The structure and mechanism of formation of C5H9O+ from ionized phytyl methyl ether

The recent proposal that ionized phytyl methyl ether[C16H33(CH3)C=CHCH2OCH3+.] undergoes an allylic rearrangement to ionized isophytyl methyl ether [CH2 = CHC(C16H33)(CH3)OCH3+.] before elimination of an alkyl radical is discussed. Both literature precedent and new results in which the structure of the [M - C16H33−]+ fragment ion is established by comparison of its collision-induced dissociation mass spectrum with the spectra of isomeric C5H9O+ ions of known structure are inconsistent with this proposal. The forma Hon of CH3CH=CHCH=O+CH3 by loss of a γ-alkyl substituent without skeletal isomerization rather than CH2=CHC(CH3=O+CH3 after allylic rearrangement is explained in terms of a mechanism that involves two 1,2-H shifts, followed by σ-cleavage of the resultant ionized enol ether, C16H33(CH3)CH-CH=CHOCH3+.