The mechanisms of the homogeneus, unimolecular elimination kinetics of ethyl 4‐chlorobutyrate and 4‐chlorobutyric acid in the gas phase

Abstract

AbstractEthyl 4‐chlorobutyrate, which is reexamined, pyrolyzes at 350–410°C to ethylene, butyrolactone, and HCl. Under the reaction conditions, the primary product 4‐chlorobutyric acid is responsible for the formation of γ‐butyrolactone and HCl. In seasoned vessels, and in the presence of a free‐radical inhibitor, the ester elimination is homogeneous, unimolecular, and follows a first‐order rate law. For initial pressures from 69–147 Torr, the rate is given by the following Arrhenius expression: log k1(s−1) = (12.21 ± 0.26) − (197.6 ± 3.3) kJ mol−1 (2.303RT)−1. The rates and product formation differ from the previous work on the chloroester pyrolysis.4‐Chlorobutyric acid, an intermediate product of the above substrate, was also pyrolyzed at 279–330°C with initial pressure within the range of 78–187 Torr. This reaction, which yields γ‐butyrolactone and HCl, is also homogeneous, unimolecular, and obeys a first‐order rate law. The rate coefficient, is given by the following Arrhenius equation: log k1(s−1) = (12.28 ± 0.41) − (172.0 ± 4.6) kJ mol−1 (2.303RT)−1. The pyrolysis of ethyl chlorobutyrate proceeds by the normal mechanism of ester elimination. However, the intermediate 4‐chlorobutyric acid was found to yield butyrolactone through anchimeric assistance of the COOH group and by an intimate ion pair‐type of mechanism. Additional evidence of cyclic product and neighboring group participation is described and presented.