The fragmentation of alkynes preferentially occurs at the carbon–carbon bond between the α and β carbon of the alkyne bond to generate a 3-propynyl cation (or propargyl cation). In terminal alkynes, there is the only type of fragmentation that yields the 3-propynyl cation. The unsubstituted 3-propynyl cation exhibits a peak at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted. For example, 2-pentyne fragments into methyl-substituted 3-propynyl cation, which subsequently features as a mass signal at the mass-to-charge ratio of 56. The 3-propynyl cation is resonance-stabilized, and the corresponding mass signal is intense in the mass spectrum of alkynes. Figure 1 shows the fragmentation of 1-pentyne into an ethyl radical and a resonance stabilized 3-propynyl cation.
Figure 1: Fragmentation of 1-pentyne molecular ion to yield the 3-propynyl cation.
Another fragmentation pathway observed in terminal alkynes is the loss of hydrogen from the terminal sp carbon. This results in a mass signal with a mass-to-charge ratio of one unit less than the molecular ion. This M–1 peak is very intense and becomes the base peak in the mass spectra of many alkynes, so it can be used to identify alkynes.
Figure 2: The mass spectrum of 1-pentyne.
Alkynes typically fragment to form the 3-propynyl cation or propargyl cation, which is resonance-stabilized.
The alkynes can be either terminal or internal.
In terminal alkynes, the 3-propynyl cation signal appears at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted and appears at a higher mass-to-charge ratio.
Another type of fragmentation frequently observed is the loss of the hydrogen atom from the terminal alkyne carbon bearing the triple bond.
So, terminal alkynes exhibit an intense mass signal at a mass-to-charge ratio of 1 u lower than the molecular ion of the analyte molecule. This peak is denoted as the M–1 peak.
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