Mass-Spectral Fragmentation of Phenyl-n-Alkanes

Abstract

The fragmentation mechanisms for the 1-phenyl and 2-phenyl alkanes were found to differ from those having substituted two or more carbon atoms from the terminal carbon atom. The latter series of compounds have two competing primary fragmentation reactions involving loss of either alkyl group. The loss of the smaller alkyl group has the lower appearance potential, but the competing reaction to lose the larger alkyl group increases in yield more rapidly at the higher electron energies, and it has greater intensity at 70 eV. The ratio of intensities of these two ions correlates with the relative position of the phenyl group from the center of the chain for phenyl alkanes of different carbon numbers. Subsequent decomposition of these primary ions is by successive loss of olefins of C3 through C6. The 2-phenyl compounds differ in that subsequent skeletal decomposition of the CH3C+HC6H5 is less probable, thus making this ion the major ion instead of C7H7+. The 1-phenyl alkanes decompose directly to C7H7+ for the lower-carbon-number compounds, while the larger molecules form toluene ion followed by hydrogen-atom loss to C7H7+ and other fragmentation reactions. Partial isomerization to the 4-phenyl compounds is suggested. Attempts to explain the higher yield of the higher-energy primary path for the internally substituted compounds within the framework of current theoretical models were unsuccessful. It is suggested that the usual method of applying the quasiequilibrium assumption is not valid for molecules of this size and degree of localization of charge and energy.