THERMAL ISOMERIZATION OF 1-METHYLCYCLOBUTENE AT LOW PRESSURES

The homogeneous, gas-phase, thermal isomerization of 1-methylcyclobutene to 2-methyl-1,3-butadiene has been studied in a 12-1. vessel at 150° for initial pressures from 1.1 to 0.004 mm and from 1.0 to 0.001 mm at 175°. In this pressure range the first-order rate constant decreases as the initial pressure is lowered: at 0.024 mm, the rate constant at 175° has decreased to about one-half the rate constant at 1 mm, and at 150° to approximately four-sevenths the value at 1 mm. At 175° the rate constant for 0.0015 mm has decreased to about one-sixth that at 1 mm. The decrease in rate constant for 1-methylcyclobutene at 175° occurs about 1/20th to 1/25th the pressure observed by Hauser for a comparable decrease of the cyclobutene rate constant at 150.4°. The curve showing the relative falloff of the rate constant with decreasing initial pressure of 1-methylcyclobutene at 175° lies close to the curve for the results of Frey and Marshall for 3-methylcyclobutene at 123.5˚. The present results are compared with the predicted dependence of the rate constant for 1-methylcyclobutene on pressure, calculated by Elliott and Frey using RRKM theory. © 1969, American Chemical Society. All rights reserved.