The mass spectra of alkynes are virtually identical to those of alkenes. The molecular ion peak is intense, and fragmentation parallels that of the alkenes.
Two differences are worth mentioning: terminal alkynes fragment to form propargyl ions (m/e = 39), and can also lose the terminal hydrogen, yielding a strong M-1 peak. These two modes of fragmentation are outlined in figure 18 for 1-butyne, and the complete mass spectrum for 1-butyne is given in figure 19.
Figure 18. Mechanism of fragmentation for 1-butyne.
Figure 19. Mass spectrum for 1-butyne.
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