze for RCCL and make the appropriate correction via the combinattion-disproportion ratio. Fortunately, our experjments show that CC18 does not react with HC1 to give CClaHup to at,least 250'. Competitive reactions would appear to ease the analysis difficulties. For example, CCL abstracts C1 atoms from hexachloroacetone, and the competition for CCL between R H and the 'ketone, which gives CCI3H-CCl4 mixtures, would seem ideal. Hautecloque6 claims to have measured the activation energy for this abstraction from the ketone by CC13 and obtains a value of about 7 kcal./mole. However, the reaction is rnuch less favorable thermochemically than the abstraction by CH8, and a lower activation energy is very unlikely. It seems that much of the CCL must have been produced by reaction of Ccl3 radicals with the C1 atoms which are also produced in the ketone photolysis Another competitive approach has been used by McGrath and Tedder,14 who photolyzed CClaBr in the presence of hydrocarbons. The mechanism is
+ CCI3Br +CC13 + Br c:c&+ RH +CClJI + R
hv
(All (A2)
+ R H -+ HBr + R R + CClaBr -+ RBr + CCls CCla + HBr -+ CClaH + Br Br
L43) (A4) (A5)
The authors claim that the chain sequence A3, A4, and A5 is unimpo,tant and t h i t thz alternative chain sequence A2 and A4 is the operative one; thus, using a mixture of two RH's and analyzing for the respective bromides, one can establish the relative rates of reactions A2 for the pair. However, they add HBr to some experiments and find that it alters the ratio of alkyl bromide products. The only possible effect of HBr is to increase the stationary Br atom concentration through reactions -A3 and A5, and a t the same time to reduce the CC13 concentration; the fact that this alters the ratio of the products proves that both chains are occurring simultaneously and that both sequences are of comparable importance. Thus it seems that the only hope for competitive techniques is to use a source which does not produce a second reactive species. Hexachloroacetone was the obvious choice, but unfortunately C1 atoms a,re produced. Other possibilities might be cCl3I or C c h S :NCC13, although there is no guarantee that C1 atoms would not be produced in the latter case. (14) B. P. McGritth and J. M.Tedder, Bull.
SOC.
chim.BeEges, '71,
772 (1962).
Volume 68,Nwmber 3 March, 1,964
