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Abstract

The second-order rate constants k (dm³ mol^-1 s^-1) for the alkaline hydrolysis of meta-, para-, and ortho-substituted phenyl tosylates 4-CH₃C₆H₄SO₂OC₆H₄-X in aqueous 0.5 M Bu₄NBr have been measured spectrophotometrically in a wide temperature range. The log k values for ortho-substituted derivatives at various temperatures together with meta- and para-substituted derivatives were analyzed using the modified Fujita-Nishioka equation log k_m,p,ortho = c₀ + c_1(m,p,ortho)σ° + c_2(ortho)σ_I + c₃(1/T) + c_4(m,p,ortho)(1/T)σ° + c_5(ortho)(1/T)σ_I. In order to study the dependence of substituent effects, especially ortho inductive and resonance terms on different solvent parameters, the following equation was used: ∆log k_m,p,ortho = c₀ + c_1(m,p,ortho)σ° + c_2(ortho)σ_I + c₃∆E + c₄∆Y + c₅∆P + c_6(m,p,ortho)∆Eσ° + + c_7(m,p,ortho)∆Yσ° + c_8(m,p,ortho)∆Pσ° + c_9(ortho)∆Eσ_I + c_10(ortho)∆Yσ_I + c_11(ortho)∆Pσ_I. ∆log k = log k^X - log k^H, σ° and σ_I, are the Taft polar and inductive substituent constants, E, Y and P, are the solvent electrophilicity, polarity and polarizability parameters, respectively. In data treatment ∆E = E_S - E_H2O, ∆Y = Y_S - Y_H2O, ∆P = P_S - P_H2O were used. The solvent electrophilicity was found to be the main factor responsible for changes in the ortho, para, and meta polar substituent effects with medium. The variation of the ortho inductive term with the solvent electrophilicity E_S was found to be twice smaller than that for para substituents, while the ortho resonance term appeared to vary with solvent nearly similarly to that for para substituents. The ortho effect caused by the supplementary inductive effect from ortho position was found to disappear in a solvent whose electrophilic solvating power is comparable to pure DMSO (E ≈ 4).

Keywords: Esters; Sulfonates; Phenyl tosylates; Alkaline hydrolysis; Substituent effects; Ortho effects; Solvent effects; Kinetics.

References: 40 live references.