Electronic effects of polar substituents in the gas phase unimolecular elimination of 4-substituted isobornyl acetates

Abstract

The rates of gas-phase elimination of a series of 4-substituted isobornyl acetates have been measured in the presence of propene between 209–370 °C and within a pressure range 37–153 Torr; a static system with seasoned vessels was used. The reactions are unimolecular, homogeneous, and obey a first-order rate law; electron-withdrawing substituents at C(4) cause a diminution in the elimination rate, such that ρ₁–0.69 in a Hammett treatment. The temperature dependence of the rate coefficients is illustrated for the Arrhenius equation for isobornyl acetate: log k/ s^–1=(12.82 ± 0.13)–(189.2 ± 1.5) kj mol^–1/2.303 RT. The results are consistent with a cyclic transition state in which C(2)–O cleavage is more advanced than that of C(3)–H. The major primary product of elimination of isobornyl acetate is bornylene, which partially isomerises to camphene and tricyclene under the reaction conditions. Bornylene also undergoes a retro-Diels–Alder reaction to yield the trimethylcyclopentadiene and ethylene.