Oxonium ions, mass spectra

Example The El mass spectra of 1-hexanol, Mj = 102, and 1-hexene, Mr = 84, show close similarity because the molecular ion peak is absent in the mass spectrum of hexanol (Fig. 6.40). However, a more careful examination of the hexanol spectrum reveals peaks at m/z 18, 19, 31, and 45 that are absent in the hexene spectrum. These are due to H20, HsO and to oxonium ions (H2C=OH and H3CCH=0H in this case) which are reliable indicators of aliphatic alcohols and ethers (Table 6.8). 

The mass spectrum of diethyl ether appears in Figure 14-5. The four most abundant ions correspond to the molecular ion, loss of an ethyl group, a cleavage, and loss of an ethylene molecule combined with a cleavage. All these modes of cleavage form resonance-stabilized oxonium ions. 

Despite the characteristic features in the spectra of furanoses, some caution (24) should be exercised to exclude the possibility of rearrangements when interpreting the mass spectrum of an unknown. It has been reported (25) that isomeric tetrahydrofuran and tetrahydropyran diacetates have virtually identical mass spectra due to rearrangement of the latter to give after appropriate eliminations, a common cyclic oxonium ion at m/e 71. 

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