Steric Acceleration in the Pyrolysis Kinetics of 2-Substituted Ethyl Acetates

Article abstract of DOI:10.1021/j150625a039

The rates of elimination of several cyclo and branched alkyl substituents at the β carbon of ethyl acetates have been determined in a static system over the temperature range of 360-420 deg C and pressure range of 34-337 torr.The reactions are homogeneous in both clean and seasoned vessels, follow a first-order rate law, and are unimolecular.The rate coefficients are given by the following Arrhenius equations: for 2-cyclohexylethyl acetate, log k₁ (s^-1)=(13.30+/-0.28)-(208.1+/-3.4) kJ mol^-1 (2.303RT)^-1; for 2-cyclopentylethyl acetate, log k₁ (s^-1)=13.20+/-0.26)-(207.4+/-3.2) kJ mol^-1 (2.303RT)^-1; for 3-methyl-1-pentyl acetate, log k₁ (s^-1)=(13.62+/-0.09)-(211.9+/-1.2) kJ mol^-1 (2.303RT)^-1; for 4-methyl-1-pentyl acetate, log k₁ (s^-1)=12.82+/-0.05)-(203.1+/-0.6) kJ mol^-1 (2.303RT)^-1.The present data together with those reported in the literature give a good correlation line only, when plotting log k/k₀ against Hancock's E_s^c values (r=0.916, δ=-0.121, and intercept=-0.020 at 400 deg C).This work ratifies that steric acceleration is responsible for the rate of elimination of alkyl and several polar substituents interposed by at least three methylene groups with respect to C_α-O bond of ethyl acetates.Unreported E_s^c values of several substituents are defined.