Effect of protonation exothermicity on the chemical ionization mass spectra of some alkylbenzenes

Abstract

Abstract The H 2 , N 2 /H 2 , CO 2 /H 2 , N 2 O/H 2 , CO/H 2 and CH 4 chemical ionization mass spectra of thirteen C 8 to C 11 alkylbenzenes are reported. Characteristic hydride and alkide ion abstraction reactions are observed with all reagent gases. The major fragmentation reactions of [MH] + are olefin elimination to form a protonated arene and arene elimination to form an alkyl ion. From the effect of structure and protonation exothermicity it is concluded that rearrangement of primary alkyl groups to the more stable secondary or tertiary structure occurs prior to alkyl ion formation. A detailed fragmentation mechanism for protonated arenes is proposed. The ‘effective’ proton affinity of the methane‐derived reagent system is estimated to be ∼556 kJ mol −1 .