The mass spectra of most alkenes show distinct molecular ion peaks. This is probably due to the loss of an electron in the p bond, leaving the carbon skeleton relatively undisturbed.
Alkenes usually form fragments corresponding to CnH2n+1+, CnH2n+, and CnH2n-1+ (the latter two fragments are more intense). It is very difficult to locate the position of the double bond in an alkene because of its facile migration in the fragments. For this reason, the mass spectra of alkene isomers are nearly identical and almost impossible to discriminate between. The only exception would be terminal alkenes which fragment to form an allylic carbocation with m/e = 41. The fragmentation mechanism for 1-butene shown in Figure 14 illustrates this point. The complete mass spectrum for 1-butene is given in Figure 15.
Figure 14. Mechanism of fragmentation for 1-butene.
Figure 15. Mass spectrum for 1-butene.
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