Chloronium Ions as Alkylating Agents in the Gas-Phase Ion-Molecule Reactions with Negative Temperature Dependence

Article abstract of DOI:10.1021/ja00365a005

The kinetics of the reactions Me2Cl⁺ + B = MeB⁺ + MeCl and MeEtCl⁺ + B = MeB⁺ or (EtB⁺) + EtCl (or MeCl) were studied with a pulsed-electron-beam, high-pressure mass spectrometer.At room temperature the rate constants were found to increase in the order B = benzene, toluene, isopropylbenzene, EtOH, Me2O, Et2O.At this point k become equal to the orbiting capture rate constant k₁ ca. 10^-9 molecule^-1 cm³ s^-.NH3 and Me3N were alkylated at orbiting capture rates.The temperature dependence of the rate constants for B = toluene, Me2O, and Et2O was examined.The rate constants were found to increase with decrease of temperature.This increase continued until the rate constants reached the magnitude of the orbiting rate constant k_L.The rates remained approximately independent of temperature below this temperature.At low temperatures the collision-stabilized Me2Cl⁺B and MeEtCl⁺B could be observed.The temperature dependence of the equilibrium Me2Cl⁺ + toluene = (Me2Cl-toluene)⁺ was measured and led to the corresponding ΔH^o and ΔS^o.The reaction Me2Cl⁺ + benzene = Me-benzene⁺ + MeCl was found to have positive temperature dependence.On the basis of the above data it is suggested that the reactions Me2Cl⁺ + B = MeB⁺ + MeCl have an internal barrier in the potential energy of the reaction coordinate.This barrier protrudes above the energy level of the reactants (Me2Cl⁺ + B) for B = benzene.This leads to positive temperature dependence.For all other B, the top of the internal barrier lies below the level of the reactants and sinks lower, roughly in the order of increasing basicity of B.This lead to negative temperature dependence (toluene, isopropylbenzene, Me2O, Et2O).For B = NH3, MeNH2, Me3N, the barrier is so low that the reactions have orbiting capture rates equal to k_L.Alkylation of bases B by chloronium ions like Me2Cl⁺ might have considerable utility in mass spectrometric analysis by chemical ionization.Ethers can be distinguished from alcohols and tertiary amines from primary and secondary amines.The alkylated ethers and the tertiary amines have no protic hydrogens and therefore do not form strongly hydrogen-bonded adducts.