Unimolecular reactions of ionised 4-methoxyheptane

Abstract

The dissociation of metastable ionised 4-methoxyheptane has been examined by means of extensive ²H-labelling experiments. Only two significant fragmentations are observed, corresponding to loss of either methanol or propane. Each of these processes involves a hydrogen transfer step in which a pronounced site-selectivity is observed. However, the site-selectivities found for the two fragmentations are distinctly different. Expulsion of methanol, which is characterised by a rather small kinetic energy release, takes place after a unidirectional 1,4-hydrogen transfer to oxygen has occurred; this step is subject to a very large isotope effect. In contrast, loss of propane involves ejection of an intact propyl group, together with an α-hydrogen atom from the second propyl group, via a formal 1,2-elimination. Primary and secondary isotope effects of comparable magnitudes intervene in propane loss from ionised methoxyheptanes in which either or both propyl groups carry ²H-labels on the α-carbon atom. The possibility that ion–neutral complexes are important in the reactions of these C₈H_18–nD_nO⁺˙ species is discussed.