Mass spectrometry of alkylbenzenes and related compounds. Part I. Gas‐phase ion chemistry of alkylbenzene radical cations

Abstract

From the vantage point of an organic chemist, many previously obscure fragmentation and isomerization processes of gaseous organic ions have become chemically reasonable reaction pathways. The term scrambling, used formerly as a synonym for mechanistic dilemmas concerning crazy atoms, has almost vanished from mass spectrometry literature in recent years, mostly because of the recognition of well-defined ionic intermediates and reaction mechanisms occurring before, during, or even after the actual cleavage of a molecular bond. The conceptual links between organic mass spectrometry and general organic chemistry have grown in many ways. In this respect, positively charged aromatic species have played a key role, owing to their inherent stability. The investigation of alkylbenzenes, in particular, has been a central focus of mass spectrometry since its application to the analysis of petroleum and gasoline in the mid-twentieth century, and has provided much insight into the intrinsic stability and reactivity of organic ions. Besides, and in connection with, the evergreens of organic mass spectrometry (e.g., the C7H7+/C7H8+' ion problem and the McLafferty rearrangement), new information has emerged during three decades of investigating the mass spectrometry of alkylbenzenes. This two-part review summarizes the mass spectrometry of alkylbenzenes and related compounds. Part one collects the gas-phase chemistry of ionized alkylbenzenes and part two describes that of protonated alkylbenzenes. This is in part due to the different relations to organic chemistry of the open-shell, radical ions (M+') on the one hand, and closed-shell ions such as [M+H]+, on the other. These two classes of ions also have different relevance to analytic applications of mass spectrometry. The radical ions of alkylbenzenes are discussed in Part I, owing to their historical priority and their broad occurrence in standard electron ionization (EI) mass spectrometry. Protonated benzene and alkylbenzenes (al