A Marcus Treatment of Rate Constants for Protonation of Ring-Substituted α-Methoxystyrenes: Intrinsic Reaction Barriers and the Shape of the Reaction Coordinate

摘要

Rate and equilibrium constants were determined for protonation of ring-substituted α-methoxy-styrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted α-methyl α-methoxybenzyl carbocations at 25℃ and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by 14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO_2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Bronsted coefficient a for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents, and the Bronsted coefficient β for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in these Bronsted coefficients with changing reaction driving force, (partial deriv α)/(partial deriv ΔG°_(av)) = (partial deriv β)/(partial deriv ΔG°_(av)) = 1/8Λ = 0.011, is used to calculate a Marcus intrinsic reaction barrier of Λ = 11 kcal/mol which is close to the barrier of 13 kcal/mol for thermoneutral proton transfer between this series of acids and bases. The value of α = 0.66 for thermoneutral proton transfer is greater than α = 0.50 required by a reaction that follows the Marcus equation. This elevated value of β may be due to an asymmetry in the reaction coordinate that arises from the difference in the intrinsic barriers for proton transfer at the oxygen acid reactant and resonance-stabilized carbon acid product.