6.9.4 Fragmentation of Alkenes

The molecular ion of alkenes, is usually distinct especially in compounds containing multiple double bonds.  Alkene fragments, like alkane fragments are situated in clusters 14 units apart.  In alkenes, the C_nH_2n-1 and CnH2n peaks are more intense than the C_nH_2n+1 peak of alkanes.

         The presence of double bonds also allows for the production of resonance-stabilized cations.  Allylic cleavage results in an allylic cation.

         Determining the position of the double bonds in the sample molecule is especially difficult and usually requires reference spectra because of double bonds migration.  Cyclic alkenes also undergo a retro-Diels-Alder fragmentation by the following mechanism.

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Section 6.9.5

©Dunnivant & Ginsbach, 2008