Abstract

AbstractA mechanistic investigation of the gas‐phase reactivity of the halomethyl anions XCH2− (X = Cl, Br) towards a mixture of the corresponding halomethane and ammonia was performed using Fourier transform ion cyclotron resonance mass spectrometry. The interpretation of the experimental data is supported by high‐level density functional theoretical (DFT) calculations for the chlorine‐containing systems (X = Cl). When the specific isotopomer AXCH2− (AX = 35 Cl, 79Br) is allowed to react in an atmosphere of CH3X and NH3, the exclusive formation of the isotope cluster of the halide anions AX− and BX− (BX = 37Cl, 81Br) is observed. However, the intensity ratio I(AX−)/I(BX−) exceeds significantly the value expected from the natural relative isotope abundances and depends linearly on the pressure ratio p(NH3)/P(CH3X). The experimental results are interpreted in terms of three competing reaction mechanisms: (i) The by far dominating process is the more than 70 kcal mol−1 exothermic one‐step SN2 substitution of AXCH2− on CH3X, generating haloethane AXCH2CH3 and X− isotopomers, the latter in the proportion of their natural abundances (direct SN2). The experimentally observed excess of AX− stems from two minor reaction pathways: (ii) in a secondary reaction, the halide X− in the primary product anion‐molecule complex [AXCH2CH3 … X−] * of the SN2 substitution induces a 1,2‐elimination, leading to the formation of the AX− isotopomer (two‐step SN2/E2). (iii) Finally, ACH2− can react with ammonia by consecutive endothermic proton transfer (PT) from NH3 to AXCH2− and a very exothermic SN2 substitution of the resulting amide on AXCH3 leading to CH3NH2 and an excess of AX− which depends linearly on p(NH3)/p(CH3X) (PT/SN2). Theoretical calculations show that in the case of [ClCH2−…NH3] *, the PT/SN2 reaction has no stable intermediate. Therefore, it is concluded that this reaction is not a two‐step but a one‐step process.

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