Onium reaction

The loss of alkenes from aliphatic onium ions via onium reaction comprises scission of the C-X bond and concomitant transfer of a hydrogen from the leaving alkyl moiety to the heteroatom, and a merely phenomenological description of this reaction has already been included in the preceding schemes. 

Example The spectra of A7-ethyl-A -methyl-propylamine (Fig. 6.6), tripropylamine, and tributylamine (Fig. 6.43) also exemplify alkene loss from immonium ions via onium reaction. Oxonium ions are involved in the fragmentation of dieth-ylether and methylpropylether (Fig. 6.7) and many others have been published. [36,143,160-163] Below, the fragmentation of butyl-isopropylether is shown  

In case of immonium ion fragmentations, the large difference of proton affinities, APA, between imine and alkene clearly favors the formation of immonium ion plus neutral alkene, whereas imine loss is restricted to highly energetic precursors. 

In decomposing oxonium ions the situation is quite different, i.e., the preference for alkene loss is much less emphasized and aldehyde (ketone) loss is gaining importance. The observed changes are in good agreement with the postulated mechanism of the onium reaction. [143,167] The alternative pairs of oxonium ion plus alkene and aldehyde plus carbenium ion may be formed with a preference for the first one, because APA is comparatively small (20-60 kJ mol ) or even zero, e.g., for the acetone/isobutene pair [143,166,167] 

The loss of alkenes from aliphatic onium ions via onium reaction comprises scission of the C-X bond and concomitant transfer of a hydrogen from the leaving alkyl moiety to the heteroatom, and a merely phenomenological description of this reaction has already been included in the preceding schemes. This seemingly simple reaction has to be regarded as a multistep process that involves ion-neutral complex (INC) intermediates. The properties of ion-neutral complexes are discussed in detail below (Chap. 6.12). Here, we restrict ourselves to the application of this concept to achieve a full and consistent explanation of the behavior of onium ions. 

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The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

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