THERMAL-DECOMPOSITION OF LABELED CYCLOHEXENE - COMPLEX-REACTION INVOLVING A FORBIDDEN 1,2-HYDROGEN ELIMINATION

The pyrolysis of 3, 3, 6, 6-tetradeuteriocyclohexene is complex and involves four primary unimolecular processes (in a ratio of 20: 1: 0.1: 0.02). retro-Diels-Alder (log k1= 14.93 ± 0.79 -(65.2 ± 2.8)/θ), D2elimination (log k2= 12.63 ± 0.76 -(61.6 ± 2.7)/θ), HD elimination, and H2elimination (θ = 2.303RT kcal/mol). The first two reactions are symmetry allowed. If the last two reactions are concerted, they are symmetry forbidden. The experimental activation energies for processes 3 and 4 are 6 ± 3 kcal/mol larger than that for process 2. Various mechanisms which may account for reactions 3 and 4 are discussed. Secondary reactions producing cyclohexadienes, benzene, H2, and HD were also observed at low conversion (~2%); any D2produced by secondary processes was masked by reaction 2. The secondary reactions were initiated by reaction 9 with an activation energy of approximately 48 kcal/mol followed by reactions 10-16. Since the secondary rate of production of H2was much faster than reaction 4, the secondary production rate could be equated to the rate of formation 0f benzene from 1, 3-cyclohexadiene. © 1979, American Chemical Society. All rights reserved.