Reactivity of mono-halogen carbene radical anions (CHX–˙; X = F, Cl and Br) and the corresponding carbanions (CH2X–; X = Cl and Br) in the gas phase

Abstract

The gas-phase reactions of mono-halogen substituted carbene radical anions, CHX^–˙(X = F, Cl and Br) and the corresponding carbanions, CH₂X^–(X = Cl and Br) with halomethanes and organic esters have been examined with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The chlorine and bromine containing (radical) anions react by S_N2 substitution with the parent chloro- and bromo-urethanes, whereas an S_N2 and/or a B_AC2 reaction occurs with the methyl ester of trifluoroacetic acid and dimethyl carbonate. The main features of the results are: (i) the S_N2 substitution of a given carbene radical anion with CH₃Cl or CH₃Br is less efficient than this reaction of the corresponding carbanion, (ii) the radical anions react less efficiently with dimethyl carbonate than the carbanions, (iii) the S_N2 substitution is less important for the radical anions than for the carbanions in the reactions with the two carbonyl compounds, (iv) for both types of ions, the B_AC2 pathway becomes relatively more important as the halogen atom is changed from chlorine to bromine. These findings are discussed in terms of the thermodynamics of the overall processes in combination with considerations of the potential energy surfaces which can describe these gas-phase processes.