Joint theoretical and experimental study of the gas-phase elimination kinetics of tert-butyl ester of carbamic, N,N-dimethylcarbamic, N-hydroxycarbamic acids and 1-(tert-butoxycarbonyl)-imidazole

The gas-phase elimination kinetics of the title compounds were carried out in a static reaction system and seasoned with allyl bromide. The working temperature and pressure ranges were 200-280°C and 22-201.5 Torr, respectively. The reactions are homogeneous, unimolecular, and follow a first-order rate law. These substrates produce isobutene and corresponding carbamic acid in the rate-determining step. The unstable carbamic acid intermediate rapidly decarboxylates through a four-membered cyclic transition state (TS) to give the corresponding organic nitrogen compound. The temperature dependence of the rate coefficients is expressed by the following Arrhenius equations: for tiert-butyl carbamate logk₁ (s^-1) = (13.02±0.46) - (161.6 ± 4.7)kJ/mol(2.303RT)^-1, for tert-butyl N-hydroxycarbamate logk₁ (s^-1) = (12.52 ± 0.11) - (147.8 ± 1.1) kJ/mol(2.303 RT)^-1, and for 1-(tert-butoxycarbonyl)-imidazole logk₁ (s^-11) = (11.63 ± 0.21)-(134.9 ± 2.0) kJ/mol (2.303 RT)^-1. Theoretical studies of these elimination were performed at Møller-Plesset MP2/6-31G and DFT B3LYP/6-31G(d), B3LYP/6-31G(d,p) levels of theory. The calculated bond orders, NBO charges, and synchronicity (Sy) indicate that these reactions are concerted, slightly asynchronous, and proceed through a six-membered cyclic TS type. Results for estimated kinetic and thermodynamic parameters are discussed in terms of the proposed reaction mechanism and TS structure.