Mechanisms of elimination reactions. Part III. Rate coefficients for elimination from substituted 2-phenylethyl arenesulphonates with potassium in t-butyl alcohol

Abstract

The rate coefficients for elimination from 24 substrates in the series 2-Z-phenylethyl p-Y-arenesulphonate with potassium t-butoxide in t-butyl alcohol at 40 °C have been correlated with the Hammett equation. The reaction constants ρ_z decrease linearly with the electronic properties of the group Y only for the most electron-withdrawing substituents, indicative of a shift to transition states with less carbanion character. The reaction constants ρ_Y become less positive as Z becomes more electron-withdrawing, suggesting that increased C_β–H bond breaking is accompanied by decreased C_α–X bond breaking, and a shift to a transition state with greater carbanion character. This result supports earlier secondary deuterium isotope studies in establishing a new interpretation of the reactivity ratio k_OTs/k_Br as a measure of C–X bond breaking.