Loss of methyl from [H2CC(OH)CH3]+˙ ions prepared by electron impact ionization of unstable 2‐hydroxypropene

Unstable 2‐hydroxpropene was prepared by retro‐Diels‐Alder decomposition of 5‐exo‐methyl‐5‐norbornenol at 800°C/2 × 10−6 Torr. The ionization energy of 2‐hydroxypropene was measured as 8.67±0.05 eV. Formation of [C2H3O]+ and [CH3]+ ions originating from different parts of the parent ion was examined by means of 13C and deuterium labelling. Threshold‐energy [H2CC(OH)CH3]+˙ ions decompose to CH3CO++CH3˙ with appearance energy AE(CH3CO+) = 11.03 ± 0.03 eV. Higher energy ions also form CH2COH+ + CH3 with appearance energy AE(CH2COH+) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C2H3O]+ ions containing the original methyl group and [CH3]+ ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate‐determining isomerization [H2CC(OH)CH3]+˙ →[CH3COCH3]+˙, followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.