Evaluation of the substituent effect by using activation parameters and isokinetic relationships: an attempt to provide experimental primary standard for comparison with quantum chemically computed results in the field of structure–reactivity relationships

Abstract

In order to be able to compare quantum chemical calculations to experimental data, related to linear free energy relationships, one needs to be able to extract intrinsic (or internal) reaction constants from measurements carried out in various solvents. δ Δ H ‡ and δ Δ S ‡ reaction constants were calculated for series of reactions from the dependence of Δ H ‡ and Δ S ‡ activation parameters on substituent constants, and their ratio was found to be equal to the β isokinetic temperature (δ ΔH ‡ /δ ΔS ‡ =β), which is the temperature of intersection of the log( k/ T) vs. 1/ T plots of the reaction series. The δ Δ H ‡ and δ Δ S ‡ reaction constants can be divided into two terms referring to the bond formation ( δ Δ H int ‡, δ Δ S int ‡) and to the solvation ( δ Δ H ext ‡, δ Δ S ext ‡). The δ Δ H ext ‡/ δ Δ S ext ‡ ratio is equal to β ext, the isosolvent temperature, which is the temperature of the intersection of the Eyring-plots of a reaction measured in different solvents. As the internal contribution of Δ S ‡ is regarded to be independent of the substituents (δ ΔS int ‡ ≈0), with the full knowledge of β ext, the value of δ Δ H int ‡ can be calculated. The given reaction constants together with isokinetic and isosolvent temperatures were evaluated for some solvolysis reactions of carboxylic and sulfonic acid derivatives. The solvent effect influences markedly the value of δ Δ H ‡ and δ Δ S ‡. The β ext isosolvent temperatures deviate only moderately (±200 K) from the temperature of the kinetic measurements ( T). The change of the free energy of activation with the substituents ( δ Δ G ‡) is not a good approximation of δ Δ H int ‡ in those cases when the absolute values of ( β ext− T) and δ Δ S ‡ are great. The reaction constants based on activation parameters allow a more detailed discussion of the substituent effect, and the influence of substituents on bond formation and solvation can be separated.