Gas-phase reactions between O(-.) and C 6H 5F: On the acidity of fluorobenzene and 1,4-difluorobenzene

The gas-phase reactions of negative ions (O(-.), NH 2 (-) , C2H5NH(-), (CH3)2N(-), C6H 5 (t-) , and CH3SCH 2 (-) ) with fluorobenzene and 1,4-difluorobenzene have been studied with Fourier transform ion cyclotron resonance mass spectrometry. The O(-.) ion reacts predominantly by (1) proton abstraction, (2) formal H 2 (+.) abstraction, and (3) attack on an unsubstituted carbon atom. In addition to these processes, attack on a fluorine bearing carbon atom yielding F(-) and C6H4FO(-) ions occurs with 1,4-difluorobenzene. Site-specific deuterium labeling reveals the occurrence of competing 1,2-, 1,3-, and 1,4-H 2 (+.) abstractions in the reaction of O(-.) with fluorobenzene. Attack of the O(-.) ion on the 3- and 4-positions in fluorobenzene with formation of the 3- and 4-fluorophenoxide ions, respectively, is preferred to reaction at the 2-position, as indicated by the relative extent of loss of a hydrogen and a deuterium atom in the reactions with labeled fluorobenzenes. The NH 2 (-) , C2H5NH(-), (CH3)2N(-), C6H 5 (-) , and CH3SCH 2 (-) anions react with fluoroberuene and 1,4-difluorobenzene only by proton abstraction. The relative importance of H(+) and D(+) abstraction in the reaction of these anions with labeled fluorobenzenes indicates that the 2-position in fluorobenzene is more acidic than the 3- and 4-positions, suggesting that the literature value of the gas-phase acidity of this compound (ΔH acid (o) = 1620 ± 8 kJ mol(-1)) refers to the former site. Based on the occurrence of reversible proton transfer between the CH3O(-) ion and 1,4-difluorobenzene, the ΔH acid (o) of this compound is redetermined to be 1592 ± 8 kJ mol(-1).