Kinetics of the gas‐phase elimination of isopropyl α‐halo acetates and the taft correlation

The gas‐phase pyrolysis of three secondary acetates in the temperature range of 290‐350°C and the pressure range of 40‐236 mm Hg reveals a homogeneous first‐order law and unimolecular elimination. The rate constants are given by the following Arrhenius equations: isopropyl fluoroacetate: isopropyl chloroacetate: isopropyl iodoacetate: The differences in rates I > Br > Cl > F apparently question previous assumptions where the pyrolyses of esters follow the same sequence as acid strength. The experimental data give a good correlation with σ* values, where ρ* = ‐1.85 at 330°C. Some kind of discrete bond polarization at the CαO bond of the acetate in the cyclic transition state seems to be involved in this type of reactions.