Mechanism of reaction of hydroxide ion with dinitrochlorobenzenes

Article abstract of DOI:10.1021/ja00001a034

Formation of dinitrophenoxide ion from 2,4- and 2,6-dinitrochlorobenzene (2,4- and 2,6-DNCB) and OH^- (OD^-) in 70:30 and 80:20 (v/v) DMSO-H²O (D₂O) is accompanied by extensive ¹H NMR line broadening of unreacted substrate and exchange of arene hydrogen with D₂O, which is quantitative at the 3-position of 2,4-DNCB. Unproductive Meisenheimer complexes are detected spectrophotometrically in the course of reaction. For reaction of 2,4-DNCB, the Meisenheimer 3-complex is formed first and then the more stable 5-complex can be detected and characterized by NMR spectrometry. There is no hydrogen exchange of Meisenheimer complexes or dinitrophenoxide ions and their ¹H NMR signals are not broadened. These results do not fit the classical mechanism of single-step nucleophilic addition, but they, and the kinetic results, are fitted by a reaction scheme involving single-electron transfer from OH^- to give a charge-transfer complex of OH^? and a radical anion which collapses to give Meisenheimer complexes and aryl oxide ion. This scheme is consistent with MO calculations by the AM1 method.