Mechanisms of the homogeneous, unimolecular gas‐phase elimination kinetics of triethyl orthoacetate and triethyl orthopropionate

Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the temperature range of 291–351°C and pressure range of 80–170 Torr. The elimination reactions of these orthoesters in seasoned vessels are homogeneous, unimolecular, and follow a first‐order rate law. The reaction products are ethanol, ethylene and the corresponding ethyl ester. The Arrhenius expressions of these eliminations were found as follow: for triethyl orthoacetate, log k 1 (s −1 ) = (13.76 ± 0.09) − (187.6 ± 1.1) kJ mol −1 (2.303 RT) −1 ( r  = 0.9993), and for triethyl orthopropionate, log k 1 (s −1 ) = (13.63 ± 0.07) − (193.3 ± 1.8) kJ mol −1 (2.303 RT) −1 ( r  = 0.9992). A reasonable mechanism of these elimination is to consider that the COCH 2 CH 3 bond, as C δ + … δ − OCH 2 CH 3 in the TS, is the rate‐determining step. The nucleophilicity of the oxygen atom of OCH 2 CH 3 may abstract the hydrogen of the adjacent CH bond for a four‐membered cyclic structure to give the corresponding unsaturated ketal. The unstable ketal intermediate decomposes, in a six‐membered cyclic transition state, into ethylene and the corresponding ethyl ester. Copyright © 2008 John Wiley & Sons, Ltd.