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Abstract

The second-order rate constants k₂ (dm³ mol^-1 s^-1) for the alkaline hydrolysis of substituted alkyl benzoates C₆H₅CO₂R have been measured spectrophotometrically in aqueous 0.5 M Bu₄NBr at 50 and 25 °C (R = CH₃, CH₂Cl, CH₂CN, CH₂C≡CH, CH₂C₆H₅, CH₂CH₂Cl, CH₂CH₂OCH₃, CH₂CH₃) and in aqueous 5.3 M NaClO₄ at 25 °C (R = CH₃, CH₂Cl, CH₂CN, CH₂C≡CH). The dependence of the alkyl substituent effects on different solvent parameters was studied using the following equations:
 
    ∆ log k = c₀ + c₁σ_I + c₂E_s^B + c₃∆E + c₄∆Y + c₅∆P + c₆∆Eσ_I + c₇∆Yσ_I + c₈∆Pσ_I
    ∆ log k = c₀ + c₁σ* + c₂E_s^B + c₃∆E + c₄∆Y + c₅∆P + c₆∆Eσ* + c₇∆Yσ* + c₈∆Pσ* .
 
∆ log k = log k^R - log k^CH3. σ_I and σ* are the Taft inductive and polar substituent constants. E, Y and P are the solvent electrophilicity, polarity and polarizability parameters, respectively. In the data treatment ∆E = E_S - E_H2O , ∆Y = Y_S - Y_H2O , ∆P = P_S - P_H2O were used. The solvent electrophilicity, E, was found to be the main factor responsible for changes in alkyl substituent effects with medium. When σ_I constants were used, variation of the polar term of alkyl substituents with the solvent electrophilicity E was found to be similar to that observed earlier for meta and para substituents, but twice less when σ* constants were used. The steric term for alkyl substituents was approximately independent of the solvent parameters.

Keywords: Esters; Alkyl benzoates; Alkaline hydrolysis; Substituent effects; Solvent effects; Kinetics.

References: 34 live references.