In the gas-phase reactions of halonitro-and dinitrophenide anions with X (X ϭ F, Cl, Br, NO 2 ) and NO 2 groups in ortho or para position to each other with selected C-H acids: CH 3 CN, CH 3 COCH 3 , and CH 3 NO 2 , products of the S N Ar-type reaction are formed. Nitrophenide anions are generated by decarboxylation of the respective nitrobenzenecarboxylate anions in ESI ion source and the S N Ar 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 NO 2 derivatives, the main ionic product is the respective [NO 2 -Ph-CHR] Ϫ anion (R ϭ CN, COCH 3 , NO 2 ). In the case of Br derivatives, the main ionic product is Br Ϫ ion because it has lower proton affinity than the [NO 2 -Ph-CHR] Ϫ anion (for R ϭ CN, COCH 3 ). For some halonitrophenide anion C-H acid pairs of reactants, the S N Ar reaction is competed by the formation of halophenolate anions. This reaction can be rationalized by the single electrontransfer 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 -NO 2 to -ONO rearrangement followed by the NO