2752
Kinetics of the Thermal Isomerization of Bicyclo [ 1.1.1]pentane and 1,3-Dimethylbicyclo[ 1.1.1] pentane R. Srinivasan
Contribution from IBM Watson Research Center, Yorktown Heights, New York. Received December 20, 1967 Abstract: The thermal decomposition of bicyclo[l.l.l]pentane (I) in the vapor phase in the temperature range from 553.0 to 582.0°K led to 1,Cpentadiene as the only detectable product. The kinetics of the decomposition obeyed the first-order rate equation up to 93 conversion. The rate of the reaction was unaffected by a 17-fold increase in the surface-to-volume ratio of the reaction vessel. The rate constant at 576.1"K was studied as a function of pressure. At 0.25 torr, the rate constant had fallen off to almost one-half of its value at 35 torr, the highest pressure that was used. The value at infinite pressure was estimated to be about 8 % higher. From the temperature dependence of the reaction, the first-order rate constant, at about 5 torr, was observed to fit the equation: k = 1 0 1 ~ ~ 2 4 e ~ 4 9 ~ ~sec-1. 0 0 * s oThe 0 ~ Rthermal T decomposition of 1,3-dimethylbicyclo[l.l .l]pentane (11) was studied at a pressure of about 3 torr in the temperature range from 568.2 to 594.3"K. The kinetics of the process fitted a first-order rate equation, the rate constant being given by 1016.*0e-53,000*2000/RT sec-1.
T
he kinetics of the thermal decomposition of a number of small-ring bicyclic hydrocarbons, including bicyclo[l .l.O]butane, 1,2 bicycl0[2.1.0]pentane,~*~ bicyclo[2.l.l]hexane,~ bicyclo[2.2.0]hexane, and bicyclo[3.1 .O]he~ane,~have been reported. The only missing member of this group is bicyclo[ 1.1. Ilpentane (I). We report here the kinetics of the thermal decomS"3
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position of two members of this ring system, including the parent hydrocarbon and its 1,3-dimethyl derivative (11). The thermal cleavage of both cyclopropane and cyclobutane, as well as many other molecules which incorporate either of these ring systems, has been explained in terms of a diradical intermediate.' The recent interpretation of the reverse reaction, uiz. the photochemical closure of dienes t o give bicyclic compounds in terms of the same diradical as an intermediate,8 lends added interest to the measurement of the kinetics of the thermal reactions in these systems.
Experimental Section Bicyclo[l.l.l]pentane(I) was prepared both by the Hg(3P1)-sensitized isomerization of 1,4-pentadienes and by the photolysis of bicyclo[2.1.IIhexan-2-0ne.~The latter method gave a product that was easier to purify by vapor phase chromatography. It was hence (1) H. M. Frey and I. D . R. Stevens, Trans. Faraday SOC.,61, 90 (1965). (2) R. Srinivasan, A . A. Levi, and I. Haller, J . Phys. Chem., 69, 1775 (1965). --, (3) M. L. Halberstadt and .I. P. Chesick, J . Am. Chem. Soc., 84, 2688 (1962). (4) C. Steel, R. Zand, P. Hurwitz, and S. G. Cohen, ibid., 86, 679 (1964). ( 5 ) R. Srinivasan and A . A. Levi, ibid.,85, 3363 (1963). (6) H. M. Frey and R. C. Smith, Trans. Faraday SOC.,58, 697 (1962). (7) S . W. Benson, J . Chem. Phys., 34, 521 (1961); S. W. Benson and P. S. Nangia, ibid., 38, 18 (1963); E. M. O'Neal and S. W. Benson, submitted for publication. (8) R. Srinivasan and I
