Mass Spectrometry: Alkene Fragmentation

Alkenes lose one electron from the unsaturated π bond upon ionization and form stable molecular ions. Further fragmentation of alkenes occurs through three different reaction pathways. The most prominent fragmentation is the cleavage at the allylic position. The resultant allylic carbocation is resonance stabilized. In the mass spectra of terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. In the internal alkenes, where there are two choices of allylic cleavage, the cleavage preferentially generates a more substituted alkyl radical as a co-product. Figure 1 shows the fragmentation of 2-methyl-1-pentene at the allylic position.

Figure 1. Fragmentation of 2-methyl-1-pentene at the allylic position.

Another fragmentation pathway occurs at the carbon-carbon bond adjacent to the unsaturated bond, yielding an alkenyl carbocation and a corresponding alkyl radical. The fragmentation preferably occurs to generate a more stable alkyl radical. For example, the fragmentation of 2-methyl-1-pentene can occur on either side of the unsaturated bond, as shown in Figure 2. The mass signals of resultant alkenyl carbocations are shown in Figure 3. The relative intensity shows fragmentation leads to propenyl cation and propyl radicals are favored.

Figure 2. Fragmentation of 2-methyl-1-pentene at the adjacent carbon of unsaturated bond.

Figure 3. Mass spectrum of 2-methyl-1-pentene.

The alkenes possessing γ hydrogen with respect to the unsaturated bond undergo McLafferty rearrangement. The McLafferty rearrangement occurs when the γ hydrogen transfers to the double bond in a cyclic six-membered ring. This leads to the cleavage of bonds between the α- and β-carbons and thereby produces two fragments: a lower molecular weight alkene radical cation and a neutral alkene. This rearrangement, significant in mass spectrometry, helps generate specific fragment ions, aiding in identifying the structure of alkenes. Figure 4 shows the fragmentation of 2-methyl-1-pentene via McLafferty rearrangement.

Figure 4. Fragmentation of 2-methyl-1-pentene via McLafferty rearrangement.