Gas-phase acidity of CH3X [X =P(CH3)2, SCH3, F, Cl, Br, or I] compounds

Abstract

Proton abstraction from (CH₃)₃P, CH₃SCH₃, CH₃Cl, CH₃Br, and CH₃I in the gas phase has been investigated with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The gas-phase acidities of the three former compounds are reported to be ΔG°_acid[(CH₃)₃P] 1 606 ± 8 kJ mol^–1; ΔG°_acid(CH₃SCH₃) 1 616 ± 8 kJ mol^–1; and ΔG°_acid(CH₃Cl) 1 628 ± 12 kJ mol^–1. The competition between proton abstraction and S_N2 substitution in the reactions of different anions with CH₃F, CH₃Cl, CH₃Br, and CH₃I has been determined, Based upon these kinetic experiments, it is concluded that the gas-phase acidity increases down the series CH₃F, CH₃Cl, CH₃Br, CH₃I. The reactions of (CH₃)PCH₂^–, ClCH₂^–, and BrCH₂^– ions with various molecules are described.