Unimolecular reactions of ionised ethers

Abstract

The 12.1 eV 75 °C electron ionisation mass spectra of 25 saturated dialkyl ethers containing between six and eight carbon atoms are reported and discussed in energetic terms. Dissociation by α-cleavage of M⁺˙ typically gives rise to intense peaks in these spectra. However, rearrangement processes, leading to eventual 1,3-O-alkyl shifts and subsequent ethyl radical loss, are also important for ionised ethers in which both alkyl groups have primary structures. Other significant ions in the spectra of n-C_n H_2n+1 OR ethers include those formed by loss of ROH or C_n H_2n–1˙ from M⁺˙. Ionised ethers containing one secondary alkyl group tend to dissociate by α-cleavage of this group, or to a lesser extent by loss of the corresponding alkane, without undergoing such extensive rearrangements. Distonic ions and ion–molecule complexes play key roles in the isomerisation and dissociation steps involved in these fragmentations. Simple cleavages dominate the spectra of ethers containing a tertiary alkyl group. These ethers show very small or negligible M⁺˙ peaks. Ions corresponding to rupture of the C–O σ-bond, with charge retention by the hydrocarbon fragment, appear in the spectra of some ethers. There is evidence, however, that these ions may originate, at least in part, by secondary dissociations of the primary daughter ions, rather than by direct decomposition of M⁺˙.