15.4 : 质谱法：烯烃碎裂

Upon ionization, one electron from the pi bond of alkenes is displaced to form molecular ions.

As in the case of 2-methyl-1-pentene, the mass signal of the molecular ion is prominent for alkenes.

The popular fragmentation pathway in alkene molecular ions is cleavage at the allylic position since the allylic fragment is stabilized by resonance.

An alternate fragmentation occurs at the carbon-carbon bond adjacent to the unsaturated bond, resulting in alkenyl carbocations and the corresponding alkyl radicals.

Alkenes having a γ hydrogen with respect to the double bond undergo McLafferty rearrangement to yield a low molecular weight alkene radical cation and an alkene.

It is challenging to distinguish the positional and geometrical isomers of alkenes using the almost identical mass signals.