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Mechanism of α-Amino Acids decomposition in the gas phase. Experimental and theoretical study of the elimination kinetics of N-Benzyl Glycine Ethyl Ester

Abstract:

The gas-phase elimination kinetics of N-benzylglycine ethyl ester was examined in a static system, seasoned with allyl bromide, and in the presence of the free chain radical suppressor toluene. The working temperature and pressure range were 386.4-426.7 °C and 16.7-40.0 torr, respectively. The reaction showed to be homogeneous, unimolecular, and obeys a first-order rate law. The elimination products are benzylglycine and ethylene. However, the intermediate benzylglycine is unstable under the reaction conditions decomposing into benzyl methylamine and CO2 gas. The variation of the rate coefficients with temperature is expressed by the following Arrhenius equation: log k1 (s-1) = (11.83 ± 0.52) - (190.3 ± 6.9) kJ mol -1 (2.303RT)-1. The theoretical calculation of the kinetic parameters and mechanism of elimination of this ester were performed at B3LYP/6-31G*, B3LYP/6-31+G**, MPW1PW91/6-31G*, and MPW1PW91/6-31+G** levels of theory. The calculation results suggest a molecular mechanism of a concerted nonsynchronous six-membered cyclic transition state process. The analysis of bond order and natural bond orbital charges implies that the bond polarization of C(=O)O-C, in the sense of C(=O)Oδ-⋯Cδ+, is rate determining. The experimental and theoretical parameters have been found to be in reasonable agreement. © 2010 American Chemical Society.