Aromatic Nucleophilic Substitution (SNAr) Reactions of 1,2- and 1,4-Halonitrobenzenes and 1,4-Dinitrobenzene with Carbanions in the Gas Phase

In the gas-phase reactions of halonitro- and dinitrophenide anions with X (X = F, Cl, Br, NO2) and NO2 groups in ortho or para position to each other with selected C-H acids: CH3CN, CH3COCH3, and CH3NO2, products of the SNAr-type reaction are formed. Nitrophenide anions are generated by decarboxylation of the respective nitrobenzenecarboxylate anions in ESI ion source and the SNAr reaction takes place either in the medium-pressure zone of the ion source or in the collision chamber of the triple quadrupole mass spectrometer. In the case of F, Cl, and NO2 derivatives, the main ionic product is the respective [NO2-Ph-CHR]− anion (R = CN, COCH3, NO2). In the case of Br derivatives, the main ionic product is Br− ion because it has lower proton affinity than the [NO2-Ph-CHR]− anion (for R = CN, COCH3). For some halonitrophenide anion C–H acid pairs of reactants, the SNAr reaction is competed by the formation of halophenolate anions. This reaction can be rationalized by the single electron-transfer mechanism or by homolytic C–H bond cleavage in the proton-bound complex, both resulting in the formation of the halonitrobenzene radical anion, which in turn undergoes –NO2 to –ONO rearrangement followed by the NO· elimination.