3346
JAMES
N. BUTLERAND ROSALIND BARROGAWA
kdis:/kcom is proportional to T-0.85for the reaction in isooctane and to T-0.6for t h a t in glycol. The balance of products was again lowj4"as in the photolysis of azoethane' and a ~ o m e t h a n e e.g., , ~ ~ we could not account for about of radicals when the reaction proceeded at - 191'. Photolysis of CF3N2CF3 in 2,3-dimethylbutane shows excellent balance of products a t T 3 -30". A t -191' the ratio iBFaH 2CzFs):'Nz was again low (about 1.74). A11 these observations resemble those reported for the interaction of ethyl radicals,' namely (1) the increase in k d i s ' k c o m when glycol replaces isooctane as a solvent; (2) the increase of k d i s 'kc,, with decreasing temperature which seems to be represented b y a function Fa; ( 3 ) convergence of the kdis:'kcom values a t the lowest temperatures to a limit which seems to be common for all the investigated solvents. It appears,
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(4) (a) In reply t o t h e referee's question, we stress t h a t t h e balance is expected t o be l o w a t higher temperatures a n d in t h e presence of scavengers, since t h e reaction of primary radicals with t h e solvent radicals a n d t h e scavenger gives products other t h a n C H I , CsHa, and CaHs Such reactions, we believed are unlikely a t liquid nitrogen temperature, a n d therefore t h e low balance obtained in those experiments puzzles us. ( b ) R. E. P e b b e r t a n d P. Ausloos, J . Phys. Chem., 66, 2263 (1962).
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therefore, that these characteristic features of the radical combination and disproportionation are typical of radical interactions in solution. There is one interesting qualitative difference in the behavior of azoethane and methyl-ethyl azo compound. I n the photolysis of solid azoethane the value of k d i i k,,, was found to be lower than t h a t obtained a t the same temperature in other solvents. I t was suggested' t h a t this result may have been due to .the crystal lattice effectively freezing the radical and preserving their initial orientation which favors the combination. In the methyl-ethyl system the k d i s 1 kcomratio was again lower in the solid azo compound than.in other solvents, although to a much smaller degree. X s this azo compound is not as symmetrical as azoethane. the crystal lattice may have had less influence on the orientation of the radicals and thus combination would not be favored to such an extent. Acknowledgment.-In conclusion, we wish to acknowledge the financial support of this study by the Office of Ordnance through Grant DA-ORD-31-12461-G72 and by the National Science Foundation.
DEPARTMENT OF CHEMISTRY, UXIVERSITY OF BRITISHCOLUMBIA, YASCOUVER 8, B. C . ]
The Thermal Decomposition of Cyclobutane at Low Pressures BY JAMES N. BUTLERAND ROSALIND BARROGAWA' RECEIVED OCTOBER 15, 1962 The thermal decomposition of cyclobutane has been studied using gas chromatography for analysis a t pressures from 43 mni. t o 1.7 X mm. a t a temperature of 449'. Ethylene is the product of a homogeneous first-order decomposition. Propylene and 1-butene are formed in small amounts by zero-order reactions. The shape of the curve of first-order rate constant for ethylene formation as a function of pressure is consistent with the Kassel theory, if s is taken to be 14, or the Slater theory, if n is taken to be 16. varied in purity from 99.1 t o 99.97,, the principal impurity being Introduction a CScompound, probably 2-pentene. The samples contained less T h e thermal decomposition of cyclobutane in the gas than 0.01'; of ethylene and less than 0.01ci of CB and Ca comphase has been shown to be a homogeneous unimolecupounds. Pressures were measured with a wide-bore manometer and lar r e a ~ t i o n . " ~ The primary product is ethylene, cathetometer, or with a McLeod gage. Thermal transpiration although traces of C3, C1, and C6 compounds have been corrections were negligible compared to the other experimental detected. 3 , 5 The first-order rate constant decreases errors a t pressures above 0.05 tnm. Xt low pressures, correca t low pressures, as predicted by the Kassel or Slater tions based on Liang's e q u a t i o i ~ 'were ~ used. The parameter 9 was estimated to be 20 by using Liang's correlation with collision theories6 of unimolecular reactions, but there has been diameter. The largest corrections ( a t 2 X rnm.) were 30L',. some disagreement as to the shape of the fall-off curve. Samples were expanded into a 300-1n1. bulb a t room ternperaValues for the effective number of normal modes conture a t the conclusion of a run and analyzed by gas chroniatogtributing to the decomposition have been q ~ o t e d ~ , ' . raphy. ~ The composition of the gas was the same within experimental error whether the sampling time was 20 sec. or 10 min. which vary from 8 to 20. and whether the gas was expanded directly into the bulb or Experimental diluted first with approximately 1000 times as much nitrogen and The furnace and associated equipment have been described p r e ~ i o u s l y . ~Spherical reactors of 100-nil., 1-I., and 5-1. volumes were made from Pyrex flasks. One thermocouple was used in the small reactor, and four thermocouples in the large reactors. The temperature variation over the surface of the 5-1. reactor was less than 3 " , and the drift in temperature during a 2-hr. run was less than 0.6'. Cyclohutane was prepared by the photolysis of cyclopentaand purified by gas chromatography. The samples used ~~
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( I ) This paper is based on t h e M.Sc. Thesis of Rosalind Barr Ogawa,
Birkheck College, hfalet S t , London U'. C I , England. ( 2 ) C . T C.enaux and W 0 . Walters, J . A m . Chem. Soc , 73, 4197 (1951); F. Kern and W . I1 Walters, Proc. S a t l . Acad. Sci. U . S . , 58, 937 (1952) [:3) C 1'.Cenaux. F. Kern, and IX'. D . Walters, J . A m . Chem. Soc., 76, €1106 (19.53) (1) H. 0. Pritchard, I
