Aromatic nucleophilic substitution reaction of 2,4-dinitrohalogenobenzene and ethyl α-cyano-phenylacetate benzyl anion

The aromatic nucleophilic substitution reaction of 2,4-dinitrohalogenobenzene (DNHB; halogeno =F, Cl, Br) with ethyl α-cyanophenylacetate (ECPA) benzyl anion has been studied by means of UV and IR spectrophotometry as well as gas chromatography. The rates of intermediate formation and product formation were determined respectively. In contrast with common S _N Ar reactions, the formation rate constant of the intermediates, substituted cyclohexadienates, was in the order of halogen =F>Br>Cl, whereas the rate constant of product formation was in the order F>Cl>Br, which revealed that the departure of the halogen atom was the rate-limiting step for the overall reaction in the presence of a sterically hindered nucleophile. These reactions could be catalyzed by the solvated electrons. The corresponding reaction intermediates, radical species, were detected by ESR and the reactions were proved to proceed mainly by a solvated-electron induced radical chain reaction mechanism.