The reaction of 1,2-Dichloro-4,5-dinitrobenzene with hydroxide ion: roles of Meisenheimer complexes and radical pairs

The reaction of 1,2-dichloro-4,5-dinitrobenzene (DCDNB) with aqueous OH- produces (after acidification) 2-nitro-4,5-dichlorophenol with loss of NO2 . Nevertheless, with > 2 mol L-1 OH-, only DCDNB was recovered due to the formation of the long-lived 3,6-dihydroxy Meisenheimer complex (M2-), and that in acid, reverted to the starting material. Fast formation of monohydroxy Meisenheimer complex (M1-) can be followed in DMSO:H2O 7:3 v/v and rate constants for its interconversion with DCDNB and for formation and return with M2- complex were estimated, with evidence for these reactions in DMSO:H2O 1:1 v/v and H2O. The rapid hydrogen exchange in OD-/D2O limits the use of 1H nuclear magnetic resonance (NMR) spectroscopy in identifying intermediates. 1H and 13C NMR signals of M2- complex were observed in DMSO-H2O-KOH. There is evidence for the formation of free radicals in DMSO:H2 O 4:1 v/v, and overall kinetics in more aqueous medium were treated in terms of the transient existence of anionic radical pairs. (author)