Thermal decomposition of cyclohexa-1,4-dienes. Part IV. cis-3,6-Dimethyl-, trans-3,6-dimethyl- and 3,3,6,6-tetramethyl-cyclohexa-1,4-diene

Abstract

Within the range 263–314° the thermal decomposition of cis-3,6-dimethylcyclohexa-1,4-diene proceeds by a unimolecular path to yield only p-xylene and hydrogen. The rate constants fit the Arrhenius equation, k= 10^12·05 exp(–39,950 cal./RT) sec.^–1. trans-3,6-Dimethylcyclohexa-1,4-diene decomposes only at appreciably higher temperatures. Its decomposition, which yields mainly toluene and methane was studied in the range 345–444°, and evidence is presented to support the hypothesis that the reaction proceeds by a radical chain mechanism. A preliminary study of the pyrolysis of 3,3,6,6-tetramethylcyclohexa-1,4-diene suggests that the decomposition occurs by a predominantly non-chain radical process.