Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Eleven p-substituted benzaldoximes (p-XC₆H₄CH=NOH, where X = H, CH₃, CF₃, F, Cl, Br, OCH₃, N(CH₃)₂, COOCH₃, CN, NO₂) have been synthesized and their dissociation constants determined in 10% (v/v) aqueous dioxane at 35 °C. Under the same conditions, the pseudo-first order rate constants k_obs of their reactions with p-nitrophenyl acetate (PNPA) were measured at pH values from 7.8 to 10.8 and at concentrations c_oxime ranging from 0 to 4.00 × 10^-3 mol l^-1. The kinetic model and mechanism of the said reaction was proposed by means of mathematical statistical modelling of the dependences of k_obs on pH and c_oxime. The mechanism involves a pre-equilibrium (k_-1/k₁) in which PNPA forms a tetrahedral intermediate (THI) with the deprotonated form of oxime. In the given medium, THI is in equilibrium with the non-reactive conjugated acid THIH (dissociation constant K_a,THIH) which is stabilized by intramolecular hydrogen bond. Depending on pH, the rate-limiting step consists either in formation of THI from educts (pH < pK_a,oxime) or in its spontaneous (k₂) and oxime-catalyzed (k₃, general acid catalysis) decomposition to products (pH > pK_a,oxime). Evaluation of substituent effects on dissociation constants (K_a,oxime) of the oximes showed that there is no direct conjugation between the substituent and the reaction centre (the found reaction constant ρ(K_a,oxime) = 0.91). The transmission coefficient of the transfer of these effects through C=N-O grouping corresponds approximately to one bond. The reaction constants in the Hammett equation obtained from the regression model are: ρ(k_-1K_a,THIH/k₁) = 1.29, ρ(k₂K_a,THIH) = 0.20 and ρ(k₃K_a,THIH) = 0.67. These reaction constants have been discussed with the regard to the reaction mechanism suggested.

Keywords: Esters; Hydrolysis; Kinetic model; Oximes; Substituent effects; Acidity; Catalysis; Dissociation constants.

References: 65 live references.