A combined field ionization kinetics, collision-induced, and stable isotopic labelling study of the ethene and ethyl (and subsequent) eliminations from the molecular ion of 2-ethylbutanoic acid

Abstract

It is shown by field ionization kinetics that the ethene elimination from ionized 2-ethyl-butanoic acid is the most dominant channel at molecular ion lifetimes ⩽10 −9 s. This channel, however, becomes rapidly less important with respect to ethyl elimination at molecular ion lifetimes ⪢10 −9 s. Both eliminations occur without any detectable exchange between hydrogen or carbon atoms from different positions as shown by specific 2H- and 13C-labelling. The same observations are made for molecular ions decomposing in the metastable time frame of 10 −6 to 10 −5 s. On the basis of collision-induced dissociation experiments, it is demonstrated that ∼95% of the (MC 2H 5) + ions have the structure of carbonyl oxygen-protonated crotonic acid which, in line with the 2H- and 13C-labelling, are formed by a successive, irreversible hydrogen shift from C-3 to the carbonyl oxygen and cleavage of the C-2C′-3 bond to eliminate ethyl. The remaining ∼5% of the (MC 2H 5) + ions have the structure of carbonyl oxygen-protonated methacryclic acid. In line with the 2H- and 13C-labelling results, these ions are generated by a successive, irreversible hydrogen shift from C-3 to the carbonyl oxygen, migration of the C(OH) 2 group from C-2 to C-3, a hydrogen shift from C-3 to C-2, and eventual cleavage of the C-2C′-3 bond to eliminate ethyl. Further metastable decompositions of the (MC 2H 5) + ions correspond to eliminations of molecules of water, C 2H 2O, and C 2H 4O. The water molecule contains the original hydroxylic hydrogen and one of the hydrogen atoms of C-3. The eliminated C 2H 2O molecule contains the C-1 and C-2 atoms, while the eliminated C 2H 4O molecule contains the C-3 and C-4 atoms. Combined with the obtained 2H-labelling results, strong support, if not evidence, is provided for the intermediacy of ion/molecule complexes during the eliminations of C 2H 2O and C 2H 4O from the (MC 2H 5) + ions.