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• W2068429036 abstract "Abstract Numerous reactions catalyzed by acids show a rate maximum in moderately concentrated acids [1]. This maximum would proceed from the decrease of the water activity when the concentration of acid increases. In fact, our study of the hydrolysis of thioamides in aqueous sulphuric solutions, reveals that the origin is only the difference of the variations of solvation energies between the protonated from (XH+) and the transition state (CT*). This difference is evaluated by acidity functions. Thioamides were chosen because solvent effects do not exits with aliphatic compounds. 1. Protonation. The 2H (= −log h2) acidity function [2] governs the protonation of thioamides. The calculated function is almost identical to the experimental functions H‴o and HT relative to tertiary amines and thioamides respectively [1, 3]. For a given thioamide, the same pK value is obtained whatever the wavelength in UV spectroscopy and the technical method (UV, 1H NMR) may be, (pK (thioacetamide) = −2.51; pK (thiobenzamide) = −3.20). The value, τ = 2, of the apparent solvation ratio [2] means that the protonated from gets a solvation energy lower than the one of the oxonium cation result confirmed by the corresponding values of entropy variations. 2. Hydrolysis. By calling H* (= −log h*), the acidity function governing the formation equilibrium of the translation state from the neutral form, we demonstrate that the rate law can be written: We find that the H* function is identical to the 4H function (4H ∼ HA [1]) and so, the solvation energy of the transition state (τ = 4) is stronger than the one of the protonated form (τ = 2); in effect, we get: From the definition of the acidity function and the one of the transfer activity coefficient, this result means: were Δot is the variation of transfert free enthalpy linked to the variation of solvation energy. In the acidity range, where the inequality hXH+ ⋙ KXH+ is verified, relation (1) gives: This relation is verified by the experimental values. The rate decreasing whatever the water activity may be. The rate maximum occurs when the concentration of acid corresponds to the half-neutralization of X. The decrease of the reaction rate proceeds only from the different natures of acidity functions H* and HXH+, when the concentration of acid increases the variation of solvation energy of the protonated form is lower than the one of the transition state (inequality (2))." @default.
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• W2068429036 date "1980-01-01" @default.
• W2068429036 modified "2023-09-24" @default.
• W2068429036 title "Variations of solvation energies of the protonated form and the transition state in the acid hydrolysis of thioamides" @default.
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• W2068429036 doi "https://doi.org/10.1016/s0020-1693(00)92274-6" @default.
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