The molecular ions of cycloalkenes undergo fragmentation via a retro-Diels–Alder reaction.

The reaction proceeds via the cleavage of two carbon-carbon bonds in the cycloalkene to yield ethene. The remaining part of the cycloalkene structure is a dienyl radical cation with a molecular weight of 28 u lower than the molecular ion. This fragmentation pathway is similar to the fragmentation of cycloalkanes releasing ethene, differing only in the resultant radical cation. An alkyl species forms from the cycloalkane, whereas a dienyl species originates from the cycloalkene.

Retro-Diels–Alder reactions of cycloalkenes are also similar to the McLafferty-rearrangement-type fragmentation observed in acyclic alkenes with hydrogen on the γ carbon. In both cases, cleavage occurs at two positions, yielding an ethene molecule and an alkenyl species. In acyclic alkenes, the cleavage occurs at one carbon-carbon bond and a carbon-hydrogen bond, along with rearranging hydrogen from one carbon to another. In cyclic alkenes, cleavage occurs at two carbon–carbon bonds without any atomic rearrangement. Figure 1 depicts the fragmentation of cyclohexene via a retro-Diels–Alder reaction.

Figure 1: Fragmentation of cyclohexene via a retro-Diels–Alder reaction.

In addition to the retro-Diels–Alder reaction, branched cycloalkenes exhibit fragmentation via cleavage of the side chain to form a cycloalkene cation. For example, the fragmentation of 1-methyl-1-cyclohexene via a retro-Diels–Alder reaction and side-chain cleavage are shown in Figure 2.

Figure 2: Fragmentation of 1-methyl-1-cyclohexene via (top) a retro-Diels–Alder reaction; (bottom) side-chain cleavage.

The corresponding signals of the isoprenyl radical cation and the cyclohexenyl cation are evident in the mass spectrum of 1-methyl-1-cyclohexene shown in Figure 3.

Figure 3: Mass spectrum of 1-methyl-1-cyclohexene.