Gas-phase reactions between O− and C6H5F: On the acidity of fluorobenzene and 1,4-difluorobenzene

AbstractThe gas-phase reactions of negative ions (O−, NH−2, C2H5NH−, C6H5−, 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−, C6H5−, and CH3SCH−2 anions react with fluorobenzene 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 = 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 of this compound is redetermined to be 1592 ± 8 kJ mol−1