Theoretical study on the mechanism of the gas-phase elimination kinetics of alkyl chloroformates

The theoretical calculations on the mechanism of the homogeneous and unimolecular gas-phase elimination kinetics of alkyl chloroformates-ethyl chloroformate (ECF), isopropyl chloroformate (ICF), and sec-butyl chloroformate (SCF)-have been carried out by using CBS-QB3 level of theory and density functional theory (DFT) functionals CAM-B3LYP, M06, MPW1PW91, and PBE1PBE with the basis sets 6-311++G(d,p) and 6-311++G(2d,2p). The chlorofomate compounds with alkyl ester C_β-H bond undergo thermal decomposition producing the corresponding olefin, HCl and CO₂. These homogeneous eliminations are proposed to undergo two different types of mechanisms: a concerted process, or via the formation of an unstable intermediate chloroformic acid (ClCOOH), which rapidly decomposes to HCl and CO₂ gas. Since both elimination mechanisms may occur through a six-membered cyclic transition state structure, it is difficult to elucidate experimentally which is the most reasonable reaction mechanism. Theoretical calculations show that the stepwise mechanism with the formation of the unstable intermediate chloroformic acid from ECF, ICF, and SCF is favoured over one-step elimination. Reasonable agreements were found between theoretical and experimental values at the CAM-B3LYP/6-311++G(d,p) level.