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

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

The 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.