15.5 : 质谱法：环烯烃碎裂

Cycloalkene molecular ions undergo retro-Diels–Alder fragmentation.

As an example, consider the cyclohexene molecular ion, which fragments into a butadienyl radical cation and ethene.

This fragmentation pathway is analogous to the McLafferty rearrangement observed in acyclic alkenes. In both cases, the cleavage of two bonds occurs to yield an alkene and an alkene radical cation as the products.

In cyclic alkenes, the reaction proceeds without a rearrangement of the hydrogen atom. However, in acyclic alkenes, the rearrangement of hydrogen from one carbon to another occurs.

Another fragmentation observed in branched cycloalkenes is the loss of the side chain from the molecular ion.