The nitric oxide catalyzed positional isomerization of 3-methylene-1,5,5-trimethylcyclohexene and the stabilization energy in the 3-methylenecyclohexenyl radical

The gas phase, nitric oxide catalyzed positional isomerization of 3-methylene-1,5,5-trimethylcyclohexene (MTC) into 1,3,5,5-tetramethyl-1,3-cyclohexadiene (TECD) has been studied for temperatures ranging between 296° and 425°C. The major reaction was first order with respect to nitric oxide and to MTC. The major side product, mesitylene, usually amounted to less than 10% of the TECD isomer formed. Only at high temperatures and large conversions has up to 20% been observed. Conditioned pyrex or quartz vessels coated with KCl have been used. The nitric oxide catalyzed isomerization is apparently a homogeneous process, as demonstrated by the insensitivity of the observed rate constants towards a 15-fold increase in the surface to volume ratio of the reaction vessels. However, a residual, presumably heterogeneous, thermal isomerization of the starting material could not be eliminated. Good mass balances were obtained for both NO and hydrocarbons. After correcting for the thermally induced conversion the observed rate constants for the nitric oxide catalyzed isomerization yield log k₁ (1 mole^?1 sec^?1) = (10.7 ± 0.2) – (37.3 ± 0.9)/θ where θ is 2.303 × 10^?3 RT (kcal mole^?1). Plotting log k₁ versus the ratio of the starting materials (MTC/NO)₀ it was found that for temperatures ≥ 365°C the rate constants were systematically too high. Using extrapolated values for the higher temperature range yields the more reliable corrected Arrhenius equation log k = 8.6 – 31.7/θ. The reaction mechanism is outlined and the implications with respect to the stabilization energy generated in the MTC? radical intermediate and the activation energy of the backreaction MTC? + HNO are discussed. Using for the activation energy E_?1 of the backreaction (R? + HNO) a literature value of 9.2 ± 0.9 kcal mole^?1 reported for the cyclohexadiene? 1,3? system, this yields 23.4 ± 2 kcal mole^?1 for the stabilization energy in the methylenecyclohexenyl radical, which is to be compared with the corresponding values for the allyl (10.2 ± 1.4), methallyl (12.6 ± 1) pentadienyl (15.4 ± 1) and cyclohexadienyl (24.6 ± 0.7) radicals. The pre-exponential factor agrees well with the value of (8.4 ± 0.2) reported by Shaw and co-workers for the similar reaction of NO with 1,3-cyclohexadiene. It is noteworthy that HNO, acting as sole hydrogen donor in the system, is surprisingly stable under the reaction conditions used. Nitrous oxide, HCN, H₂O and N₂ are observed in the product mixture of experiments carried out to high conversions at higher temperatures.