Competing electron transfer, proton abstraction and nucleophilic substitutions in gas-phase reactions of (radical) anions with chloro- and bromomethanes

Abstract

The product ion distributions and rates of the gas-phase reactions of two series of (radical) anions with chloro- and bromomethanes (CH3Cl, CH2Cl2, CHCl3, CCl4, CH3Br, CH2Br2, CHBr3 and CBr4) were determined with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The first series consists of anions (HO−, CH3O−, C2H5O−, C3H7O− and CH3S−), for which the corresponding neutral radicals have a relatively high electron affinity (EA > 150 kJ mol−1). The second series consists of (radical) anions (CH2S−·, CH2(DOUBLE BOND)CHCH2−, CH2(DOUBLE BOND)C(CH3)CH2−, C6H4−· and C6H5−), for which the corresponding neutral species have a relatively low electron affinity (EA ≤ 100 kJ mol−1). These (radical) anions react mainly with the halomethanes to afford (i) halide ions, (ii) halomethyl anions with the same number of halogen atoms as in the parent halomethane and (iii) halomethyl anions with one halogen atom less than the parent substrate. The last process involves nucleophilic attack on a halogen atom and is particularly important in the reactions with substrates containing three or four halogen atoms. The halide ions may arise by a number of different pathways, such as SN2 substitution, α-elimination, halogen attack followed by dissociation of the thus formed halomethyl anion and overall dissociative electron transfer. The SN2 process is held responsible for the formation of halide ions in the reactions with monohalomethanes, whereas α-elimination is likely to be of importance only for the reactions with trichloro- and tribromomethanes. Attack on a halogen atom followed by dissociation of the ion generated initially is likely to be important if CCl4 or CBr4 is the substrate. Electron transfer is only a dominant pathway in the reactions of the CH2S−· ion with the halomethanes. The occurrence of electron transfer in the reactions of this ion with CHCl3, CCl4 and CHBr3 is evidenced by the formation of minor amounts of stable halomethane radical anions in addition to the generation of CH2SCl− or CH2SBr− ions and abundant halide ions. The interplay between the various possible reactions is discussed on the basis of thermodynamic considerations and the rates of the overall processes.