Resonance Effect in Hammett Relationship. IV. Linear Free Energy Relationship based on the Normal Substituent Constants

A set of normal substituent constants, σ0, was derived from the rates of alkaline hydrolysis of m- and p-substituted ethyl phenylacetates in 60% (v/v) aqeuous acetone at 25.0°C. The obtained values of σ0-constants were generally identical with Taft’s values within experimental uncertainty. On the basis of the derived σ0, substituent effects on the general electrophilic reactions were correlated excellently by the equation, logk⁄k0=ρ(σ0+rΔσR+) where ΔσR+ corresponds to the exaltation of Brown and Okamoto’s σ+ from σ0. The similar treatment could be applied also to the nucleophilic reactions. The utility of this equation for the estimation of resonance contribution and for the consideration of reaction mechanisms are discussed.