Abstract
Various types of isokinetic (isoparametric) relationships in heterolytic reactions were summarized and critically analyzed. It was presumed that the series of substrate reactivity is reversed after passing the isoparametric point, and the bimolecular reaction mechanism changes to unimolecular: SN2-SN1, SN2-E1, SE2-SE1, SE2-SN1, and SN2(SSIP)-SN2(C+). Three particular cases of isoparametric relationships are discussed: (1) isoentropy (ΔS≠ = const) which reflects formation of contact ion pair; (2) isoenthalpy (ΔH≠ = const) which reflects formation of space-separated ion pair; and (3) isoenergy (ΔG≠ = const), when ΔH≠ = ΔG≠ = ΔEr. The rate of heterolysis in cyclohexane does not depend on the substrate nature, and a universal minimal rate of heterolysis exists, k25 ≈ 10−10 s−1, τ1/2 = 220 years. There is no nucleophilic assistance by the solvent in unimolecular heterolysis.
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