GAS-PHASE REACTIONS BETWEEN O- AND C6H5F - ON THE ACIDITY OF FLUOROBENZENE AND 1,4-DIFLUOROBENZENE

The gas-phase reactions of negative ions (O-., NH2-, C2H5NH-, (CH3)2N-, C6H5-, and CH3SCH2-) 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 NH2-, C2H5NH-, (CH3)2N-, C6H5-, and CH3SCH2- 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 (DELTAH(acid)degrees = 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 DELTAH(acid)degrees Of this compound is redetermined to be 1592 +/- 8 kj mol-1.