Mechanisms of alkane CH-activation: The 5/6 effect, single-factor compensation effect, strongest reactant and earliest transition state. A puzzle of Shilov reaction

Abstract

The results on the investigation of alkane activation kinetics and mechanisms in aqueous and acidic media, including theoretical and experimental approaches to the problem, proposed by the author are summarized. The new concepts are introduced: a “direct kinetic study of CH activation”, “visible and hidden reagent preactivation”, the “strongest reactant and earliest transition state” (ETS), the “5/6 effect” (the ratio of CH bonds splitting constant rates in the с-C₅H₁₀/с-C₆H₁₂ couple), “single-factor compensation effect” (SCE), and a “refined compensation temperature”. The compensation effect (CE) exists in reality, but it is necessary to exclude the influence side factors, for example, to go from rate constants to the 5/6 effect to observe its purest form. The 5/6-CE combination as an example SCE is a new instrument for studying CH-activation mechanisms. Temperature dependences of (5/6) values for the reagents in aqueous solution (Pt^II, MnO₄⁻, HMnO₄, HOCl, HOONO, HVO₂ – Ru^IV, OH, SO₄⁻) and sulfuric acid (Pd^II, Hg^II, CrO₃, Mn^III, Co^III, NO₂⁺, OH⁺, 1-adamantyl cation, C₁₄H₁₁⁺, СН₂OH⁺, SO₃H⁺) are found. The only influencing factor on the (5/6) value is conformational strain difference in the C₅ and C₆ rings, which in a series of one-type reactions decreases with increasing the reagent activity. Four mechanisms are revealed: H-atom abstraction by anionic (I⁻), uncharged (I⁰) and cationic (I⁺) oxygen-centered “O-reactants”, and a bifunctional incorporation into CH bond of electrophilic cation – base adduct (II⁺). Various SCE lines (the entropy change versus enthalpy changing) for these groups are found. They converge in a “SCE pole” (the first example of ETS). Features of these mechanisms, differences of Pt^II, Pd^II, Hg^II acidocomplexes as the reagents and a surprising similarity between the Shilov reactant Pt^IICl₃(H₂O) and OH radicals in the water are discussed. The probable general cause of CEs in various processes is compensation effects in thermodynamics of interparticle interactions.