Reactivity of p-Substituted Benzaldoximes in the Cleavage of p-Nitrophenyl Acetate: Kinetics and Mechanism

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 kobs of their reactions with p-nitrophenyl acetate (PNPA) were measured at pH values from 7.8 to 10.8 and at concentrations coxime 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 kobs on pH and coxime. 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 Ka,THIH) which is stabilized by intramolecular hydrogen bond. Depending on pH, the rate-limiting step consists either in formation of THI from educts (pH < pKa,oxime) or in its spontaneous (k2) and oxime-catalyzed (k3, general acid catalysis) decomposition to products (pH > pKa,oxime). Evaluation of substituent effects on dissociation constants (Ka,oxime) of the oximes showed that there is no direct conjugation between the substituent and the reaction centre (the found reaction constant ρ(Ka,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-1Ka,THIH/k1) = 1.29, ρ(k2Ka,THIH) = 0.20 and ρ(k3Ka,THIH) = 0.67. These reaction constants have been discussed with the regard to the reaction mechanism suggested.