Gas-phase reactions between O − [rad] and C 6H 5F: On the acidity of fluorobenzene and 1,4-difluorobenzene

Abstract

The gas-phase reactions of negative ions (O − , NH − 2, C 2H 5NH −, C 6H 5 −, and CH 3SCH − 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 C 6H 4FO − 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, C 2H 5NH −, (CH 3) 2N −, C 6H 5 −, and CH 3SCH − 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 (δ H° acid = 1620 ± 8 kJ mol −1 refers to the former site. Based on the occurrence of reversible proton transfer between the CH 3O − ion and 1,4-difluorobenzene, the δ H° acid of this compound is redetermined to be 1592 ± 8 kJ mol −1.