Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part V. Abnormal deactivation by some ortho-substituents: the direct field effect

Abstract

The rates of pyrolysis of 1-arylethyl acetates containing the 2-CF₃, 3-CF₃, 4-CF₃, 2-NO₂, 2-F, 2-Cl, 2-Br, and 2-l substituents have been measured at temperatures between 330·6 and 409·6 °C. A value of +0·565 is obtained for σ⁺_3–CF3 which gives a much better correlation of the data for all electrophilic aromatic substitutions and related reactions, than does the value of 0·52 derived from the solvolysis of t-cumenyl chlorides; use of the latter value should be discontinued. The o-trifluoromethyl substituent is no more deactivating than the meta-substituent and this is attributed to direct stabilisation through space of the electron-deficient reaction centre by the fluorine lone pair. The effect of the o-nitro- and o-halogen-substituents are consistent with the linear correlation of kinetic data for 1-arylethyl acetate and 1-arylethyl methyl carbonate pyrolysis (for which further evidence is given) but the recent use of such correlations to derive σ⁺(ortho) values is shown to be invalid. A survey of reactions with electron-deficient transition states for which the ortho-halogen substituent effects have been measured shows that where reaction occurs at the α-side chain atom, and where the solvent is either absent or of low polarity, these substituents deactivate more than from the meta-positions; this is rationalised in terms of the direct field effect. The greater deactivation by the o-nitro-substituent relative to the para, parallels that in protodesilylation but not nitration and chlorination which is however not inconsistent with recent theories of substituent effects.