EMS Account: Alkane eliminations from ions in the gas phase

Abstract

A review of reactions involving the elimination of an alkane from a variety of even and odd electron ions in the gas phase is presented. Particular attention is focused on the mechanisms of these reactions and the role of ion–neutral complexes. Alkane eliminations from open shell species derived by ionisation of ketones, ethers, alcohols, amines, alkanes and alkyl-substituted cycloalkanes are considered, together with ethane loss from the distonic ion CH2=O+CH2CH2•. These reactions are, without obvious exception, complex-mediated. Elimination of alkanes from an assortment of closed shell ions, including protonated acetaldehyde, dialkylmethylene immonium ions, onium ions, quaternary ammonium cations, alkoxide anions, enolate anions and deprotonated amines are also discussed. These reactions tend to be complex-mediated when heterolytic bond cleavages are possible. When such cleavages are not energetically feasible, alkane eliminations can take place by asynchronous, concerted mechanisms. Several factors affect the relative rates of alkane elimination compared to those of competing alkyl radical loss in the dissociations of these radical-cations; these factors include the internal energy of the ion, the size of the ionic and neutral partners, the initial orientation of these components and the binding energy of the complex. In addition, primary and secondary isotope effects influence both alkyl radical and alkane elimination. When alkanes are eliminated from radical-cations, reaction between the partners is often quickly overtaken in importance by direct separation of the components if the internal energy is raised either by increasing the energy of the ionisation process or by prior isomerisation over an energy barrier. This trend demonstrates that the attractions between the partners are easily overcome by quite small amounts of excess energy in the system. Typical binding energies between the ionic and non-polar neutral partners lie in the range 0–30 kJ mol−1 at distances at which covalent forces cease to be significant. Illustrative examples are given to show how studies of reactions involving ion–neutral complexes offer a means of obtaining information on the dynamics of short range interactions between ions and neutrals, including the motions of the partners relative to each other.