The mechanism of neutral amino acid decomposition in the gas phase. The elimination kinetics of N,N-dimethylglycine ethyl ester, ethyl 1-piperidineacetate, and N,N-dimethylglycine

The gas-phase elimination kinetics of the ethyl ester of two alpha -amino a cid type of molecules have been determined over the temperature range of 36 0-430 degreesC and pressure range of 26-86 Torr The reactions, in a static reaction system, are homogeneous and unimolecular and obey a first-order ra te law. The rate coefficients are given by the following equations For N,N- dimethylglycine ethyl ester: log k(1) (s(-1)) = (13.01 +/- 3.70) - (202.3 +/- 0.3)kJ mol(-1) (2.303 RT)( -1) For ethyl 1-piperidineacetate: log k(1)(s(-1)) = (12.91 +/- 0.31) - (204.4 +/- 0.1)kJ mol(-1) (2.303 RT)(- 1) The decomposition of these esters leads to the formation of the correspondi ng alpha -amino acid type of compound and ethylene, However, the amino acid intermediate, under the condition of the experiments, undergoes an extreme ly rapid decarboxylation process. Attempts to pyrolyze pure N,N-dimethylgly cine, which is the intermediate of dimethylglycine ethyl ester pyrolysis, w as possible at only two temperatures, 300 and 310 degreesC. The products ar e trimethylamine and CO2. Assuming log A = 13.0 for a five-centered cyclic transition-state type of mechanism in gas-phase reactions, it gives the fol lowing expression log k(1)(s(-1)) = ( 13.0) - (176.6)k) mol(-1) (2.303 RT)- 1. The mechanism of these alpha -amino acids differs from the decarbonylati on elimination of 2-substituted halo, hydroxy, alkoxy, phenoxy, and acetoxy carboxylic acids in the gas phase. (C) 2001 John Wiley & Sons, Inc