The mass spectra of alkynes are virtually identical to those of alkenes. The molecular ion is usually more abundant, and fragmentation parallels that of the alkenes. Two differences are worth mentioning: terminal alkynes fragment to form propargyl ions (m/z 39), and can also lose the terminal (or an a-) hydrogen, yielding a strong M - 1 ion. These two modes of fragmentation are outlined in Scheme 8 for 1-butyne, and the complete mass spectrum of 1-butyne is given in Figure 11.
Scheme 8. Mechanism of fragmentation for 1-butyne.
An alternative way to describe the loss of hydrogen radical from an alkyne would involve a 1,2-hydride shift (converting a vinylic radical cation to a more stable allylic radical cation) that subsequently loses hydrogen radical to give the M - 1 ion. This alternate mechanism is outlined in Scheme 9.
Scheme 9. Alternate mechanism of fragmentation for 1-butyne.
Either mechanism provides an explanation for the formation of the M - 1 ion. The mechanism in Scheme 9 does, however, provide a plausible explanation for the loss of hydrogen radical from internal alkynes (such as 2-butyne) the mass spectrum of which does indicate a relatively abundant M - 1 ion.
Figure 11. Mass spectrum of 1-butyne. (Adapted from NIST Mass Spec Data Center, S.E. Stein, director, "Mass Spectra" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, June 2005, National Institute of Standards and Technology, Gaithersburg MD, 20899 (http://webbook.nist.gov))
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