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

TLDR

The study discusses how the derived equation can estimate resonance contributions and inform reaction mechanisms. The authors derived normal substituent constants σ₀ from alkaline hydrolysis rates of substituted ethyl phenylacetates and showed the approach could also be applied to nucleophilic reactions. The σ₀ values matched Taft’s within experimental uncertainty, and they enabled an excellent correlation of electrophilic reaction rates via log(k/k₀)=ρ(σ₀+rΔσR⁺).

Abstract

Abstract 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+rΔ\barσR+) where Δ\barσR+ 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.