Gas-phase unimolecular chemistry of ethyl butyl ketone cations

Mass spectra of the four isomeric ethylbutyl ketones, C2H5CO‐n‐C4H9 (1), C2H5CO‐iso‐C4H9 (2), C2H5CO‐sec‐C4H9 (3) and C2H5 CO‐t‐C4H9 (4), are reported. The structure of the fragment ions produced from precursors of both high and low internal energy were identified by collision experiments in a six‐sector mass spectrometer. Metastable molecular ions 1+˙ and 2+˙ behave identically in expelling CH3˙, C3H6 and C2H5˙ to produce the same fragment ions: [C2H5C(OH)CHCHCH3]+, [C2H5C(OH)CH2]+˙ and a mixture of [CHOHCHCHC2H5]+ and [CHOHCHC(CH3)2]+, respectively. The key isomerization step between 1+˙ and 2+˙ is a 1,2‐enol–olefin shift. Metastable molecular ions 3+˙ and 4+˙ give essentially ethene and ethyl losses, respectively; the fragment ions are [C2H5C(OH)CHCH3]+˙ and [t‐C4H9CO]+. Keto–enol isomerization also occurs to a significant extent for ions 1+˙ and 2+˙ and 3+˙. In contrast, 1,2‐ or 1,3‐ethylhydroxycarbene migrations are only marginal processes.