LINEAR FREE-ENERGY RELATIONSHIP INVOLVING ORTHO SUBSTITUENTS IN GAS-PHASE REACTIONS .17.

Rate constants, and in most cases Arrhenius parameters (Ea and ∆S‡ values), have been determined in the gas-phase pyrolysis of sixteen 1-arylethyl benzoates, sixteen 1-arylethyl acetates, and sixteen 1-arylethyl methyl carbonates. In each case the rates for the pyrolysis of the meta and para isomers were correlated in a Hammett plot using σ+ constants. Furthermore, linear free-energy relationships involving ortho substituents were obtained in these gas-phase reactions when the log (k/k0) for the pyrolysis of 1-arylethyl methyl carbonate was plotted as a function of log (k/k0) for the pyrolysis of 1-arylethyl acetate or 1-arylethyl benzoate, each including ortho substituents. This demonstrates that proximity effects are minimal or nonexistent in these reactions. From this study the following σ0 + substituent constants have been obtained: o-MeO, -0.411; o-Me, -0.233; o-Br, +0.551; o-Cl, +0.452; o-NO2, +0.749. © 1969, American Chemical Society. All rights reserved.